GENETIC AND SOMATIC
EFFECTSOF IONIZING RADIATION
United Nations Scientific Committee on the Effectsof Atomic Radiation
1986 Report to the General Assembly, with annexes

UNITED NATIONSNew York, 1986

NOTE

The report of the Committee without its annexes appears as Official Records of theGeneral Assembly, Forty-first Session, Supplement No. 16 (A/41/16).
The designations employed and the presentation of material in this publication do
not imply the expression of any opinion whatsoever on the part of the Secretariat of theUnited Nations concerning the legal status of any country, territory, city or area. or of its
authorities, or concerning the delimitation of its frontiers or boundaries.

UNITED NATIONS PUBLICATION
Sales No. E.86.IX.9
ISBN 92-1-142123-3004800P

ANNEXC
Biological effects of pre-natal irradiation

CONTENTSParagraphs

INTRODUCTION

J-7

Paragraphs

IV. THE FETAL PERIOD ............ .
A.

I. BASIC INFORMATION ON THE
HUMAN EMBRYO AND FETUS ...
The main phases of pre-natal
development .................. .
B. Normal del'elopment of the humanembryo, particularly of the CNS ..
I. Gross measuremencs ....... .
2. Cellular phenomena .•......
3. Biochemical development ....
C. Epidemiology and aetiology ofhuman abnormalities ........... .
I. Terms and concepts ....... .
2. Birth prevalence of congenitalabnormalities ............. .
3. The aetiology of human congenital abnormalities ....... .
D. Conclusions .................. .

Effects on the gonads and reproduction ...................... .
I. The mouse .......•........
2. The rat .................. .
3. Other animals ............ .
4. Interspecies comparisons ... .
Other miscellaneous effects ..... .
Conclusions .................. .

185-201
185-189
190-194
195-196197-201202-212213-219

INTERNAL IRRADIATION ...... .

220-266

Experimental data ............. .
I. Tritium ..•................
2. Radioactive nuclides of iodine
3. Phosphorus-32 ............ .
4. Sulphur-35 ............... .
5. Selenium- 75 .............. .
6. Fission products .......... .
7. Lead-203 ................. .
8. Thallium-204 ..•...........
9. Transuranic nuclides ....... .
Conclusions .................. .

225-260
225-239240-243244-245246
247
248-249250251252-260261-266

VI. THE ROLE OF MODIFYING FACTORS ........................... .

267-305

8-65

A.

II. THE PRE-IMPLANTATION PERIOD
A.
B.
C.

9-1 I

B.
12-40
13-23
24-30
31-40

1II. THE PERIOD OF MAJOR OR GANOGENESIS .....•..................

95-183B.

c.

................

A.

51-58
59-65

68-84
85-89
90-94

I. Skeleral malformations ......
2. Malformations of the eye ....
3. Malformations of the CNS ...
4. Other types of malformations
Data from man
Conclusions ...................
I. Experimental animals ......•
2. Human observations ........

V.

46-50

Experiments in 1·itro ........... .
Experiments in vim ............ .
Conclusions .................. .

A. Data from animals .............

C.

41-58
42-45

66-94

97-151
97-104105106-148
149-151
152-173
174-183
174-181182-183

184-219

B.

Radiation quality .............. .
I. Neutrons ................. .
2. Helium ions .............. .
3. Negative ;r mesons ......... .
Combined actions ............. .
I. Various chemical substances
2. Radioprotective and radiosensitizing treatments ...... .
Conclusions .................. .

268-279269-274
275-278279
280-300
280-290

VII. TUMOUR INDUCTION .......... .

306-394

A.

B.

C.

A.

Data from animals ............ .
I. The mouse .............•..
2. The rat .................. .

291-300
301-305

307-342
310-328329-339263

Paragraphs

3.

The dog .................. .

340-342

B.

Data from man ............... .
I. Atomic bomb survivors .... .
2. Medical exposures ......... .

343-376
350-353354-376

C.

Conclusion~ ... , . , , ........... .

377-394377-383384-394

I.
2.

Data from animals ........ .
Data from man ........... .

Paragraphs

B.

Effects and periods of maximum
sensitivity . , .................. .
C. Risk estimates ................ .

412-418
419

IX.

CONCLUSIONS ................ ..

420-435X.

RESEARCH NEEDS ............. .

436-444
!'age

VIII.

RISK ESTIMATES ........ ,.......

395-419

General .... ,..................

395-411A.

Introduction
I. A study entitled "Developmental effects of irradiation in utero" was presented in annex J of the 1977
UNSCEAR report [U2]. It reviewed most of what was
known at the time concerning such effects, both inexperimental animals and in man, but since theamount of information on the former exceeded by far
that on the latter, the study dwelt largely on theexperimental animal data.
2. Since then, new information has become available,
relating particularly to man. which has highlighted theneed to focus discussion on the human experience andto draw on any data that might set more precisely inperspective the effects and risks of irradiation in utero.
Moreover. effects such as tumour induction, whichhave been discussed elsewhere. need to be re-examinedin light of the new data if overall risk estimates for
man are to be attempted. These considerations haveprompted UNSCEAR to undertake the present study
entitled "Biological effects of pre-natal irradiation".
3. As reflected by its title, the study is meant to be
more comprehensive than the earlier one, including
effects such as tumour induction and other long-termsequelae of irradiation which were specifically excluded
from the 1977 study [U2]. It is also aimed at drawing
conclusions concerning the potential for practicalapplications, insofar as the data will allow. As theprevious study adequately covered the fundamental
aspects of the subject, the present one will simply
point out how the new data would alter previously
drawn conclusions. (Data pertaining to genetic damage
induced by irradiation in utero. damage induced
before fertilization, and some special effects such as
the induction of XO females, are to be found in annex
A to the present report.)
4. In the 1977 report [U2] the subject was divided,
mainly as a matter of convenience. into generalchapters covering the methodological aspects and themechanisms of action of radiation effects in utero.
The substantive data were 1i1ostly reviewed in separate
sections covering the pre-implantation period. theperiod of major organogenesis, and the feta! period.
Within each section. lethal. teratological. developmental and other effects were considered, information
on animals and man being grouped for the purpose of
showing qualitative similarities. Such a treatment may
not always adequately reflect the complications of the264

Tables,,, ........... ,,, ..... ,,, .. , .... ,.

342

References ......................... , .. , .

352

biological models and the variability of the data. but
any alternative sub-division of the subject matter
would also have its limitations. The format used did,
however, offer the advantages of setting the observations in some reasonable sequence with respect toembryonic and feta) development, thereby allowing
individual discussions of each major class of effect, by
drawing together all observations pertaining to thesame stage of development.
5. In the present study, the matter is reviewed again
by pre-natal stages in animals and man, in order tofacilitate projections across species. This way ofordering the subject does not imply that all suchprojections may be legitimate and justified. On thecontrary. as emphasized repeatedly in the 1977 study,
such projections (and particularly the more quantitative ones) should not be undertaken without due
regard for the specific characteristics of the biologicalsystems under comparison.
6. As in the 1977 study, the present one will consider
separately subjects such as modifying factors, internal
irradiation and tumour induction because the relevant
information does not apply specifically to any developmental stage but often refers to the whole pre-natal
period.
7. Although the main aim of the study is to review
the published data with a view to assessing radiation
effects in man, this is made difficult by the nature ofmost of the work available. which is usually directed
towards studying the mechanisms of development
rather than the radiobiologically important variables,such as dose and time. It should be noted thatquantitative information on dose-effect relationships
and on the influence of dose rate and radiation quality(which is the most valuable for the Committee's own
purposes) is still scanty. particularly at the low doses.
Moreover. it is often buried under observations on thepathogenesis of malformations or the morphology ofmalformed tissues. This. together with the lack ofsystematic information on species. tissues and effects.
may in some instances cause this new study to fallshort of its aim. However. in view of the interest thatthe field continues to command. of the new data
available, and of the relatively high risk that exposureof the conceptus in utero might entail under certain
conditions. the re-consideration of old data and thediscussion of new data is amply justified.

I.

BASIC INFORMATION ON THE HUMANEMBRYO AND FETUS

8. In order to set the following text in perspectivewith respect to its main aim. it is important to review
some basic information on the normal development of
the human embryo and fetus. (As a general reference
on this subject see [H27].) After a short introduction
on the main phases of pre-natal development invarious mammalian species. therefore, the present
chapter will focus particularly on the human central
nervous system (CNS). which is known in man to
be most susceptible to radiation-induced pre-natal
damage. New data on the gross development and thebiochemical events taking place in human ontogenesis
will be outlined. together with modern acquisitions on
the histogenesis of the brain structures. For the same
purpose. a short section will also be devoted to the
etiology and epidemiology of malformations in man,as a background to the risk evaluations discussed later(see chapter VIII).

generally characteristic of irradiation in the preimplantation phase. while neo-natal death and malformations are associated particularly with irradiation
during organogenesis. Irradiation during the feta!
stage does not normally lead to gross malformationsin the teratological sense. but rather to maldevelopment of tissues and defects of growth. particularly in
the CNS and gonads. At high doses. it may lead todeath. A systematic description of pre-natal effects ofradiation supporting these conclusions is to be foundin the 1977 UNSCEAR report [U2). That reportemphasized, by means of examples referring to different animal species and various types of effects. thatthe outcome of a given radiation exposure in respect
to both dose and time is highly dependent on thedevelopmental characteristics of each species. Thiscondition makes it difficult to extrapolate the type anddegree of radiation-induced effects across species andto formulate generalized and particularly quantitative
conclusions. However, as the developmental physiology
is similar. some useful comparisons may be made.
B.

A.

THE MAIN PHASES OF PRE-NATAL
DEVELOPMENT

9. The development of the conceptus in mammals is
usually (and very schematically) divided into threemajor phases: the pre-implantation phase. the periodof major organogenesis and the phase of feta! development. Implantation of the early embryo into theuterine mucosa marks the separation between the firsttwo periods. Although implantation is given in thereports at a certain day post conception (p.c.). itshould be remembered that the complete process,from the initial contact of the blastocyst with theuterine wall to its firm attachment through the erosionof the uterine epithelium by the trophoblast and theestablishment of a placental blood flow. may last from
one to one and a half days in the mouse and a few
days in man. A clear-cut separation between theembryonic and the feta! periods is even more difficult
since the transition is marked by the end of differentiation and the growth of the newly formed organs in
an animal which has attained the characteristic morphological features of the species. It is estimated that by theend of the embryonic period. that is by the 8th postovulatory week, the human embryo (measuring about
30 mm in crown-rump length and weighing 2-2. 7 g)
already possesses more than 90% of the more than
4500 structures described in the adult body [08].
10. Thus. the conventional limits of the three developmental periods. which are given for various animal
species in Table I. should only be regarded as roughapproximations. A tabulation of times for the development of morphological characters in some mammals
is also to be found in [S84] and [M57].
11. The division into the three phases mentioned
above corresponds approximately with significant
differences of developmental events in the embryo. It
is also suitable for a description of radiation effects,
which are very different in nature and degree within
the three phases. For example. death in utero is

NORMAL DEVELOPMENT OF THE HUMANEMBRYO, PARTICULARLY OF THE CNS

12. In order to assess whether or not there are
periods of high susceptibility in human development
in regard to any toxic agent, it is important to time theeffects induced in relation to the menstrual cycle or to
fertilization. A precise mapping of developmental
events is still, however. a major unresolved issue inhuman embryology, because experimental studies are
not possible and because one must rely on approximate
estimates of the date of conception. Gross measurements, histological investigations and biochemical
analyses on autopsy material have usually been
employed to this end, although. more recently, direct
measurements in vivo have become possible. Up-todate observations on the early development of thehuman brain and information on the staging system of
human embryology are to be found in several references[M49. 07. 08]. The last of these also provides anannotated list of the chief sources of recent information on the development of various structures of thehuman body.
I. Gross measurements
13. It should be realized that neither the weight nor
the length of an embryo are an adequate guide to
developmental status and therefore should not betaken as a "stage". which is a term based on theoverall morphology of the embryo and not on any
single measurement. Kobyletzki and Gellen [K27]
investigated the phase of development of 555 humanembryos (morphologic development, fresh weight,
crown-rump length) obtained from legal terminationsof early pregnancies and correlated it with theduration of pregnancy, age and weight of the mother,parity and number of abortions. They found it
impossible to correlate the phase of development inany single case to a reasonable degree of precisionand. as a consequence, to predict periods of gestation
where the product of conception might be at a higher
risk of exogenous embryotoxic agents.
26514. The normal and abnormal development of early
human embryos was also studied by Nishimura andcollaborators. In a first publication [N 5] containing
data on 1213 intact embryos, standards of normal
development (with respect to crown-rump length,
body weight and external form) were establishedwhich were thought to be more reliable than thestandards usually cited. Remarkable variation wasnoted between clinical age and these growth indicators.
It was concluded. therefore. that the general developmental stage was more reliably established by suchobjective parameters than by the clinically establishedage. Dead embryos were more frequent in womenwith genital bleeding during pregnancy. The prevalencesof externally malformed embryos were 2.35% (44/ 1870)
at Carnegie stages [08] 16-18. and 2.12% (46/2169) atstages 19-23 [N 14]. Malformations observed included
exencephaly, cyclopia, myeloschisis, cleft lip andvarious limb abnormalities. It was noted that theincidence of most of these defects was far higher thanthat observed in newborn infants. In a second paper[N6] reporting on 90 normal specimens from healthy
pregnancies at Carnegie stages 7-13, they correlated
clinically assessed age and growth and found remarkable individual variability in embryos at stages 11-13,
but an overall accordance with observations publishedby other investigators.
15. Jakobovits et al. [J9] also addressed the problemof establishing reliable standards of the rate of growthof human embryos and fetuses. The material examined
comprised 354 embryos and fetuses ranging from 20
to 200 mm crown-rump length, obtained by therapeutic abortion in three different countries. Comparison of the measurements taken with the widely used
standards of Streeter [S55] (see Table 2) indicated thatthe discrepancy observable at the embryonic stages
diminishes gradually into the feta! period and eventually
becomes quite insignificant. The discrepancies may be
explained on the ground that Streeter's material [S55]contained data on a significant number of spontaneousabortions in the early embryonic period when the feta!
age may be misjudged by the possible occurrence ofmenstruation after fertilization. The embryos would
then be judged to be younger than their real age.
When the number of embryos below the standard
morphological age for their ovulation age is comparedwith the number of those above standard. it is foundthat the morphological development of a large proportion of malformed embryos is below standard[Nl4). The paper by Jakobovits et al. [J9] discusses
the sources of variability between and within series
and concludes that the standards of intra-uterine
growth must be considered of limited value mcorrelating age and measurements in single cases.

16. Skidmore [S43] also examined the crown-rumplength of 483 fixed human embryos (Carnegie stages
6-23) and calculated the median and predicted mean
length. The results were compared with those ofothers and found to be in good agreement. Weightstandards for the whole body and organs in humanfetuses below 500 g (or 200 mm of crown-rumplength) were described by Tanimura [Tl5].
266

17. In a study of a few hundred staged humanembryos examined for a variety of gross and microscopic morphological features, Moore et al. [1\130)investigated the significance of a relationship between
estimated post-ovulatory age and size or maturity ofthe embryos. as well as the timing of the developmentof major landmarks. They concluded that the embryo
develops to I mm at the 28th post-menstrual day,after which its crown-rump length increases at the rateof approximately 0.7 mm per day up to 80 mm. Coitaland post-ovulatory ages estimated from menstrual
data correlated significantly (P < 0.001) with crownrump length and the Carnegie stage system. However,the post-ovulatory age, even after adjustment for lackof synchrony between menstrual period and time after
ovulation. showed high variability (standard deviation,
10.5 days) indicating poor predictability for single
specimens. On the other hand, morphological featuresappeared within a narrow interval. The data showedthat during the portion of embryogenesis studied (upto about 100 days of post-ovulatory age), there is a
close synchrony of events, each of which occurs very
rapidly in an apparently stereotyped sequence withlittle statistical variation. The time of normal development may be determined accurately if the date of asingle or most likely fertile coitus is known. To assess
embryonic age in individual cases on the basis of
menstrual data, however. is a very unreliable systemof dating.

18. With the development of new ultrasonic techniques. it is now possible to measure in viva embryonicdevelopment as a function of pre-natal age in order tocompare such findings with length/age tables incurrent embryological usage and to score for thepresence of congenital malformations [C21, C22].
Drumm and O'Rahilly [D 16] made one such comparison on a sample of 44 normal women studied
between 33 and 86 days from ovulation. They concluded that, in the embryonic period proper (i.e., up
to 8 post-ovulatory weeks). the measurements in vivo
give. a slightly greater length for a given age than thosenormally assigned in embryology, possibly due todifferences between in vivo and post-mortem length or
to imprecision in estimating the data from anamnestic
findings. At later stages in the period investigated(early feta!) a good correspondence was found between
in vivo ultrasonic and morphological measurements
on fixed specimens.
19. Dobbing and Sands [D 17] analysed. as a function of pre- and post-natal age. the changes ofbiparietal diameter, head circumference and brain
weight in m·an. The shape of the curves was shown tobe different for each parameter and the differenceswere compared and justified. The significance of thesefindings in relation to limiting influences. such as
nutritional deficiencies. and to the capacity forrecovery, was discussed. Jordaan [J7] also investigated
the relationships between body and brain weight as afunction of intra-uterine age in man and showed thatthe ratio of brain/body weight is a useful index forappraising the quality of growth in cases at risk of
intra-uterine growth retardation.

20. Burdi et al. [BI4] examined organ weight patterns
in the course of human feta! development. They
studied 80 singleton abonuses assessed as being
morphologically normal and typical for age, coveringa period between 13 and 31 weeks of fertilization.
They obtained regression formulae characterizing the
growth of JO selected fetal organs relative to total
body weight (Table 3). According to these authors,
feta! body weight (rather than the more commonlyused crown-rump length or age since last menstruation)
appears to be the best reference parameter forassessing the growth of the various organs and forworking out quantitative formulae for such growth.
Data analysis showed that, of the JO organs monitored, the brain was the organ the weight of which ismost highly correlated with the changes of eithercrown-rump length or total body weight.
21. In a series of 72 autopsies, Friede [F8] timed
some readily identifiable histological landmarks usefulfor dating cerebellar development between 24 weeks ofgestation and 13 months of life and corn paredhistological events in the cortex with overall weightdata for the cerebellum. This work shows that by the
9th post-natal month the human cerebellum has
attained only 50% of its adult weight, and up to about
this time active growth of the molecular cell layer
takes place to its adult dimension. The doubling of thecerebellar weight after the 9th post-natal month isprobably due more to a continuing process of myelin
formation in the white matter than to continued cellproliferation in the cortex.
22. Table 4, taken from O'Rahilly [08], summarizes
information of a quantitative and morphological natureon the sequence of developmental stages in the humanembryo. The author, among other points, emphasizes
that the most useful single measurement for assessing
the age of an embryo or fetus is the crown-rump length,
which is found to agree closely with that determined
ultrasonically [D 16]. Alternatively, the greatest lengthof the embryo may be used. which is a practicablemeasurement from 2 post-ovulatory weeks (stage 6)through the remainder of the embryonic and also in
the fetal period [011]. The 23 stages mentioned inTable 4 refer to the embryonic period only, because
no staging system has yet been devised for the fetus.
Finally, embryonic ages are now expressed in postovulatory weeks or days: little or no attention is paidin modern embryological work to menstrual data.
23. The ectodermal tissue that will eventually form
the various brain structures may be identified in thehuman embryo at about 6 days after conception [07].
It appears as a neural plate which develops into aneural groove at about 18 days. By day 20 [M49] thethree major portions of the brain (the prosencephalon,
the mesencephalon and the rhombencephalon) can be
identified. The neural groove becomes a neural tube atabout 22 days p.c. The open ends of this structuresoon close. As the tube grows in length and thickness.
it flexes, particularly at the rostral end, while otherstructures (otic vesicle, optic vesicle) begin to appear.
By stage 15 (about 5 weeks p.c.), the neural tube iswell differentiated and the embryonic cortical zones,
which will finally give rise to the cerebral hemispheres,
begin to form [S21].

2.

Cellular phenomena

24. The cellular events leading to the formation of
the brain cortex in mammals have now been describedin considerable detail [S2 l ]. In order to understandthese events it is necessary to have in mind themorphology of the embryonic cortical zones mentioned
in the previous paragraph and shown in Figure I. The
following zones may be recognized: (a) a ventricularzone, made by the ventricular cells which replace thespongioblasts and the germinal cells; (b) a marginalzone, the outermost cell-sparse zone from which cell
nuclei are excluded during their movements; (c) anintermediate zone. developing between the previous
two, as postmitotic neurons move outwards andafferent axons grow between regions: and (d) asubventricular zone. situated at the junction between
the ventricular and the intermediate zones, containing
cells giving rise to the macroglia and special classes ofneurons.

Figure I. Semi-diagrammatic drawing of the development of
the basic embryonic zones and the cortical plate. Abbreviations:CP, cortical plate; I, Intermediate zone; M, marginal zone;
S, subventrlcular zone; V, ventricular zone.
[S21]

25. According to the descriptions of Sidman andRakic [R23, S21], which integrate previous observations, the whole process of cell migration andcortical development in man may be summarized in a
number of stages, as in Figure II {approximate
gestational ages in weeks given in parentheses):
Stage I (7-JO). This is characterized by the initialformation of the cortical plate, starting with the
migration outward of post-mitotic ventricular cells toform a new accumulation of cells (neocortical plate)which is several layers of cells deep by the end of thestage.
Stage II ( I 0-11 ). There is a primary condensation of
the cortical plate which becomes thicker and morecompact. Neurons in the plate are immature and theirelongated axis is oriented perpendicularly to thesurface of the cortex. Migration of cells to the corticalplate from the ventricular zone through the fibre-richintermediate zone slows down by the 11 th week.
Stage III (11-13). The cortical plate becomes subdivided between an external zone of cells with densely
packed nuclei and an inner zone with widely spaced
large nuclei. The maturation of early migrated cells
and the addition of new immature neurons from a
new wave of migration accounts for this bilaminate
structure of the cortical plate.

267

STAGE 1z:

STAGE lI

<6wks

6- IOwlr.~

6•8wks10-llwh...

11·1'3wks

13-15wkt< l6wks·······

Figure II. Semi-diagrammatic drawing of the human cerebral wall at various gestational ages
listed in fetal weeks below each column. The stages refer specllically to an area arbitrarily
chosen midway along the lateral surface of the hemisphere. Because there Is a gradient of
maturation, as many as 3 of 5 stages of cortical development may be observed in different
regions of neocortex in the same letal brain. The extreme right column shows a large
interruption In the intermediate zone to signify that the lull thickness of the cerebral wall has
increased compared to earlier stages. The curve in the lower part of the figure representsroughly the lime and relative magnitude of the waves of prollferation and migration ofneuronal cells. Abbreviations: CP, cortical plate; Im, intermediate zone; I.Im and O.lm, innerand outer intermediate zones, respectively; M, marginal zone; SGL, subplal granular layer; S,
subventrlcular zone; V, ventricular zone; wks, age In feta! weeks.
[521)

Stage IV (13-15). There is a progressive thinning of
the ventricular zone as many of its cells moveout wards and less of the remaining continue to divide.
The cortical plate becomes at the same time morehomogeneous in appearance, due possibly to an
enlargement of the neurons. As relatively few cells
enter the conical plate. its separation from theintermediate zone appears sharper. The intermediatezone has in the meantime become over 5 mm thick.

zone persists as a germinal area and continues to
produce medium-sized and small neurons as well as anincreasing number and variety of glial cells, and thatthese processes of cell production may continue up to
the middle of gestation. Cell migration. on the other
hand, may continue throughout the first year of extrauterine life, but the extent and the timing of theseprocesses of cell production and migration are still
uncertain.

Stage V ( I 6-after birth). This stage, lasting well into
post-natal life is not precisely defined and includes alarge range of interrelated developmental events.
There is controversy as to the amount and length ofneuronal migration during this stage. Although proliferation of the ventricular zone is decreased by thefifth month, many neurons which had been generated
earlier are still in the process of migrating to thecortical plate. There is evidence that the subventricular

26. The curve in the lower part of Figure II is meant
to represent. roughly, the two waves of proliferationand migration of neuronal cells, the first one being of
shorter duration and involving less cells than thesecond. These waves are taking place in the humancerebral cortex around stage I and stages II-IV.
respectively.

268

27. ln the developing cortex of the cerebellum, cell
relationships arc very similar, but the migration of
granular cell neurons proceeds from the externalsurface inwards past the dendrites and somas of the
Purkinje cell's, guided by the radial arrangements of
the Bergmann glial fibres. There arc also other special
migration patterns applying to some zones of theCNS.
28. It is not yet clear whether cell migration might
involve the same mechanisms during the initial stagesof cortex development, or during later stages whencells move along distances of several millimetres.
However, it is now well established that migratingneurons find their way to the cortex by following theradially oriented fibres of the glial cells as guides. The
later generated cells take positions external to those of
their predecessors and the final position of cellelements along the radial sectors may be influenced byafferent axons. Cell surface properties may be involvedin controlling migration. but their nature still remains
undefined [R24].
29. Another paper by Rakic [R9) added imponantdetails to the basic phenomena described, which are ofsignificance in respect to the specificity of origin andinterconnections of neurons. A systematic analysis of
the neuron division and migration carried out by3 H-thymidine labelling in the Rhesus monkey has
made it possible to establish that successive generationsof neurons originating in a given location at theventricular surface migrate along the same glial fibres
and settle themselves eventually in the same radial
cortical column. Thus, the glial fibres, in addition to
directing neuronal migration, may be of importance inpreserving the topographical. and therefore functional.
relationship of clonally related neurons. They are also
important in reproducing the mosaicism of the ventricular zone in which neurons originate at the level ofthe expanded surface of the cerebral cortex. It is notknown whether the killing of glial cells may impair theorderly migration of the neurons. In summary, althoughmigration of cells from one part of the body to another is
a common phenomenon in ontogenesis, there are
unique features associated with cell migration taking
place in the developing brain. First, as it only occurs
after the last cell division. it involves neurons that will
not reproduce any longer. Secondly, it involves active
cell movement following selected pathways along theglial fibres. Finally. it exhibits precise space and timedependencies [R24].
30. In addition to neuroblast division, migration anddifferentiation. neuronal death has been shown to playa role in shaping the final morphology and function of
the adult brain. Various types of programmed neuronal
death have been described in various animal species[C 17]. One is related to the target area to beconnected by certain types of neurons and is to someextent adjustable to the size of this area: another isrelated to the elimination of errors that may occur inconnecting various areas of the brain; a third relates
to the selective response of neurons to circulating
hormones; and a fourth is under tight genetic control.
Although there is doubt whether, and to what extent.
all these types may be present in the developing

mammalian brain, it appears that programmed deathof neurons is directed to the adjustment of eachneuronal population to the size or functional needs of
its projection field and to the elimination of neurons
whose axons have grown to the wrong target or to thewrong region within the target area. Other regressive
events leading to the elimination of some neuronalconnections initially formed, without death of theparent cell, are also known, and they appear to be
directed to the fine tuning of neuronal wiring. All
these phenomena are now recognized to play a majorrole in the final maturation of the CNS. but nothingis known yet about their possible relationship or
importance to radiation-induced cell killing.

3.

Biochemical de,·elopment

31. Brain maturation may also be followed by
mapping of biochemical events as a function of time.
Howard et al. [Hl4] studied the content of desoxyribonucleic acid (DNA) in 28 human feta! brains at
between 10 and 31 weeks of gestation from surgical
specimens obtained in Sweden and in the UnitedStates. The cerebrum and cerebellum were analysed
separately and, in addition to DNA and ribonucleic
acid (RNA). cholesterol was also estimated as anindex of the biochemical maturation of the organs.
The total content of DNA in the cerebrum increased
exponentially between JO and 14 weeks of gestation,
after which the rate gradually declined to a linear
function; there was no sign that growth would stop up
to 31 weeks of gestation (Figure III). By contrast. theDNA content of the cerebellum kept increasing
exponentially throughout the same period (Figure IV).
300

100
en
E

<
z

C

..., 30

•

<
a:

co
.....
a:
.....
u

O

Specimens from UppsalaSpecimens from Baltimore

10

3

10

14

18

22

26

30

GESTATION AGE (weeks)
Figure 111. The Increase of total cerebral DNA during development of the human fetus. Ordinates: DNA In mg on alogarilhmlc scale; abscissae: age In weeks from the onset of thelast menstrual period, estimated from body weights according
to Streeter. Each circle represents one brain.
Data from [H14].

269

30

10

en

.s.
<t:

z

0

3-

a:

c,::

_,
_,

w
a:,
w
a:
w

•
O

u

Specimens from UppsalaSpecimens from Ba 1timore

0.3

10

14

18

22

26

30

GESTATION AGE (weeks)
Figure IV. The Increase In total cerebellar DNA during devel·
opment of the human fetus. Each circle represents one brain.
Data from [H14].

On the assumption that the increase in DNA may
roughly correspond to an increase in the number ofnuclei, the different patterns of growth of the cerebrum
and cerebellum were interpreted to show that in someareas of the cerebrum cell division may be terminatedearlier than in the cerebellum. These data also
showed, in agreement with morphological studies, thatin man there may be considerable post-natal cell
division of neuronal precursors.
32. In the cerebrum, total RNA increased with DNA
and tissue weight. The RN A/DNA ratio. taken to be
proportional to the mean RNA per cell. showed noappreciable change at the time of maximum DNA
growth between JO and 14 weeks of gestation.
Thereafter, as the rate of DNA accumulation declined,
the RNA/DNA ratio rose progressively, probably
reflecting an increased cell differentiation. The ratio oftissue weight to total DNA, considered to be proportional to mean cell size or mean cell territory, increased
exponentially up to 30 weeks. The cholesterol/DNA
ratio. i.e., the amount of cholesterol per cell, increased
exponentially throughout the age range studied, ingood relation with the increase in cell membranesurface area associated with the developing dendritic
arborization. Consistently different patterns for all theabove quantities were observed in respect of cerebellar
development.
33. The quantitative growth of the human brain
was also followed by Dobbing and Sands [D 12];139 human samples. ranging in age from JO weeks
gestation to 7 years post-natal, were studied, together
with 9 adult brains. The whole brain and the threemajor regions (forebrain, stem. cerebellum) were
examined separately for weight. DNA. cholesterol and270

water content, in order to describe quantitatively the
spurt in brain growth. The weight of the whole brain
and its parts followed a sigmoid trend with time, witha cut-off point between 18 and 24 months of post-natal
life for whole brain and forebrain. The cut-off point
for the cerebellum was a little earlier than that of thewhole brain. The cerebellum appeared to start growing
later and to come to a plateau earlier than the rest of
the brain. Cellularity (as expressed by the amount of
DNA per g of tissue) in the forebrain and in the stem
decreased as a function of age up to birth, due todisproportionately rapid growth of cell size, cellbranching and myelination. In the cerebellum. however,cellularity increased until after birth, owing mainly toa rapid rate of cell multiplication.
34. Total cell number increased in an approximately
sigmoid fashion as a function of time in the wholebrain and all its regions. In the whole brain the cut-off
point marking the levelling-off of the sigmoid curve
occurred at around 18 post-natal months. Cerebellar
cell growth started later and finished earlier. while theforebrain curve bent over at about 2 years of age.
Figure V shows a semi-logarithmic plot of total DNA(proportional to total cell number) in the humanforebrain between about JO weeks of gestation and
4 months post-natal. The first of the two growthphases was thought to reflect rapid neuronal proliferation. the later, shallower. part was attributed to .glial
proliferation. The sharp bend of the curve at about
18 weeks of gestation does not imply that neuronal
proliferation is stopped completely at this time. northat glial cell division does not occur before, butrather that the rate at which the two processes proceed
is sufficiently different to be reflected in the overall
curve. The similarity of these data with the earlier
ones reported by Howard et al. [H 14] and shown in
Figure III is remarkable. Figure VI summarizes, forthe three different brain regions. the values of total
DNA content as a function of fetal and post-natal
ages, expressed as percentage of adult DNA values.
3

!
Q.

i

2

C

10

20
WEEKS

30

Birth

4
2
MONTHS

Figure V. Total DNA-P (proportional to total number of cells)
In the human lorebraln from ten gestational weeks to fourpost-natal months, showing the two-phase characteristics ot
pre-natal cell mutlipllcalion.
[D12]

100

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-;;;
>

...

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......
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75

0

CII

...... 50
a,

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u

Forebrainc.

Stem
Cerebellum

Q.

<
z
25
0

C

2

Birth

4

3

AGE (years)
Figure VI. Relative values for DNA-P (proportional to total
number of cells) in three human brain regions.
[012]

35. The cholesterol content is taken as an index of
myelination. For cholesterol. the growth spurt wasless sharply defined and extended for whole brain.
forebrain and stem well into 3 or 4 years after birth,
possibly up to 5 years. The cerebellum seemed toaccumulate this substance faster compared with otherregions of the brain.
36. The paper by Dabbing and Sands [D 12] adds
considerably to the previous one by Howard et al.
[H 14]. It establishes a growth spurt for human brain

development centred around the time of birth. but
extending from mid-pregnancy well into the second
post-natal year and beyond. This is at variance withwhat happens in other species: for example. thegrowth spurt is essentially post-natal in the rat, prenatal in the guinea pig and perinatal in the pig. Data
referring to the timing of the brain growth spurt invarious mammalian species taken from another paperof Dabbing [Dl3J are shown in Figure VII. Dabbing
and Sands [DJ2] commented on the length of human
feta! brain development in comparison with that ofother species and on the timing of the various
processes in relation to noxious influences such as
malnutrition. They specifically drew attention to thestriking correlation between the high rate of multiplication of neurons between JO and 18 weeks ofgestation and the period of maximum sensitivity forradiation-induced small head size and mental retardation shown by Miller and Blot [M20] in children
exposed in utero during the bombing of Hiroshima
and Nagasaki. Mole [M 10) also emphasized thiscorrelation. Although it has long been known that thetime of major organogenesis of a structure coincides
with its maximum sensitivity to irradiation [U2], theobservation of Dabbing and Sands [D 12) is importantin human radiobiology because it provides a cellularbasis for the occurrence of mental retardation.
37. Dabbing and Sands [Dl8] investigated themechanisms for the vulnerability of developing rat
brain. Starting from the notions that the brain is
especially vulnerable to growth retardation during the
growth spun, and that any deficit imposed at this timecannot be recovered at later stages. even if the

6

,__

:r
<.:>

LLI

:,:

Lo..

0
,__ 4

z:

L...I

u

a:
LLI

c..

2

-30

-20

-10

Birth

AGE

+10

+20
AGE

Figure VII.
The total brain growth spurts of 7 mammalian species expressed as first-orderderivative curves of the increase in weight with age. It should be noted (Figure VI) that different
regions of the brain develop at different times. Weights are expressed as weight gain as apercentage of adult weight for each unit of time. The units of time for each species are as follows:
guinea pig, days: rhesus monkey, 4 days; sheep, 5 days; pig, weeks; man, months; rabbit, 2 days;
rat, days.

[D13]

271

restrictive action 1s over. they attempted to study
whether growth retardation might be due to achronological delay of the growth spurt complex or.
instead, to a lesser extent of the growth spurt
occurring at a fixed chronological time. They examined,
as a function of time before and after birth, thechanges in weight, DNA and cholesterol concentrationin the brains of either normal or undernourishedgrowth-retarded animals and showed that these components of the brain growth spurt were determined by
the chronological and not by the develupmental age.
Thus, the brain has a once-only opportunity to growcorrectly and when such opportunity is restricted thedamage directly or indirectly induced may not berecovered at later stages.
38. In a re\·iew of the effects of early nutrition on
CNS growth, Winick et al. [ W I3] pointed out that theseries of critical periods occurring during brain development make this organ particularly sensitive to lackof n utriems. During the post-natal proliferative growthof some mammalian species, including man, malnutrition may interfere with cell division. In the pre-natal
life of the rat, maternal under-nutrition blocks cell
division in the feta! brain, whereas uterine vascularinsufficiency does not. This suggests that there may bedifferent types of intra-uterine growth failure. In man,severe malnutrition retards placental cell division andmay affect feta! brain development. However, thestudy of these effects is difficult because of the lack ofappropriate biochemical markers to assess fetal growthin utero. A later chapter by Dabbing [D 13] also
reviewed the problem of brain vulnerability to the lackof nutrients in relation to structural development.
39. In a short communication, Edwards [E7] pointed
out that true microcephaly is difficult to produce
experimentally because most noxious agents retard
general body growth as well as brain and skull
growth. However, some noxious agents such as certain
viruses (rubella, cytomegalovirus), antimitotic agents.
hyperthermia and radiation. act by destroying proliferating neuroblasts, and the surviving cells areunable to make up for the deficit through additional
divisions. The subsequent proliferation of glial cells
and production of myelin are also reduced in proportion to the neurons surviving. The net result is a brain
which is normal in shape and histology, but deficientin size and possibly in funccion. The lack of correlation between the size of the brain and the deficit inmental activity has also been commented upon by
Dobbing and Sands [D 18].
40. Erzurumlu and Killackey [E5] discussed thepresence of critical and sensitive periods in neurobiology, extending che concept from the morphologicalto the functional development of the CNS. In theirview, the word "critical" should be reserved for normal
development, to denote those periods during which
the action of a specific external or internal condition
or stimulus is required for the normal progress ofdevelopment; ··sensitive" periods. however, are times
in development during which a given system is highly
susceptible to the effects of harmful agents. This term
refers. therefore, to the realm of teratology. In
connection with such critical and sensitive periods, the272

authors discussed the effects of nutritional and hormonal deficiencies. of the exposure to toxins or otherharmful agents, and of stimuli deprivation during
development. They showed that it is at the time of thefastest rate of growth of a given structure thatvulnerability is at its maximum. They also illustrated,with practical examples. a general theoretical approachto the problem of critical periods and an analysis of asystem's organization and rate of development inrelation to such periods.

C.

EPIDEMIOLOGY AND AETIOLOGYOF HUMAN ABNORMALITIES

41. To keep the findings in a proper perspective when
discussing radiation-induced developmental defects inman. it is important to know the natural birthprevalence of congenital anomalies. The brief outlineof the epidemiology and aetiology of congenital
anomalies given below is based on general references[C 11. C I 4, C 18, 15, K32. L4, M45, N9, N JO. S38, U5,
V5, V8] which contain compilations of numerous
original literature reports. in addition to original
material. No effort is made to review individual
reports independently. since the object of the exercise
is simply to provide some indications of the natureand order of magnitude of the baseline occurrence ofhuman anomalies.

I.

Terms and concepts

42. The term "congenital anomaly" is used by theWorld Health Organization (WHO) [W26] in the International Classification of Diseases (ICD) to designate
structural (or morphological), biochemical and functional developmental disturbances which occur in thefetus (in this context a conceptus, irrespective of theduration of pregnancy prior to the complete expulsion
or extraction from its mother, is called fetus [W26])
from conception until birth. and which are present atbirth, whether detected at that time or not. The term
"congenital anomaly" is a very broad one andincludes a number of categories of developmental
disturbances. as follows: congenital abnormalities;
genie defects, including inborn errors of metabolism
and chromosomal aberrations; late intra-uterine infections and consequent damages, the so-called fetopathies; idiopathic intra-uterine growth retardation;
immunological diseases. e.g., mocher-fetus rhesus
blood group incompatibility: mental retardation andbehavioural deviations: congenital defects of the senseorgans and other handicaps: congenital hernias: andtumours.
43. The congenital abnormality is a structural defect,gross or microscopic, that is present at birth, whether
diagnosed at that time or not. Recently, a pathogenetically oriented and practical classification has
been recommended [S77] for defects of morphogenesis(i.e., congenital abnormalities) as follows:(a)Malformation: a morphological defect of anorgan, part of an organ, or larger region of the
body resulting from an intrinsically abnormal
developmental process:(b)Disruption: a morphological defect of an organ.
part of an organ. or a larger region of the bodyresulting from the extrinsic breakdown of, or aninterference with. an originally normal developmental process;(c)Deformation: an abnormal form, shape, or position of a part of the body. caused by mechanicalforces:(d)

Dysplasia: an abnormal organization of cells into
tissue(s) and its morphological result(s). that is. theprocess (and the consequence) of dys-histogenesis.

44. Pathogenetically, it is necessary to separate
isolated and multiple patterns of morphological defects
within congenital abnormalities [Cl8]. For example:
isolated cleft lip, with or without cleft palate, may
represent a homogeneous group of congenital abnormalities of similar origin. However, cleft lips withother. different. congenital abnormalities may occur
as part of multiple congenital abnormalities in a
number of diseases of different origin. Thus, thecombination and joint evaluation of isolated andmultiple congenital abnormalities is the cause ofserious confusion both in research and in clinicalpractice.
45. Isolated congenital abnormalities are the consequences of developmental disturbances of single
localized errors in morphogenesis. This category
involves single, complex (more abnormalities in one
organ or organ system). sequential (one defect with its
subsequently derived secondary defects in later morphogenesis) and polytopic field defects (a pattern of defects
derived from the disturbance of a developmental
field). A multiple congenital abnormality is a concurrence of two or more morphogenetic errors ofdifferent sites in the same person. Within this category.
syndromes (patterns of multiple anomalies thought tobe causally related), associations (non-random patternsof two or more anomalies presumed to be pathogenetically related) and random combinations areusually distinguished.

2.

birth than later, owing to spontaneous recovery.
Fourth, a large group of congenital abnormalities suchas congenital dislocation of the hip or phenylketonuria
has only a "theoretical" birth prevalence because, due
to neo-natal screening, at least in some countries, thedevelopment of these conditions may be prevented by
special treatment or diet. The method of scoring (birthor death certificates, hospital records, special anomaly
registries. surveillance or monitoring systems), thevariable sizes of the samples, the variability of thedefinition and classification of congenital anomalies
and the diagnostic criteria between different observers,
differences in reference unit (affected individual orcongenital anomaly) are important factors influencing
the estimates. To these should be added other confounding demographic factors (age. parity). ethnic
group, the family genetic background. and the socioeconomic characteristics of the parents (e.g., social
class). Finally, there is the additional variability
related to the geographical location. the season of theyear, and the time span covered by the investigators.
All these factors explain the difficulty in comparing
the figures reported in the literature.
47. It is not surprising, therefore, that the figures
found or cited in the references given [Cl 1, Cl4. CI8,
15, K32, L4, M45, N9, N 10, S38, U5. V5, V8] are veryvariable from a low of 1.0% of all liveborns to a highof 8.5% in total births (i.e .. still- and live-births) in the
United States [M45]. Recent Hungarian estimatessuggest that, for the group of conditions designatedby ICD codes 740 to 759 (in chapter XIV of the
ICD entitled "Congenital anomalies"), the prevalences
are 6.1 % and 6.0% in total births and live-births,
respectively. If the entities covered by the ICD codes 550(inguinal hernia). 553 (umbilical hernia) and 227-228(congenital tumours)-which many teratologists consider as congenital anomalies-are included, then the
figures become 7.2% and 7.3%. in live- and totalbirths, respectively. In the prospective United States
study (M45] which aimed at as complete an ascertainment of all congenital anomalies as possible (and inwhich the Iiveborn children were followed up to one
year of age), the prevalence (including ICD entries
740-759, 550, 552. 227-228) was 8.5% of total births.

Birth prel'alence of congenital abnormalities

46. The baseline birth prevalence of all congenitalabnormalities in man is difficult to assess for a varietyof reasons. First. as is well known, the affected newborns seen at birth are only the survivors of pre-natal
selection, i.e., of a much greater number of abnormal
fetuses which occurred at one or another stage of prenatal development. Therefore. only their point prevalence at birth (and not time incidence) can be
determined for a majority of congenital abnormalities:
recently, the former has been designated as birthprevalence. Second, some classes of abnormalities(e.g .. coarctation of aorta, teeth abnormalities) cannot
be scored at birth or are fewer at birth than after a few
years, i.e .. their point prevalences would be higher by
50 to 100% if cases could be followed up for a few
years. However, the rate of these late-diagnosed cases
may permit estimates of prevalence at birth. Third, theprevalence of other abnormality types (e.g .. ventricularsepta) defect or undescended testicle) may be higher at

48. It is worth stressing the fact that in both the
Hungarian and the United States data, musculoskeletal and skeletal anomalies are the predominant
contributors to the total. Foremost among them,especially in Hungary, is congenital dislocation of thehip. Anomalies of the integument constitute about
10% of the total in the United States and about I o/c in
Hungary. (For detailed comparisons between the
Hungarian and the United States data, see chapter I of
the annex A to the present report.) Since a truly
representative figure, applicable in a global context,
for the prevalence of all congenital anomalies isdifficult to arrive at. and in view of the differences
existing between different studies (some, genuine
differences between populations and some due todifferent degrees of ascertainment), a round figure of
6% of total births for the prevalence of congenital
anomalies with medical consequences. is adopted in
the present study. to provide a general frame ofreference for discussions. It is realized that this273

estimate might become higher if anomalies ascertainedlater in post-natal life, minor structural blemishes orconditions of little medical significance, low birthweight. mental retardation and other congenital handicaps (not always accurately ascertained) are also
included.
49. As a result of progress in epidemiological expertise, birth prevalences of specific types of congenitalabnormalities are Jess variable. In fact, a number ofwell-defined and easily diagnosed congenital conditions (e.g., cleft lip, spina bifida. anal atresia) haveapproximately similar birth prevalences in several
countries. Furthermore. birth prevalences of different
types of congenital abnormalities show a surprisingly
narrow range of random fluctuation within the same
country or region [19]. Congenital anomalies areusually listed according to the anatomical structures
affected and they are grouped together. although they
may have quite different pathogenetic mechanisms.
Therefore, a pathogenetically oriented classification
system is very much needed.
50. The approximate birth prevalences of all isolatedcommon major congenital abnormalities are shown in
Tables 5, 6 and 7. Table 5 is based on a study of aseries of consecutive births in 24 centres organizedby WHO in 1961-1964 [S38]. In this study, a similar
approach was used in all the participating centres andthe outcome of a total of 421,781 pregnancies wasinvestigated. The study indicated geographical variations of birth prevalences of congenital abnormalities
depending on ethnic-genetic, territorial-environmental,
demographic and social (health service, etc.) circumstances. Some congenital abnormalities were notinvestigated in the WHO study. In these cases, thedata of other important epidemiological studies weretaken into consideration. These figures are indicatedby asterisks in Table 5 and the references are in [Cl8].
Table 6 shows the data from Hungary where birthprevalences were obtained by the same method (adhoe epidemiological studies) and by the same staff in a
population of 10.7 million between 1963 and 1974.
Over this period, the prevalence of congenital abnormalities was studied continuously. The variability ofthe estimates as a function of time was surprisingly
small. Table 8 (modified from a review of Villumsen[V8]) expresses the proportion of congenital abnormalities in various anatomical structures as a percentage
of all abnormalities scored, and shows a range ofvalues which is comparable to that seen in Tables 5
to 7.

3.

The aetiology of human congenital abnormalities

51. As mentioned earlier, there are a number of waysof classifying congenital abnormalities. e.g., according
to their severity (lethal, major, mild. minor), occurrence(common. moderately frequent, rare). teratologicalmanifestation (zygopathy, embryopathy. fetopathy)
and aetiology. Their classification according to aetiology, which is considered below, distinguishes between
those of simple genie origin; those due to the interaction between hereditary, mainly polygenic, liability
and environmental triggering factors; those due to274

chromosomal aberrations; those brought about by theaction of environmental. including teracogenic andmaternal, factors: and finally. a heterogeneous classof abnormalities of unknown origin [K32].
52. Mutant genes, autosomal or gonosomal. recessive
and dominant, may be the cause of congenitalabnormalities, mainly congenital malformation syndromes. The occurrence of each single mutant gene in
the population at large may be small. but since thesemutant genes are numerous (a few thousand havebeen described in the McKusick catalogue [M47)),
their overall effect in the population may be important. Their variability in penetrance. their rarity. andthe differences in clinical severity of their manifestations (expression) account for the difficulties in
estimating the frequency of the relevant malformations.
Figures between 0.5 and JO per 1000 births have been
provided, but the extreme values may reflect overassignment or incomplete ascertainment of cases to this class.
Taking intermediate values of 2.25 per 1000 [Cl I] or
5 per 1000 children born with a serious disorder[W27], it may be calculated that of the order of 3.8
and 8.3 (i.e .. as an average round figure 6%) of allcongenital abnormalities may be of genie (or monolocal. monogenic or Mendelian) origin in North
America and Europe (2.25/60 = 3.8%: 5.0/60 = 8.3%;
average between 3.8 and 8.3% = -6%). Inclusion ofstill-births in these estimates would make little change
as the major part of congenital abnormalities in thestill-born is not of genie origin.
53. To the second class of abnormalities belong all
those for which twin and familial studies indicate acomplex aetiology explained by the multifactorialthreshold model [Cl8]. These congenital abnormalities
depend on a polygenic disposition interacting with avariety of mostly unknown environmental factors. For
congenital abnormalities belonging to this class. thefindings pointing to genetic disposition in the various
major types and the genetic mechanism underlying
their appearance have been discussed by Carter [CJ I].
According to a WHO study [S38]. there are 11 mostcommon congenital abnormalities. that is, those exceeding the rate of I per 1000 births. Their detailed
break-down is given in Table 5. Comparable data for
Hungary. where the situation has been followed
carefully, are given in Table 6 [CI8]. In this country.
for example, the total prevalences of congenital
anomalies are 73.6 and 71.8 per 1000 total or livebirths. respectively [Cl9). Table 6 shows that, for 9
out of the 11 conditions. the most likely explanation is
the multifactorial aetiology. The overall point prevalence of these 9 in Hungary is 54.0 per 1000 births,
that is 73% of all congenital abnormalities in thatcountry. as shown in Table 6 (54.0/73.6 = 73%). (Thefigure of 54.0 is obtained by adding the prevalences
for: neural tube defects (2.9), ventricular septum
defects (2.1 ). cleft lip± cleft palate ( 1.0), congenital
hypertrophic pyloric stenosis (1.5), undescended testicles (3.6). hypospadias (2.2), congenital dislocation of
the hip (28.0). structural talipes equinovarus (l.3) andcongenital inguinal° hernia ( 11.4) ). It should be bornein mind. however, that the number of cases withtreated congenital dislocation of the hip is extremely
high in central and eastern Europe. Recently figures ofthe order of about 5 per 1000 for congenital dislocation of the hip have been published in westernand northern Europe. Taking this into consideration,
the proportion of the 9 isolated common congenitalabnormalities becomes 42% of all congenital abnormalities, as a minimal figure (54.0-28.0 + 5.0 = 31.0
om of 73.6 is 42%). Furthermore, only structural
talipes equinovarus was evaluated within groups ofvarus. valgus and other deformities of feet with afigure of 28.8 per 1000 births. Also, about 3.5% ofmale new-barns have undescended testicle(s) at birth,
but nearly 80% of these descend spontaneously within
the third month. Finally. some moderately frequentisolated congenital abnormalities (e.g., cardiovascularmalformations with a birth prevalence of 5 per 1000
(i.e .. one-half of 10.6 minus 2.1 )) may belong to thisclass. Taking all these considerations into account. a
total prevalence of 30 per 1000 births, i.e .. about 50%
of all congenital anomalies, appear to be a reasonableestimate for this class (i.e., 50% of 60/ 1000).
54. In the third class belong those due to chromosomal abnormalities. The most recent UNSCEAR
review of the total frequency of chromosomal abnormalities in newborns (which includes a total of 67,014
karyotyped cases) [US] points to a birth prevalence of
0.63%. Not all of these would carry visible malformations, but assuming that roughly 50% do, it may be
estimated [W27] that the contribution of this class tothe total abnormalities in man may be of the orderof 5% (3.1/ I 03 + 60/ l OJ = 5% ). This figure would
negligibly increase when still-births are included. It
hardly needs to be stressed that chromosomal abnormalities are more frequent early in pregnancy. The
review [US] indicates that about 15% of recognizedconceptions will lead to spontaneous abortion (early
and intermediate feta) deaths) and about one-third of
the aborted fetuses will carry some kind of chromosomal aberrations [Hl8, H21]. Germinal chromosomal
mutations occur in about 5% of recognized conceptions [H22, W26]. The rate of embryonic loss between
conception and recognized pregnancy is unknown, butis generally agreed to be very high [K39]: of the orderof 40% between conception and 20 weeks of pregnancy[M7] and of the order of 10% between 10 and
18 weeks [G 16]. These figures indicate that mostconceptual losses must occur before pregnancy has
been diagnosed, and often before the first missed
period. The occurrence of abnormalities in the spontaneously aborted fetuses is also much higher thanthat seen in new-borns. which points to the role offeta) death in selecting morphologically and chromosomally abnormal fetuses.
55. The fourth class of abnormalities is that induced
by environmental factors, many of which have beensuspected (but less often proved) to induce teratogenic
effects in utero. The list that follows is taken from
Kalter and Warkany [K32], who gave a detailed
discussion with references. Among environmentallyinduced congenital abnormalities, it is worth separating maternal conditions from teratogens, because theimportance of the former is probably underestimated,while that of the latter may be exaggerated. Maternaldiseases (e.g., diabetes mellitus, phenylketonuria, somehormonally functioning tumours and perhaps hypo-

gonadism) belong to the group of maternal conditions.
Maternal infection (e.g., rubella and varicella which
can cause embryopathy and fetopathy. and cytomegalovirus and toxoplasmosis, which may damage
the fetus) is a condition which is intermediate between
the maternal and the teratogenic factors. because theinfectious agents may sometimes cause congenitalabnormalities without affecting the mother. Considering all maternal factors together. probably 3.5% ofcongenital abnormalities could be attributed to thisgroup of causes.
56. Among chemicals. pharmaceutical drugs seem tohave the greatest practical significance as teratogens.
At present. the list of teratogenic drugs involves
thalidomide, androgens. anticancer and immunosuppressive cytotoxic and antifolic-acid derivates, synthetic
female sex hormones. anticonvulsants (including
hydantoin, trimethadion, valproic acid and otherderivatives), anticoagulants, large doses of vitamin A
and its derivatives, lithium and penicillamine. The
mild effect of tetracyclin (teeth discoloration), and thereversible goitre after excess maternal ingestion ofiodide or propylthiouracil, have also been discussed.
However, the suspicion of teratogenicity for many ofthese drugs has not been confirmed. Among nutritional factors. the ingestion of alcoholic drinks during
pregnancy has the greatest importance owing to the
fetal alcohol syndrome. Of a long list of environmental substances, at present only mercury is considered to be a demonstrated human teratogen [K32].
57. Specificity is an important attribute of teratogenesis because nearly all known teratogens causecharacteristic multiple congenital abnormalities, i.e.,
syndromes. Many other conditions or agents may, ofcourse. have non-teratogenic effects on the fetus: forexample, hyperthyroidism and lupus erythematoides
as well as occupational exposure to anaesthetics may
increase the rate of feta! death. while active andpassive smoking during pregnancy may cause asignificant reduction of weight in newborns. To give
an approximate figure for the birth prevalence ofenvironmentally induced anomalies is very difficult
because any agent administered in high amounts could
be toxic and could retard the development of humanfetuses or cause feta) loss. On the other hand, several
environmental factors could have a triggering effecton a number of common or moderately frequentcongenital abnormalities of polygenic origin. Estimatesof the causal role of teratogens vary between l and 5%of all congenital abnormalities. Thus, it is estimatedthat roughly 6% of congenital abnormalities might becaused by environmental (including maternal) factors.
58. In summary, it is clear that, on the basis of theiraetiology. the congenital abnormalities can be subdivided into those due to (a) major genes, 6% of the total
prevalence of 60/1000; (b) multifactorial causation,
50% of the total: (c) chromosomal anomalies, 5% of
the total; and, (d) environmental. including maternal
factors. 6% of the total. It can be inferred that about
30% of the abnormalities scored at birth have noknown cause at present.
275

D.

CONCLUSIONS

59. The development of the mammalian conceptus
may be divided approximately into three major
phases: the pre-implantation phase, lasting from fertilization to the settling of the embryo into the uterine
mucosa; the phase of major organogenesis, which
extends in man to approximately the 8th week postovulatory; and the phase of feta! development, lastingfrom about 9 weeks until birth. The timing anddevelopmental details of each phase. however. differ
greatly in various species and may considerably
influence the outcome of a given radiation exposure.
Quantitative extrapolations across species are therefore unwarranted.
60. The developing human brain appears to bevulnerable to irradiation. In order to assess theexistence and extent of special sensitivity periods. it isimportant to map the development of the nervous
system in relation to time. To this end. time after
ovulation is a more reliable parameter than time after
menstruation. There is, however, no single quantitative parameter to describe the developmental age ofan embryo or fetus. Although the length of theembryo is statistically well correlated with its age anddevelopmental status. it is, in any single case, oflimited value for predictive purposes. It has, however,
the advantage of being directly measurable in vivousing non-destructive ultrasonic techniques. These
give information that is in close agreement withmeasurements taken on embryological samples. There
are standardized systems of staging embryonic development, based on the overall morphology of theembryo, by which the major landmarks of organogenesis may be characterized with sufficient precision.
61. The formation of the human brain has recently
been described in considerable detail according to themajor stages of organogenesis, the cellular kinetics of· the developing structures and the overall biochemical
maturation of the various structures as a function oftime. The major features are: the division of undifferentiated neural cells in the ventricular zone; themigration of post-mitotic cells to the cortical zones
according to precise sequences of time and programmed spatial arrangements; the establishment offunctional connections between neurons and glial cells
and between neurons in various areas of the brain;
and the presence of selective mechanisms to adjust thesize and functions of the nerve cells in relation to theirareas of projection and to neuronal wiring. All these
phenomena and their temporal sequence determine, toa large extent, the final outcome of a radiation
exposure of the developing brain.
62. It has been confirmed that the period of maximum
sensitivity of the brain structures corresponds to theperiod of maximum activity in neural cell production
between about 8 and 16 weeks post-fertilization inman. It is also apparent that brain histogenesis andmaturation take place in various animal species atdifferent times and that in man the maturation of the
CNS extends from the embryonal well into the feta!
and even the post-natal ages. This accounts for thedifferent sensitivity of the brain structures in various
276

species to the damaging action of radiation. Also,
different parts of the brain develop and mature atdifferent times. so that one should expect different
periods of sensitivity for the functions associated witheach of these structures. In contradistinction with what
happens in rodents, the development and maturationof the human brain cortex lasts relatively longer thanthe major organogenesis of the brain: this may explain
why. in man, disturbances of higher nervous functions
are relatively more common after irradiation than isgross teratological damage.
63. The following concepts, derived from what iscurrently known about human brain development.
structure and function, should be emphasized for theirimportance in respect of radiation effects. First. theneurons are perennial cells: they are generated once
and for all in the course of cortex histogenesis andtheir inactivation by the action of radiation (or any
other embryotoxic agent) is likely to result in permanent loss of mental functions. Secondly. brain structures are arranged in anatomical centres related to
specific functional activities; inactivation of thesecentres is likely to result in a deficiency of the specificfunctions. These functions are not recovered byrepopulation of glial cells. Thirdly. the brain functions
are critically dependent on the establishment of special
connections between neuronal cells in various parts of
the CNS. A programmed sequence of events allows
the cellular architecture to be established according toprecise spatial and temporal arrangements. Interference
by radiation, or other agents, in such a highly
structured developmental plan is likely to result in a
disruption of the neuronal connections. with therelated functional loss.
64. The incidence of congenital abnormalities in thehuman species is highly dependent on the method andthe time of scoring. They are found in about 6% of
new-born individuals. Very roughly, they may beclassified according to their etiology into those of
simple genie origin (about 6% of all malformations atbirth): those having a complex multifactorial origin(about 50%): those due to chromosomal defects(about 5%); and those known to be associated withvarious environmental factors (around 6%). Finally.
there is a large class comprising about one-third of allabnormalities visible at birth which includes all those
without an apparent cause. Radiation-induced malformations must be viewed against this background ofnaturally occurring conditions.
65. Children seen to be malformed at birth are only
the survivors of a much greater number of malformed
embryos or fetuses that have died at some stage in theintra-uterine development. If scoring is done on
grown-up children the incidence of malformed ones ishigher than at birth. Incidence figures are highly
dependent on the method of scoring, on the size of thesample and on diagnostic criteria. The ethnic group.
the familial genetic background. the age. parity andsocial class of the parents, the geographic location. theseason of the year are also important sources ofvariability for malformation incidence. For the purpose of this study it is assumed that approximately 6%of all new-born children are affected by structural

abnormalities that are sufficiently severe to disturb
their physical well-being and in so~e cases to decrease
their viability.

II.

THE PRE-IMPLANTATION PERIOD

66. I~ is very difficult to study, in man, events taking
place in the conceptus before implantation. This isbecause there is no way of knowing whether fertilization has taken place, until the most sensitive radioimmunoassay tests show an increased concentration ofhuman chorionic gonadotropin (HCG) in urine, thus
indicating trophoblastic activity. It is widely believed.
however. that many pregnancies end in embryoniclosses before pregnancy has been clinically diagnosed
and often before the first missed menstruation [G 16,
K39. M7]. These considerations explain why direct
human observations of the pre-implantation stages arevery rare for both normal development or after
embryotoxic treatments. Of necessity, therefore, onemust rely on observations in experimental animal
systems. The short duration of the pre-implantation
phase relative to the total duration of pregnancy inman, and the high rate of embryonic loss during thisphase. must be kept in mind in order to assess
appropriately any radiation effects that mav be
induced during this period of development. The ~copeand the limitations of model systems in teratology
have been discussed in a paper by Beck [B29].
67. From its previous analysis. based exclusively on
animal data. UNSCEAR concluded [U2] that death ofthe embryo was the most conspicuous effect to be seen
a_fter irradiation in the pre-implantation phase. Reducuon of body growth and induction of malformations
were not reported after irradiation at this stage ofdevelopment. Chromosomal damage in the irradiatedblastomeres. leading to degeneration of the primitive
cells. is the major mechanism responsible for thekilling of the embryo. Substantial differences insensitivity
were noted dependino0 on the various•
species and as a function of time from fertilization in
the early segmentation stages. In the mouse, therodent species to which most data referred. data ingood agreement pointed to a risk coefficient for killingof zygotes soon after fertilization, of the order of
I Gy- 1. The following text updates these conclusions
by review of the new findings on in vitro and in vivo
systems.
A.

EXPERIMENTS IN VITRO

68. Since the 1977 UNSCEAR report [U2], theculture in vitro of fertilized mammalian oocytes up to
defined stages in their development has been widelyused in many laboratories, both as a means to
inv.estigate mechanisms of damage in the pre-implantati?n. stage and as a useful test of effects induced byradiation when administered alone or in combination
with ?ther en:ibryotoxic agents. The main advantagesof this techmque are: incubation under well-definedcon?itions:. ~areful timing of the treatments; good
plating efficiency and capacity for further differentiation; and possibility to reimplant the blastocysts

into foster mothers to follow their further development [J 15, S 19]. The reports dealing with mechanisms
will be examined here, while the work in which invitro culture of embryos has been used simply as a testfor embryotoxicity will be reviewed in VI.B.
69. Alexandre [A3] irradiated 2-cell or morulae
mouse embryos (x rays, doses from 0.5 to 10 Gy) andcultured them in order to follow the percentage ofembryos that would reach the blastocyst stage atvarious times post-irradiation. The most significant
effect for irradiation of 2-cell embryos was theinhibition of primary differentiation shown bv the
failure of the embryo to proceed to the blastocyst
stage. No embryo reached such a stage after a dose of
4 Gy. X rays administered to advanced morulae did
not inhibit primary differentiation. although they did
affect. eventually, the hatchability of the blastocysts.
In a subsequent paper [A4]. the same author showedthat the reduced capacity to reach the stage of blastula
after irradiation at the 2-cell stage was the result of
early killing of cells. In fact, blastocysts from irradiatedembryos contained a smaller number of cells. A lackof inner cell mass and its derivatives in further
development was the end-effect of irradiation for boththe 2-cell and morula-stage embryos.
70. Two papers by Pedersen et al. [Pl. P2] showedthat early mammalian embryos possess the enzymes
required to perform excision repair of DNA damaged
by ultraviolet (UV) light. Embryo cells maintained arela_tively constant level of excision repair capacity
until the late feta! stages, when such capacity was lost.
However. there seemed to be no simple relationshipbetween these changes in repair capacity and the
known changes in the radiation sensitivity of embrvos
and fetuses.
·
71. In a small series of experiments, Jacquet et al.
[JI] followed, as a function of time after fertilization.
the development in vitro of cultured mouse embryosthat had been irradiated with a series of x-ray doses(from 0.25 to I Gy) 2.3 or 6.7 hours after induced
ovulation. The authors concluded that the sensitivitvof the mouse egg was high at the pronuclear stag~
before DNA synthesis began, when doses of 0.2 Gy
were sufficient to increase pre-implantation loss.
These results in vitro parallel earlier in vivo findings
by Russell and Montgomery [R20] who reported thatthe sensitivity to radiation-induced killing and tochromosome loss was very high shortly after sperm
entry, and again at the early pronuclear stage. but
beca_me low in a late pronuclear stage. Jacquet's
findings [JI] are at variance, however. with those of
Schlesinger [S4] who found no difference in theradiosensitivity at any time during the first day after
fertilization.
72. Jacquet et al. [JlO] also reported on a preliminary~tud~ of the synthesis of DNA in mouse zygotes. They
1rrad1ated, with a range of x-ray doses (0.25 to
1.0 Gy), superovulated mice at the time of fertilization
at the beginning and at the maximum of DNA
synthesis. and at the beginning of the first cleavage
metaphase. On the day following irradiation theembryos were collected and classified as either uncleft277

or 2-cell embryos: the latter were followed for afurther 7 days. The pronuclear stage was found to be
the most sensitive: irradiation at this time resulted inhigh mortality prior to first cleavage and at the 8-cell(1 Gy) or morula (0.5, 0.25 Gy) stages. Hatching of
the blastocysts and attachment to the bottom of theculture Oasks with the formation of a normal innercell mass (which phenomena are referred to by theauthors as "implantation") were also impaired. Cytogenetic studies on metaphase chromosomes also confirmed the well-established notion [U2] that embryonic
death is attributable to chromosomal damage.
73. Dornon [D JO] analysed the cell-cycle-dependentradiosensitivity of BDF I mouse 2-cell embryos. The
cell cycle of these embryos lasts 18 hours and ischaracterized by a Jong G 2 + M of 14 hours and an
S phase of 4 hours. a figure that is shorter than inother cases [M47, S70]. The author followed. as afunction of time, the changes of the dose needed toprevent 50% of the cells from reaching the blastocyst
stage (LD 50 ) and found that the LD 50 varied within afactor of about 6. The LD 50 of the embryos was about
2 Gy during the S phase and increased sharply in early
G 2 ( - 6 Gy); resistance gradually dropped (to - I Gy)
as they progressed to M. Dornon concluded that theposition in the cell division cycle at the time ofirradiation is a major determinant for the in vitro
response of the early embryo to irradiation. The rangeof variation of the radiosensitivity values observed
within the cell cycle appears. indeed. much larger than
the range of radiosensitivities of pre-implantationembryos in different developmental stages (see Table 4,annex J, in [U2]). One-cell-stage mouse embryos werealso found to be more sensitive than two-cell stages inexperiments by Ku and Voytek [KI8].
74. Dornon [DI I] also measured the dose required
to prevent development of 50% of the embryos toblastocysts during 5 days of culture in vitro ofpronuclear mouse embryos irradiated at various times
during the cell cycle. This dose was found to varyfrom I to 2 Gy during the time from G I to the firstcleavage. Pronuclear embryos were found to be moresensitive to radiation than embryos in the 2-cell stage[D JO]. However. when radiosensiti vity was related to
the number of blastomeres. the pronuclear embryos
appeared to be as sensitive as the 2-cell ones. It was
concluded that the proportion of cells of various cellcycle ages in the conceptus governed radiosensicivity
during the early cleavage stages.
75. Yamada et al. [Y3] addressed the problem of therapid changes in radiosensitivity, observed in thefertilized ovum during the time from fertilization to
first cleavage, by the use of a mouse in vitro
fertilization system. They found that the LD 50 • as
defined by Dornon [D 15], varied rapidly and extensively
over a range of about 40 to 400 R. Sensitivity was
relatively low soon after sperm entry and became progressively high (40 R) 4 to 6 hours after insemination,
just before pronuclear formation. In the later pronuclear stage (12 hours after fertilization) sensitivity
became low (400 R). only to rise again just before firstcleavage.
278

76. In another paper by the same group [M48].
mature sperm, eggs just before insemination andzygotes in the early pronuclear stage were exposed tox rays ( 40 R) and then cultured in vitro. The control
non-irradiated zygotes, and those derived from irradiated sperm and eggs. progressed to the metaphasestage in over 90% of the cases by 15-17 hours from
insemination. By this time. however, about 40% of the
I-cell eggs were still at the pronuclear stage. following
irradiation in the early pronuclear phase. There wasoften a delay of chromatin condensation in one of thetwo pronuclei. In agreement with these observations,the incidence of chromosomal aberrations in firstcleavage metaphases of zygotes was much higher(20%) when irradiation took place on the zygotes thanwhen it was carried out in sperm (2.9%) or unfertilized
eggs ( J l.0% ).

77.

A series of papers by a group in the Federal
Republic of Germany dealt with mechanisms ofradiation damage during the initial phases of mammalian embryonic development. One of these papers[Mll] showed that pre-implantation mouse eggs
irradiated with x rays or neutrons in the G 2 phase of
the 2-cell stage contained micronuclei and chromosomal aberrations when they reached the 4- and 8-cellstages. Micronuclei were more numerous in the 8-cellembryos, while chromosomal aberrations (particularlyof the chromatid type) appeared more numerous in
the 4-cell stage. As micronuclei derive from acentric
fragments following chromosomal aberrations, thesequence of events,would thus be logically explained.
78. Other papers [M32. M34] described the cell
kinetic events following irradiation of the G 2 phase of
the 2-cell mouse embryo. Doses of x rays between 0.12
and 1.88 Gy were delivered to such embryos. and theirdevelopment up to blastocysts was followed as afunction of time. A dose-related impaired hatching of
blastocysts. and a reduced number of cells per
blastocyst. were observed. Recovery and survival ofirradiated embryos was also found. Moreover, normal.
healthy mice could be produced by blastocyst transfer
into foster mothers even after 0.94 and 1.88 Gy. A
radiation-induced G 2 block of the cells, leading to a
disturbance of cell progression along the cycle, wasdescribed. Hypodiploid cells were seen in later cell
cycles. presumably related to the loss of genetic
material induced by chromosomal aberrations. A finer
analysis of the radiation-induced G 2 block after
irradiation of embryos in various stages of development and of the resulting effects was also publishedby the same group [M 12].
79, The DNA amounts and cell cycle phases werestudied in mouse embryos after administering a doseof l.88 Gy to 1- or 2-cell mouse embryos [Ml3].
Blastocyst formation was more impaired (28% ofcontrol) following irradiation of the I-cell than of the
2-cell embryos (73% of controls) and cell death was
mostly responsible for the impairment of the embryonicdevelopment. Cell death was well correlated with thepresence of micronuclei after irradiation, and bothphenomena were most evident after irradiation of the
I-cell zygote than of the 2-cell embryos.

80. The cell proliferation in vivo and in vitro of preimplanted mouse embryos was investigated from 2 to
144 hours after conception [S70] by measuring thenumber of cell nuclei, the DNA content of the cells
and the mitotic and labelling indices. After the firstcell cycle the length of the S phase remained constantat about 7 hours, while G 1 and G 2 varied considerably.
The increase in the number of cells was exponentialfrom 31 to 72 hours after fertilization, but the rate of
growth during this time was slower in vitro than inviva. Also, the proliferation rate slowed down as thedevelopment proceeded to blastocyst. Although, in
general. development was judged to be slightly fasterin vivo, the phenomena were found to be very
comparable under both conditions: this is evidence
that the technique of in vitro culture is suitable forstudying early kinetic events in the developing embryo.
81. Molls et al. [M47] compared, by using cytofluorometric techniques. the progression through the first few
cell cycles of pre-implantation mouse embryos of the
Heiligenberger and NMR 1 strains. Ova fertilized invivo during a short mating period were followed up to
about 4 days from conception and the duration of thecell cycle phases was estimated at various times
directly from the DNA histograms. Differences between
the two strains were mainly in the length of theG 2 + M phases. In both strains. the length of S
increased from the first to the second cell cycle and
the G 1 periods of the second and third cycle were very
short. The increase in total cell number was exponential
by the third cycle, with some differences in the rate ofproliferation between the two strains. By the end of
the pre-implantation development, when embryos
reached the stage of hatched blastocysts, growthstarted to decline. The differences found were considered to be strain-specific and not due to themethods used for DNA measurements or to thedifferent degree of synchrony of the embryos m thetwo strains.
82. In experiments on the preservation of mousestocks by low-temperature storage of early embryos.
Whittingham et al. [Wl I] kept 8-cell embryos inliquid nitrogen at -196° C. They were exposed tovarious levels of background radiation ranging from
1.8 to 84 times the ··normal" background, for periods
ranging from 6 to 29 months. The capacity of theseembryos to reach the blastocyst stage in vitro or toreach fetal or live-born stages when transferred torecipient females were the end-points used to assess
the effects. Comparisons with non-frozen or shortterm frozen controls were also made. The lowtemperature treatment itself was responsible for amarked loss of viability, so that when all data werepooled, only about 20-30% of the embryos originallyfrozen were recovered as fetuses or live-boms. By
comparison with this high rate of loss. the effect ofbackground was absent or non-significant. Only theembryos kept for 29 months at 84 times the control
background showed a slight reduction of implanted
and live fetuses. However. even this effect was of
doubtful significance in view of the lack of any
relationship with the time of storage.
83. When assessing the usefulness of techniques of
assay in vitro for exposure of embryos in vivo, directcomparisons by the evaluation of similar end-points
are of importance. ln this respect, it is appropriate tocite a paper [F2] which escaped the previous UNSCEAR
review [U2]. In these experiments, 2-day-old mouseembryos of the 129/SvSJ strain were obtained byhormonally-induced superovulation. They were treatedwith increasing exposures from 220-kVp x rays (from
6 to 1455 R) either in vitro or directly in the
superovulated females. Embryos were then removed
from these females within 2 hours and cultured invitro up to blastulation under conditions similar to
those used for the embryos irradiated in vitro.
Blastulation occurred in about 80% of cultured shamirradiated control embryos. It was found, in essence,that under all conditions the inhibition of blastocyst
development by irradiation was dose-related. However.
the exposure required to inhibit blastulation of 50% of
the embryos was about 300 R for irradiation in vitro
and about 600 R for irradiation in vivo.
84. After 48 hours of culture. embryos that had
blastulated were again transplanted into pseudopregnant females which were killed JS. days later forscoring of implantation scars, resorptions and fetuses(weight, gross abnormalities. skeletal abnormalities).
Approximately 70% of implanting control blastocysts
developed into fetuses. After in vitro exposures inexcess of 73 R, significantly fewer fetuses developed.
Exposures of the order of 170 R resulted in no fetal
development at all, and those of about 270 R resulted
in no implantation. Following in vivo irradiation, feta]
development occurred up to 388 R. No more or nodifferent gross and skeletal abnormalities were observed
significantly above the control level after either invitro or in vivo exposure. As to weight changes, animpairment occurred in fetuses developing from transplanted embryos irradiated in vitro with 73 R or more.
It was concluded that the lower susceptibility toradiation damage found for embryos irradiated invivo was due either to direct embryo shielding or to aprotective physical or metabolic mechanism provided
by the maternal tissues and operating within the
2-hour interval between irradiation and transfer toculture. However, the details given in the paper [F2]about radiation dosimetry and the use of exposures.
rather than absorbed doses, render any quantitative
assessment of these data rather problematic.

B.

EXPERIMENTS IN VIVO

85. By comparison with reports on irradiation invitro, the reports on in vivo treatment of preimplantation stages are few. In a series of papers on
the effects of gamma rays and helium ions onembryonic rats (0.5-6.0 Gy). Ward et al. [WI] exposed
pregnant animals on one of days 4 to 9 of gestation(the pre-implantation stage ends in the rat at about
day 7) and scored the embryo survival at 20 days p.c.,
as a function of the embryo position in the uterus.
They found that embryos located in the ovarian or in
the cervical ends of the uterus had higher rates ofmortality than their litter-mates implanted in the
middle part of the uterine horns. This conclusion wastrue for both radiations and for all irradiation times.
The influence on survival of the uterine position279

increased with the dose of radiation and with thenumber of implants. so that with high doses and acrowded uterus the probability of survival of embryosin the least advantageous positions could be as low as
one-half of that of the litter as a whole. Irradiation
under hypoxic conditions also led to a high rate ofkilling of embryos implanted in the cervical end of theuterine horns: this suggested that the selective distribution of intra-uterine deaths could not be attributed
to a state of relative hypoxia. The non-random
distribution of the killing effect was in contrast withthe random occurrence of the gross morphologicalabnormalities to be seen in the irradiated litters.
86. In another experiment [S4]. pregnant mice were
exposed to JOO R of x rays at various hours in the firstday of gestation and the fetuses were scored at termfor death during the pre-implantation or early postimplantation stages. An increased rate of both typesof death was induced by the treatment, accompanied
by an increase in the mean feta! weight and adecreased incidence of malformations. Changing theirradiation time did not affect survival of the animals
at term. At various times after irradiation. theembryos were tested by the eosin Y supravital staining
method to assess cell viability. The relative frequencyof viable and dead cells in normal or irradiatedembryos was found to be comparable at I. 2 and 3days. At 4 days of gestation. an increased proportionof dead cells was noted in both the control andirradiated embryos: this was taken as evidence of theoccurrence of cell death as a normal mechanism in thecourse of mammalian development. At this time.
irradiation caused a decrease in the percentage ofembryos reaching the blastocyst stage and a decreasein the number of cells per embryo. The authors notedthat the lack of a significant increase in the percentage
of dead cells in irradiated mice could be due to thehigh variability of the findings masking the killing
effect of radiation or to the fact that cell death may
becom·e manifest only after the fourth day of gestation.
This research is broadly in agreement with many otherexperiments [U2] confrrming that embryonic death is
the main effect found after pre-implantation irradiation. However, the single exposure used, the shorttime tested. the lack of a precise timing for mating.
and the non-specificity of the techniques used. renderthese conclusions of rather limited significance.
87. Roux et al. [R21] investigated the effect of 0.05.
0.1, and 0.25 Gy of gamma radiation administered insingle doses to Wistar rats 9 or 16 hours after mating.
At 9 hours. DNA synthesis had not yet begun. and at
16 hours was still taking place in the ovum. The
pregnant females were killed on the 21st day ofgestation for counting of the total number of implantation sites, the number of live fetuses. resorption sites(embryonic death between 7 and 12 days) and deadfetuses (death after day 12). Live fetuses were also
weighed and examined for malformations of the headand skeleton. The pre-implantation embryonic mortality. the mean weight of the fetuses and the sex ratio
were not significantly changed by the radiation treatment. There were also no differences in the type andincidence of malformations in the various experimental groups. However. mortality (both embryonic280

and feta!) was increased by the radiation exposure,without any significant difference between the two
irradiation times. At 0.05 Gy the increased mortality
only occurred at the fet al stage; at 0.1 Gy both theembryonic and feta! mortality were enhanced. but thelatter more than the first: at 0.25 Gy the stage ofconceptus at death depended on irradiation time.
because embryonic mortality was always increased.
although much more in the 16-hour group. while feta!
mortality was only increased in the 9-hour group.
These results are in agreement with previous ones inshowing that mortality, rather than malformationinduction. is the prevalent effect of irradiation during
the pre-implantation stage. They are. however. at
variance with the previously discussed notion [U2]that pre- and early post-implantation death is generally
seen under such conditions of exposure. As to thedependence of mortality on irradiation time, thepresent series [R2 l] shows a complex relationshipbetween dose and time at irradiation.
88. Aldeen and Konermann [A2] treated pregnant
mice by a single exposure of 300 R x rays I. 2 and3 days p.c. and dissected the uteri 6 days p.c. The
mean number of implantation sites was found to be
9.67 per female in the controls and 8.00. 6.63 and7.00 per female in animals irradiated 1. 2 and 3 daysp.c., respectively. Among 22 implants that were
examined histoiogically after irradiation on day I p.c.,no living embryo could be detected: the same \Vas trueof 19 implants on day 2 and 11 implants on day 3
after irradiation. The percentage of resorptions waslowest on day I after irradiation (31.8%) and highest(94. 7%) on day 2. Cytological and histochemical
criteria were used to assess the state of the decidua.
which was either normal or only marginally affectedon day 2 after irradiation. concomitantly with thepeak of embryonic deaths. From this the authors
concluded that direct embryonic killing, and notmaternal effects. were involved in the damage
described.
89. Zhou and Wang [23] submitted mouse embryosin the pre-implantation period to single or fractionatedexposures of radiation (gamma rays. 100 R) .. Among
early effects, they reported an increased mortality (asevidenced by an increased number of dead fetuses andresorption sites) and a decreased mean feta! weightof irradiated litters.

C.

CONCLUSIONS

90. The latest information on the effects of embryo
irradiation during the pre-implantation stages does
not include data for man. In the last few years,
techniques of embryo culturing in vitro have been
widely employed in experimental animals to study avariety of problems, mostly dealing with the mechanisms of radiation damage during this stage. For suchproblems, in vitro culture offers significant technicaladvantages and results that are qualitatively comparable with those obtainable in vivo. The similarity of
the findings between in vitro and in vivo analyses does
not extend. however. to the quantitative aspects ofradiation damage: conclusions drawn in vitro may noteasily be extrapolated to in vivo irradiation. Anothercondition that impedes generalizations across species
is that the latest data refer only to mice and rats.
91. Experiments in vitro have made it possible toascertain that the sensitivity of the embryos is highest
for irradiation of the pronuclear stage. They showquite conclusively that there are oscillations in the
response of the embryos related to the cell division
cycle. Doses as low as 0.2 Gy have been reported tocause statistically significant embryo killing at thetime of maximum sensitivity before DNA synthesis
begins. However. the variability of doses required tokill one-half of the cultured embryos (LD 50) spans afactor of about JO among different phases in the
mitotic cycle and different mitotic cycles. The formation of blastocysts is more damaged when irradiation
takes place at the I-cell than at the 2-cell stage, andthere is a good correlation between the death of theembryo after irradiation and the occurrence in it ofchromosomal abnormalities. Irradiation also disturbs
the kinetics of cell progression. mostly through avariation of the duration of G I and G 2 phases.
92. Irradiation of the 2-cell embryo results in a doserelated block to proceed to the blastocyst stage. Dosesof a few Gy are needed to stop completely 50%
blastocyst formation. As the embryo maintains areasonable degree of synchrony during the first few
segmentation divisions, its sensitivity may be analysed
as a function of cell cycle phases. Under these
conditions. the changes of embryo sensitivity result in
LD 50 values ranging from I to 2 Gy (irradiation in M
and S) and 6 Gy (irradiation in G:?). A G 2 block of thecell cycle has also been described for irradiation at thisstage. Hatching of blastocysts is, in general. a moresensitive end-point. Cell killing by induction of chromosomal aberrations appears to be the cause of a decreased
rate of growth of the irradiated embryo and. ultimately.
of the embryonic mortality. As the number of cells in theembryo increases. the dose needed to stop embryonicdevelopment to a given stage appears also to increase.
However. for irradiation at a given stage. the apparent
radiosensitivity of the embryo to killing increases withthe time intervening between irradiation and thescoring of the effect.
93. Experiments in vitro have documented the existence of some strain-related variability in the rate ofprogression of the embryo through the initial developmental stages in the mouse. There may also be
differences in the intrinsic radiosensitivity of earlymouse embryos of the same strain irradiated in vitro
or in vivo. the latter condition being apparently lessdamaging. Transplantation into foster-mothers ofembryos irradiated and cultured in vitro allows thesurviving embryos to complete their development tonormal animals. Such experiments have confirmedthat loss of viability is the only effect to be seen fortreatment during the pre-implantation stages at thetwo-cell stage or later. Weight changes in the survivors
could be secondary to the killing of some of theconceptuses in polytocous animals. Radiation-induced
teratological effects in excess of the natural level havenot been documented.

94. Experiments on rodent embryos irradiated invivo have confirmed the above in vitro conclusions. atleast qualitatively. Embryonic killing emerges in vivo
as the major effect of irradiation. It appears to be due
to an action on the embryos that is direct and not
mediated through disturbances of the formation of theplacenta. In vivo. relatively small doses (0.05 Gy) havebeen reported to result only in fetal mortality. but
after higher doses (0.1 Gy) embryonic mortality is also
affected. Oscillations of embryonic radiosensitivity
would appear, on the whole. to be Jess prominent forirradiation in vivo: this could. however. be due to thelesser degree of synchrony that obtains under these
conditions.

III.

THE PERIOD OF MAJOR
ORGANOGENESIS

95. In its 1977 review [U2] UNSCEAR concluded
that the most characteristic (but by no means theonly) type of damage to be seen in mammalianembryos irradiated during the period of major organogenesis was the induction of malformations. The
review discussed the conditions under which teratoJogical damage became apparent and stated that, atleast qualitatively. similar classes of malformations
occurred in various animal species upon irradiation ofcomparable developmental stages. Inter- and intraspecies variability was also reviewed on the basis of
the data then available. The review discussed thepresence and timing of the periods of maximum
sensitivity for certain classes of malformations andobserved that these maxima usually coincided with the
major phase of morphogenesis of the relevant anatomical structures. It observed that, although thedose-response relationships for a given type of malformation were of the sigmoid type. grouping ofembryologically or topographically related malformations or expressing the teratological damage as theratio of malformed to normal animals. irrespective ofthe type and degree of malformations carried, tended
to generate quasi-linear dose-effect relationships. Datathen available concerning rodents (the projection ofwhich to man would. however. be unwarranted)
tended to yield estimates of the absolute increase ofincidence of malformed fetuses per unit absorbed doseof the order of 5 10- 1 Gy-1 of low-LET radiationdelivered at high dose rate. In man, an estimate ofincidence of severe mental retardation per unit absorbed
dose was tentatively set at 10- 1 Gy- 1, based on thedata from Hiroshima and Nagasaki. Other epidemiological surveys in man, after doses in the region of
0.01-2.0 Gy. were only of indirect value in excluding
the possibility that at such doses the human embryo
could be 10 times more sensitive than implied by theincidence of malformations at higher doses [U2].
96. The information that has since become available
has added considerably to our knowledge of phenomena induced by irradiation in experimental animals.
particularly in the CNS. New data have also been
produced on the occurrence of mental deficiencies inchildren irradiated during the explosions at Hiroshima
and Nagasaki. All the relevant material on malforma281

tions of the CNS in experimental animals is being
reviewed here, partly because it would be difficult andrepetitive to separate out from the various series data
referring to either embryonic or fetal stages; andpartly because mental retardation in man (which
occurs especially at times which are at the borderline
between the two developmental stages) is also treated
here. It is recognized. of course. that cases of mental
deficiency may also appear in children irradiated attimes which are considered well within the fetal stageof development. However, by discussing all data
together it is hoped to convey a more coherent pictureof the malformations observed and of their incidence
as a function of dose and time. The reader is referred
to chapter I for a brief description of normal CNS
development in animals and man.
A.

DAT A FROM ANIMALS

I. Skeletal malformations
97. Single exposures from 6°Co radiation. in therange from 50 to 500 R, were given to about 600
pregnant Swiss mice at 9, 10, 11 and 12 days ofpregnancy [P3]. Starting from day 13, and up to day
18 p.c., the embryos and their placentas were weighed
and examined. For irradiation on day 9, exposures
below 150 R produced no significant deviations fromcontrols. whereas higher exposures were lethal to theconceplus. For irradiation on day 10, an exposure of
JOO R was the threshold for effects on the weight ofthe placenta. Malformations became apparent at
125 R. At 150-175 R, feta} mortality was over 60%
and surviving fetuses were malformed. At 200 R, fetal
loss approached 100%. On day I I the feta-placental
complex was reduced in weight at 75 R. At 200 R, prenatal death was over 60%. and at 300 R it was close to100%. On day 12 the weight of the conceptus andplacenta declined between 125 and 450 R. Malformations were seen at 150 R. and 500 R produced total
loss of the fetuses. The data were compounded into an
overall model of action accounting for dose, time andtype of effects.
98. Tribukait and Cekan [T3] carried out experiments on C3H mice, primarily to study the form of
dose-effect relationships for malformations brought
about by irradiation with single, fractionated or
protracted doses during the period of major organogenesis. Pregnant females irradiated at 9 days p.c. with180- or 250-kV x rays were killed at 18 days p.c. for
examination of implantations. resorptions, living anddead fetuses and fetuses with gross external abnormalities. Skeletal and visceral malformations were also
specifically studied, but the incidence of this latterclass was too low and its scoring was later omitted.
Single doses in the range of 0.125 to 2.5 Gy weredelivered. In total, 50 malformed out of 1929 nonirradiated fetuses were found; 1320 malformations
were seen in 3558 irradiated animals.
99. The frequency of malformations belonging tovarious classes was very different when all irradiatedand non-irradiated animals were compared. In thevertebral column and ribs the frequencies were 32 and282

44% in non-irradiated and irradiated. respectively;
umbilical hernia was, respectively, 14 and 6%; digital
malformations 8 and 6%; anophthalmia, and microophthalmia and other malformations of the eye 8 and
18% (although coloboma was mostly seen in theirradiated mice). Tail malformations were. in bothcases. 18%. Dose-response curves for resorptions andmalformations were of the sigmoid type, with a very
low incidence of the effects up to 0.75 Gy and a steep
rise thereafter. Resorptions. in particular, with acontrol incidence of 10% did not show any clear trendwith dose, up to about l Gy. Taking the various
classes of malformations separately, absolute increases
of incidence per unit absorbed dose of between 9 and
54% Gy- 1 were observed in the steep region of thecurves. Pooling all malformations. a 57% increase
per Gy resulted, with practically JOO% malformed
animals at 2 Gy. For doses below 0.75 Gy, theabsolute increase in the percentage of malformed micewould be 0.1% per 0.01 Gy (Figure VIII). Expressing
the effect as number of malformations per fetus
produced a similar pattern, with an initial increase to
1.4 malformations per animal up to 0.75 Gy and afurther, steeper rise to 3 malformations per fetus up to
the highest doses used.

JOO. Dose-fractionation experiments were also carriedout [T3] in which the embryos received a total of
1.75 Gy, split into 2 fractions delivered at intervals
between 15 minutes and 8 hours. Even taking into
account that a decrease in the sensitivity of the systemoccurred over this time (resorptions dropped from 45 to
34% and malformations from 45 to 21 % following single
doses of 0.875 Gy) an interval of 15 minutes brought
about a 30% reduction in the incidence of both effects,
followed by an increase at 4-6 hours and a further
decrease to near control values at 8 hours. This pattern isreminiscent of the two-dose recovery curves commonlydescribed for many mammalian cells and tissues.
Different responses as a function of fractionation time
were noted for the various classes of malformation.
those of the eye being apparently more spared by dose
splitting than the skeletal ones.
JO I. Further experiments were conducted [T3] inwhich the same dose of I. 75 Gy was delivered at dose
rates of 0. 72, 0.055 and 0.015 Gv min- 1• The total
incidence of malformations increased from 55 to 80%
between the highest and the lowest dose rates tested. but
different malformation types were differently affected,
the external and the ocular malformations being
practically unchanged and the skeletal ones essentiallydoubled. This finding is at variance with the majority ofdata discussed in the 1977 report [U2], but it isconsistent with the notion that dose-rate changes andfractionation may affect various malformations differently. Cell kinetic data obtained by whole embryo
preparations were also given in [T3]. but the significanceof such findings in relation to the problems at hand is
not clear.
102. A thesis by Knauss [K&] contains accurate dataon the dose-response relationships for mouse skeletal
malformations induced by pre-natal exposure to radiation. Mice of the Heiligenberger strain were exposed at
7, 10 and 13 days p.c. to 150-kV x rays in the range ofALL MALFORMATIONSRESORPTIONS
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malformatlons (right) Incidence In mice irradiated at 9 days p.c. The number of observations Is
given for each experimental group.

[T3]

5-250 Rand examined on day 19 p.c. for the presence ofminor variants, abnormalities or major malformationsof the skeleton,· after staining with Alizarin S. The
various types of effect were examined separately or
jointly as a function of dose. The 7th day p.c. was thetime at which the sensitivity appeared to be at its
maximum; the minimum was at 13 days p.c. By
considering each class of malformation (they weredivided according to the various sections of theskeleton), quite pronounced threshold-type exposureincidence curves were obtained. The data were also
treated in such a way as to obtain estimates of fetuses
without abnormalities or malformations (excluding thevariants). They were plotted, as a function of dose. as in
Figure IX. That figure shows that exposures of theorder of about 80 or 140 R would be needed to induce
malformations in 50% of the irradiated fetuses at 7 or
10 days, respectively. The difference is due mainly to thewider threshold observed at 10 days p.c. Theseexperiments are to be regarded as a further confirmationof the non-linearity of the dose-response relationship forthe induction of malformations at the most sensitive
stages in development.
103. These and other experiments were later included
in a review by Konermann [K 12] from which Figure X istaken. The experimental points in this figure are basedon 500 to 800 mice each. By plotting the incidence ofgross skeletal malformations as a function of x-ray
exposure on a semi-logarithmic graph. it is shown thatthe spontaneous incidence of 0.6% malformations isincreased by a factor of 3 after an exposure of 5 R on
day 7 p.c. The increase is due essentially to thecontribution of malformations in the skull and thecervical column. The decrease in incidence from 5 to

25 R is attributed to the confounding influence ofembryonic selection. An exposure of25 Ron day 10 p.c.
doubles the incidence of malformations seen in thecontrol mice. which is, however, unaffected by an
exposure of only 12.5 R.
104. In the course of work on sensitivity analysis for an
in vivo screening test for teratogens, Russell [R 19]
explored the possibility of using homeotic shifts in thequantitative configuration of the axial skeleton. Having
ascertained that about 9 days p.c. was the optimum time
for induction of changes in certain quantitative featuresof the axial skeleton of BALB/c mice. she irradiatedpregnant mice at this particular stage ( 12.5, 25, 50 and
100 R). Fetuses were removed just before delivery
and their axial skeleton was analysed. This inbred
mouse strain shows a high degree of variability forseveral quantitative characters such as the numbers of
ribs. costo-sternal junctions, pre-sacral vertebrae andsternebrae. Even the lowest exposure tested caused
clearly discernible shifts in 3 of the 4 quantitative
characters mentioned. By contrast, the morphologicalabnormalities of the axial skeleton showed muchhigher thresholds: of 10 abnormalities scored, none
was induced at 12.5 R, and I, 5 and 10 were induced
at 15, 50 and 100 R, respectively.
2.

Malformations of the eye105. Cellular changes in the developing eye ofembryonic mice irradiated on day 8 p.c. with 0.9 Gyof x rays were described by Balla and Michel [B l, B3].
The end-points scored on days 9 and 10 p.c. included
eye size, mitotic activity, cell death and chromosomal
283

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7 p.c.
Irradiation on day 10 p.c.
0 Irradiation on day 13 p.c.

A I rra di at ion on day

lLJ

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30
20
10

5 25

50

100 125 150 175

200

250

300

EXPOSURE ( R)
Figure IX. Percentage frequency of fetuses with normal skeletal structure, Including variants, afterx Irradiation on days 7, 10 or 13 p.c., expressed In per cent of the total number of living fetuses on
day 19 p.c.
[K12]aberrations. The size of the eye vesicle was significantly
reduced in the irradiated embryos. but was restored
24 hours later, except for a significant reduction of theJens size. In agreement with these findings. the total
number of cells and the mitotic activity of the vesicle
were decreased at the time of observation. The
incidence of chromosomal aberrations in anaphasecells was increased in the irradiated eyes. as was theincidence of morphologically assessed cell death.
Similar experiments were conducted on embryos at
the 9th day of gestation. Cell death was scored atvarious times post-irradiation, up to 24 hours, and indifferent regions of the developing eye. The pattern of
necrosis differed from that seen, for irradiation on
day 8. and the rapid recovery occurring by 12 hours
pointed to a high capacity for repair of this tissue[B2].

their main aspects and for their qualitative features,the effects observed on such animals are similar to
those produced in man. Hicks and D'Amato [HIS], ina review of the lethal and non-lethal cell changes of
the nervous system, have attempted to show the goodcorrespondence existing in respect of the underlying
mechanisms between the laboratory animal and thehuman data. Brizzee et al. [B9] also published a brief
review of data on the structural nervous system effects
caused by irradiation in ucero in both animals andman. In the following paragraphs. new data on thissubject will be reviewed, with special reference tothose appearing since about 1977. Further data on
brain damage by chronically ingested radionuclides,
particularly by tritium [V2, Zl, Z2], are discussed inchapter V.

(a)3.

106. Although normal events in the development of
the human nervous system have been mapped to areasonable degree of precision (see chapter I), data on
the effects of radiation directly on human material arevirtually impossible to obtain. Thus, our knowledge ofthe pathogenesis of radiation-induced changes relies
entirely on observations in experimental animals.
mostly rodents. It may reasonably be assumed that, in

284

The mouse

Malformations of the CNS

107. Kameyama et al. [K3] studied the effect of 25 or
I00 R x radiation on CF- I mouse embryos exposed at
10. 13 or 15 days of gestation, in an attempt to obtain
quantitative evidence of early effects on the undifferentiated matrix cells of the developing telencephalon.
Of all gestational ages. the 13-day-old embryos were
found to be the most sensitive in terms of lengtheningof the cell-cycle time in thymidine-labelling experiments. An exposure-effect relationship between JO and

100
50
40
30
20

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Irradiation on day 10 p.c.

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0 12.5 25

50

150

100

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EXPOSURE (R)
Figure X.

The incidence of gross malformations in the mouse skeleton after various exposureson days 7 or 10 p.c.
[K12]

150 R was also established at this age for mitotic delay(G 2 + l/2M) of the matrix cells. The delay was foundto be directly related to the logarithm of exposure.
Extrapolation of this function to the lowest exposuresuggested that the apparent threshold for the effectwould be slightly lower than 10 R, thus confirming thedetectability of cell kinetic disturbances (the overallsignificance of which is. however. difficult to assess) at
this exposure level. Other data on the cell cycle andradiosensitivity of matrix cells in the brain of themouse fetuses at 13 days p.c. were also reported in an
abstract by Hoshino and Kameyama [H28].
108. Hayashi et al. [H2. H4. but particularly H9]described pathological changes observable by lightand electron microscopy in the telencephalon of CF-1
feta! mice exposed to 25 R or JOO R on days 10, 13
and 15 p.c. The detectability and incidence of cellswith cytoplasmic inclusions by light microscopy wasincreased when earlier developmental stages were
exposed. By electron microscopy, such inclusions could
be divided into cytoplasmic degeneration and phagocytosed neuronal cells: they preceded the appearanceof nuclear changes in the irradiated cells. Precise times
for the occurrence of all these changes, as a functionof dose and irradiation time, could be established.
109. Kameyama [K30] noted that the changes in theradiation response of the matrix cells were notapparently related with the decrease of the cells'
mitotic activity as a function of time. He pointed outthat the most sensitive period for these cells is when

the cortical plate begins to appear. According to theauthor, this developmental stage in the mouse corresponds. approximately, to the 8th to 10th week ofgestation in man, based on the histogenic state of thecerebral cortex.
110. Working on two different mouse strains (CF-!
and ICR), and with different gestation times (13 and15 or 17 days), Hoshino et al. ([Hl2. Hl3]. summarizedin [H 18]). studied the changes in the architecture of
the cerebral cortex induced by 25 or 100 R. To do so,they mapped the position ofthymidine-labelled neurons
reaching various layers of the cortex by 4 weeks ofextra-uterine age. They recognized the presence ofspecific changes in the various layers. The nature of
the changes was found to depend on irradiation time.
their severity on dose. In parallel experiments by
Hayashi et al. [H 10], where conical thickness, celldensity and dendritic arborization of pyramidal cells
were observed. it was found that 25 R did not cause adecrease in the number of cortical cells, but resulted ina developmental effect on the pyramidal cells whose
dendritic arborization was retarded. Ten roentgens.
given at 13 days p.c. to ICR mice. reduced thebranching indices marginally [H 11].
111. Schmahl and his collaborators published avaluable series of papers in which the mechanisms andconsequences of radiation-induced damage to the
CNS were analysed. In a first contribution [S7]. theyreported on experiments in which NMRI mice received
2.0 Gy of x rays on day 12 p.c. and were examined on
285

day 18 p.c. for lesions of the CNS. Cell clusters(rosettes), presumably originating from the ependymal
layer, were seen at this time. together with areas of
necrosis close to the ventricular surface in the subependymal layer. which is the site of reparative cellproliferation. These lesions had morphologically typicalaspects and evolved after birth into cysts. with flattening of the cortical wall, without reactive processes
such as glial scarring. Cyst formation was accentuated after birth and evolved into porencephaly with
communication between the ventricular lumen and thesurface of the cerebral cortex. The histogenesis ofthese events, with special emphasis on the topographical differences in the developing brain, wasdescribed also in another paper [S40]. Abnormalitiesof the nervous tissues were often accompanied bymalformations of the overlying skull. such as dysdifferentiation of the cartilage. formation of hyperostosis, and disturbances of the calcification processes[S58].
112. Another paper by the same group [SS] attempted
an analysis of cellular mechanisms by a joint evaluationof pathological and cell-kinetic events in the mousetelencephalon at late organogenesis. A dose of 0.95 Gy
killed virtually all matrix cells in G 1, G 2 and M, and50-75% of those in the S phase, the survivors being
responsible for the subsequent regeneration of thedamaged tissue. Cell labelling allowed the differentiation of two different pools of cells surviving in S,
which contributed in different ways to the regeneration
processes of the telencephalic roof after radiation
injury. A later study on similarly treated mice [S49]described the generation times. potential doublingtimes. growth fraction and cell cycle stages of theneuroblasts and their alterations following irradiation.
113. Heinzmann [H3] studied, in the mouse, thedysplasia of the third cerebral ventricle induced by
1.9 Gy of x rays on the 12th day of gestation. Perinatal observations and observations at 20 months ofage were conducted by gross, microscopic and ultrastructural techniques. Brain reduction and ventriculocele were carefully described and the genesis of
the lesions discussed in its possible patho-physiologicalmechanisms. This author also described [Hl9] grosschanges and alterations in the fine structure of thechoroid plexus of mice, induced by doses of 0.95 and
1.9 Gy of x rays given to the 12-day fetus. Thesechanges were found to persist throughout post-natal
life up to the age of 20 months.
114. In related studies [H25], the same author
described the normal functional activity of the mousebrain, from the 13th day of gestation up to post-natal
ages, by injection of tritium-labelled deoxy-0-glucose
and radioactivity measurements. He showed that
tritium uptake differed considerably between tissue
samples taken from various regions of the brain at
various developmental times. He also compared thenormal pattern of glucose incorporation into the brain
structures with changes induced by x irradiation (0.9
and 1.9 Gy) given on the 12th day of gestation. It
is difficult, however, to correlate such biochemical
findings with the morphological effects of irradiation
286

which consist in a hydrocephalus of dose-related
severity. Other data on the biochemistry of the mousebrain irradiated in utero have also been reported[VIO].
115. Morphological effects (cell necrosis) in various
parts of the brain 6 hours after irradiation with fastneutrons (single dose, 0.5 Gy) on C57BL/6 mice
between the 13th and 18th day of gestation werereported [A 15, V9]. Animals similarly irradiated on
day 18 were examined 3 weeks after birth, at whichtime their body and brain weight was significantly
decreased, for content and synthesis of DNA. RNA,histones and other proteins in brain and liver. Thesebiochemical experiments suggested that damage to thedeveloping brain by neutrons was associated with the
inhibition of synthesis of certain histone fractions
and non-histone proteins, probably resulting from adecreased capacity of amino-acylation by transfer
RNA [A 16]. (Timmermans et al. [TI I], Tanaka et al.
[Tl2]. Van Beuningen et al. [VIO] and Deroo et al.
[D l] have also reported preliminary studies of various
aspects of the biochemistry of the rat brain after
irradiation in utero. Review of these data will only bepossible after their publication in full.)
116. In experiments by Konermann [K34), mice werechronically irradiated in utero at various developmental stages (5-10, 11-15, 6-13 days p.c.) with a rangeof exposure rates from 40 to 80 R/day. These treatments induced compensatory proliferation reactions
during the formation of the brain glia. and suchreactions were more marked the more brain development was retarded [K 11 ]. The lipid synthesis during
the period of pre-myelination gliosis was also studied byspectrophotometric and histoautoradiographic analysisof the uptake of 3H-mevalonic acid, 3H-glucose and
32 P-sodium-phosphate in the brain. During the premyelination period. it was found that there was a very
strict correlation between the lipid synthesis and theformation of glial cells. Such correlation was independent of the irradiation period and of the exposure rateand, since over-proliferation occurs at high doses, arise of incorporation with increasing brain retardation(and therefore with dose) was documented. In various
brain sections. in parallel with an increase of myelin
density, a tighter packing of the oligodendrocytes
responsible for brain myelination was also shown.
From an overall evaluation of the findings. it was
concluded that lipogenesis during pre-myelination
occurs indirectly as a result of cell proliferation whichleads. in the most developmentally retarded brains, tonon-coordinated, over-compensatory reactions [K34].
117. Konermann and Schwald [K 13] irradiated albino
inbred mice at 13, 16, 18.5 and 22 days p.c. with
4 doses of x rays of between 0.5 and 3.0 Gy, andstudied, by means of a microphotometric technique,the formation of the tigroid substance up to 80 days
post-natally. The formation of this substance isthought lo be related to, and reflected in, thematuration of the neurons. An irregular decrease withrespect to controls of the tigroid density, which was
dose- and stage-related, was observed in the irradiatedanimals. There were fluctuating responses also inrelation to the various brain regions examined (cortex,
thalamus. cerebellum, hippocampus, gyms dentatus,
nucleus motorius trigemini) which were most pronounced during the critical periods of post-natal brain
maturation of the various structures. The decrease of
tigroid density was compensated at long term in the
animals irradiated with the smallest dose, but withincreasing doses the changes became either not compensated or progressive. Since the magnitude of thelate responses decreased when the embryonic age at
irradiation increased from 13 to 22 days p.c., theresponses must either be extended through several cell
generations or be induced to a lesser degree in the latepre-natal stages. In parallel with the responses of the
tigroid substance during the second and third week
after birth, there were also changes in the total content
of RNA in the brain. which were again most pronounced
for the earliest ages at irradiation. The overall
meaning of these changes appears to be a retardationand stabilization of the brain maturation, but theirsignificance remains to be fully evaluated and interpreted.
118. Treatment of mice in utero with fractionateddoses between 3 X 1.05 and 3 X 1.33 Gy on gestationaldays 11-13 gave rise to rosette-like clusters of primitive
cells resembling ependymal cells, dispersed within thebrain cortex wall. These lesions appeared to beparticularly abundant in the females. suggesting thatthe radiation treatment acted on differentiation steps
that were specific for one of the sexes and fully
developed already by the beginning of the feta! periodprior to gonadal differentiation. It was speculated thatlesions in the X chromosome would be involved in thedevelopment of such a specific dimorphic action. andthat this pattern might only become apparent in theforebrain because of the relationship existing in thisorgan between cell necrosis and rosette development[S60].
119. Dimorphic lesions were described not only inthe forebrain, but also in the cpithalamus. as grossmalformations appearing in the mouse at 18 days p.c.,following a three-day fractionation treatment at doses
between 0.95 and 1.35 Gy/day on days 11-13 p.c.
[S61]. Two specific types of lesions regarding. respectively. the dorsal diencephalic sulcus and a narrowing of the epithalamus were recognized: it was shownthat the second (and more severe) type of malformation is rather more common in female fetuses. Dosedependencies for the two lesion types were also
described. The sexually dimorphic reaction pattern ofthese thalamic malformations cannot be explained by
the fact that the relevant structures are in a different
developmental stage in the two sexes at the timeof irradiation: nor with a disturbance of perinatal
hormone imprinting mechanisms in the CNS occurringin females more than in males. The authors believethat these malformations should be viewed together
with other changes described in the neo-cortex [S60]and are the result of retrograde (cortico-thalamic)trans-synaptic degeneration, secondary to the corticallesions. Epithalamic dysfunction in the immediate
neo-natal period would then be responsible for thedeath of animals carrying preferentially the second
type of lesions, which are the female animals.

120. In order to find some justification for such apreferential effect on female animals, Weber andSchmahl [W 19] studied also the developmental profiles
in mouse brain of the activities of a number ofenzymes (alpha-galactosidase, hypoxanthine-guanine
phosphoribosyltransferase and glucose-6-phosphate
dehydrogenase) which are linked with the X-chromosome. The inactivation of this chromosome, whichtakes place at an early stage of development. leads toidentical enzyme activities in male and female animals
between 15 days p.c. and 64 days of post-natal age.
These enzyme activities were studied in the brains ofanimals that had received 1.05 Gy three times, at 11.
12 and 13 days p.c. Treatment of the animals with thisdosage (but not after 3 X 0.95 Gy or 3 X 1.15 Gy at
the same ages) resulted in death of predominantly
female offspring within 48 hours of birth [S59]. Whenthe hypothesis was tested that such preferential
mortality might be attributable to a reactivation of theinactive female X-chromosome in brain cells, it was
found that. although the radiation-induced differencesin enzyme patterns were quite remarkable, none ofthem was specific to either of the two sexes. It
appeared unlikely, therefore, that the two phenomena
might be at all correlated.
121. Schmahl et al. irradiated pregnant NMRI miceon gestational days 11-13 with 3 X 1.05 Gy and foundthat only the female offspring had an increased postnatal mortality within 2 days of birth [S59]. It should
be noted that this effect was not observed with
3 X 1.18 Gy. However, within this narrow dose range.
the only neuropathological difference between thetreated groups was the occurrence of rosette-likelesions after 3 x 1.05 Gy. which were more abundantin females than in males. This was the reason toassociate the post-natal mortality effect with rosettelike lesions in the brain [S60]. Weber and Schmahl[W20] investigated the weight of the brain. its protein
content and the activities of two enzymes, acetylcholinesterase and Na.K-ATPase. at various postnatal ages up to 64 days. The treatment resulted in a
50% reduction of brain weight, as compared with
controls. with accompanying alterations of the protein
content. The activity of acetylcholinesterase was increased in treated offspring at all times, but thedifferences from control levels were not very high. The
activity of Na,K-ATPase was changed slightly, but
not significantly. Brain weight. protein content andenzyme levels were not different between male andfemale animals. either control or irradiated. Thus,
these findings do not help in explaining the sexspecific difference.
122. Konermann [K 11] exposed mice during organogenesis (6-13 days p.c.) or fetogenesis (14-18 days p.c.)
to gamma rays ( I 0-80 R/day) and studied the growthretarding effect of the treatment on the brain. He
found that under these conditions the capacity forcompensating the brain growth deficit became smaller
the later in development that irradiation took place.
Even in the absence of appreciable embryo or fetus
killing, the growth-retarding effect of the exposure
continued at long term. Daily fractionated treatments
at 6-10 days or 5-9 days p.c. resulted in death as wellas in some deficit of brain weight. The uptake of287

tritiated thymidine into brain DNA at birth was
decreased in irradiated as compared with non-irradiatedanimals, but at the end of the first and at the end of
the second weeks in neo-natal life it appeared to be
increased. At this latter time the amount of uptake
followed an increasing function of dose and, therefore,of the brain growth retardation. This abnormal
response was interpreted as an overshoot in therecovery processes, due to the fact that microneuron
and glial cell proliferation, which occurs during earlypost-natal development. i., stimulated in inverse proportion to the radiation-induced killing of neurons.
123. Other papers by Konermann et al. [K 12, K36]
summarized a variety of post-natal effects induced by
pre-natal and perinatal x irradiation of the albino
mouse brain with doses of 0.25-0.5 Gy. 8iphasic
patterns of increased brain cell proliferation occurred
during the time of myelination gliosis after acute or
daily fractionated exposure to radiation. Such overproliferation was, however. insufficient to make up for
the Joss of brain weight and was only confined to glial
cells, so that the organ could not recover its specificfunctions. Other damages in the lipid uptake, myelination and formation of the tigroid substance werealso reported. Biochemical and histochemical responses
fluctuated with time. while structural and cytoarchitectural damage persisted in post-natal brain. Thechanges described were induced by irradiation throughout the advanced stages of intra-uterine and the postnatal development in the mouse, as might be expectedfrom the normal pattern of development of the CNS
in this species. Disruption of cerebellar architecture inmice given various doses of gamma rays from 1ncs atbirth (0.5 to 7 Gy) or on day I 7 p.c. (2-4 Gy) was also
described by Sasaki [S79]. The sensitivity of feta) miceappeared to be lower than that of mice irradiatedpost-partum. The changes described were of different
severity and persisted at 900 days of age.
124. As to functional effects. l'vlinamisawa and Sasaki[M42] studied the electrocorticogram (ECG) of hybrid
mice irradiated at I 7 days p.c. or at birth with x rays
(300 R). The ECGs were recorded on the freely
moving animals by permanently implanted electrodes
fixed over the surface of the visual cortex when the
animals were 6-8 or 24-26 months of age. From thepattern of changes observed, the authors concluded
that the adult and aged mice, irradiated when in utero
or at birth. showed ECG responses that were characteristic of later ages in control groups. This wasinterpreted as evidence of an accelerated "aging" ofthe ECG pattern brought about by the radiation
treatment. In its 1982 report [U5], UNSCEAR commented at length on the impossibility of defining"aging" at the tissue or organ level by means of any
morphological or functional test.

(b)

The rat and the guinea-pig

125. In an attempt to reconcile differences between
morphological and teratological findings after x irradiation, Berry and Rogers [B 15] provided a description of
the development of the rat cerebral cortex, studied bylabelling with tritiated thymidine between the 16th day

288

in utero and birth. They were able to establish that theneuroblasts present on days 16-17 migrated to infragranular layers V and VI: those found on day 18 tolayer IV: and those found on days 19-21 to layers II
and III. The ependymal cells retained a physicalconnection with the superficial and deep strata of thecortex by means of their long processes, which
provided a channel through the cortex along which
the neuroblasts migrated. Not until completion of
migration did these cells separate from the ependymal
processes to start differentiation. Therefore, in thisspecies also the formation of brain cortex occurs fromthe inside out. as was later shown in primates andman (see I. 8.2).
126. In contrast with the paucity of data on thedevelopment of the cerebrum in the rat [815. 830,
838], there is relatively more data available on
cerebellar development [A 12-A 14. LIO]. It may be
deduced from the whole of the information cited thatthe growth spurts of the cerebrum and cerebellum arevery similar in the rat and the mouse; and that while
the cerebrum shows a sharp peak of mitotic activity
around 14- I 5 days p.c .. the cerebellum shows its mostactive wave of cell production 6-10 days after birth.
Any study that does not take into account thepeculiarities of each structure is therefore bound to beinaccurate, considering the developmental complexityof the phenomena described.
127. Berry and Eayrs [830] described the effect ofx irradiation of the fetus (200 R) on the histogenesis of
the cerebral cortex and the morphology of neurons.
Exposure at 19 days of gestation resulted in widespread destruction of cortical cells I 2 hours after
irradiation, with signs of recovery and differentiationof the surviving neurons at 24 hours. Evidence wasalso found that under these conditions the ependymal
layer had lost its capacity to contribute further. as in
the normal situation. to the hisiogenesis of the cortex.
A variety of morphological cell abnormalities were
described in the irradiated neurons. The germinal layer,
the mantle layer and the actively migrating cells
appeared to be selectively destroyed, while cells
which had completed migration remained relatively
unharmed. The distribution pattern of dendrites
appeared to be little affected by irradiation. although
the number of primary dendrites and the amount ofbranching was less than normal. suggesting that thecapacity of these cells to establish interconnections
with incoming axons was greatly reduced.
I 28. According to Semagin [S9], exposure of pregnant
Wistar rats from day 16 of pregnancy to delivery into
a 6°Co irradiation field between 0.0008 and JO R/day(8 hours/day) reduced the whole-body weight of the
new-born animals and increased significantly theabsolute weight of the brain. The effect could not be
attributable to oedema. Thickening of the cerebral
cortex (in the absence of any microscopically detected
destructive changes) was also described. The author
attributed this finding to a stimulating effect of lowdose-rate irradiation on the cerebral cortex and was
obviously aware of the peculiarity of this conclusionin the face of other experimental and clinical data.
The same author [S75] irradiated 26 non-inbred rats

in utero on the 18th day of pregnancy ( 1 R) and tested
them (together with 25 controls) for conditioned
reflexes, starting at 45 days of age. In the irradiatedanimals, there was a slowing down in the consolidationof conditioned reflexes and a weakening of such reflexes.
Conditioned natural reflexes were found to be more
pronounced than in the controls, a phenomenon
explained by an impairment in the co-ordination of
movements. Irradiated animals also showed an increasein the magnitude of intersignal reflexes during conditioning of differentiated responses. In the author's
opinion all these findings are evidence for the harmful
effect of the radiation treatment.
129. Inouye [II] studied. in the cerebral cortex of rat
embryos exposed to 200 R at 17 days p.c.. thedevelopment of histopathological changes from I hourp.c. to late in post-natal life. Cell killing of the brain
cortex was described as an early change which later
resulted in profound disturbances of the fibre formation of the corpus callosum, with reduction of its size.
These defects were also recognizable in irradiatedanimals which reached adulthood. As to the cortex,
disturbances in cell migration were seen, resulting
later in a hypoplasia of layers II-IV, abnormal
branching and irregularity of the dentritic processes ofpyramidal cells in layer V. Tridimensional reconstructions of various sections of the brain showed marked
hypoplasia of the cortex, the basal ganglion and thethalamus.
130. Following a suggestion that there may be
species differences in the relative contribution of thedifferent layers of the neocortex to the development of
the corpus callosum. Jensen and Altman [J5] madeuse of the sterilizing effect of x irradiation given at\'arious pre-natal ages to kill the late-generated neurons
in the rat. They found that the late-generated neurons
residing primarily in the supra-granular layers are
essential for the formation of the corpus callosum.
Reyners et al. [R29] irradiated with x rays on the
15th day p.c. rat embryos with doses of 0.1-0.5 Gy.
and examined them at various times between I monthand 2 years after irradiation. They found that thedepth of the cingulum bundle, a myelinated substructure of the corpus callosum, was a sensitive
indicator of radiation damage at doses of 0.2 Gy. The
authors traced the origin of the anatomical damage toa disorder in the sequence of morphogenetic events,
rather than to a process of radiation-induced cellkilling.
13 I. In experiments involving the use of mes
gamma rays (1.75 ± 0.08 Gy) administered to pregnant
Sprague-Dawley rats on gestation days 16-19 (day I
was considered to be the morning after mating),
Jensen and Killackey [J 13] investigated the subcorticalprojection of cortical neurons that had been caused totake up an ectopic position by the radiation treatment.
The study was carried out by a labelling techniquewith horse radish peroxidase applied when the rats
had reached 30-80 days of age. It showed that theradiation treatment given before. during. or after the
production of neurons destined for layer V in the neocortex and projecting to the spinal cord resulted intheir abnormal location. One ectopic location, typicalof rats irradiated on or before day 17. was in clusters
beneath the cortical white matter bordering the dorsomedial aspect of the lateral ventricle. The otherlocation was between layer V and the pial brain
surface, which was seen in rats exposed at any time
between day 16 and 19. In spite of their abnormal
position. all these neurons projected to a sub-corticaltarget appropriate for level V. indicating that neither
the migratory path nor the final position of theneurons was essential for specifying their target.
Furthermore. the fact that peri-ventricular ectopias
occurred after irradiation on day 17 and earlier
suggested that the projections of the cortico-spinal
neurons are determined early in their individual
ontogeny. prior to migration. In view of the high doseused, the importance of this paper lies more with the
implications for mechanisms, rather than with any
consideration of risk.
132. Adult rats exposed in utero to x rays (0.09 to
0.49 Gy) on day 15 p.c. were killed at various ages forelectron microscopic analysis of different brain parameters selected to provide quantitative information.
The x-ray treatment reduced the body size of bothnerve and glial cells in the central cortex and produced
an increased packing density of these cells. The
reduction in cell size appeared to be the primaryfactor in these changes because, when the data wereappropriately corrected, the number of cells per unit
volume of cortex was not significantly modified by theradiation treatment. It appeared as though the brain
had suffered a sort of miniaturization process, rather
than a loss of cells as a result of cell killing. Defects ofsynaptic arborization were also described. A doserelated decrease of blood capillaries in the cortex was
reported from these studies. which. when appropriately
corrected for the reduction in cortex thickness, became
statistically significant at the dose of 0.49 Gy [R25].
133. A comparison of the neuronal damage inducedby irradiation in chick and rat embryos was attemptedby Schneider and Norton [S8]. They carried out ananalysis of cells at the telencephalic-diencephalicborder on neurons which show morphologic similarities
in the two species. Analysis involved nuclear size, thecontent of cytoplasmic acetylcholinesterase and thepresence of several branched. spiny dendritic processes.
These authors were able to establish that l Gy in
the 7-day chick embryo had about the same effect as1.25 Gy in the 15-day rat fetus. They interpreted thesimilarity of the response of an avian and a mammalian
species to mean that maternal factors in mammals
may be of little significance for the radiation responseof these structures.
134. A fair amount of information has also beenreported in regard to cerebellar damage. Das [D3]exposed 18 days p.c. rat embryos to x rays ( 170 R) andcollected specimens of the cerebellum at various times
thereafter, from I hour to 30 days post-natally. Sixhours after irradiation, the author described pyknosis
of cells in the external granular layer along theposterior part of the cerebellum. The neuroblasts. thatwould later develop into Purkinje cells, were foundarrested in their migratory pathway. Some recovery of
the external granular layer was seen at later stages. but289

an abnormal clustering of neuroblasts resulted as apermanent architectural abnormality during postnatal development. Malformation of the folia in the
cerebellum, and the smaller size of this organ, were
described as the long-term sequelae of the treatment.

in the molecular layer of the cerebellar decline. Thiswas statistically significant only in animals given I Gy
at 6 days of development and amounted to about
17%. There was no gross cortical abnormality to beobserved in the cerebellum.

135. Given that development of some brain structurescontinues in the rat well after birth [D 19) it is not
surprising that similar changes could also be inducedby post-natal irradiation of the rat 5 days after birth.
According to Korogodin and Korogodina [K 14).
animals exposed to various doses of 60Co gamma rays,
and killed at 30 days of age. showed disorganizationin the location of the Purkinje cells which was doserelated with a maximum at the low lethal doses. Hereagain. the increase of disorganization was accompanied
by a decrease of the cerebellar weight and an
impairment of some electrophysiological parameters.

139. Concerning behavioural functional effects, experiments were reported in which rats were exposed at
17 days of gestation to x irradiation (150 R, 200-kVp)
and males were reared after birth under enriched(EC). standard (SC) or impoverished environmental
conditions (IC) for 30 days after weaning [S57].
Standard conditions implied large cages with threeor four rats: impoverished conditions implied isolated
cages with animals housed singly: and enriched
conditions implied cages where many animals lived
together and had various objects to play with, such as
ladders, tubes, balls and boxes. The irradiated animals.
together with an appropriate number of controls, werethen tested in a maze for their performance, using astandardized behavioural technique. The effects ofradiation and of the different environmental conditions were both significant in initial, repetitive andtotal error scores and running time. At later times. it
was found that the differences between the EC-SC and
EC-IC were significant in irradiated rats. whereas only
the EC-IC difference was significant in controls. A
20% decrease in brain weight was present in irradiatedanimals, as compared with controls. X-irradiatedanimals, compared with controls, revealed a decrease
in total protein. protein per gram of cortex, totalbenzodiazepine and muscarine receptor bindings andmuscarine cholinergic receptor binding per milligramof protein in the cortex. However. all these indiceswere not significantly different in animals reared
under different environmental conditions. Other experiments by the same group had shown that at 200 R theradiation effects were very evident and those of theenvironment virtually absent [K35], while at 100 R
irradiation was essentially insignificant and environment very important in respect of the performance of
irradiated microcephalic animals [S73). The conclusionfrom the experiments on the intermediate 150 R
exposure was that environmental enrichment is a
useful tool to alleviate the learning impairments
induced by irradiation in pre-natal animals.

136. In experiments by Inouye [16), a quantitative
analysis was attempted of the cerebellar malformationinduced by exposure to x rays ( 100 or 200 R)
administered to pre-natal rats on one gestation day
from 16 to 21 p.c. The changes described were thoseto be seen at 60 days of post-natal age in surviving
animals. After the highest exposure, there was a slight
reduction of the weight. but not of the width. of theorgan. The dorso-ventral length of the cerebellum was
reduced. the more so the earlier x irradiation hadtaken place. A description of the diameters andarrangements of hemispheres and lobules, as alteredby the radiation treatment, was provided. Histologically. an ectopic appearance of Purkinje cells ineither the granular cell layer or the white matter wasseen in rats treated on days 20 or 21 p.c .. but not forirradiation at earlier stages. The size and the architectural disposition of the cerebellum after 100 R were
only mildly altered.
137. Inouye and Kameyama [17) studied the effects
on the matrix cells in the cerebellum of rats exposed toseveral single doses of x rays ranging from 0.03 tol Gy. Exposure took place at 0. 3. 6, 10, 13 andJ7 days of post-natal age. Animals were killed 6 hours
after exposure and the organ was processed forscoring of the frequency of pyknotic cells in t~eexternal granular layer along the primary fissure.
Pyknosis was significantly increased by the lowest
dose level of 0.03 Gy. At all irradiation times, theincidence of pyknotic cells increased linearly with dosein the range of 0.03 to 0.25 Gy and then increased
more rapidly at higher doses, suggesting a linearquadratic trend. The highest response was seen inanimals irradiated at 6 and 10 days of age.
138. A quantitative analysis of cerebellar development was performed by Inouye and Kameyama [I 10]
on F334/DuCrj male rats exposed between O and17 days post-partum to single whole-body doses of
0.25, 0.5 and I Gy of x rays. When these animals
reached the age of 8 weeks. they were killed andmeasured for body, brain and cerebellum weight.
None of these parameters, nor their ratios, appeared
to have been altered by the treatment. Microscopically,
the only effect reported was a reduction of cell counts

290140. In other experiments by Tamaki and Inouye[T8), a facilitation of the avoidance behaviour was
found in rats that had been exposed to x rays (200 R)
on their 17th day of gestation. The primordial
hippocampus of these animals showed extensive cellular
damage. The facilitation described was thought to be aconsequence of a strong tendency to running induced by
the shock and was related to the hippocampal
damage.
14 I. Bornhausen et al. [B 17) studied the learning
ability of rats and reported data on animals irradiatedin utero on days 6-9 of pregnancy with 4 daily dosesof 0.01 Gy. When tested at 4 months of age. using
behavioural tests in which the performance requirements were gradually increased. the animals showed asignificantly impaired function, particularly when they
were required to perform the most difficult tests.

Preliminarv data were also reported by the sameauthor [B40] on rats irradiated in utero on day 13 ofpregnancy with doses of gamma rays from 137 Cs in theregion of 0.15-0.9 Gy. In addition to showing a large
variability of performance from one animal to another,
the data indicated that performance deficits depended
more on the time of exposure than on dose.
142. In another paper [M 17], the locomotor damage
present in adult rats after irradiation in utero (days14-17 p.c.) was studied. Alterations in gait were foundafter 125 R given on days 14-16. Exposures of 50 Ron
day 14 and 125 R on day 17 induced no effect on
locomotion. Hopping or alternating, waddling gait,
accompanied by abnormal positions of the hind feet.
resulted from such treatments. By histological examination of the brain. these effects could be traced
back to lesions of the corpus callosum and thetelencephalic commissures.
143. Hudson et al. [H5] reported that neo-natal
x irradiation induces, at 7 days of age, changes in themetabolism of monoamines in the brain of rats. The
concentrations of norepinephrine and serotonin increased in all the regions of the CNS showing rapid
growth and proliferation of the axons, but not in theregions of the cell bodies from which these transmitters originate. The increase of the concentration
was attributed to the increase in the rate of synthesis.
These changes were to be seen up to about 3 weeks ofage, but the monoaminergic system returned tonormal when brain maturity was attained. Suchchanges were interpreted as reflecting a disturbance inthe density of nerve endings in the regions where theseterminate due to the radiation-induced disturbance incell proliferation. Cerda et al. [CIO] irradiated young
rats at 5. iO or 25 days of post-natal life with doses of
1-8 Gy of 60Co gamma radiation and followed the fateof the DNA strand breaks in the brain cells between 1
and 180 minutes of irradiation. The number of breaks
induced per gray was found to agree with earlier dataon small intestine and spleen mouse cells. It was also
found that by 30 minutes after treatment the radiationinduced double-strand breaks in the brain were
repaired in all age groups tested.
144. Wanner and Edwards [W 12] reported on acomparison of the effects of 6°C gamma radiation andhyperthermia in inducing retardation of brain growthin the guinea-pig. To this end, they exposed pregnant
animals on the 21st day of pregnancy to gamma-ray
doses of 0.32-3.75 Gy delivered at dose rates of 0.043,
0.069 or 0.125 Gy/minute; or to 42.5-43.5° C for one
hour. The offspring were killed within 24 hours ofbirth for examination of the viscera and weighting of
the body and brain. Doses of 0.04-0.99 Gy produced adose-related. irreversible reduction of the brain weight.
but had little effect on the weight of the body.
Hyperthermia also resulted in microcephaly of theoffspring. The relative effectiveness of the two agentswas such that a dose increment of 0.525 Gy produceda brain weight loss equivalent to an elevation of thematernal tern perature of l ° C. The dose thresholddetected by this assay system was between 0.05 and0.10 Gy of gamma radiation.

(c)Primates

145. In view of the difficulties in obtaining samples
from irradiated human fetuses. observations on primates are of particular interest. Brizzee et al. [B8]reported on the gross and microscopic effects in somebrain structures (visual cortex. hippocampus andothers) of squirrel monkeys irradiated at each trimester of their pre-natal life with 6°Co gamma
radiation (2 Gy). The total duration of pregnancy inthese animals is 165 ± 5 days. In the visual cortex,
animals irradiated during the last two trimesters andexamined 2 days after birth showed a thinning and adecrease in the number of neurons per cubic millimetre of tissue, although neither of these effects was
statistically significant. The spines in the dendrites of
the giant Meynert neurons also showed some alterations. In the hippocampus, the thinning of the cortex
and the loss of neurons in the irradiated monkeys was
statistically significant, as 'wl!re the alterations in themorphology of dendrites. In the three animals irradiatedin the first trimester, gross abnormalities of the brain,
urinary system and limbs were seen, but no microscopic observations of the brain were yet available. It
is to be hoped that. with an improvement of themorphometric techniques, the careful quantitativeevaluation of similar data on primates will facilitate
the projection to man of findings from the widely
studied rodent species.
146. In another paper by Brizzee and collaboratorson the effects of pre-natal irradiation of the squirrel
monkey [BI OJ, various morphological and functional
end-points were evaluated. Data were reported on 78
pregnant females. either controls or irradiated with
60Co gamma rays (0.1, 1.0 and 2.0 Gy). Irradiation
took place at 95 ± 5 days of gestation. Among all
females. 55 live births and 23 infant deaths wererecorded. Up to the time of reporting, the sex ratio
and the percentage of infant mortality were notsignificantly different between controls and irradiated.
On day 2, and during 30 days after birth, body weight.
crown-rump length and head circumference weredecreased in the offspring receiving 1 and 2 Gy.
Various nervous reflexes were also disturbed, as wereother tests of behavioural exploration and general.motor activity. Temperature was significantly lower.
and the respiration rate significantly higher, in theseanimals. Brain. cerebellum, pituitary and kidneyweights were significantly lower after 2 Gy on day 2
after birth. Dendritic spine counts of cortical motor
neurons were also significantly reduced after the same
dose. The percentage of B lymphocytes with surface
lgM and the total blood lymphocytes were decreased
after l.O Gy at 1 year of age.
147. The same group [02] reported on the effects
of pre-natal 60Co irradiation on the visual acuity.
maturation of the retinal fovea and the striate cortex
of squirrel monkey offspring receiving between 0.1
and 1.0 Gy at 80-90 days of gestation. The visualacuity of animals receiving 0.5 and 1.0 Gy wassignificantly lower than that of control animals or of
those given 0.1 Gy: these changes were scored in the
periods from birth to 30 days and from 30 to 90 days.
Morphometric evaluations of the retina at 90 days of291

age showed 1hat pre-natal exposure, with a dose of
1.0 Gy. resulted in a significan1 difference in foveal
cone density. In the brain there was a corresponding
thinning of the foveal striate cortex at 90 days after
1.0 Gy. The cy1oarchitecture of the cortex was also
altered.
148. Finally. in another paper [01], the effects of 0.5
and 1.0 Gy of gamma rays from 60Co given to squirrel
monkeys pre-natally between 80 and 90 days p.c. werereported. The animals were studied between birth and3 months of age for a number of morphological andfunctional parameters. The circumference of the head,the crown-rump length and (to a lesser extent) thewhole-body weight grew at a significantly slower rate
after irradiation. The nervous activity of the monkey
irradiated at both doses appeared to be less accurate
and complete by comparison with that of the controls;
the time required for the performance of reflexes andneuromuscular co-ordination was also lengthened. In
visual orientation, discrimination and reversal learning, irradiated animals had a significantly decreased
percentage of correct responses. Spontaneous lightdark activity was significantly increased in the dark
session over that of control. Plasma cortisol was alsosignificantly increased in the animals receiving I Gy.

4.

Other types of malformations

149. A previously undescribed class of malformationsof the skull was reported by Schmahl et al. [S58] on
NMRI mice exposed in utero in the late organogenesisstages ( I 1-13 days p.c.) to either a single (200 R) or
fractionated (3 X 160 R) x-ray exposure. The developmental stage at irradiation appears to be a criticalfactor for observation of these effects, because irradiation during the early feta! period (14-16 days p.c.) did
not influence the development of the neurocranium,
irrespective of the dose applied. The malformations
consisted of: (a) small nodules of cranial hyperostosis
in up to 90% of the offspring: or (b) an excessive
formation of bony tissues. a heterotopic chondrification of the neurocranial capsule in the parietal andoccipital bones. Both classes of abnormalities occurredin the presence of a completely normal developmentof other cranial and facial bones. Malformations oftype (b) extended deep into the forebrain and could beseen histologically only in about JO% of the offspring.
A growth disturbance of both the mesenchymal skull
primordium and the brain was thought to be at theorigin of these overgrowth phenomena, in the sensethat brain malformations producing a lower intracranial pressure may lead in turn to an abnormal
growth activity of the skull sutures, stimulated byirradiation in a hyperplastic direction. For theseeffects, see also a paper by Meyer [M44].
150. Fractionated irradiation of pre-natal mice inlate organogenesis at 11-13 days p.c. induced amarked incidence of hypotrichosis and alopecia in theoffspring and led to severe ulcerative dermatitis,
starting at 2 months of age. This effect had a marked
prevalence for animals irradiated at least on the above
days ( other fractionation patterns were also tested
extending over 11-16 and 14-16 days p.c.). There was292

no apparent dose dependence for such an effect within
the range of 2.4 to 7.2 Gy total dose, as incidences ofulcers of between about 39 and 48% were observed
among the irradiated animals. No sex dependence wasapparent. nor was any dependence on the conditionsof the animals' housing. Internal organs showed no
special sign of alterations. except for a higher incidenceof amyloidosis. Keeping animals under germ-free
conditions also did not alter the disease pattern orincidence [S62]. The intra-cutaneous administration ofskin extracts from affected animals into non-irradiatedones resulted in a marked infiltration of leukocytes.
suggesting that the pathogenesis of the lesions may
involve a radiation-induced skin dysplasia which
is followed by an endogenous leukotactic activity
resembling human psoriasis [S63].
151. An observation by Michel and Fritz-Niggli[M8] should be mentioned for its general methodological relevance. These authors studied the effect that
confinement and restraining of pregnant mothers(which is often necessary in the course of experimentalwork) may have on the incidence of malformations in
the developing fetus. NMRI pregnant females on day 8
were restrained for 2 to 36 minutes: the fetuses they
carried on day 13 had a significantly higher incidenceof malformations of all types (but not of any single
class). The effect was attributed by the authors to ahormonal discharge of the hypophysis-adrenocorticalaxis induced by the stress of confinement.

B.

DAT A FROM MAN

152. Millier and O'Rahilly [M57] published a histological study of a twin at stage 13 showing an openingof the neural tube, believed to be the earliest example
of purely cerebral dysraphia recorded so far. The
paper is valuable for its careful description of timingof morphological events and mechanisms resulting inanencephaly in man and for a comparison, by meansof synoptic tables, of events and stages leading tosimilar types of malformations in the rat and mouse.
153. The information accumulated since 1977 on theoccurrence in man of severe mental retardation
following exposure at Hiroshima and Nagasaki is ofgreat interest and must be reviewed in depth. In 1978.
Ishikawa [12] published what is essentially a casereport on the long-term follow-up of four patients
exposed pre-natally to the atomic explosions andfollowed for their psychological symptoms over a longtime. These individuals had been exposed between 2
and 8 weeks of gestation at distances between 780 and
1180 111 from the hypocentre and their mothers had all
shown signs of acute radiation sickness. The subjects
were between 1.3 and 2.0 kg of weight at birth andwere later found to be microcephalic, and physiologically and mentally retarded. Degenerative signs
and diseases. as described in the "Epidemiologicalstudy on microcephaly" (T5) were also found in thesechildren. This small survey showed essentially that thestate of these children was clinically serious and their
prognosis over a long observation period of about
20 years should be considered as very severe.

154. Ishimaru et al. [I 12] published a re-analysis of
the anthropological measurements obtained at age 18
in children exposed in utero (and their controls) in
Hiroshima and Nagasaki. The paper reported, as afunction of dose, the height, body weight and headand chest circumference of these children. There was astatistically significant linear decrease of head andbody size with T65 dose for both sexes and both cities(except for height in Nagasaki females), but the interrelationships between the various effects shown was
not analysed. There was no apparent correlation
between pre-natal age at exposure and head circumference. It should be noted. however, that headcircumference was not adjusted for body size, and prenatal age was treated as a continuous variable,
without regard to discrete embryological events.
155. A discussion of the terms ''microcephaly" and"mental retardation" is to be found in a paper by
Dobbing [D2!] who pointed out that such terms areused for conditions of different clinical severity invarious reports. Radiation is very effective in producing small head size and mental handicap, but it is
only one among many other agents that may cause
similar effects. Actually. the precise developmental
time at which an agent responsible for such effects is
applied is often more important than the nature itselfof the agent. Contrary to other teratological malformations, which are induced by a precise, momentary
exposure to such agents. microcephaly and mental
handicaps are characterized by relatively longer periodsof developmental vulnerability lasting hours or days inrodents and other fast-growing species, and weeks ormonths in man and slowly growing animals. Unlike
teratological defects which result in permanent lesions,brain growth disturbances may to some extent be
remedied, and much more so the later the affectedstage is and the earlier any remedial action isundertaken.
156. A study by Otake and Schull [03] describes
epidemiological findings in atomic bomb survivors in
relation to dose and time. thus allowing some roughestimate of the rate of incidence of mental retardation.
After exclusion of a few children from the in utero
sample at the Radiation Effects Research Foundation(RERF). on account of wrong or incomplete information on date of birth and exposure of the mothers.
the sample on which the study was based comprisedl S99 children ( l 2S l at Hiroshima and 348 at Nagasaki).
For the purpose of the study. mental retardation was
defined as incapability "to perform simple calculations,
to make simple conversation, to take care of himself
or herself. or if he or she was completely unmanageable or had been institutionalized". Most of theretarded children were never enrolled in public schools.
but among the few who were. the highest IQ was 68.
157. The stages of pregnancy at the time of thebombing were re-assessed and expressed as the number
of weeks elapsed from the presumed time of conception.
The dose to the fetuses was expressed in terms ofmaternal kerma T65 rad, modified by the related
attenuation factors for position of the fetus within the
body of the mother (both at present under revision).
because this was likely to remain. for some time. theonly system of individual dose assessment. The alter-

native provisional "free-in-air" doses available in 1983
did not appear to change greatly the form of the dose
relationships or the estimates of incidence. A detailed
account of the findings is given in Table 9.
158. Table 9 and Figure XI show a dose-related
increased risk of mental retardation for gestational
ages 8-15 weeks and also, to a lesser degree. for ages
16-25 weeks. Table 10 shows the statistical parameters
for the evaluation of the relationships between mental
retardation and feta! absorbed dose from reference[I 11]. It shows that the absolute excess risk per unit
absorbed dose for exposure at 8-15 weeks in bothcities combined, when zero-dose cases are excluded, is0.38 ± 0.09 Gy-1: this value is about 4 times greaterthan for exposures at 16 weeks and over. Statisticalanalysis showed that there was little or no suggestionof a non-linear component in the dose-response
function at 8-15 weeks. The calculated regression
suggests no threshold. Actually, a threshold of> 40 mSv
may be excluded only with a probability of 0.20 [P 17).
This is not true. in general, for the later exposure
times, particularly after 25 weeks of conception, buthere the dose-response relationship is weak and not
significantly different from zero. Exclusion of 4 mentally retarded cases non-radiation-related (3 with
Down's syndrome and 1 with infantile encephalitis)
did not alter the above findings appreciably. Althoughchildren exposed in utero in the range between 2000
and 3000 m from the hypocentre were not included inthis revision, ancillary evidence showed that there wasno reason to suggest that the frequency of mental
retardation in this group (approximately 5/60 I or
0.8%) would be different from that found in thecontrol group (9/1085 or 0.8%). As to the difference
between observations in the two cities. the baseline
values of 0.6% incidence of mental retardation at
Hiroshima and 1.6% at Nagasaki are not significantly
different but, if so. could not be explained on any
known ground. Differences in other irradiation groups
also remain unexplained. Moreover. if the absoluterisk of mental retardation per unit dose seen in the
more numerous Hiroshima series were used to predict
the incidence among those exposed at Nagasaki. given
the average dose received by that group, 4 to 9 cases(all ages included) would be expected to appear in that
series; 4 cases were actually found.
159. Concerning any possible correlation between
mental retardation and small head size. it has beenreported [W22. W23] that about 10% of individuals
having a head size two or more standard deviations
below the mean of all those in the study sample werealso mentally retarded. Data on the relationshipbetween mental retardation and small head size are
given in [04]. This subject has been discussed separately
by Mole [M9. MIO, M21]. It appears that among thementally retarded children. about 60% had a smallhead size. Thus, the association between the two
conditions is not very close, owing presumably to thefact that loss of neuronal cells 1:. compensated to someextent by over-proliferation of the glia. which may
restore the size. but not the function. of the irradiatedbrain. There is no unequivocal evidence that othertypes of malformations may be increased in thispopulation of children exposed in utero [M5 l, P 16].
293

BOTH CITIES COMBINED

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DOSE CATEGORY (Gy)Figure XI. The frequency of mental retardation among children exposed In utero during the atomic
explosions In Japan, as a function of dose category and gestallonal age. (Both cities combined.)[111]160. The existence in this series of a well-defined
period of maximum susceptibility of the brain, taken
in conjunction with the changing form of the doseeffect relationship as a function of the time at
exposure, were discussed in the study by Otake andSchull [03] in light of the findings reviewed in chapter I,
section A. The evidence presented there shows that bothproliferation and migration of neurons in the humanforebrain are largely completed by the 16th week ofgestation, although glial cell production may still
continue in the brain cortex up to much later stages indevelopment, with some uncertainty as to the timing
and the actual extent of such processes. Also, any
distinction between effects on cell proliferation. differentiation and migration is hard to make since these
are not taking place at the same time but different
cells in different stages are found in the cortex at thesame time of development.
161. The apparent linearity of the response at
8-15 weeks warrants some discussion. In view of thenon-stochasticity of the phenomena, which is reflected
almost invariably in the animal experiments by thecurvilinearity (upper concavity) of the dose-response
relationships for malformations induced at well-defineddevelopmental stages (see III.A), the finding of alinear trend with dose appears surprising. There are,however, possible explanations for it. Among them:
the fact that many cellular events (division, differentiation, migration) may be variously contributing tothe observed effect: the fact that the period between 8
and 15 weeks of gestation is very long and that thesensitivity of the neuroblasts and their functions may

294change considerably during this time; the fact that theclinical conditions grouped under the definition of"mental retardation" may be of different severity. andthat severity may change with dose: the fact that. inclinical judgement. severe mental retardation is anarbitrary cut-off point in the lower portion of anormal distribution curve; and the possible effect of
the environmental conditions under which retarded
individuals were kept from birth to diagnosis. whichmight themselves have affected the outcome of any
given case. If any of the processes or conditions
envisaged were characterized by a curvilinear relationship with dose. but if each of these relationships had
different linear or quadratic components, it would still
be possible for them to combine in a way that theoverall effect might be seen as apparently linear.
However, in the present state of knowledge, thelinearity of dose-effect relationships from 8 to 15 weeks
of gestation must be taken as observational evidence,since our capacity to interpret it in precise radiobiological terms may be inadequate given the complexityof the underlying phenomena.
162. The data reported by Otake and Schull (03)show quite conclusively that the apparent absolute
excess incidence of severe mental retardation per unit
dose, under the exposure and observational conditions
described, is of the order of 4 10-1 Gy-1 at the peaksensitivity period, a figure greater than that given
by UNSCEAR in its 1977 report [U2] which wasaveraged over the whole pregnancy and based on an
earlier and much less refined analysis of the epidemiological evidence. They also show that the develop-

mental stages at 16 weeks or later, which coincide in
man with the period of synaptogenesis. are distinctly
less sensitive to the radiation insult (risk factor about
10-1 Gy-1) and that the form of the dose-effectrelationship for mental retardation induced at thesestages does not exclude curvilinearity. This latter
observation would be expected with a non-stochastic
type of damage: the decreasing susceptibility with timeis to be seen in the framework of the repeatedly
discussed concept of maximum sensitivity periods andthe time for the changing form of the response agrees
closely with the nature and timing of developmental
events in the brain cortex (see Figure V).
163. The results of an autopsy performed on a
microcephalic child exposed in utero to the atomic
bomb deserve some attention. even though they are so
far only available in an abstract [N 12]. As previously
discussed, disturbance of neuronal migration may be an
important cause of radiation-induced mental retardation. The morphological consequence of disturbed
migration is. of course. the presence of heterotopic graymatter in the brain white matter. The importance of
this case (the dose was estimated at 1.75 Gy) is due tothe finding of massive inclusions of heterotopic graymatter in the brain hemispheres. This observation
confirms in man observations in animals discussed in
the 1977 UNSCEAR report [US]. showing the similarityof the basic mechanisms leading to brain abnormalities
and the consequent functional defects in all mammals.
164. Another aspect of these findings which deserves
mentioning is the relatively high frequency of mental
retardation in the control children (of the order of
I%) over lower values reported from other sources.
There is, of course a well-known geographic andethnic variability in the natural incidence of malformations (see I.C.2). to which the high rate of firstcousin marriages in Japan at the time of the bombing
may have contributed to some extent. Aside from this.
however, the possible effect of malnutrition has been
discussed. which was known to be prevailing in Japan
towards the end of the Second World War [04]. There
are data on animals indicating that malnutrition may
itself produce abnormal development of the brain andbrain functions [D18. Wl3. W32]. but no clear effectof synergism with radiation has so far been documented. The extent of malnutrition in the pregnant
mothers. and its effect separate from that of radiationin producing mental disorders, would be difficult toquantify and resolve.
165. Another possible confounding factor could beanaemia induced in the pregnant mothers that were
exposed to a relatively high radiation dose. Whileindirect effects on the fetus through the mother's stateof health may not be definitely ruled out, particularly
at high doses [U2], there are at least some grounds forbelieving that they may not have played a very major
role. Actually. it is known that, to a large extent. lackof haemoglobin concentration or red blood cells in themother's circulating blood may be compensated byphysiological adaptation of the haemodynamic conditions. In addition. mothers carrying congenitalanaemic diseases are not known to produce anabnormally high prevalence of mentally retarded off-

spring. It seems likely. therefore, as a first approximation. that the high incidence of mentally handicappedchildren after irradiation in utero may be attributed toa direct action of radiation on the developing nervous
structures of the conceptus.
166. A preliminary analysis of intelligence test scoreson individuals exposed in utero at Hiroshima andNagasaki has also been reported [S38]. In the wordsof the authors. the following is a summary of the major
findings: "First. neither tests of skew nor graphicalrepresentation of the data suggest a comminglingof distributions such as might arise through theinadvertent inclusion of a qualitative different groupof individuals, that is unrecognized cases of mental
retardation. Indeed. the cumulative distribution suggests
a general phenomenon, a shift in the distribution ofscores with exposure. Second, there is no evidence ofa radiation-related effect on mental retardation or
intellection generally for those individuals exposed in
the first eight weeks of life. Third. the mean test scores
but not the variances are consistently significantly
heterogeneous among exposure categories for thoseindividuals exposed at 8-15 weeks after conception.
and less heterogeneous for those groups exposed at
16-25, or 26 or more weeks of age. Fourth. regression
analyses indicate that among those in utero individuals
exposed either at 8-15 or 16-25 weeks of gestational
age a significant decrease in intelligence test score
occurs with increasing exposure. This obtains whether
the eight clinically diagnosed cases of mental retardation to which allusion has been made are or are notincluded. However, the shape of the dose-response
curves appears different: it is linear-quadratic for the
8-15 week group and linear for the 16-25 week group.
This is the opposite of the findings with respect toclinically diagnosed mental retardation in the Revised
PE-86 sample. Finally, fifth. within the most sensitive
group, that is. individuals exposed 8 to 15 weeks afterconception. and with the better fitting model, thelinear-quadratic. the diminution in intelligence score is21-26 points per gray ... ·· (at l Gy). Further analysis
and evaluation of these data will have to be deferred
until a definitive publication appears.
167. In the present state of knowledge. the action ofradiation on the cortical brain function could be
visualized as a dose-related shift towards lower valuesof the normal distribution of "intelligence" as assessedby conventional test scores. Such a shift would take anincreasing percentage of individuals at the lower endof the distribution into a region where they would beclinically defined as mentally retarded. As might be
expected of a true non-stochastic effect, both theprobability of entering such a region and the severityof the retardation would thus be dose-dependent.
However. as the clinical definition of mental retardation includes conditions of different severity, it would
be difficult to predict the shape of the dose-response
relationship for mental retardation. If such an interpretation is accepted. then "severe mental retardation"
may be viewed as the extreme consequence of a doserelated loss of mental fuction. Accordingly. irradiation
at low doses (say, 0.1 Gy of low-LET radiation) would
not result in an increased probability of perfectly normal
individuals becoming severely mentally retarded, but295

rather in an increased probability that individuals who
would otherwise develop to be at the lower tail of the
distribution of intelligence test scores may suffer
damage that would turn them into mentally retarded.
according to the clinical definition of this term.
168. In view of the non-stochastic nature of thedamage caused by irradiation in utero. one would
expect conditions of lesser clinical importance thansevere mental retardation to be present in the cohortsof Hiroshima and Nagasaki exposed pre-natally. In
addition to the intelligence test scores mentioned
above, it is known that clinical evidence collectedfrom these children over the years is available inrespect of a number of other topics, as follows:
anamnestic observations. such as the occurrence ofconvulsions. diplopia. blurring of vision; observations
on major landmarks of physical development. such as
the ages when the children were able to stand up, towalk, to speak; ,information on school performance
during elementary training (including periods ofabsence from school on account of disease or othercauses). number and types of subjects taken at schoolwith relevant scores over the years, performance as afunction of age as the children moved to higher
classes, behaviour of the children while attending
school: and information on grip dynamometer studies
and on repetitive action tests. All these data should
give a fairly good representation of the development,
neuro-muscular coordination and higher nervous function in the children [S78]. It is to be hoped that thismaterial will be appropriately collated and analysed,
because it may give information on man that is of thehighest interest and obtainable directly.
169. Granroth [G8] reported a study based on the
Finnish Register of Congenital Malformations, which
has been in operation since 1963. Defects of the CNS(710 cases from 1965 to 1973 comprising: anencephaly.
199; spina bifida. 239: hydrocephaly, 216: microcephaly, 41; hydroanencephaly. 15) and polydactyly(259 cases. to provide an internal control by amalformation with a strong genetic background) were
included. The study was based on a total number of
621,026 deliveries. The effect of radiation exposure(diagnostic x rays administered for medical reasons)on the occurrence of malformations was evaluated interms of relative risk in a matched-pair case control
study aimed at evaluating the association of defects
with different types of x-ray examinations received by
the mother before or during pregnancy. There was no
statistically significant increased risk of malformations
to the fetus from pelvis examination of the motherprior to pregnancy. As to examinations received
during pregnancy, the risk of malformations after
chest x rays was not significant, while for feta! x rays
the risk of developing CNS malformations was 2.2 times
higher than for those not receiving any examination.
For polydactyly. the odds ratio was 1. For abdominal
examination of the mother. the relative risk was foundto be between 2 and 3 during the various trimesters ofpregnancy, but the number of examinations was verysmall and did not allow any meaningful calculation to
be made. In a careful discussion of these findings. theauthor concluded that the few associations foundbetween congenital CNS defects and diagnostic x-ray

296

examinations could be explained either by chance (nostatistical significance) or by the fact that the defectitself was the probable reason for the x-ray examination. It turned out, in fact, that, although the specificreasons for the examinations were not recorded inevery case, the great majority of examinations were
performed because of clinical suspicion of eithermaternal or feta! anomaly.
170. Preliminary data have also been reported of aprospective clinical study started in 1967 to provide
advice to parents whose children were exposed in earlypregnancy to diagnostic x-ray examinations [N2. N7].
In the course of the study. embryological observations
after abortion are carried out and the children
irradiated in utero are followed up clinically andgenetically. Up to 1982. more than 200 cases had beencollected and studies performed on 73 children up to
10 years. One publication dealing with this series [N8]provides useful information as to the time at which
irradiation occurred, the doses received. chromosomal
analyses of 28 irradiated children. and the instances inwhich spontaneous or induced abortion occurred. But
it gives no information as to the number and type of
malformations found in the irradiated children.
171. Further data [N 11] on the 73 children followed
to 10 years showed that, of those exposed in the dose
range below I00 mGy, there were two malformations(3.4%) that could not be associated with radiation
because exposure occurred before organogenesis.
Another malformation (hypospadia and spina bifida
occulta) was found on an aborted fetus: here theexposure time might correspond with the time ofdifferentiation of the organs involved. However, themalformations found are not rare among thoseoccurring naturally. In the dose range above 100 mGy,
two embryological abnormalities were found at termination of pregnancy. Cytogenetic investigations on
30 exposed children were negative as were otherinvestigations of 21 biochemical genetic markers
carried out on a selected group of 19 children. The
results of a wide range of pediatric and laboratory
investigations appeared, on the whole. to be similarly
negative.
172. The search for a possible increased incidence ofdisturbances in human development among children
born in areas of high background has continued. Ina paper from China, Lu [LI] reported on a surveyof hereditary ophthalmopathies and congenital ophthalmic malformations in the Guangdong province,
comparing two areas where the annual cumulative
dose equivalents to the whole body were about 2.5
and I mSv. respectively. In the area of low background. 66 out of 13.987 children were found to carryeye malformations. to be compared with 99 out of
13,425 children living in the high-background area.
The difference was not statistically significant. Twocontributions from Ujeno examined the relationships
between gonad dose in areas of Japan and theincidence of various conditions. The first paper [U6]dealt with the presence of Down syndrome and visible
malformations among subjects born over a three-year
period (1979-1981) in 46 prefectures, where the dose
rates of natural background radiation varied between

about 0.4 and 0.9 mSv per year. Over 457 .397 subjects
included in the survey. there were no statisticallysignificant correlations between dose and the incidenceof the conditions examined. The second paper [U7]covered 11 prefectures having a6out the same spreadof gonadal dose rates and 1,899.477 births between
1974 and 1976. The following chorionic diseases were
examined: hidatidiform mole, malignant hidatidiform
mole and chorionepithelioma. Of these. only the firsttwo showed a statistically significant correlation withgonadal dose rate. However, when all diseases werepooled together the regression on dose was found to
be non-significant.
173. Evidence of a decreased weight of female
offspring born of mothers irradiated (mostly bythorax fluoroscopy) in the course of pregnancy wasreported by Ilina [18]. but her findings are notsufficiently documented. Findings of doubtful significance on the incidence of human malformations
were also reported in an abstract in 1972 [El]. Sincebrain maturation continues beyond birth in man, dataon exposure of juveniles are also of interest in a
general sense. such as those reported by Glazunov andTereshchenko [G 14] and by Meadows et al. [M3].
Doses of many grays are. however, involved in thisexperience, and these are far beyond those of interestin the present context. In addition, the relatively low
number of cases involved makes them unsuitable forthe purpose of risk assessment.

C.

l.

CONCLUSIONS
Experimental animals

174. The current data on the irradiation of experimental animals during organogenesis have added
significant details to the overall picture drawn by
UNSCEAR in its 1977 report, but have not strikingly
changed its main conclusions. They have confirmedthat the effects resulting from irradiation of the
mammalian conceptus during the phase of majororganogenesis are characteristically teratological. They
have also confirmed that such effects, most common
in experimental animals. are essentially not seen inman. Teratogenic effects develop during the intrauterine life as the growth of the organs proceeds, andthey persist. if compatible with extra-uterine survival.
during post-natal development. Malformations may
also be accompanied by growth disturbances of thevarious structures or of the whole body. The period of
main organogenesis of each structure (for example.
the skeleton. the brain, the eye) coincides with theperiod of maximum sensitivity of that particular
structure to the induction of malformations, in thesense that the dose needed to produce a given level ofincidence of each malformation is lowest when the
spurt of cell multiplication occurs in the relevant
structure. Increasing the dose either before or after the
peak sensitivity period may, however, result in thesame type and degree of effect.
17 5. As organogencsis proceeds at different rates indifferent animal species, it is to be expected, and it isactually found, that malformations of given structures
occur in each species at a characteristic time fromfertilization. There is. therefore. a very pronounced
dependence of each kind and degree of malformation(for a given acute dose to the embryo) on the animal
species and its developmental state at the time ofirradiation. Spreading of the same dose over a longer
period of time by fractionation or protraction tends toreduce the frequency of occurrence for each class ofabnormalities below that expected from an equal
exposure at the most sensitive stage.
176. The most recent data have added to theevidence that the form of the dose-effect relationshipfor each specific type of malformation is generally
sigmoid. with the malformation rate per unit dose
increasing as a function of dose. This evidence is inagreement with the expectation for effects of the nonstochastic type. It applies to the majority of cases
when effects were accurately quantified over a sufficiently large range of doses. which happens particularly in the mouse. It also applies to effects asdifferent as skeletal abnormalities or major malformations. disturbances of brain cell kinetics or pyknosis of
cerebellar matrix cells. When irradiations at different
developmental ages were compared, as for mouseskeletal malformations, the curvilinear form of therelationship was found to become more pronounced
as development proceeds. This is also in agreementwith the postulated non-stochasticity of the induction
mechanisms.
177. Abnormalities of the mouse skeleton have been
particularly well analysed. as they may be scoredrather effectively and are fairly easily quantified.
Although, in general, sigmoid dose-incidence curves
adequately describe their induction pattern, the precise
shape of the relationship is different for malformations
found in different parts of the skeleton. Accordingly,
dose-fractionation and dose-protraction regimes affe~t
the various classes to different degrees. with resultsthat may be difficult to predict in terms of the overall
responses for the whole skeleton. To give someestimates of incidence. at the time of maximum
sensitivity: in one strain of mouse acute exposures ofthe order of 5 R were found to triple the spontaneous
incidence. although there was no further proportional
increase, but rather a decrease, from 5 to 25 R. Whenthe effect was examined in terms of the frequency ofanimals having a normal skeletal structure. no significant deviation from normal could actually be detected
up to at least 25 R. In another strain the quantitative
configuration of the axial skeleton appeared to be
shifted by an exposure of 12.5 R.
178. Malformations of the CNS have been thoroughly
investigated, although in this case the quantitative
analysis of dose-response relationships has taken
second place to the study of pathogenctic mechanisms.
From the overall evidence available in experimental
animals. it appears, however, that no effects having
clearly pathological connotations have been reported
for doses in the brain structures lower than about
0.1 Gy of low-LET radiation. The fairly extensive setsof observations on mice, rats and primates shows that,297

depending on the time of pregnancy at irradiation,different structures may be affected, although thepattern appears to be fairly uniform in all species
when irradiation takes place at similar stages.
179. In very schematic terms (but with many intermediate forms due to the fact that cells in thedeveloping brain may be at different stages of development at any one time). damage to the developing
brain cortex may be induced by a variety of mechanisms.
Irradiation at the time of active division of theneuroblasts results in the production of fewer neurons
and therefore in a loss of function. Damage to theneuroblasts may to some degree be compensated by a
hyperplastic reaction of the glial cell which might
restore the size, albeit not the function, of theirradiated structures. During the formation of thecerebral cortex, radiation may interfere with the
migration of the neurons (by this time. cells are in a
post-mitotic stage). Failure of the cells to reach theappropriate cortex layers causes them to be left behindin the white matter as displaced islands of gray matter,thus disrupting the final brain architecture. Radiation
may also disturb the establishment of neuronal connections in neurons that are in the course of migration
and are taking up their final position in the cerebral
cortex. An impairment of the number of dendritic
processes of the neurons, and of their secondarybranching. results from irradiation interfering withsynaptogenesis.
180. There are indications that each process may bedifferently affected by irradiation at the various
developmental stages, the sensitivity of the neurons
decreasing as their maturation proceeds: however, nosystematic investigation of this point has been undertaken. In parallel, other processes of maturation, suchas the myelination of the fibres or the formation of the
tigroid substance in the neurons, may be altered. Glial
cells may also be affected in both number and function.
Qualitative data Qn these latter mechanisms are stillinsufficient and quantitative observations only refer tooverall structures and not to specific cell types. In
other sections of the CNS. such as the cerebellum,
essentially similar effects have been described: however, they take place at times and with mechanismsthat are characteristic of these structures.
181. Functional deficits resulting from structural
damage in experimental animals have not been studied
systematically and so far there has been little attempt
to correlate precisely the lesions in given brain
structures with the resulting loss of the functions
under their control. In rodents. disturbances ofconditional reflexes. impairment of learning ability
and locomotor damage have been reported after
various, but mostly fairly high, doses causing, or
expected to cause. gross structural damage. Relatively
minor functional deficits induced by irradiation inutero have been shown to be alleviated by stimulationof nervous and behavioural activity post-natally,
although the more severe damage induced by the higher
doses appears to be largely, if not wholly, irreparable. In
primates, impairment of nervous reflexes, behaviouraland motor activities, visual acuity and orientation and298

neuro-muscular co-ordination have been described,
but the documentation in this respect is only indicative
and therefore insufficient.

2.

Human obsenations

182. In man, teratological effects of the type describedin experimental animals are uncommon. As to otherabnormalities, a significant step forward has recently
been made with a re-evaluation of clinical findings
and doses in children exposed in utero during theatomic bombing of Hiroshima and Nagasaki. The
latest data show that mental retardation is the most
likely type of developmental abnormality to appear in
the human species. Severe cases of mental retardation
have been included in the study sample. The association between mental deficit and small head size is notvery close. In essence, analysis as a function of timeshows that the probability of radiation-related mental
retardation is essentially zero with exposure before
8 weeks from conception. is maximum with irradiation
between 8 and 15 weeks (at the time of rapid neuronalproduction and migration). and decreases between
16 and 25 weeks when glial cell proliferation andsynaptogenesis occur. After 25 weeks and for doses
below I Gy no case of severe mental retardation has
been reported. Analysis as a function of dose showsthat the incidence of severe mental retardation isapparently linear for irradiation between 8 and 15 weeks
and curvilinear (concave upward) between 16 and
25 weeks. On the assumption that the induction of theeffect is linear with dose, the probability of induction
per unit absorbed dose at the time of the peaksensitivity is of the order of 4 10- 1 Gy- 1 (feta! dose)and of the order of 10- 1 Gy- I between 16 and
25 weeks from conception.
183. These findings are important for a number ofreasons. First, they emphasize that the most common
types of malformation to occur in man are those of
the brain cortex, resulting in severe mental deficit.
Secondly, they extend to man the notion of a close
correlation between the time of most active division of
precursor cells of the cortical neurons and the time of
maximum sensitivity of the brain cortex to radiationinduced malformations. Thirdly, they show that similar
mechanisms of action as those experimentally seen inanimals may be operating in the developing humanbrain. with due allowance for the time scale overwhich the relevant events occur in different species.
Finally. they allow reasonable quantitative estimatesof risk to be made for the induction of radiationinduced severe mental deficit in man. While not all
aspects of the reported findings may be explained at
the present state of knowledge. it is expected that they
may have a significant impact in radiation protectionin the exposure of pregnant women for occupationalor medical reasons. Further developments in respectof functional disturbances of lesser severity thanmental retardation may also come from studies
currently under way on this cohort of people. Other
newly reported observations on radiation-induced
malformations in man are much less informative andessentially negative in showing any effect.

IV.

THE FETAL PERIOD

184. The 1977 UNSCEAR report [U2] reviewed
much information concerning irradiation during the
feta! stage of development, pointing out the heterogeneity of the observations. It appeared that lethal
effects and growth disturbances had been commonlydescribed for irradiation at high doses, while teratogenic effects became increasingly difficult to document
macroscopically as feta! development proceeded. Effects
on the eye. the CNS and the gonads were the mostcommonly described consequences of irradiation during
the feta! stage, in addition to a large variety of othereffects. e.g., the haematological. The most recent
literature has added considerably in some areas. suchas the effects on the CNS and on the gonads, but little
elsewhere. The effects on the CNS have been consideredin III.A, although they actually start to occur at theborderline between the embryonic and the feta!
period in man. and extend well into the latter. The
present chapter groups the effects on the gonads andon the reproductive system (including external andinternal exposure) and all other miscellaneous effects.
It should be pointed out that in some experimental
series, embryonal as well as feta! exposures areincluded.

A.

EFFECTS ON THE GONADSAND REPRODUCTION
1.

The mouse

185, In 1978, following other experiments previously
discussed by UNSCEAR [U2], Nash and Sprackling[Nl] reported on the reproductive capacity of animals
belonging to 13 inbred mouse strains that had been
exposed continuously. for a varying number of generations, to 60Co gamma rays. Over the period theexperiments lasted, the exposure rate changed between
about 7.8 and 0.33 R per 22-hour day. Ancestral anddirect exposure values were determined when the
animals were removed at weaning from the irradiationchamber and tested for their reproductive performance,
using standard tests, to measure percentage fertility,
mean litter size and ratio of living to total embryos.
Each female (irradiated or control) was classified assterile or fertile. The latter were killed at the 10th dayof pregnancy for scoring of the pregnancy outcome.
This included counting, for each litter, the number ofresorptions. the number of dead or live embryos, andthe percentage of live to dead embryos.
186. The data showed [N 1] that for ancestral exposuresof 1100 R or greater nearly all females were sterile, but
the incidence of sterility at exposures from zero to
200 R was little affected. Response to direct irradiation
was found to be more pronounced, with extensivesterility (> 70%) at 200 R or above and no fertile
females above 900 R. In the range of exposures below
those that induced complete sterility, the overall
fertility curves were found to be sigmoid, an observation indicating that this damage is of a cumulative
type. Eleven out of the 13 strains studied showed asignificant decrease in fertility with increasing ancestral

or direct irradiation. Analyses of variance were carriedout to detect the presence of linear, quadratic andcubic components in dose-effect relationships. or
possible strain differences which might be reflected on
total litter size or on the ratio of living to totalembryos. In each instance, strain effects were found tobe significant. Litter size was more likely to beaffected by radiation than the ratio of live to totalembryos. Interpretation of these results is madedifficult by the fact that the results of similar
experiments reviewed previously by UNSCEAR [U2]
do not compare in design with this series, so that it is
not clear to what extent the differences might be
attributable to the different designs or to the methodsof scoring and analysis.
187. Adult female mice of the strain A/J received
tritiated water by injection on the day of fertilization,
so that the tritium content in the body water was
brought promptly to levels of about 310, 31 and3.1 kBq/ml. They were also started at the same timeon drinking water containing about 480. 48 and4.8 kBq per ml of tritium to maintain the initial levels
during the entire periods of pregnancy and lactation.
Female F I offspring thus exposed from conception to
14 days of age were killed at this time for counting thetotal number of oocytes in serial sections of theirovaries. It was calculated that the whole-body doses
accumulated in the 33 days from conception to killingwere from 0. 79 Gy to 0.0079 Gy at the highest andlowest levels of contamination, respectively. The
numbers of primary oocytes were lowered by thefollowing percentages with respect to control: at
0.79 Gy. more than 90%: at 0.079 Gy, 30%: at
0.0079 Gy, 13%. It must be noted that the lowest levelof tritium contamination is substantially below thosepreviously reported to cause measurable biologicaleffects in mammals. The dose-effect relationship was
said to be exponential and without threshold within
the dose interval referred to above. indicating that, ina female mouse exposed continuously from conception
to 14 days of post-natal age, the level of tritium that
may be expected to result in a 50% loss of the numberof germ cells is about 74 kBq per ml body water,corresponding to 0.12 Gy [08].
188. After reporting that a single injection of 5 MBq
tritiated water/g body weight, given to pregnant miceon gestation day 9, reduced the reproductive performance of the offspring [T7], Torok and Kistnerexamined this effect systematically as a function of theactivity administered and of the day of administration[Tl]. The injection time (a single intraperitoneal
administration of 5 MBq/g to the mother, corresponding to a pre-natal dose of about 1.3 Gy) was varied
between 5 and 13 days p.c. In addition. three activity
levels were tested at this latter time (5, 2.5 and0.625 MBq/g, corresponding to pre-natal doses of 1.3,
0.6 and 0.2 Gy, approximately). At 2 and 3 months ofage the offspring were mated, in single pairs, with
control mice of the other sex for 10 days consecutively
and then separated. Mice were also set up for
histological analysis of the gonads. Fertility was not
influenced by treatment on gestation days 5 or 7, but
starting from day 9 p.c. in females, and from day 11
p.c. in males, a 50-100% decline by comparison with299

controls was found. Administration of tritium on
gestation day 13 had the greatest effect. Male fertility
was less affected. At 2 months of age, all males and
40% of the females were infertile. while at 3 months
female fertility declined and male fertility tended
to improve. These observations were broadly inaccordance with histological findings. Analysis as afunction of dose showed that the fertility of both sexes
was affected by doses in excess of 0.5 Gy, while. at aconstant dose rate. 3.3 Gy from HTO would beneeded to decrease reproductive performance. The lateorganogenesis stages were found to be the mostsensitive under these conditions.

I 89. Torok et al. [T7] reported a significant decreasein the weight of brain and genital organs at 4.5 months
in the offspring of NMRI mice given a single intraperitoneal injection of tritiated water (2.6 MBq/g) at
9 days of pregnancy. The number of oocytes in theovaries was reduced and the seminiferous epitheliumof the male mice was considerably damaged. These
mice had been found to be fertile at 2 months of age.
while at the same age doses twice as large affected thefertility of females but not that of males, and doses
four times as large resulted in sterility of both sexes.
At 18 months of age, the ovarian tumour incidence ofthe controls (14%) was increased almost 5-fold (67%)
by the administration of about 10 MBq/g of tritiatedwater. As to effects before or at birth. about 20 MBq/g
at 7, 9 or 11 days p.c. resulted in 100% peri-natal
mortality. increased resorption and stunting. but
negligible incidence of gross external or skeletal
malformations. Administration of tritium in excess of
37 MBq/g caused maternal death. In the offspring.
damage to the prosencephalon and the gonads stoodout generally as the most prominent histologicalfinding.
2.

The rat190. Coffigny and his group [C5] reported on
experiments in which pregnant (14 to 21 days p.c.) andnew-born rats of up to 2 days of age received a single
dose of 1.5 Gy of 60 Co gamma radiation. Neo-natal
mortality, body weight and adult brain weight were
investigated. Among the effects found, germ cellkilling was particularly notable. This became thesubject of another publication [C7] in which theobservations were extended to animals irradiatedbetween 10 days p.c. and 6 days after birth, with thesame dose being delivered. When the male animals
irradiated in utero reached adulthood, they werestudied for testis weight and histological appearance.
for the weight of the epididymis and seminal vesicles.
and for testosterone plasma concentrations. Testis
weight was found to be slightly reduced by irradiation
at day 15, but progressively more so at later times,
with a maximum between 18 days p.c. and 3 days after
birth. A reduced number of spermatogenic cells in theseminiferous tubules accounted for the reduction inmass of the organs. The weight curve of the epididymis
followed, in the main, that of the testis but the weightloss of the seminal vesicles was rather moderate.
Testosterone secretion was not adversely affected.
suggesting that the endocrine function of the testis is.
on the whole. rather resistant, as might be expected.
300

191. The same conclusions were drawn from anotherstudy [C6] in which the adrenal activity of male andfemale rats was tested after 1.5 Gy administered on
day 15 of gestation. the time when the hypothalamic
nuclei and the pituitary and adrenal glands begin todifferentiate. When the irradiated animals reachedadulthood. their brain mass was severely decreased,
both absolutely and in relation to the whole bodymass. The weight of the pituitary and adrenal glands
was also reduced absolutely, but not in relation to thewhole-body weight. The functional activity of the
hypothalamo-pituitary-adrenal axis. as measured byadrenal and plasma corticosterone levels. their diurnal
variations, their response to stress or pharmacologicalchallenge were found to be normal, in spite of thegross lesions of the brain and gonads.
192. Reported changes in the fertility of females
irradiated during the stage of major organogenesis are
scanty. Shapiro [S53] noted the occurrence of sterility
in 36% of female animals that had exposures of 50 R
at 9 days p.c. Tests conducted by Kakushine andPlodovskaya [K29] showed that 70% of rats exposed
to 250 R on day 20 p.c. were sterile.
193. Pietrzak-Flis [P5] investigated the effects of low
doses of tritiated water or organically bound tritium
ingested chronically. on the number of oocytes in theovaries of contaminated rats. After adjusting the3
H concentration in water and food to levels thatwould give similar total ingestion, female rats were
exposed starting 5 weeks before mating with a nonex posed male and -up to the 21st day of life of theiroffspring. Female offspring of the F I and F 2 generation
were sampled at 3 weeks of life for tritium analysis
and oocyte counts. There was a close similarity of thedoses calculated in different tissues between animals
receiving tritiated water or organically bound tritium.
although the contribution of the non-exchangeable
tritium to the total dose was on average 3 to 7 times
higher in animals fed tritiated food than in thosedrinking tritiated water. A lower (although not statistically significant) number of oocytes wa~ found incontaminated females than in controls. On the basis ofsome simplifying assumptions, it was calculated thatthe mean dose to the ovaries was 73 mGy. This led
to a 10% loss of oocytes, both in F 1 and in
F2 animals. It appeared. therefore. that the total
absorbed dose, rather than the form under which
tritium was administered. was the variable thatmattered most for oocyte survival.
194. In a subsequent series of experiments [Pl5], thesame author exposed rats continuously to a constantactivity of tritium in the drinking water or toorganically bound tritium from the time of conceptionof the F I generation through maturity. She calculated
that the two groups of animals exposed to tritiatedwater had a mean absorbed dose rate in the ovaries of
7.25 ± 0.37 and 14.73 ± 0.79 mGy/day. respectively.
The group receiving tritium-contaminated food received
on average 4.84 ± 0.25 mGy/day. Female offspringwere killed when 21 or 71 days old for oocyte
counting. The reduction of the number of oocytes in
the animals exposed to organically bound tritium waslarger (for the same dose rate) than in the animals

exposed to tritiated water. and the dose-rate dependenceof the reduction of small oocytes was of an exponential
type. The damaging effect of tritium appeared to behigher between birth and 3 v.-eeks of age than between
21 and 71 days of age. The highest sensitivity totritium irradiation was seen in small oocytes and in
oocytes with one complete layer of follicle cells:
disappearance of these highly sensitive cells resulted.
in turn. in an increased relative frequency of growing
and large oocytes.

3.

Other animals

195. Fractionated x-ray exposures (0.115 Gy per
fraction given twice weekly up to 2 Gy) wereadministered to 12 female bonnet monkeys either
between 48 and 104 days or between 77 and 133 days
p.c. Follicle counting in the animals' ovaries was
performed between 3 months and 1.6 years of theirpost-natal life. Animals exposed in the first group had
a number of follicles approaching that of nonirradiated controls (about 2000), while the others
revealed severe ovarian damage. abortive oogenesis.
and reduction of the follicle counts to less than onethird of the normal. No other damage in any organ
was found except in the ovary [AS, A6].
196. Data concerning gonadal damage by pre-natal
irradiation are also available in the cow [E3]. Cows
bearing conceptuses of varying ages from 40 ± 5 days
p.c. (when gonadal sex differentiation occurs) to
270 ± 10 days (about 2 weeks before birth) were
irradiated once with 60Co gamma rays (300 R). an
exposure known to be just below that likely to causematernal death and gross abnormalities in the productof conception. This resulted in an absorbed dose in
the feta! gonads estimated to be about I Gy. Whenfemale offspring reached about 10 months of age,
their ovaries were histologically analysed to count thefollicles in different stages of maturation. Primordial
follicles were significantly lowered (to 64% of control)
for exposure between 70 and 90 days of gestation.
Since at this stage the germ cell population ischaracterized by a high proportion of mitotically
active oogonia. it was deduced that this class of cells is
the most vulnerable to radiation. Follicular development, however (as shown by counts of growing andvesicular follicles), was apparently unaffected byirradiation at all ages tested. Cows irradiated prenatally under the same conditions at 80 ± 10 or
130 ± 20 days p.c. had a fertility rate and a reproductive performance that were not different from controls
over at least five years of post-natal life and threepregnancies.

4.

Interspecies comparisonspremature end of reproductive life. Despite wellknown differences between the radiosensitivity ofhuman and mouse oocytes (see annex A), the authors
attempted an extrapolation between the two species,
using tritium data from monkeys. They concluded
that pre-natal irradiation was likely to result inpremature menopause in women and could possibly
follow "relatively low exposures··.
198. Two papers [09 and E2] attempted someinterspecies comparison of gonadal cell sensitivity inutero under internal and external conditions of exposure, respectively. Dobson et al. [09] exposed micefrom conception to 14 days of age to tritium indrinking water and established that the concentration
required to cause 50% loss of oocytes under these
conditions was about 74 kBq/ml of body water.
delivering about 0.0056 Gy/day. Female germ cells ofsquirrel monkeys required an even lower level (about
19 kBq/ml) for the same end-point. Over the full
153 days of gestation of the species, this level would
correspond to a total dose of 0.168 Gy. which might
further drop to 0.056 Gy if the sensitivity of theoocytes were confined to the last third of gestation.
These data are in sharp contrast with the well-known
radioresistance of primary oocytes in monkeys andman to acute x-irradiation exposures. It must be
assumed, therefore. that the germ cells in the feta!
primate are going through a highly sensitive period ofshort duration. occurring probably around the middleof the last trimester of gestation in the monkey, atwhich time doses of less than 0.05 Gy are sufficient tocause 50% killing. It appears that in mice this stage ofhigh sensitivity is maximum between 5 and 19 days
post-partum. a time at which susceptibility is also highin respect of the toxicity of the polycyclic aromatichydrocarbon 3-methylcholanthrene. Similarities between rodent and primates. and between radiation andchemically related sensitivity, suggest that there may
also be a highly vulnerable period for the oocytes of
the human fetus located approximately at the middleof the third trimester of pregnancy, which might
represent a special hazard in relation to chronic
exposure.
199. Erickson [E2] irradiated continuously, throughout gestation. mice, rats and guinea pigs at rates of
0.01 to 0.03 Gy/23 h day. Pigs were also irradiatedcontinuously for 108 days of gestation at 0.0025 Gy/
23 h day. In all species. the gonads were sampled atbirth or at 6 days of age and the numbers of germ cells
per organ were estimated as a function of dose. The
0 0 and extrapolation numbers (n) were calculated to
be as follows. on the basis of a single-hit multi-target
model:
Pig

Rat

197. When Dobson et al. [025) examined the effectof tritium on fertility by measuring the life-long
reproductive capacity of treated mice, they found that
reproduction was less affected than germ cell number.
For doses causing early oocyte losses of 80-90%, thenumber of offspring only decreased by 20%. However,
initial oocyte deficiencies of about 50% resulted in amalefemalemale
femaleDo(Gy)

n

0.28
0.27
2.75

0.8
3.2
0.3
0.8

1.59

The low value of the extrapolation number suggeststhat in both males and females (but especially in theformer} the germ cell response is dominated by single
events. The value of 0.3 for the extrapolation number
in male rats could also be attributed to heterogeneity301

in the composition of the irradiated cell population
with respect to their sensitivity to radiation exposure.
200. After irradiation at 0.01 Gy/day throughout
pregnancy, the germ cell counts were reduced to thefollowing percentage of the controls in the species
tested:Tora/ dose

Pig
Guinea pig
Rat
Mouse

Percent of control

(Gy)

Male

Female

1.08
0.62
0.21
0.20

I

5

41
50
71

71
90
87

These data show that the pig is by far the mostsensitive species with respect to continuous irradiation
during the pre-natal period, but this conclusion does
not take into account the large interspecies differencesin the length of the developmental period of the maleand female germ cells. The paper [E2] provides usefuldata for the relevant comparisons, collated from otherreports and from original work (references are to befound in publication [E2]). The data are summarizedin Table l l.
201. Erickson 's data [E2] show that in spite of thelarge variation of the kinetics of primitive germ stem
cells in the various species, the pig remains the mostsensitive species even when total doses (rather thandose rates) are used as the basis for comparison. The
data also show that male germ cells are more readily
killed by protracted exposure than female ones as aresult of both their kinetics and of the protracted
period of vulnerability. However. the data do notoffer a satisfactory account of the relatively lesser
susceptibility to acute than to protracted regimes ofirradiation, because they may not entirely explain how
the refractoriness to acute exposure of a significant
segment of the primitive germ cell population might
be overcome by chronic exposure. The differences
shown in Table 11. and various arguments put
forward in the paper, point to the need to extend
consideration of this subject and of the relevantinterspecies comparisons much more than has been
the case so far. Sensitivity to mitotic delay. in addition
to cell killing, will have to be explored further and in asystematic manner in future work.

B.

OTHER MISCELLANEOUS EFFECTS

202. A number of other miscellaneous effects havealso been described. Thus. for example. Swiss albino
mice irradiated in utero at days l l 1/4, 14 l/4. 16 l/4
and 18 l/4 p.c. with 60Co gamma rays (150 or 250 R)
showed a dose-related decrease in the number ofgoblet cells in the crypts (and to some extent also inthe villi) of the gut at I day and l, 2, 4 and 6 weeks
post-partum [M46]. Chajka and Lobko [Cl6] studied
rats irradiated with x rays at 12-14 days offetal age andinvestigated from day 13 to 22 the formation of theparamesonephral tracts. describing in detail disturbances in the development of the vagina.
203. Gerber and Maes [G4] investigated the radiosensitivity of blood-forming stem cells in the bone302

marrow and the spleen during the feta! hepatic period( 18 days p.c.) and during the post-natal and adultphases of haematopoiesis. On the basis of theircapacity to produce spleen colonies in heavily irradiatedtransplanted mice. the stem cells isolated from thefeta! liver had a D 0 of 1.29 Gy, as compared with
0.70 Gy and 0.86 Gy for the spleen and marrow cellsof adult animals. respectively. Together with a decreasein the D 0 values after weaning. the percentage of cells
in the S-phase and the rate of division of transplanted
cells in the irradiated hosts also decreased. There was,however, no significant difference in the D 0 values
between haemopoietic cells in all phases of the cell
cycle and the 3H-thymidine-sensitive cells in the
S phase.
204. Weinberg and her group [W24. W25] described
disturbances of B6D2Fl mouse and HA mouse [W33]haematopoiesis found at various times of pre- andpost-natal life and induced by irradiation (0.5-3.0 Gy,
60Co) of the animals at 10.5 days p.c. They consisted
essentially of disruption of feta! liver cellularity,morphology and haematopoietic cell concentration
extending to 14 weeks of extra-uterine age. The sameauthors also examined haematopoietic alterations in
the beagle dog irradiated during mid-gestation at
33 days p.c. (an age corresponding approximately tothat of the 10.5 day-old mouse) with 0.9 Gy from
6°Co gamma irradiation [W21]. Various blood indiceswere investigated at 42-59 days of gestation. Differences
between irradiated and non-irradiated animals included
a decrease of nucleated cell counts in the peripheral
blood between 44 and 49 days. followed by anovershoot at later times: changes in the size andhaematopoietic activity of the spleen: an overall
decrease in haematopoietic functions of the feta! liver:and an increased activity of the bone marrow. The
experiments left undecided whether the observedchanges might be due to direct radiation damage on
the migrating stem cells or to a radiation-induced
defect of the intercellular interactions in the microenvironments that regulate haematopoiesis.
205. In other experiments [M2], pregnant sows were
exposed to whole-body gamma radiation at rates of
1.5, 3. 9. or 20 R per day (exposures actually lasted
22-23 hours per day) for 108 days of their average
gestation period of 112 days. After 3 weeks ofcontinuous irradiation, all groups had depressedleukocyte counts which were generally related to the
exposure rate. Counts remained low throughout thewhole irradiation period and they did not completely
recover even 3 weeks after discontinuation of exposurein post-natal life. As to the effects on the piglets, thosethat had been exposed to 3.2 R/day showed no effecton the leukocyte system, and those in the 7 R/day
group had a moderate Jymphocytopenia at birth
which rapidly disappeared. This differential effectbetween adult and feta! animals took place in spite of
the fact that the fetal leukocyte system is functionallymature during the last 5 weeks before birth.
206. To facilitate extrapolation of the above data,
it should be recalled that in man the first blood cells
and blood vessels are found in the extra-embryonicmesenchyme at about I 8 days from conception. Thisfirst period of haematopoiesis is succeeded and gradually replaced by blood formation within the embryo.
chiefly in the mesenchyme of the liver, during thesecond month of development. At still later stages,
starting from the 11 th week on. a third period of
haematopoiesis commences in the bone marrow andthe lymph nodes. There is considerable overlapping
between the three periods and it is only the last type of
haematopoiesis that persists normally in adult life[H27). Data obtained directly on human samples are,however. very rare. In one paper, no post-natal
abnormality appeared to be associated with feta!
exposure in the range of 0.01-0.03 Gy of 498 children
born during 1963-1977 and examined when 5-19 years
old. There were after birth 5 deaths due to accidents
or acute infections. but the 493 survivors were normal
in respect to stature, peripheral blood counts, mental
development and, in females, menarche [LJ]. In
contrast. 25 children exposed in utero to diagnostic x
irradiation after the 8th month of intra-uterine life toestimated doses of about 0.03-0.2 Gy showed significantly lowered leukocyte counts at 2 days and I year
after birth [Z4].
207. Konermann [K33] reported a post-natal studyof the liver of mice in which he analysed, as a functionof dose and time. the growth response of the organ in
animals that had been irradiated in utero. The animals
were exposed to between 10 and 60 R/day during
different periods of their pre-natal life. as follows:
blastogenesis (l-5 days p.c.): organogenesis (6-13 days):fetogenesis (14-18 days); early embryogenesis(6-10 days); and late embryogenesis (l 1-15 days). In
addition to weight. the content of DNA and, the3 H-thymidine uptake into liver cells were followed for
up to 11 weeks after birth. All irradiated animals
showed a periodical increase of the proliferative liver
growth, especially between the end of the first and the
beginning of the second post-natal week. This proliferation decreased with the age of the irradiatedconceptuses. from a clear over-compensation during
blastogenesis to poor response during fetogenesis.
There was also a dose dependence of the effect which(during organogenesis) was biphasic. in the sense thatcompensatory proliferation was small at JO R/day.
maximum at 40 R/day, and declining at 60 R/day.
Overall. these responses were interpreted as attempts
by the organ to attain a pre-determined normal size,
but appeared to differ in many respects from thehypenrophic regenerative changes induced by postnatal liver damage. First. pre-natally induced proliferation affects, at the same time and to similar
extent. both the haematopoietic and the parenchymal
cells of the feta! liver, which are considered to befunctionally independent components of the organ.
Secondly, this compensatory growth is maximum atsimilar developmental stages and the maximum is not
greatly dependent on the radiation dose and on theage at irradiation. Finally, radiation-induced growth
stimulation of the liver appears to take place in theabsence of any change or even in the presence of adecreased overall body weight.
208. About 100 beagle dogs exposed to 60Co gamma
radiation (270-435 R) at 55 days p.c .. or at 2 days
post-natal age, were examined at 7 days and at 2 and4 years of age for effects of radiation on the kidney[J2]. Morphometric studies showed an arrest ofdevelopment and maturation of the organ. For prenatal exposure, the density of mature glomeruli was
45% less than in non-irradiated controls. while forpost-natal exposure the density was only 24% less. At
2 and 4 years, a smaller reduction was observed in thesame direction (37 and 32%, respectively). Glomeruli
that were mature at the time of exposure underwent
progressive sclerosis, characterized by proliferation of
the mesangial cells and increase of the mesangial
matrix, leading to a reduction of the lumen of thecapillaries and the complete obliteration of many
glomeruli. Thus, about 21% of the animals died fromchronic kidney failure before 4 years of age: thiscondition was more common in male (19/58) than infemale dogs (2/41), pointing to a major sex differenceof unexplained origin.
209. It was also reported in the same series ofexperiments [L2] that an exposure of 100 R at 55 days
p.c. resulted in about 80% of the animals having
missing teeth, compared with about 57 and 26% inanimals irradiated at 2 days p.c. or in control dogs.
The upper and lower premolars were the teeth mostfrequently absent.
210. Developmental toxicity evaluations do not often
include functional deficits, but are mostly limited togross effects such as embryo lethality. malformations
and growth disturbances. In a methodological contribution, Andrew [A 7] proposed the study of developmental enzyme patterns as a means of directly
evaluating the integrity of metabolic competence ofdeveloping organ systems. The full complement of
enzyme activities is usually developed in mammals ina characteristic sequence: alterations from this normal
pattern may occur at three periods. namely, the latefeta), early neo-natal and weaning times. The author
argued that qualitative or quantitative changes in thepatterns of development of one or more key enzymes
in a tissue might be taken to indicate developmental
toxicity. Some contributions reported along thoselines in respect to radiation toxicity were alreadydiscussed in the 1977 UNSCEAR report [U2].
211. Some papers from the USSR dealt with disturbances of the enzyme levels in the liver of rats
irradiated in utero. The first [S51] reported a reproducible decrease of the glucose-6-phosphatase activity
observed in 21-day old fetuses that had been irradiatedwith 2 Gy on day 9 p.c. As was later shown. thisdecrease may be removed by the administration ofexogenous thyroxine [K25]. The second paper [S52]reported increased tyrosine aminotransferase activity
in new-born rats irradiated with 0.5 Gy on the 9th day
after conception. This effect correlates with the statusof the cyclic-AMP system. It was proposed that theabove effects in the liver of the rat at term. following
irradiation during early organogenesis. may result
from changes in the amount of inductors [K25].
212. Brain weight and enzymatic activities were also
investigated by Weber et al. [W8]. In an attempt todevelop more sensitive end-points of damage by prenatal irradiation, NMRI mice received a range of303

doses (x rays, 0.24 to 1.9 Gy) on the 12th day ofgestation. They were then sampled at various times
between birth and 64 days of age to investigate theweight of their brain and the levels of acetylcholinesterase and of Na,K-A TPase in the organ. Brain
weight was related to the enzyme activities in a nonlinear fashion: for both enzymes there was a relatively
flat dependence up to a brain weight of 350 mg(corresponding to a control age of about 17 days) anda steeper relationship as the brain weight increased.
The weight of the brain was reduced in increasing
proportion to dose to reach about 40% of control
after the highest dose. The data were interpreted toshow that, in spite of this reduction, the developmental maturation of the organ was not retarded.

C.

CONCLUSIONS

213. As the growth of the anatomical structures
established--during the phase of major organogenesis
proceeds during feta! development, fewer teratologicalmalformations are being produced for the sameradiation dose. However, more subtle changes, often
documented at the microscopic level. and growthdefects of the developing structures, are commonlydescribed. During feta} development, the periods ofmost active growth of anatomical structures are also,
in general. those that are most susceptible to radiation
action. Approximate dose-effect relationships may beestablished for both external and internal exposures inthe best documented series.
214. Among the many effects produced by external
irradiation of feta! animals, those on the developing
gonads have been particularly well described for avariety of animal species to a reasonable degree ofquantitative detail. In the mouse, studies were made ofboth the inactivation of gonadal cells and the ensuingloss of reproductive performance when the animals
reached sexual maturity. For external irradiation.
female fertility was not appreciably affected up to
200 R of ancestral irradiation, but sterility increased
conspicuously when the same exposure was delivered
directly to a developing animal. Pronounced sigmoid
relationships appeared to prevail for the impairmentof reproductive capacity, and a good degree of interstrain variability has been documented.
215. In the mouse. experiments on internal tritium
irradiation were reported after long-term exposuresfrom conception up to 14 days of age. as well as for
single treatments delivered at various times during
development. Under the first condition of exposure.
the activity of tritiated water needed to cause a 50%
loss of oocytes in the female gonads was estimated atabout 74 kBq per ml of body water (corresponding toa uniform whole-body dose of about 0.12 Gy) with anapparently exponential relationship between absorbed
dose and oocyte killing. For single injections, the latestages of organogenesis were the most sensitive inrespect to loss of fertility. In both sexes, fertility wasaffected by doses in excess of 0.5 Gy. but male gonads
tended to recover at longer times from treatment.
while female fertility continued to decline.
304

216. In the rat. external radiation exposure resulting
in doses of 1.5 Gy caused measurable male germ cellkilling. which persisted long after birth without
evidence of any adverse effect in the endocrine
function of the gonads. or of the hypothalamopituitary-adrcnal axis. Tritium. given as tritiated wateror as organically bound tritium over many generations
to developing animals. caused a loss of oocytes in thefemale gonads for doses of the order of 0.07 Gy. In
the bonnet monkey, fractionated exposures up to a
total of 2 Gy for periods of about 50 days during thelatter part of pregnancy resulted in a consistent Joss ofoocytes. which persisted into adult life. The same istrue for cows irradiated in utero at doses of 1 Gy
during the phase of active mitotic division of theoogonia at 70-90 days p.c. However, there was noevidence in these animals that fertility and reproductive performance would be affected, at least over the
initial period of their reproductive life.
217. Valuable interspecies comparisons of the sensitivity of the developing gonads under internal andexternal radiation exposure have also become available.
For one species of monkey, there is evidence that thegerm cells go through a highly sensitive period ofshort duration around the middle of the last trimesterof gestation when doses from tritiated water, of theorder of 0.05 Gy, might cause one-half of the oocytes
to die. For external irradiation. comparisons ofvarious species showed the existence of a wide scale ofsensitivity values. It is clear that the intrinsic radiosensitivity of the gonadal cells, together with theirkinetic characteristics. determine in the various species
the apparent sensitivity of the irradiated germ cells. It isnecessary. therefore, to make comparisons between
species, not only in respect of dose-rate, but also interms of total dose received by the germ cells during
the whole sensitivity period. When this is done,paradoxical effects such as an apparent higher responseof the developing testis to protracted than to acuteirradiation are found. This is indicative of the existenceof phases of high sensitivity in the development of themale gonadal cells. an effect known to occur also in
the adult testis. Much remains to be done to resolveand clarify these inter-species differences upon feta!
irradiation.
218. A variety of miscellaneous effects have also
been described for irradiation of feta! stages. The
blood-forming system has been closely investigated.
but detailed analysis of the radiation action is madedifficult by the transition from the hepatic to the adulttype of haematopoiesis which occurs before birth.
Acute doses of a few tenths of Gy, or continuous
exposures to a few roentgen per day, are necessary toelicit consistent changes in the blood-forming organs
and the peripheral blood of the fetus. The persistenceof the changes observed throughout the post-natal
ages. and their significance in respect of normal
haematopoiesis. are difficult to evaluate. Time- anddose-related hyperplastic reactions of the irradiatedfeta! liver. both in its haematopoietic and parenchymal
component, have also been described. They may
generally be interpreted as compensatory regeneration
stimulated by the radiation-induced killing of cells.
But they have a distinctly different character from the'regeneration phenomena induced by post-natal liver
damage. Kidney lesions may also be induced byirradiation during the feta! stages. This damage
primarily affects the capillaries and is mostly manifested by glomerulosclerosis resulting in chronic kidney
failure during extra-uterine life.
219. Finally, attention should be drawn to recent
attempts to develop tests of toxicity induced in
irradiated feta! organs by measuring the level ofenzyme activity and the development of the characteristic adult enzyme pattern. These tests are different
from macroscopic or microscopic methods for scoringof malformations because they aim at documenting
functional. rather than morphological. derangementsfrom normal. While they may turn out to provide
more sensitive techniques of assaying the radiationinduced damage in utero, their full methodologicalpotential, and their relevance to clinically documented
detriment, remain to be established.

V.

INTERNAL IRRADIATION

220. The information available to UNSCEAR up to1977 was insufficient for statements of general validity
regarding the effect of radioisotopes taken in by themother and irradiating conceptus. Given the chronic
nature of the exposure, any evaluation of the effects
and risks must be based on a precise knowledge of thedose accumulated at each particular stage in any given
developmental structure, but the achievement of sucha task for all nuclides and all possible conditions ofadministration is still far away. UNSCEAR identified
a number of variables which may interact in the
production of any end-effect. They are: the physicaland biological characteristics of the nuclide: its chemical
form: the route and the dosage schedule of administration: the kinetics of the radioactive material in themother-fetus complex: the developmental stage at thetime of treatment in relation to periods of maximum
sensitivity: the species and age-related variables: and.
finally. the role of maternal exposure, particularly athigh doses.
221. Information in this field that has come to theattention of UNSCEAR since about 1977 has added
somewhat to the previous knowledge. However, this ishardly significant compared with what is required to
produce widespread generalizations relating the levelof uptake of radioactive substances by the mother tothe probability of effects that might be expected in theconceptus. The following paragraphs summarize themain new findings regarding the various nuclides.
222. The methodologies for the study of placental
transfer have been reviewed [S 16]. Moreover. factors
influencing the transfer of non-radioactive substances
injected into the mother have been discussed by
Klinkmilller and Nau [K7] who have compiled a
review of the transplacental passage of drugs throughexperimental observations carried out on aborted
children. Some information on the placental transfer
of proteins in the human species is also to be found ina paper by Dancis et al. [D2].

223. Sticve [S66. S68]. Roedler [R27] and Roedler et
al. [R28] reviewed experimental literature (see also areport by Ahrens et al. [Al I] for a bibliographic list)
and discussed the general aspects of exchange andtransfer mechanisms of radioactive substances between
the mother and the conceptus as they vary with thetime of intra-uterine development, and the various
physical and biological factors governing such transfers. It is obvious from these analyses that the need todetermine experimentally the transfer coefficients andtheir inter-species variability prevents any straightforward extrapolation from animal to man. Examples
drawn from experiments using tritium. iodine, calcium
and caesium were given to illustrate the complexity ofthe problems involved in assessing the rate andamount of transferred radionuclides. Transplacental
transfer of iron in animals and man have also been
discussed [F9. T14]. Data referring specifically totritium have been provided by Moskalev et al. [M 15].
and a compilation of data for the radioactive nuclides
of iodine, iron, strontium and caesium has also been
made available to UNSCEAR (S8 I].
224. Ovcharenko (010] examined the transfer ofradioactive substances from the mother to the newborn through lactation and concluded that the rate oftransfer is governed by a number of factors similar to
those affecting the passage across the placental barrier.
In particular. he reported on an inverse relationshipbetween the rate of transfer and the mass number ofthe radionuclides. in addition to a profound influenceof the animal species and the body weight of theoffspring. He pointed out, however, that the main
factor in the amount of radioactive material passed on
to the offspring through the milk is the level ofradionuclide in the mother's blood.

A.

EXPERIMENTAL DATA
1.

Tritium

225. Yamada et al. [YI] investigated the response of
pre-implantation BC3F I mouse embryos to chronic
exposures of both tritiated water or 60 Co gamma
rays. Embryos were cultured in vitro in chemicallydefined media containing tritium oxide at activity
levels from 3.7 to 74 MBq/ml. Taking the progressionof 50% of the embryos to the blastocyst stage as theend-point for survival, the activity needed to achieve
this end-point (LD 50 ) was found to be about 4.9, 8.5
and 16 MBq/ml for pronuclear, early 2-cell and late2-cell embryos, respectively. In comparison with thechronic 60Co treatment. the RBE of the beta radiationof tritiated water was calculated to be between 1.0 and
1.7. On the basis of other experiments on the inductionof chromosomal aberrations in mouse zygotes fertilized
in vitro, the RBE values for tritium were reported tobe 1.8 and 1.5, relative to 60Co gamma rays and to
x rays, respectively [M56].
226. In another study [S 18], an attempt was made tocompare the outcome of tritiated water and tritiatedthymidine treatments on pre-implantation mouse embryos cultured in vitro. Also in this study. the end305

point scored was the ability of the embryos to reachthe blastocyst stage; tritiated thymidine was about1000 times more effective than tritiated water ininhibiting blastocyst formation. The treatment withtritiated water was particularly effective in inhibiting
the late stages of blastulation, while thymidine blocked
the rapid cleavage stages. Thymidine was concentrated
in the nuclei through DNA incorporation by a factor
of about 1000. In very general terms. and under
specific assumptions applying to the system analysed,
it was concluded that there was a reasonable agreement between the dose per cell nucleus and theinhibition of blastocyst development. such that about
19 kBq/ml of tritiated thymidine corresponded toabout 19 MBq/ml of tritiated water. Observations on
the development of pre-implantation mouse embryos.
and a discussion of effects in relation to path lengthand energies of the electrons emitted by 3H and 35S,
are to be found in a paper by McQueen [Ml].
227. Spindle and Pedersen [S74] investigated theeffect of 3H-thymidine on the development of earlymouse embryos by counting the number of embryos
forming blastocysts, trophoblast outgrowth, inner cellmasses and differentiated primary endoderm andectoderm. Embryos were cultured from the 2-cell stage
continuously for 8 days in the presence of 3H-thymidine
in various concentrations. Concentrations as low as
7.4 Bq/ml reduced the percentage of embryos forming
the two differentiated primary layers. At 370 Bq/ml.
the embryos developing to the three post-blastocyst
end-points were also reduced. Exposure of the embryos
for 1 day at various developmental stages produced
effects that were Jess severe than for continuous
exposure for 3 or 8 days. The sensitivity of theembryos differed with stage: the high sensitivity of theinner cell mass to 3H-thymidine. while persisting
through the late blastocyst stage. declined progressively
thereafter. Autoradiography experiments showed thatthe radiosensitivity changes of the embryos or of theinner cell mass were generally related to their capacity
for incorporation of thymidine into the cell DNA.
228. Yamada and Yukawa [Y4] fertilized BC3Fl
mouse oocytes in vivo or in vitro and then cultured
the embryos in vitro up to the stage of blastocyst in
the presence of chronic irradiation from tritiatedwater or thymidine, or under acute x-ray exposure in
the range of 10-600 R. Taking the development of theembryo to the blastocyst stage as the end-point. thesensitivity of the embryos to the /3 radiation oftritiated water was confirmed as in Yamada et al.
[Yl], while the LD 50 for tritiated thymidine was given
as about 0.63 and 3.96 kBq/ml for pronuclear and late2-cell embryos, respectively. The highest sensitivity tokilling by acute x irradiation occurred at the beginningof the pronuclear formation.
229. Some papers dealt with tntmm uptake andturnover in pregnant and feta! animals. Saito et al.
[S34] exposed pregnant mice of the Heiligenberger
strain to tritium in drinking water as methyI-3Hthymidine at the level of 18.5 kBq/ml. The new-born
animals were either nursed by their mothers that
continued to drink the tritiated thymidine, or by foster
mothers drinking non-radioactive water. The authors
306

measured in the offspring, at various ages, theincorporation of tritium into the small molecularcomponents of the acid-soluble fraction, lipid, RNA,DNA and proteins. At the time of birth. the specificactivity of the isotope in the DNA was highest in theheart and lowest in the thymus. Soon after birth, thetotal radioactivity per gram of tissue declined with ahalf-life of 2.5-2.9 days in the spleen. liver, intestine,
stomach, thymus. lungs, kidney, heart and brain in themice nursed by the non-contaminated mothers. By
about 2 weeks post-partum. a slower component oftritium elimination was emerging, mainly due to the
DNA-bound isotope. On the assumption that the total
and DNA-bound activity is uniformly distributed inindividual organs and tissues, the cumulative absorbed
doses in various organs were worked out for the first
4 weeks after birth. These doses were found to behighest in the spleen ( 1.15 mGy) and lowest in thebrain tissue (0.13 mGy). The tritium ingestion frommilk in suckling animals was found to be a rather
minor source for dose accumulation in the DNAbound tritium of the cell nuclei of various organs.
Similar data were also presented in another paper by
the same group [S35].
230. Lambert and Phipps [L7] infused SAS/4 micecontinuously during pregnancy at four activity levels,
such that the new-barns would be contaminated withorganically bound tritium in concentrations of
10.4-130 MBq/kg body weight. Total doses in utero
were calculated to be in the four injection groups 0.15,
0.4, 0.76 and l.72 Gy. The tritium content of variousorgans was measured in the mice at frequent intervals
up to about 2 months of age. and again when the
animals died. Full pathological investigations were
performed. Early effects were seen, such as reductionof litter size, changes of the sex ratio. and neo-natal
mortality, particularly at the two highest activity
concentrations. These effects were judged to be moresignificant overall than late ones, such as a reductionof the long-term survival time which appeared in thehighest activity group. There was an indication of anincreased incidence of neoplastic and non-neoplastic
lesions, possibly related to the intra-uterine dose;
however, the significance of the various causes ofdeath could not be judged without a full statisticalanalysis. which was not performed.
231. Wang and Zhao [W34] performed an experiment in which the effectiveness of /3 radiation fromtritiated water and gamma rays from 137 Cs wascompared in respect to their capacity to reduce
cerebral development in Wistar rats. Females were
injected with HTO in various concentrations on thefirst day of pregnancy and provided from then on withtritiated drinking water. so as to maintain the tritium
concentration in body water approximately constantthroughout pregnancy. The intra-uterine dose rates in
the five groups of animals examined were estimated tobe 0, 0.005, 0.012. 0.02 and 0.038 Gy per day. Other
pregnant females were irradiated during 22 hours per
day throughout gestation with a 137 Cs source at 7 dose
rates between zero and 0.12 Gy per day. There was agood negative correlation between the size of thecerebral mass at birth and the cumulative radiationdose in the two series. Approximately the same

reduction of the brain weight was produced by
0.22 Gy of /3 radiation delivered over the whole
pregnancy as by 0.47 Gy of gamma radiation (RBE
about 2.14). At higher total cumulative doses (0.84 Gy
and 1.3 Gy, respectively) the RBE was found to belower (1.55). A significant decrease of the whole-bodyweight at birth was only found for cumulative gamma
doses of 1.76 Gy or higher. No dose dependence wasseen at lower dose levels.
232. In experiments by Lyaginskaya [L9), rats were
given single intraperitoneal injections of tritium oxide(33 MBq/g) on days 4, 11 and 17 of pregnancy. This
treatment was said to have resulted in feta! exposuresof about 4.12. 3.00 and 2.50 Gy. respectively. by theend of the pre-natal life and 412. 420 and 478 Gy by
the time of weaning. The descendants of threesuccessive generations of the injected animals showeda reduction of life span and an increased frequency oftumours. The average life span in the various groups
was as follows:
Life span (days)
Trearment
rime

First
generation

Secondgenerarion

Third
generation

4th day
I Ith day17th dayControls

378
556
705

394

910

1060

280
288
618
1020

705
735

Thus, the greatest reduction of life span was shown by
the third-generation descendants and by the offspringof animals treated early in pregnancy. The overalltumour incidence increased in the offspring from thefirst to the third generation and was again highest in
the offspring of animals injected on day 4.
233. Gerber and Maes [G3) raised pregnant BALB/cmice on food enriched with tritiated thymidine fromthe day of conception until delivery. for a total tritium
intake of 4.8 MBq. At birth. and again at 4 different
times up to 109 days of age, the tritium content wasmeasured in the DNA, protein and supernatantfraction of various parenchymal organs. It was foundthat a total of 2.0-2.5% of the total activity in the neonatal mouse was incorporated into the DNA. while aslightly larger amount entered the proteins and lipids
together. Most organs showed two metabolic components of the DNA, one with a half-life of the order of
10-20 days and another (amounting to 10-30% of thetotal activity incorporated into DNA) with a very slow
turnover. of the order of 100 days.
234. Data along similar lines were also reported in
the pig. after exposure of pregnant sows to tritiatedwater (about 19 and 55 MBq/liter) during pregnancy
and for 43 days after birth, up to a total of 120 days
[V7). Some of the new-born pigs were left with themother and the others were foster-fed by noncontaminated sows in order to follow the tritiumoxide and organic tritium in different organs. This
allowed the study of the loss of activity after birth. themetabolism of tritium under conditions of continuing
exposure and the importance of uptake through milk.
The turnover time of tritium oxide in the adult sow
was found to be about 10 days. that in younger pigs
about 8 days. There were, in addition. components ofslow turnover. amounting to less than 5% of thetotal. At equilibrium. the ratio of specific activity oftritium oxide in organic matter to that in tritiatedwater ingested was about 0.7 for body water in adultand new-born pigs and about 0.14 for organic tritiumin most tissues. The ratio was higher (0.22) in the brainof the new-borns than in other tissues. The turnovertimes. in days, for organically bound tritium were as
follows: brain, 59: muscle, 28: kidney, spleen andpancreas. 22: liver and intestine. 17. It was estimated.
in conclusion. that the contribution to the integral
tissue dose due to organic tritium was between 0.3 and0.7 that from tritium oxide alone, although in thebrain this contribution may equal or even exceed thatfrom tritium oxide.
235. Carsten [C 15) described a broadly based programme designed to evaluate the somatic and genetic
hazards of continuous exposure to tritiated water for
two generations in the mouse. The levels used were
between about 11 and 110 kBq of tritium per ml ofdrinking water and the relative biological efficiency
was obtained by comparing the effectiveness of such atreatment with that from continuous exposure to 137Cs
gamma. The end-points for comparison were dominantlethal mutations, chromosomal aberrations in the cellsof regenerating liver. increased sister chromatid exchanges in bone marrow cells and changes in the bonemarrow cell and stem-cell numbers. The RBE for
HTO ingestion for all these effects was assessed to be
between I and 2. compared with external gamma
exposure.
236. A series of papers dealing with the effects ofchronic exposures to tritium on brain development
has been published by Zamenhof et al. [V2. Zl, 22).
Female rats (F 0) were given tritiated cfnnidng water(110 kBq/ml) from 60 days of age until and throughout pregnancy [Zl]. The maximum concentration ofradioactivity in these animals was reached at 30 days
and in the blood at 42 days after the beginning of thetreatment. Under these conditions. the average dose to
the cell nuclei was estimated to be 0.0078 Gy per day.
The consumption of water and food and the changesof body weight before and during pregnancy in theseanimals did not differ from non-contaminated controls. The course of pregnancy and time of delivery
were also normal. The highest specific acthity in the
newborn animals (F 1) was found in the nucleic acid
fraction, but the bulk of radioactivity was containedin body fluids. These animals showed no sign ofradiation-induced effects. The weight of the new-born
animals was normal. but 60% of them had haematomas,
oedemas. and subdural haemorrhages. which disappeared subsequently by the time they reached
30 days of age. No haematological changes wereobserved as a function of age. the only exception
being a significant decrease of alkaline phosphatase.
In spite of a normal brain weight, these animals had asignificantly low content of DNA. protein and protein/
DNA ratio in this organ. The administration oftritiatedwater was continued in the F I animals throughout
weaning. adolescence. and subsequent pregnancies in
the F 3• F 4 and F 5 generations. Analysis of cerebral
damage in F,F 5 rats showed that this was not moreevident than in F 2, i.e .. that the effects of tritium were307not cumulative over the generations. There were, on
the contrary, indications that brain damage might beless pronounced.
237. The same scheme of tritium administration was
followed in other experiments by the same group [Z2].
The brains of the irradiated animals were studied at
30 and 120 days post-natally within each generation.
Significant decreases were reported in weight, DNA
and protein contents of some parts of the brain and themost pronouced effects were found in the diencephalon.
Protein losses were more pronounced than those of
DNA and the effects were overall more conspicuous at
30 than at 120 days and in the fifth than in previous
generations. There was an attempt to explain these
phenomena by considering the periods of maximum
sensitivity of the brain structures involved, possible
effects on the mothers in each generation and thecapacity for repair following the radiation-induced
damage to nucleic acids.
238. In other experiments [V2]. female rats of
60 days of age were started on tritiated water(110 kBq/ml) and killed when 90 days old. The
ovaries were excised and the oocytes removed bypuncturing the follicles. Examination of the chromosomes in these cells showed that their rate of
maturation was not affected by the presence oftritium. When the offspring of these females were
examined at birth, the number of cells in the cerebrum
was only slightly decreased as compared to controls.
Other rats were exposed to tritium from 30 days
before mating to 6 days after mating. This treatment
was found to depress the decidual response. The
offspring of this group of animals had a significantly
lowered number of cerebral cells at birth. This effectwas even more pronounced when exposure wascontinued throughout the pregnancy.
239. Finally. Radwan [Rl] carried out a comparisonof chronic exposure to tritiated water or organicallybound tritium to study developmental (startle response.
righting reflex, age at eye opening) and behavioural(diurnal and nocturnal locomotor activities) effects.
Wistar rats receiYed tritium at a concentration of
37 kBq/ml in drinking water, or at about 48 kBq/g infood. or no tritium at all for 2 generations. From theoverall results. it was concluded that. for the sameactivity level. continuous exposure to the organicallybound tritium was more damaging than that totritiated water. despite the higher cumulative dosesdelivered to the brain of the animals by means ofdrinking water. This may mean that the higher tritiumincorporation into the organic fraction of the brain
after exposure to the tritiated food is responsible forthe effects observed.
2.

Radioactive nuclides of iodine240. Several reports have become available concerning
thyroidal exposure during developmental stages. Oneof them [B6] concerns the metabolism of 1311 in guineapigs during the last two weeks of gestation. The
animals were killed at various times (from 5 hours to
9 days) after injection, and the blood, amniotic fluid.
308

thyroid glands and other tissues were sampled. Overthe time indicated. the concentration of the isotope
was consistently less in the blood of the mother thanin that of the fetus. Moreover, concentration in thefetus was less than that in the amniotic fluid. Peakconcentrations of 13 11 in the thyroids of both themothers and the focuses were found at 3 days postinjection. In the feta! thyroid (per gram of tissue) theconcentration was 3 times higher. The biological halflife of the nuclide in the thyroid gland was about
3 days in the fetus and about 11 days in the mother.
Between 7 and 9 days post-injection, the 131 1 in thematernal thyroid increased, and this phenomenon wasinterpreted to show a recycling of iodine within the
pregnant animals.
241. Another group analysed the kinetics of diaplacental transfer of 131 1 in rats fed either standard or
iodine-deficient diets [S69]. Female animals werestarted on diets containing 41 or 320 ng of iodine pergram of food a few days after their birth. When they
became pregnant, they received intravenously (on eachgestation day from 17 to 20) 0.74 MBq of 13 11 andwere killed at various times (up to 24 hours) frominjection. Five femses per female were counted forradioactivity in toto and another 5 were dissected formeasurement of radioactivity in various tissues, aswell as in the blood and the amniotic fluid. The
131 I activity concentration per gram of tissue in the
fetal thyroid rose by a factor of over 1000 between day
17 and 20 of feta! age. By day 19. 35% of theradiorodine in the fetus was in the thyroid. The
concentration in the fetal thyroid never exceeded thatof the mother's, at least up to day 20. and it appeared
to decrease from day 19 to 20. The uptake of thenuclide in the thyroid of the mothers fed the iodinedeficient diet was much higher and much more rapid
than in that of the mothers kept on the standard diet.
As to the uptake in the feta! thyroid. it was found tobe higher at 2 hours in the offspring of the iodine
deficient mothers. but by 8 and 24 hours frominjection it had fallen to values lower than in thecontrol group. These phenomena were explained on
the ground that the high uptake of iodine in the
thyroid of the mothers receiving a deficient dietresulted in a decreased diaplacental transfer of thenuclide available for uptake by the thyroid of the
fetuses.
242. A third report [B7] is of relevance in thecontext of releases to the environment under normal
or accidental conditions. It refers to man. and it
contains calculations of the doses delivered to thethyroid glands of various age groups in the population(fetuses of 13-40 weeks of gestation, infants of l or
2 years. children of 5, 10. 15 years and adults). bydifferent concentrations of inhaled or ingested iodine
nuclides ( 1291. 1311, 1321, 1331). These calculations werecarried out on the basis of assumed parametersconcerning the weight of the thyroid. the biologicalhalf-life of iodine. and the amount of iodine ingested
or inhaled. For the same atmospheric concentrations.
the results indicate a greater feta! dose from inhalation,suggesting that the fetus is most susceptible to being
affected by airborne releases of the relevant radionuclides. Based on data from animal studies, suggest-ing that the thyroid sensitivity is 10-200 times higher
in fetuses than in adults. the fetus was also found to
be most susceptible to ingested short-lived radioiodine. As to the longest-lived 1291, inhalation in adults
and ingested milk in the young infants are the routes
likely to deliver the greatest thyroidal doses.
243. The above data [87] combine information on
age-dependent dose per unit uptake. and per unit
concentration in the environment (which includes agedependent diets and breathing rates) and age-dependent
sensitivity. This information is analysed separately inanother model of iodine metabolism useful for calculation of the dose to the thyroid as a function of the
feta! age in the fetus and mother descn'bed by Johnson[JI I]. This model. based on available human data.
indicates that the doses to the feta! thyroid fromradioiodine taken in by the mother range from below
the dose observed in the mother's thyroid to a factorof 3, at most, above it. The composition of the dietwill determine whether ingestion or inhalation is themost important pathway. As to the sensitivity of thehuman feta) thyroid, Johnson and Myers [J 12] haveargued that there is no evidence to support large
differences in the effects of 13 11 and external irradiationin post-natal human thyroids, although differences by
a factor of up to 10 cannot be ruled out. Doses to the
feta! thyroid from chronic radio-iodine uptake by themother over the gestation period would not besignificantly different from those to the mother's
thyroid. For acute exposure at any time during
pregnancy, it may be calculated [J 11] that the dose to
the feta I thyroid is never more than a factor of 3 higher
than that to the mother's. A recent careful review ofthe placental transfer and feta! binding of iodine invarious mammalian species, including man, contains
valuable information for inter-species comparisons[S85]. This is generally in line with conclusions in [Jl)and [Jl2].

3.

Phosphorus-32

244. In experiments by Krishna et al. [K 17]. pregnant
mice received an intraperitoneal injection of 32p(370 kBq) at 3.5 days p.c. and were then allowed to
produce offspring. The incidence of successful copulations in this group of animals. compared with that in
non-injected controls, was drastically reduced. indicating that there were losses of all litters prior to
implantation. There was also a reduction of the litter
size at birth and at weaning. due to post-implantationmortality during the early stages of development. The
weight of the litters at birth was not apparently
changed by the treatment, but subsequent growth wasto some extent affected. These data are qualitatively inagreement with those that would be expected fromexternal irradiation during the pre-implantation stagesof development.
245. Radiophosphorus (37 kBq/g body weight) was
also injected intramuscularly into pregnant Swiss
albino mice on the 7th day after fertilization, but was
not found to affect substantially the development of
the pituitary gland, as judged by histological tests at
18 days p.c. or 14 days of extra-uterine age. Mice

injected intraperitoneally with the same level ofradiophosphorus when one day old. and autopsied atvarious times thereafter up to 42 days of age. showedsome hypertrophy of acidophilic cells. In animals
injected at 7 days, signs of cell death. followed by anincrease of acidophilic cells. were found over the sametime period. For injection at 14 and 21 days. a clear
decrease of acidophils was found. These cells seem,
therefore. the most severely affected by the injectionof 32 P [D6].

4.

Sulphur-35

246. Pregnant mice of the CBA strain were injectedwith 740 kBq of 35S on 3.5 days p.c. The treatment
had no effect on the mortality of the injected animals.
but a slight non-significant decrease of the number of
fertile matings was observed. Litter size was significantly
decreased (6.4 young per female), by comparison with
control animals not injected (7.5 young per female). A
further loss of animals took place between birth andweaning. owing to a significant increase in the neoand post-natal mortality of the offspring in the treatedgroups. The sex ratio and the whole-body weight of
the young born to injected mothers were not alteredwith respect to non-radioactive controls [S3].

5.

Selenium-75

247. Following a single administration of 75 Semethionine to pregnant female rats at 11-13 days p.c ..
the state of the endocrine system of the progeny wasevaluated at 9, 12, 18 and 26 months of age. To thiseffect the concentrations in blood of corticosterone.
testosterone and estradiol were studied. Changes wereseen to be more pronounced in the male animals[D20].

6.

Fission products

248. Senekowitsch and Kriegel [S67] compared theplacental transfer of fission products such as strontium,
caesium, yttrium and cerium in the rat and found thatthe uptake by the fetus decreased from the first to thelast nuclide. The mothers were injected intravenouslywith a standard amount of each nuclide 21 days
before pregnancy, on the day of mating. and 10 or
17 days after conception. The activity concentration in
the organs of the mother and the fetuses wasdetermined on pregnancy day 19, at which time thefeta! weight had a fairly large degree of variability,
owing. presumably. to a very long mating periodof 2 days. For each nuclide, the authors found a
statistically significant positive correlation between thedeposition in the fetus and its weight. Uptake also
depended on the day of development and was higher
the later in pregnancy administration took place. The
concentration of 13 11 in feta) organs at 8 hours postinjection showed a very large increase (by a factor of
about 1000) between 17 and 20 days of intra-uterine
age. particularly between days 17 and 18. Thyroid
activity compared with whole-body activity was 0.44%
on day 17 and it increased to 35% by day 19.
309

249. In work by Villiers et al. [VI I]. sows werechronically treated with 85Sr by intraperitoneal injection twice weekly of 185 kBq. in order to follow
uptake and retention in the pregnant mother andfetus. The state of pregnancy did not modify theseparameters appreciably. At birth, piglets had an 85 Srcontent amounting to about 0.5 to 1.0% of that of thedelivering mother. and the radionuclide taken up inutero was only excreted very slowly. Piglets were fed
after delivery by either contaminated or control sows
to study uptake through the milk. Those fed bycontaminated sows increased their radionuclide content by about three times before weaning. but excreted
it quite rapidly thereafter, so that by about three
weeks they contained the same amount of 85Sr as atbirth. Some aspects of the metabolism of 131 I in theruminant, including pregnant and lactating animals,
were also reported on by Daburon [D26].

7.

Lcad-203250. At 8-18 days of gestation, pregnant mice of the
C57BL strain received 203 Pb-nitrate intraperitoneally[D22]. The whole animals, or their excised uteri, were
subjected to autoradiography or autopsied for scintillation counting of the excised organs. Lead was foundin the embryonic and feta! tissues after injection at all
gestational ages. On days 8-11, it appeared mainly inembryonic blood. suggesting that it was essentiallyincorporated into the haemoglobin of yolk-sac erythrocytes. Starting from day 12, lead was found in liver(possibly in connection with the haematopoietic function of this organ) and in the skeletal cartilage andfrom day 14 on in the ossified skeleton. Thus. theoverall feta! concentration increased progressively
with gestational age. No uptake was observed in feta!
kidney. in spite of accumulation in proximal tubuli of
the maternal kidney.

8.

Thallium-204

251. The uptake and retention of thallium in mice
during gestation was investigated by autoradiographic
techniques with 2° 4Tl sulphate [09]. About 1.85 MBqof the nuclide were injected in the peritoneum of 15-day
pregnant females. Fifteen minutes after injection, thenuclide could be traced in the fetuses. The maximum
feta! accumulation was seen at 2-4 hours post-injection
and the minimum at the last observation time. which
was 4 days after injection. Observations at 5-15 daysof gestation showed that thallium crosses the placental
barrier at any time. It is retained by the visceral yolksac placenta during early pregnancy and also by thechorioallantoic placenta and the ammnion during lategestation.

9.

Transuranic nuclides

252. Although a fair amount of new information on
plutonium and other transuranic compounds administered to pregnant animals has become available. thisrefers more to the transplacental movement anddistribution of the substances rather than to the effects
310

induced on the product of conception. In the BALB/cmouse, Weiss and Walburg [W9. W21] measured theplacental transport of plutonium citrate after intravenous injection of three different concentrations of
2 39 Pu during the late stages of pregnancy. The animals
were killed 48 hours post-injection for tissue preparation and analysis. The authors noted that there was anincrease in the percentage of plutonium incorporated
into feta! tissue with decreasing activity down to levelsof 3. 7 kBq per animal. This was attributed to a masseffect. This pattern of relationship was also noted forthe placental tissue and was explained by the fact thata body compartment, most likely the liver. retains agreater fraction of the radioactive material at higher
levels.
253. Similar and parallel experiments [WIO] werealso performed with americium-243 administered intravenously as citrate in various concentrations (between
about 0.4 and 130 kBq/mouse) to pregnant BALB/canimals 16 days after mating. These concentrations
were chosen in such a way that the number of atoms
in each injected dose would be equal to the number of
239pu atoms in the study reviewed above [W21].
Forty-eight hours after injection. the concentrations of
the nuclide were determined in fetuses. placentas. inthe maternal femur, liver, carcass and pelt. It was
found that. atom for atom. americium was incorporated
into feta! tissues I0-25 times less than plutonium atsimilar concentrations. Here again. however. at low
dose levels the average fraction incorporated into thefetuses decreased with increasing activity administered
to the pregnant mothers. The same could be said forthe placenta and the femur. The mass effect foramericium was about a factor of 2 within theconcentrations covered, that is slightly lower than thatof 239 Pu which was nearly a factor of 4 [W2 l ]. A
scavenging effect by the maternal tissues. and particularly by the liver, was discussed as a plausible
explanation.
254. The point was made, regarding these twostudies [W IO, W2 J], that extrapolations from animal
to man should always be made at the low levels ofenvironmental human exposure, which would be
hundreds of times less than for the experiments
reviewed. It is doubtful whether activity analyses atsuch extremely low levels of contamination are at all
possible. However, the concept should be borne inmind that perhaps current environmental levels of thenuclides might be more readily transferred frompregnant mothers to the fetus than for the usual muchhigher experimental level uptakes.
255.

New-born rats from dams injected with 49 kBq
citrate kg- 1 body weight 38-53 days prior toparturition contained an average of 0.008% of thematernal injected dose per neo-nate [Tl6]. Also in
the rat Hisamatsu and Takizawa [H29] studied theplacental transfer and distribution of 241 Am by injecting the radionuclide under the form of citrate at 15 or
18 days of gestation. The animals injected at 15 days
were killed at 2, 24, 48 or 120 hours and those injected
at 18 days were killed 24 hours thereafter. An
autoradiographic study of the nuclide distribmion in
the fetus. feta! membranes and placenta showed that239pu

the deposition in the feto-placental units increased
with gestational age. Major deposition sites were theskeleton and liver. but the presence of the nuclide in
the yolk sac splanchnopleure and in the exocoelom
suggested also an important role of the yolk sac
placenta in the transfer mechanisms. The post-natal
development of rats treated with 24 1Am was studied by
Ovcharenko [013].
256. ln theguinea-pig(outbred Harley strain). Kelman
and Sikov [KS] measured the movement of 23 9 Pu invivo at 59 to 61 days of pregnancy by perfusion of the
feta! circulation, a technique which avoids the complications occurring through fetal accumulation of thenuclide. The perfusion technique also entails continuous
monitoring of the pressure, heart rate. electrocardiogram, blood pressure and respiration rate of themothers. The pregnant animals were administeredtrace amounts of tritiated water in order to monitorchanges in the maternal blood flow to the placenta
and about 1 MBq/kg of 239 Pu citrate intravenously.
This dose of plutonium is of the order of the LD 50130
for other species (about 740-3000 kBq/kg). Clearanceof plutonium from the mother to fetus was found tobe small, amounting. for example, to less than onefifth of that for inorganic mercury. One factor thatmight account in part for such a low clearance is theplacental blood flow of the mother, which is greatly
depressed in the animals treated with plutonium.
257. In an abstract by Andrew et al. [A8], valuable
data were provided on the distribution of 239 Pu in thegravid baboon. Following previous evidence that, inthe rat the placenta and the feta! membranes attain
substantially greater concentrations of the nuclide
than does the fetus. and that these observations may
not be easily extrapolated to man owing to large
interspecies differences. the authors chose the baboon
as an experimental animal. In this species, in fact.
development of the placenta is similar to that in man.
Pregnant animals at various stages of gestation wereadministered 370 kBq/kg of monomeric 239pu intravenously. The uteri and their content were thenremoved surgically 24 hours later for radioanalysis
and autoradiography. The mothers themselves wereallowed to live an additional two to six days. The
fraction of dose reaching the feto-placental unit, andthe ratios of the concentrations in the fetus to those in
the placenta. were found to be very similar in the rat
and the baboon, when similar stages of gestation werecompared. If, in addition, the morphological differences in placentation were allowed for. the autoradiographic distribution of plutonium was found to
be remarkably similar in the rat and the monkey.
258. Joshima et al. [13] studied the effects onembryonic and feta) haematopoiesis of l39pu citrateadministered intravenously to the Wistar rat (about
1.3 MBq/kg, no dose estimate available) on gestation
day 9. At this time, the yolk sac begins to form theblood islands and also selectively concentrates monomeric plutonium. The injected animals were killed 5 or
10 days after injection to measure any effects in thecirculating blood. yolk sac and feta! haematopoietic
organs. The treatment increased pre-natal mortality
and reduced maternal (but not feta!) weight gain. At

both sampling times the pregnant mothers showed adecreased number of reticulocytes and leukocytes: thered blood cells were also lower at JO days. The fetuses
showed a transient decrease in the concentration of
non-nucleated mature erythrocytes and changes in the
distribution of cell types of the erythropoietic series.
which were interpreted as disturbances in the maturation processes. The presence of plutonium also reduced
the mean weight of the yolk sac and the fetal liver. In
both these organs, and also in the spleen. the mean
number of cells was decreased, without any shift in the
distribution of the various cell classes.
259. Kelman. Sikov and Hackett reported [K6. K23]
on the effects of monomeric 23 9Pu on the feta) rabbit.
Animals at gestation days 9. 15 and 27 received
370 kBq/kg body weight of monomeric 239 Pu intravenously and were killed one day later: no estimate ofabsorbed dose was provided. Groups were alsosimilarly injected at 9 and I 5 days and killed at the28th day of gestation. One group. injected on day 9
and killed on day 28, received a four-fold higher
activity concentration. Conceptuses were examined for
viability at 10. 11 and 29 days. Each fetus from littersinjected at 9, 15 and 28 days, and killed on day 29.
was weighed, measured and examined for sex. external
abnormalities and cleft palate. Exposure to about
1.5 MBq/kg decreased feta) weight significantly. compared with that of the group receiving 370 kBq/kg.
while the smaller weight losses induced by this latter
activity concentration were not thought to be significant. Foetal mortality was significantly increased,
except in groups receiving the lowest activity on days
9 and 15. Only 5 out of 377 fetuses examined wereabnormal. This low number. and the lack of any
pattern in the induction of malformations. made any
association between their incidence and the exposure
to plutonium rather unlikely. Subtle alterations. which
could not properly be classified as malformations.
were detected in the skeleton of the exposed litters andthere was a general retardation of the skeletal development in the group receiving the highest activity ofplutonium.
260. Ovcharenko and Fomina [06] reported on theeffects of 237 Np oxalate administered intravenously
to rats. They .found an increased pre-implantationmortality in females. The progeny of these rats
receiving 11-185 kBq/kg showed a decrease in the rateof production of red blood cells after gamma irradiation. together with a prolongation of the period of
narcosis after administration of hexanol.

B.

CONCLUSIONS

261. Although a number of new contributions
regarding the effect of internal irradiation on the
mammalian conceptus have appeared since the matter
was last reviewed by UNSCEAR [U2], the information
gathered is not systematic in regard to the nuclides
tested, their chemical form, the route and schedule ofadministration, and the kinetics of transfer andmetabolism of the substances from the mother to the
feto-placental complex. Complete dose-effect relation311

ships are seldom reported. Also. data regarding
various animal species, developmental stages at treatment, and type of effects scored, are very sporadic. As
a consequence. any attempt to extrapolate between
nuclides, doses. species, developmental stages or any
other major physical or biological variable appears
premature. From the long list of nuclides for which
scattered information is available, three stand out as
having received somewhat better attention: tritium.
iodine-131 and plutonium-239. The fact that these also
happen to be. potentially. among the most importantfor human exposure under normal or accidental
conditions is probably the reason for the interest they
have received. It does not mean, however. that the
information available for them is very satisfactory.
262. For tritium, many contributions refer to irradiation during the pre-implantation stages in vitro. In theculture medium. activity concentrations of tritiumoxide of the order of 4- I 6 kBq/ml have been shown tostop the progression of 50% of the zygotes to theblastocyst stage, the variability between these values
being due in large part to the developmental stage ofthe early embryo at irradiation. Embryo sensitivity
generally decreases with increased differentiation.
Tritium in the form of thymidine is about 1000 times
more effective in producing such effects than tritiatedwater. Tritium oxide and tritiated thymidine also act
differently on various developmental stages. Tritium ismetabolized by the embryo and its organs according todifferent kinetics: the loss of the radionuclide per gramof tissue in the new-born after uptake in utero shows afast component with a half-life of a few days followed
by slower components linked to the metabolism of theorganically bound isotope. This has been establishedin both the mouse and the pig. The dose received bycells in each tissue is therefore governed by the level oftritium in the body fluids and by the specific cell
kinetic and metabolic pattern of each tissue. Wholebody doses of the order of I Gy or less from tritium
have been reported to produce changes in litter size,
sex ratio, and neo-natal mortality in the mouse. They
also cause increased tumour incidence and changes in
the life span. Sustained levels of tritium in thedrinking water of pregnant rats at concentrations ofabout 100 kBq/ml produced in the offspring somebiochemical sign of cerebral damage (low DNA andprotein content), although the weight of the brain was
normal. Continuous administration of tritiated waterover 5 generations showed no sign of cumulative
effects. Other patterns of tritium dosage at similar
concentrations also produced evidence of brain defects
which could be fqllowed well into post-natal life.
There is evidence that organically bound tritium ismore damaging than tritiated water for the sameadministered activity. The RBE for a variety of
end-points (dominant lethal mutations, chromosomal
aberrations. chromatid exchanges. alterations in bonemarrow cells) following HTO ingestion in the mouse.
compared with external gamma rays from 137Cs. wasassessed to be between I and 2.
263. Many of the experiments using radioactive
iodine appear directed to clarify the relationshipbetween the amount of activity administered to thedam and its route of uptake, on the one hand, and the312level of iodine in the feta! tissues. particularly in the
thyroid, on the other. The transplacental transfer ofradio-iodine as a function of time from the uptake by
the mother has been studied in rats and in the guineapig. So have the relative concentrations in the bloodof the mother and of the fetus and in the amniotic
fluid, and the relative uptake in the mother's andoffspring's thyroid. They have also been estimated in
man on the basis of some assumed parameters of
uptake and metabolism. As a result of using simplifying calculations, it has been provisionally concluded
from these data that for acute exposure at any time
during pregnancy the dose to the thyroid of the fetus
is never more than three times higher than that of themother. For chronic uptake over the whole gestation
period these doses would not significantly differ fromeach other.
264. Experiments on 239 Pu contamination of pregnant
mice have shown that the proportion of this nuclide
incorporated into the feta! tissues tends to be relatively
higher (a factor of 4, as a maximum) with activity
concentrations decreasing down to about 3.7 kBq per
animal. Although atom for atom. 243Am is incorporated I 0-25 times Jess efficiently than plutonium
at similar concentrations, the average fraction ofadministered activity incorporated into the fetus decreases also for this nuclide with increasing mass ofthe nuclide to the pregnant mother, by a factor of
about 2. The presence of such effects should be bornein mind when assessing the uptake of environmental
levels of transuranic nuclides, which are orders ofmagnitude lower than the experimentally tested levels.
265. Comparisons of the distribution of 239 Pu in themother and fetus in the rat and the baboon haveshown that when similar developmental stages arecompared. and morphological differences in the placenta of the two species are allowed for. the distributionof the nuclide is very similar. While existing data are
insufficient for precise estimates. no clear tendency ofplutonium to concentrate into the fetus has been
documented. There appears to be, on the contrary,some tendency of the placenta and feta! membranes totake up the plutonium that might otherwise betransferred to the fetus. Experiments on the effects ofplutonium are relatively less common than those on
its distribution. However. some haematological changes
have been documented in the rat at about 1.3 MBq/kg
of 23 9 Pu citrate administered on gestation day 9. Fetal
weight loss and mortality (but not malformations) weresignificantly induced by concentrations of monomeric
239 Pu in the range of I MBq/kg administered topregnant rabbits.
266. There appears to be an urgent need to enlarge
the data base for the nuclides of most practicalimportance with respect to uptake. distribution in thefeta-placental complex, and possible effects in theoffspring. over a larger variety of radioactive chemicals
and a wider range of nuclide concentrations. The
importance of correlating effects to tissue doses rather
than intake activities should also be emphasized. In
addition to providing much needed information ofpotential value under conditions of normal operation
and accidental exposure to radiation sources, such

t

t1
f
i

1

new data might contribute to bringing about generalized conclusions on dose-response relationships for
pre-natal internal exposure to radioactive nuclides.

VI.

THE ROLE OF MODIFYING FACTORS

267. The present chapter deals with physical, chemical
or biological factors that have been shown to modify
the radiation response of pre-natally irradiated mammals. It focuses on radiation type and energy and theeffects of combined actions, including radioprotective
and radiosensitizing treatments.

A.

RADIATION QUALITY

268. The insufficiency of information concerning theeffect of radiation quality and dose rate, in the context
of teratogenic damage and pre-natal irradiation effects
in general, was pointed out by UNSCEAR in its
previous review of the subject [U2]. It was concluded
at the time that the data only allowed some speculation about RBE factors in respect of irradiation inutero. However, this knowledge was compatible with
the notion that higher RBE factors applied to themore densely ionizing radiation and to low-LET dose
protraction, as compared with high-dose-rate and lowLET radiation. UNSCEAR called specifically for asystematic exploration of this area. and over the past
few years new information has in fact been produced.
This information is reviewed below, according to thevarious types of radiation tested.
I.

Neutrons

269. In experiments by Molls et al. [M33], preimplantation mouse embryos in the G 2 phase of the
2-cell stage were exposed in vitro to either x rays
(240 kV) or fast neutrons (average energy. 7 MeV). At
various times thereafter (up to 63 hours post-irradiationand growth in culture). these embryos were scored forthe number of cells per embryo and the number ofmicronuclei per cell. Both radiations did induce theformation of micronuclei at very low doses. and the
kinetics of induction was found to depend on radiation dose and the length of the division delay during
the first and second cell cycle after irradiation. New
micronuclei were appearing even after the third andlater post-irradiation mitoses, in accordance with thenotion that radiation-damaged cells do not all die atthe first post-irradiation division. The number ofmicronuclei per cell at 39 hours post-irradiation
appeared to follow an essentially linear increase withthe dose of x rays and a convex upward response withthe dose of neutrons. An x-ray dose of 0.06 Gy was
sufficient to double the average number of micronuclei
per cell of the control embryos; 0.03 Gy of neutrons
produced a 6-fold increase over the control values.
270. Cairnie et al. [C3] reported on teratogenic andembryolethal responses in CF 1 mice irradiated in theJanus reactor at the Argonne National Laboratory in

the United States. At this facility the mean energy ofthe neutron spectrum was approximately 0.8 MeV:
thermal neutrons and gamma rays contributed lessthan I% and less than 3%. respectively, of theabsorbed dose within the mice. Neutron effects werecompared with those of mes or 60Co gamma rays.
Irradiation took place at 3, 5, 8 and 11 days p.c.
Doses were 1, 1.5, 2.0 and 3.0 Gy for gamma rays and
0.25, 0.5. LO, 1.5 and 2.0 Gy for neutrons. Offspringwere examined on day 17 of pregnancy for theincidence of resorptions and dead or viable fetuses.
Live fetuses were also scored for gross external
abnormalities and body size. The same types of
responses were observed for neutrons and gamma
rays. For lethal effects and malformative effects
considered as a single class. the RBE in terms of
average absorbed dose to the embryo was estimated tobe 2.9 (the corresponding ratio in terms of kerma infree air was 2.5). Taking the average number ofabnormalities per fetus as the end-point, the findings
also pointed to an RBE of between 2 and 3. The
highest incidence of abnormalities was obtained forirradiation on day 11, when malformations of thelimbs were especially frequent. Approximate values of
RBE estimated for those abnormalities that were
sufficiently numerous were in the range of 2 to 3,
usually closer to 3. It was pointed out that these RBE
values were consistent with other values for effects
dependent on cell killing in vitro and in vivo which,for the particular radiation sources used, were between
2.2 and 4.
271.
Although no comparison between different
types of radiation was involved. Vogel [V3. V4]
exposed female Sprague-Dawley rats on pregnancy
day 18 to single whole-body doses (0.2 to 1.5 Gy) offission neutrons of 1.2 MeV average energy (neutron/
gamma dose ratio approximately 7. dose median LET
approximately 50 keV/µm). A dose of0.2 Gy caused asmall decrease of body weight lasting from birth toweaning. During this period. 9% of irradiated rats
died. compared with 4% of the controls. After 0.5 Gy,
about 24% of the irradiated animals died within thesame period. with a more profound body weight lossof the survivors. One Gy caused about half of the rats
to die within 24 hours of birth and over three-quarters
before weaning. with a large and significant loss ofweight in the survivors. No survivor among 95 rats
receiving 1.5 Gy was left within 2 days of birth. The
neutron LD 50 from birth to weaning was estimated tobe 0.75 Gy. Organ weight changes were followed
between birth and I week of age and they showed an
overall decrease of the measured values in irradiatedanimals. The most significant weight loss was found in
the CNS and in the testis one month after neutron
exposure. These effects were thought to be directly
induced in the irradiated fetuses and not secondarily
through irradiation of the mother.
272. Di Majo et al. [D7] compared the effects offission-spectrum neutrons (zero to 0.465 Gy, averageenergy -0.4 MeV, gamma contamination -12.5% of
total dose) with those of x rays (250 kV, 0-2.0 Gy at
0.13 Gy/min). The end-points scored included embryolethality. weight reduction and teratogenic effects(gross malformations and minor anomalies). The
313

animals used were F I hybrids of C57Bl and C3H mice,
irradiated at 7.5 days p.c. They were scored for theabove effects at 18.5 days p.c. Mortality curves foundfor both radiations were highly curvilinear. with alower "threshold" for neutrons than for x rays. The
ratio ofx-ray to neutron doses (1.4 and 0.47 Gy. respectively) to produce 20% mortality was approximately 3.
The Joss of mean feta} weight was approximately
linear with increasing dose of both radiations. and theratio of the slope regressions gave an RBE of
3.2 ± 0.8, as an average over the dose range tested.
Highly curvilinear relationships with dose were observed for major malformations after x-ray irradiation,with thresholds of the order of 1 Gy. Dose relationships for neutrons, however, showed no apparent
thresholds, so that the RBE of neutrons was very
dependent on neutron dose. The equal effectivenessdose ratio estimated at 7% incidence of severe externalmalformations was 2.4, increasing to about 3.4 ifother, more common, head abnormalities were also
included.
273. Preliminary studies of dose-response relationships for developmental damage in the CNS of themouse after pre-natal exposure to x rays. fast neutronsand 13 11 were also reported [K41]. RBE values
following x-ray or neutron exposures were establishedin relation to a number of morphological parameterssuch as the thinning of cortical plate. corpus callosum
and fimbria hippocampi. They were shown to vary
between 3 and 4, according to different times andeffects. The studies included analyses of neuronalbranching defects by computerized micro-video techniques, as a function of dose. There was evidence that"thresholds" existed in the region of 0.1 Gy for suchtypes of structural damage.
274. The effects of a single whole-body dose of
0.5 Gy of neutrons (peak energy about 3 MeV)
on mortality and body and brain weight in(DBA2 X C57BL/6)F I hybrid mice were described by
Antal and Vogel [A JO, V9]. Radiation was deliveredat 17 ± 2 days p.c. This treatment caused a significant
increase of death within two weeks after birth, from alevel of about 12% in the control animals to about
41 % in irradiated ones. The average birth weight
showed a 15% reduction in the irradiated mice. Thisweight loss was not recovered until 6 months after
birth. Brain weight at 3 weeks of age was 30-35% lessin the irradiated mice. Early mortality and brain
weight loss were not seen in mice irradiated at 1-7 days
from birth. The significance of these data, particularly
in regard to timing, was discussed in relation with theinduction of severe mental retardation of children
exposed in utero to the atomic bomb explosions. The
authors' opinion is that the time of most active
neuroblast proliferation is, in both species, the mostvulnerable period for radiation exposure.

2.

Helium ions275. In a series of papers, Ward and his group[W2-W6] reported extensively on the effects inducedby irradiation in utero by a helium ion beam
generated in a synchro-cyclotron. By suitable ridge
314

filtration, doubly charged 530-MeV monoenergetic
helium ions were modified into a heterogenous beamwith an extended Bragg peak, delivering essentially auniform radiation dose at a depth of 4- JO cm in water.
This allowed animals to be exposed rather uniformly
from a lateral direction. The LET spectrum was suchthat at 8 cm about 10% of the dose was due to tracksof LET higher than 20 keV/µm; and about 1% totracks of LET higher than JOO keV/µm. Cobalt-60
gamma rays were used as the standard radiation forcomparison.
276. In the first experimental series [W2. W4],
Sprague-Dawley rats were exposed on gestation day 8
under aerobic or hypoxic (95% helium and 5%
oxygen) conditions in the dose interval of 2-12 Gy.
The uterine content was examined on gestation day 20
for live and dead fetuses, resorption sites, weight andgross morphological abnormalities. The number ofcorpora lutea served as an index of the number ofovulations. and embryonic survival was expressed as a
per cent of live fetuses over the number of corpora
lutea. The results may be summarized as follows:
Survival
(%)

50
10

RBE
Oxygen enhancement ratio Oxygen
Aerobic Hypoxic Helium ions Gamma rays effect gain
1.0
I.I

1.2
1.4

I. 7
1.5

2.2
2.0

1.2
1.3

Thus, the high-LET component of the helium ion
beam had a negligible effect on the RBE under aerobic
conditions, but lowered the effective oxygen enhancement ratio of the beam by about 25% relative to thatof the gamma rays. Helium ions were significantly
more effective than gamma rays in stunting fetal
growth under hypoxic and aerobic conditions (1.5 and1. 7. respectively) and in producing gross morphological abnormalities under aerobic. but not underhypoxic, conditions. In fact, at 10% incidence ofabnormal fetuses, the equal effect ratios were about
1.7 and l under oxygenated and hypoxic conditions,
respectively.
277. In another paper (W3], the embryonic survival
was examined in rats exposed to split doses ofradiation delivered in two fractions at 5 and 6 days
p.c. Helium ions, which showed a single-dose· RBE(with respect to 60Co gamma rays) of 1.0, had a splitdose RBE of 1.5, in accordance with expectation if thekilling curves for helium ions had a smaller shoulder
than that of gamma rays.
278. Finally [W5, W6], Sprague-Dawley rats were
exposed to 0.5-6.0 Gy of 60Co gamma rays, or toextended Bragg peak helium ions. on days 3-10 ofpregnancy, and the uterine content was examined on
pregnancy day 20 for scoring of resorptions and deador live fetuses. The number of dead fetuses wasreferred to the number of corpora lutea to calculate
the percentage of killing. The sex of the fetuses was
also scored. For gamma irradiation, the LD 50 in utero
ranged from 1.4 Gy on day 3 to 5.15 Gy on day 6 ofgestation. For helium ions, the values ranged from 1.7
on day 10 to 4.9 Gy on day 6. Day 6 (the time of
implantation) was the most resistant stage for bothradiations, while the early organogenesis period (days

I

i.'

i

I

9 and I 0) was found to be the most sensitive. In spiteof the variation of radiosensitivity over a factor of 3 to
4 during the period of development examined, themagnitude of the fluctuations at the various times was
essentially the same for both radiations, so that theRBE of helium ions with respect to gamma rays was,on average, 1.0 during the whole period. A consistentfeature in irradiated litters was the preponderance ofmale fetuses among the survivors. for which no
explanation is available.

3.

Negative 1r mesons

279. In experiments by Michel and collaborators[M4], mouse embryos in the pronuclear-zygote stage(gestation day zero) were irradiated with either 140-kV
x rays or negative pions to a dose of 0.135 Gy. The
animals were examined when they reached the feta!
stages on gestation day 13. The developmental effects
scored included intra-uterine death, retardation of
growth and incidence of malformations. Significant
decreases in the percentage of normal implantations
were obtained with 1r mesons at the peak and withx rays. The effect of the pions at the plateau was lessimportant. Irradiation with peak pions was found to
be more effective than with x rays, by a factor of
about 1.7.

B.

COMBINED ACTIONS

1.

Various chemical substances280. Most of the data published since 1977 on the
combined effects of radiation and other agents, taking
as end-points various types of damage to embryonicsystems, have been obtained by treatment of the very
early stages of development and in vitro culture of thetreated embryos. General aspects of the interaction
between radiation and chemicals during pre-natal
development were discussed in annex L of the 1982UNSCEAR report [U5] and were reviewed by Streffer[SJ7, S36].
28 I. The same author and his group have used
extensively the in-vitro culture of the early mouseembryo to test a number of substances in combinedtreatment experiments. Following the order of publication, they reported on the interaction between
ionizing radiation and lead [M3 I, V6], actinomycin D[M29, Sl9]. phenols [M27]. sodium nitrite [M16],
cadmium [M28], ethidium bromide [M29], 2,4-dinitrophenol [M 19], sodium sulfite [M43] and caffeine[M43].
282. The combined effects of these substances withx rays were studied. taking as end-points the development of 2-cell mouse embryos to hatching of
blastocysts, the cell proliferation kinetics in theseembryos, and the formation of micronuclei as anindex of cytogenetic damage. The substances wereusually added to the culture medium directly before
x irradiation. Additive effects were found for all these
end-points when x rays were combined with mostchemical substances. This was the case for phenol,
p-nitrophenol, 2,4-dinitrophenol. sodium mtnte.
sodium sulfite, 2,4.6-trinitrobenzene sulfonate andethidium bromide.
283. A supra-additive (synergistic) effect was observedwith the combination of x rays and actinomycin D,
cadmium chloride or lead chloride on one of theparameters. Only with very high concentrations ofcaffeine did a supra-additive effect occur on the threeparameters. The supra-additive effects were generally
seen under conditions when recovery processes after
irradiation were inhibited. Of special importance is thefinding that comparatively low concentrations of lead
chloride [M31] and of actinomycin D [Sl7, S36]
induced such potentiation of the radiation effects.
284. In the field of combined actions, a few otherscattered observations have also been reported. Sikov
and Mahlum [S 12) found that the intravenous administration of 5.5 kBq of 239Pu, which by itself was ineffective. to pregnant rats at either 8.5 or 9.0 days p.c.
increased synergistically the teratologic and embryocidal effects of trypan blue injected intraperitoneally(70 mg/kg) at these times of gestation. Trypan blue isitself a teratogen in the rat. at least up to 11 days p.c.
[B29].
285. Yuguchi [Y2] treated pregnant ddN mice at
7 days p.c. with x rays (100 or 150 R) with or without
ultrasound at 1.5 or 3.0 W/cm 2 (1 MHZ) for 5 minutes.
The gross appearance of the offspring and thehistology of their eyes were examined on the 18th dayof gestation. The author found an enhancement of theembryolethal effects of 150 R by the highest dosage ofultrasound and an enhancement of the teratogenic
effects of JOOR, also by the highest dosage. The
histopathogenesis of the ocular abnormalities was
carefully described in these experiments. The sensitivity
periods in feta) mouse retina by the combined actionof the cytostatic drug 5-azacytidine and x rays were
investigated by Schmahl and Torok (S6].
286. Sadovskaya et al. [S76] and Rantseva [R22]
investigated the reproductive function and physicaldevelopment of the progeny of non-inbred irradiatedwhite mice. Females were exposed every other day to
fractionated long-wave x radiation (10.2 keV) for
31 and 82 fractions (five of which were administeredafter mating with non-irradiated males) at doses of
0.13 or 0.26 Gy per fraction. From the number ofoffspring, the coefficient of reproduction and the
body weight of the new-born. these treatments wereapparently ineffective. However. if under the same
conditions of irradiation an SHF treatment (ppm
5 MW cm- 2, 20 minutes per session) was also jointly
administered, there was a tendency towards a reduction
in fertility and a decrease in body weight. as comparedwith the effects of the two treatments given alone [L8].
287. Kusama and Yoshizawa [K37] reported briefly
on intra-uterine death, malformations. body weightand sex ratios of mice exposed to 1 or 2 Gy of gamma
radiation and 1 or 2 mg cortisone (administered in thisorder) on the 7th day of embryonic age. A dose of
1 Gy of x rays given alone produced no higher
315mortality than that in the control, but a higher
frequency of exencephaly. \Vhile both end-points were
found to be higher after 2 Gy. Cortisone alone did not
significantly alter any of the end-points scored. On thecontrary, the combined-action groups showed rates ofmortality and malformation which were twice as highas those induced by the same dose of radiation given
alone, an observation implying a synergistic interaction.
288. In another series of experiments [K38], thesame authors exposed pregnant ICR mice on their
7th day of gestation to various doses of gamma
radiation (0.5, 1.0 and 2.0 Gy) and of caffeine (0.1.
and 0.25 mg/g body weight). The drug in the combinedaction groups was administered immediately after theappropriate radiation dose. Controls with no treatment,
and with radiation or caffeine alone at the same doses,
were also included. The end-points scored on thefetuses after killing of the pregnant mothers at 18 daysof gestation were the same as in the previous series[K37]. Intra-uterine mortality increased as a functionof dose following a highly curvilinear shape (upper
concavity). There was no effect when caffeine wasgiven alone. but the combination of radiation andcaffeine resulted in a synergistic effect on mortality. A
synergistic effect was also found in respect of theincidence of malformations both minor (polydactylia.
micromelia, short tail) and serious (hydrocephaly,
exencephaly, parietal hernia, cleft palate and lip). For
induction of gross malformations, the dose-effectrelationships for radiation alone or in combination
were highly curvilinear. Body weight reduction was
thought to be a good indicator of the growthretarding effect of the treatments. but showed no signsof synergistic interaction. Finally, the sex ratio of live
fetuses in the different treatment groups was not
significantly different from control.
289. After showing that pre-conception exposure of
ICR mice increased the incidence of anomalies and oftumours (mainly lung adenomas) in the offspring oftreated mice [N4], Nomura [Nl3] tested tumorigenesis
by irradiation in utero. followed by chemical treatment with urethane. Pregnant mice of this same strain
were exposed to x rays (a single dose of 0.36 Gy) every
second day between zero and 16 days of gestation.
Neo-natal mice were also similarly irradiated within
1 day of birth and, in addition, other pregnant
females received 1.59 Gy on gestation day zero. When
21 days old, the mice received subcutaneously 5 µM
of urethane per gram body weight and were killed
5 months afterwards for scoring of tumours andmalformations. Tumours were mostly papillary lungadenomas. Survival of the animais was decreased with
irradiation on day zero. and body weight decreased
with irradiation on days 4- I 0. and particularly on
day 12 of gestation. Irradiation in utero alone did notchange the natural tumour incidence. Irradiation ondays 0-4 and 8. followed by administration of urethane,
increased the number of tumours per animal, comparedwith both irradiation alone or urethane alone. It was
speculated that unidentified events, induced by irradiation in the embryos and persisting through a large
number of cell generations, may render the cells more
susceptible to the post-natal treatment with urethane.
316

Such events may be similar to those recemly describedin cells transformed in vitro by x rays and discussed inannex B to the present report.
290. In a short Jetter, Kinner-Wilson et al. [K42]reported on 2707 children who died in I 972-1977 ofvarious malignancies, including leukaemia. and had
been exposed in utero to various types of drugs and to
x rays, separately or jointly. These children werematched to 2298 control non-exposed cases. The
increase in relative risks for "all drugs'' and forradiation given jointly were significamly (P < 0.001)
increased above the level of action of the agents given
alone. When the drugs were divided into separate
groups, only for two of the three groups (namely··sedatives" and "other unspecified drugs"), but notfor the third ("hormones"), was there a significant
association. In the view of the authors, these resultscannot be attributed to artifact. The association could.
in principle, be explained on one (or more) of thefollowing mechanisms: an effect of the illnesses
requiring the drug treatment: a true causal effect of
the treatment(s); or an association between the drugs
and another non-identified risk factor. Results are too
preliminary to allow a choice among the mechanisms
proposed.
2.

Radioprotective and radiosensitizing treatments

291. Although part of the larger subject of radiationdrug interaction, the effects of radioprotective andradiosensitizing substances are traditionally treatedseparately. A number of reports have appeared onthese subjects since the last UNSCEAR review [U2].
Radioprotective effects on embryonic systems weremainly studied by a group of research workers in India:
radiosensitizing substances were mainly investigated in
Switzerland.
(a)Radioprotective treatments

292. The drug 2-mercaptopropionyl glycine (MPG),
a -SH radioprotector, was used in the studies by Dev
and her group. A variety of end-points were used toassess the protective action. including organ andwhole-body weight changes and functional tissue
effects. Pregnant Swiss albino mice were exposed to
gamma rays (250 R) at 14.25, 16.25 and 18.25 days ofgestation. The post-partum weight of the offspring
was remarkably improved by pre-irradiation treatmentof the mothers with 20 mg/kg body weight of MPG.
Of the three ages tested, the earliest was the mostsensitive [D4). Similar observations were made on
mice receiving 150 R [D 19] and 50 R [D5] at the sametimes p.c.
293. In another series [G9], pregnant Swiss albino
mice at 11.5 days p.c. were injected intraperitoneallywith a single dose of about 5.6 MBq of BII per mouse.
Other mice received MPG (20 mg/kg) 30 minutes
before the 131 1 and daily on each subsequent gestation
day until parturition. Finally. a third group of mice
was similarly treated with distilled water. The body

weight of the litters born to these animals waschecked, starting on the day of birth and for 6 weeks
thereafter. Organ weights of the animals at 6 weeks
were also obtained. MPG afforded a modest degree ofprotection against loss of total body weight. andmarginal protection in regard to the weight of brain.
kidney, testis, spleen, thymus and liver.
294. As to tissue effects, 50 R of gamma radiationfrom 60Co, given at days 14.25, 16.25 or 18.25 resulted
in a depression of the leukocyte count of the young
1 or 2 weeks after birth, with recovery by the
4th week. Treatment of the mothers with MPG(20 mg/kg) 15-30 minutes before irradiation significantly reduced the leukocyte depression at I or
2 weeks of age [G7]. In other experiments [G 13] mice
were irradiated in utero at 14.5, 16.5 and 18.5 days
p.c .. with or without pre-treatment of the motherswith 20 mg/kg body weight of MPG. Doses of 60Co
gamma rays were 0.5, l.5 and 2.5 Gy. Male animals
were killed when 6 weeks old and the weight of theirtestes or the total number of spermatids in tubulesections were assessed. The radiosensitivity of thetestes was found to increase with increasing fetal age.
There was also some protection by MPG of the testes
according to both end-points scored, but the amountof protection was variable and not evaluated over alarge range of doses.
295.

Palyga

et

al.

[Pl3]

irradiatedembryonic(1-14 days p.c.) and feta! (15-23 days p.c.) rats while

the pregnant mothers breathed a hypoxic gas mixture
containing 10% oxygen and 90% nitrogen. A single
dose of 2.0 Gy was used. Judging by the incidence ofstill-born, and by the per cent survival 30 days after
birth, the treatment afforded a reliable degree ofprotection against lethality.
296. Turakulov et al. [T9] studied, in pregnant dogs,
rabbits and rats the specific activity and biologicaleffects of potassium iodide in various combinations
with potassium perchlorate. as a prophylactic treatment for the protection of the thyroid gland against
the effects of radioactive iodine during pregnancy.
They reported that a combination of the two drugs indoses of 0.5 and 300 mg/kg. administered at any time
during pregnancy, is biologically harmless and showsa high protective effect.

(b)Radiosensitizing treatments

297. The hypoxic cell sensitizer misonidazole wasused by Michel and collaborators in interaction
studies with low- and high-LET radiation. In one
report [M5. M6]. pregnant NMRI mice on the 8th day
p.c. received intraperitoneally various doses of the
drug (from 200 to 1000 mg/kg body weight). Single
doses of 0.01 Gy of 140-kV x rays and 15-MeV
electrons were given. The end-points investigated wereembryonic and fetal death, malformations, and growth
retardation as scored at 13 or I8 days p.c. Growth
retardation was assumed to be present when the fetal
weight was more than two standard deviations below
the average control weight. Treatment with the drug

alone at 670 or 1000 mg/kg was toxic to 13-day-p.c.
embryos. while doses of 200 mg/kg were only marginally effective. On I 8-day-old fetuses the drug alone(670 mg/kg) significantly increased the number ofmalformed living fetuses. X irradiation produced an
insignificant decrease in the frequency of normal
fetuses. while electron irradiation produced a marked
increase of resorptions. The first observation is not
surprising in view of the low doses given, but isapparently in contrast with the latter. As to combined
actions. they were thought to be "at least additive'' inthe case of electrons and "synergistic'' for the x rays.
But whether the same observations might hold atother doses and times remains to be confirmed.
Similarly. the differential effect of misonidazole inregard to x rays or electrons remains to be explained.
298. Michel et al. [M35] also reported on the
combined effects of misonidazole with negative pions(0.125-1 Gy) or 200-kV x rays (0.125-1.5 Gy). Mouseembryos on the 8th gestation day were irradiated. withor without application of the drug, 30 minutes before
exposure. The fetuses were examined on day 13 forlethality. growth disturbances or the presence ofmalformations. No significant killing effect was seenfor either radiations in the dose range up to 1 Gy:
however. the drug alone or in combination led tosome lethality. Growth retardation was significantly
present at I Gy and in combined treatments at lower
doses. Multiple severe malformations and severe
retardation were seen when a dose of 0.5 Gy was
combined with misonidazole. The frequency of teratogenic effects (all included) was significantly higher at
all doses for both the x-ray- and negative-pion-treatedanimals when irradiation took place after the administration of the radiosensitizer. There was. overall. a
synergistic enhancement of radiation effects by the
combined treatment, the extent of which varied as afunction of the radiation dose and quality and also
according to the induced effect.
299. In other experiments [MI4], pregnant mice ofstrains FIA and NMRI were exposed to 0.01 Gy ofwhole-body pion- or x-irradiation at day 8 of gestation.
Lucanthone (Miracil D), a known radiosensitizer invarious biological systems, was administered intraperitoneally 30 minutes before irradiation at 35 or
70 mg/kg. Five days after treatment, the fetuses wereobserved for developmental anomalies (growth retardation, micro-ophthalmia. exencephaly). In both strainsof mice, it was found that radiation produced asignificant increase in the incidence of abnormal
fetuses compared with non-irradiated fetuses. The
effectiveness of negative pions (peak irradiation) was
higher than for x rays with respect to the productionof teratogenic effects, the RBE being between 1.7 and1.9. The application of Lucanthone increased thenumber of damaged fetuses and led to various degrees
of sensitization, depending on the mouse strain anddosage used. Differences between strains in respect of
the incidence of malformations were explained by thepossible slight difference of developmental stages at
the time of treatment. A similar explanation was given
for the strain differences found in the same mousestrains irradiated with 0.135 Gy in association with
Lucanthone [M50].
317

300. Preliminary data were also reported by Balla etal. [B39] on the interaction between x rays (0.01 Gy)
and the cytotoxic drug vindesine (injected intraperitoneally at 0.35 mg/kg I hour prior to irradiation)in mice on the 9th day of gestation. The end-points
scored at 4, 12 and 24 hours after irradiation were
mitotic activity and cell death in the neuroblasts of the
optic vesicle. Results at 4 hours showed potentiationof the x-ray effects by a factor of 1.8. based on cell
necrosis.

C.

CONCLUSIONS

301. Knowledge of the effect of radiation type andenergy in modifying the response of pre-natally
irradiated animals is still insufficient, particularly incomparison with similar information related to othernon-stochastic effects. However. for irradiation with
fast neutrons of various energies. data are now
available on the induction of cell killing and production of micronuclei in the early embryos; for embryolethal and teratogenic effects in mice; and for lethal
effects and body or brain weight loss in rats. By
comparison with the effects of x or gamma rays, in nocase did neutrons show an RBE higher than 10 at thedoses used, down to about a tenth of a gray ofneutrons, irrespective of the neutron energy, dose
range, developmental stage at irradiation and time atscoring of the effects. In most cases reported, the RBE
values were actually lower than 5. although the values
may vary to some extent for different types of lethal
or developmental effects or malformations. Peaks ofmaximum sensitivity related to the stage of development of the various anatomical structures occur atthe same time for neutrons as for low-LET radiation,
but quantitative information is inadequate to state
whether the amplitude of the sensitivity oscillations
is consistently higher or lower than for low-LET
radiation. Moreover, experience is too limited torecognize definite changes of the RBE as a function ofdose, although somewhat higher values could be
expected at low doses. judging from the form of thedose-induction relationships of the radiations undercomparison in some of the experiments.
302. Information on the shape of dose-effect relationships for various end-points and on possible changesof this shape with radiation quality is. however.
insufficient. This makes it difficult to predict whether.
at doses of the order of I mGy or lower. the values of
the RBE might increase appreciably, as is the case formany other non-stochastic end-points. There is still no
information on the joint effect of LET and dose rate,
dose fractionation, and state of oxygenation of the
feta! or embryonic tissues. that would make it possible
to extend to effects in utero generalizations known to
be valid for other effects at the cell or tissue level in
adult animals. For example, a lower sparing effect offractionation with high-LET radiation, as comparedwith that of low-LET, has not been documented forsystems developing in utero. Similarly, data are
insufficient to say whether the RBE of different
radiations is higher under anoxic than under welloxygenated conditions for a number of end-points in
318

utero. At the same time, however, there is no
indication that the developing conceptus may behave,in respect of these major radiobiological variables, ina manner that is grossly at variance with that to bepredicted on the basis of adult mammalian systems.
303. A series of papers has reported rather extensively
on the effects induced by pre-natal irradiation with
530-MeV monoenergetic helium ions. This radiation
was slightly, but consistently, more effective than
gamma rays in stunting feta! growth under hypoxic orwell oxygenated conditions and in producing grossmorphological anomalies under aerobic exposure.
Dose splitting over 24 hours (for the same total dose
administered) resulted in higher RBE values. The
radiosensitivity of the embryo and fetus to killing inutero over days 3-10 of pregnancy showed oscillations
within a factor of three to four but on average theRBE of helium ions with respect to gamma rays over
the whole period considered was 1. Thus. there wereno major features to be gained from these experiments
in respect to sensitivity periods. effects of oxygen, or
effects of fractionation that would not be in accordancewith expectations from basic radiobiological knowledgeof gamma ray and helium ion dose-effect relationships
for cellular effects. An RBE value of 1.7 for peakrr mesons was reported for irradiation of pronuclearzygote mouse embryos scored at day 13 of pregnancy
for a variety of developmental effects.
304. The technique of in vitro irradiation and cultureof early mouse embryos has been used extensively inrecent years to test for combined actions with avariety of substances added to the culture medium. In
general, this technique produces information that ismore valuable for general toxicological than forradiobiological purposes. However. additive effects
were reported for the majority of substances tested.
including phenol, p-nitrophenol. 2,4-dinitrophenol,
sodium nitrite and sulfite, 2,4.6-trinitrobenzene sulfonate and ethidium bromide. In other cases, for somecombinations of radiation-drug dosage or for some ofthe end-points scored, synergistic effects were described.
This applied to drugs such as actinomycin D. cadmium
and lead chloride, and caffeine. The wider significanceof the findings in respect of the very low environmental levels of some of these substances, and of laterstages of development, remains to be fully clarified.
For exposures during the stage of major organogenesis, synergistic effects of radiation administered inconjunction with trypan blue, cortisone, caffeine,urethane. ultrasound and high frequency radiation
were also reported in single instances. More systematicstudies would be needed to substantiate these observations for a wider range of doses and effects.
305. Clear evidence has been gathered of a radioprotective action for the drug 2-mercaptopropionyl glycine
for a number of effects at the tissue or whole-bodylevel in pre-natally irradiated mice. On the other hand.
misonidazole, a hypoxic cell sensitizer. has beenshown to potentiate the action of radiation whenadministered in combination with low- and high-LET
radiation and for a variety of teratological anddevelopmental end-points. The same was true of another known radiosensitizer. miracil-D. which showeddifferent degrees of potentiation, depending on thestrain of mice, the dosage used and the type of
malformation scored.

VII.

TUMOUR INDUCTION

306. This chapter contains separate reviews of information on animals and on man. It will not attempt todiscuss the general problem of mechanisms of tumour
induction by radiation, which has been considered by
UNSCEAR on many occasions and most recently inannex B to the present report. It is not known atpresent whether the same mechanisms underlying
tumour induction in adult animals apply to mammals
irradiated in utero: knowledge of these mechanisms is
not sufficiently advanced to allow separate analysis ofcarcinogenesis in adult and developing animals. The
reader is therefore referred to annex B for an extended
treatment of tumour induction mechanisms which, atleast in relation to basic radiobiological aspects, are also
assumed for the time being to apply to tumours inducedby irradiation in utero. The subject of malignancy
induction following pre-conception exposure is discussed in annex A to the present report (chapter VI.
section A).

A.

doses that would not be considered high whenworking with adult animals. As a consequence. there
is a narrow range of doses to show radiation
carcinogenesis in pre-natal animals, which is a limiting
condition for obtaining precise dose-response relationships.
309. Valuable methods of studying radiation oncogenesis in cell culture have been developed in recent
years and are increasingly being used. Some of them
rely on the use of embryonic cells exposed in utero
and then tested for transformation in vitro. Suchmethods would allow. in principle. comparisons ofsusceptibility between cells of various feta! ages, of
carcinogenic responses in vitro and in vivo [Bl8, BI9.
B20], and of interaction studies between radiation andvarious inducing and promoting agents. All this could
be done taking as an end-point an effect such as celltransformation. which is thought to be closely related
to in vivo carcinogenesis. There are, at the moment.
some unsettled methodological points and difficulties
in understanding the precise relationships between
transformation in vitro and carcinogenesis in vivo.
When these difficulties are resolved, cell transformation may emerge as a very useful tool to study themechanisms of carcinogenesis in pre-natal stages. At
the moment. however, it appears from an. extensive
review of the field [B3 l] that no significant contributions are being published along the lines suggested
above.

DAT A FROM ANIMALS

307. Data on the relative susceptibility to tumour
induction of animals irradiated before or after birth.
which had appeared up to 1977, were already reviewed
by UNSCEAR in annex 1 of its 1977 report [U2].
While noting the relative scarcity of such data.
UNSCEAR had concluded that pre-natal irradiationof animals seemed to affect the growth and differentiation of tissues rather than their malignant transformation. The types of tumours induced by radiationin adult animals vary considerably with species andstrains. and this was shown to be true for irradiationin utero as well. Up to the time of the 1977 review,
there was no evidence. however, that pre-natal irradiation of experimental animals in utero could be more
carcinogenic than irradiation of young or adultanimals. The following text updates that review withrecently published data. It also examines other reports
not included in the previous analysis.
308. Carcinogenesis following pre-natal exposure,particularly in experimental animal systems. wasreviewed by Sikov [S46]. In his paper, he emphasized
the methodological difficulties of working on prenatal animals, and discussed the likelihood of anincrease in tumour incidence following exposure inutero being statistically significant at low doses withthe group size normally used for such studies. He also
drew attention to the need to consider patterns ofeffects and modifications of tumour spectra to avoid
missing important effects. On the other hand, his
paper illustrated the difficulties of increasing the doses
to produce unequivocal evidence of effects, because
irradiation of pre-natal stages may produce morphological and physiological effects in several tissues at

1.

The mouse

310. Rugh, Duhamel and Skaredoff [RIB] working
on small groups of CFI mice exposed to x rays
( 100 R) at each day between fertilization and 18 days
p.c. found that the tumour incidence in the offspring
was not statistically increased by pre-natal irradiation,although it was higher among the females, whether
irradiated or not. From these experiments. the evidence
for any increase in tumorigenesis was on the whole
equivocal, and it appeared that the long-term sequelae
of embryonic and fetal irradiation related more to
growth and total body weight (permanent stunting)rather than to tumour induction.
311. Agarwal et al. [A 1) reported on the radiation
dosimetry of an experiment in which a range of doses(0.1 to 2 Gy) was given at 10 or 18 days p.c. to mice inutero (strain not specified). Their paper is almost
entirely confined to the physical aspects of theexperiments and refers only to an increased incidenceof lymphoma and breast and lung carcinoma.
312. In the experiments of Friedberg et al. [F4)pronuclear zygotes of mouse strain CD-1, randomly
bred. were irradiated in utero by fast neutrons(0.15 Gy). Animals that survived up to 30 days ofextra-uterine life were then kept and observed until
their natural death. recording the cumulative mortality
distribution, the percentage incidence of the principal
neoplastic diseases and the mean age at death for eachtumour class. The differences found between irradiatedand sham-irradiated animals of the same sex were notfound to be statistically significant. , .....
319

313. Sasaki et al. have published a series of studies
on carcinogenic effects induced in mice by irradiation
at various intra-uterine stages. In the first of thesestudies [Sl]. (C57BL/6 X WHT)F 1 mice were exposed(200 R) at 12 or at 16-18 days p.c. and examinedthroughout their extra-uterine life for the induction ofdegenerative diseases. growth retardation and thepresence of tumours. For irradiation at the middle
intra-uterine stages, the prevailing effects to be observed
were growth retardation. several congenital malformations and amyloid degeneration. Tumours were notin excess over non-irradiated controls in this group of
mice. but there was a significant decrease of incidenceof lymphoreticular, lung and uterine tumours. Lung,
pituitary gland and ovarian tumours were significantly
enhanced after irradiation at the late feta) stages; liverand skin tumours were only slightly increased, thymic
lymphoma was not.
314. In another study [S2], data were published onmice of the same strain irradiated at 17 days p.c. (150or 300 R). within 24 hours of birth (400 R) or at
5 weeks of age (400 R). Overall. the susceptibility totumour induction increased with age, in the orderfrom pre-natal to neo-natal to young-adult mice;
however. the most remarkable changes were found in
the spectrum of tumours produced. An excess of
pituitary, pulmonary and hepatocellular tumours
developed in mice irradiated in utero, but lymphoreticular neoplasms and myeloid leukaemia were notinduced. For irradiation in the neo-natal stage, thymic
lymphoma, pituitary, ovarian, harderian gland, butparticularly hepatocellular tumours were found inexcess. Exposure at the juvenile age resulted instead ina higher incidence of ovarian and harderian glandneoplasms. The complex of these findings points tothe conclusion that the age-dependence of susceptibility
to radiation carcinogenesis differs for different organs
and tissues. The high susceptibility of peri-natal miceto x-ray-induced hepatocellular carcinogenesis foundin this series was also specifically discussed in anotherpaper [S 71] and was deemed to be related to the highfrequency of liver cell mitosis at the time of. or after,
x irradiation.
315. Two studies by the same group are so far only
available in an abstract. In the first one [S28]. groupsof B6WF 1 mice were exposed to 200-kVp x rays
( 150, 300 or 600 R) at 17 days p.c. The incidence oflymphocytic neoplasms was compared with that shovm
by mice irradiated at 1. 7, 35 and 195 days of age.
Exposure in the late feta! stages produced significantly
higher rates of such tumours. reaching about 23%
after 600 R. Susceptibility to the tumours was lower inthe feta! than in neo-natal and suckling mice, but not
lower than in the young adult mice. Data on the
breakdown of various classes of lymphocytic tumours
were also reported. In the second study [S27] B6C3F 1•
animals were exposed to x rays at 17 days p.c. and atdays 1, 7, 35 and 105 (300 R). Animals were killed
when 800 days old for study of kidney and cerebellar
damage and of tumour incidence. The carcinogenic
response at the feta! stage was far lower than that of
neo-natal mice and the tumour spectra were very
similar. In early post-natal life lymphocytic, liver andpituitary tumours developed with high incidences,320

while mycloid leukaemia and harderian gland tumours
were most elevated when irradiation took place at 35
and 105 days of age.
316. The results of other experiments by the samegroup are still in press [S72]. These experiments werecarried out on B6C3Fl mice irradiated at 17 days p.c.,within 24 hours of birth and at 15 weeks of age. Dosesof 1.9, 3.8 and 5. 7 Gy of 137 Cs gamma rays were used.
Gross and histological pathology were performed on
the animals at death. but data were not corrected forcompeting death causes. Overall, tumour incidence inirradiated animals was not significantly higher than incontrols, which had themselves about 90% tumour
incidence. However, late feta! animals were found tobe particularly prone to pituitary tumours, which were
statistically higher than in controls and had thelongest latent period of all induced neoplasms. After
1.9 and 3.8 Gy, given at 17 days p.c .. the incidence oftumours of the Jung was significantly higher than incontrols. Malignant thymic and non-thymic lymphoma
was also in excess and appeared after shorter latent
periods in feta! mice given 5.7 Gy; however, thesusceptibility was higher in early post-natal than in
pre-natal mice. The latter animals also showed ahigher incidence of liver tumours than the controls.
but not higher than that of neo-natal animals. Overall,
the tumour susceptibility of mice irradiated in the latefeta! and in the neo-natal periods was similar, but
young adults were little, if at all, susceptible to the
induction of pituitary, lung and liver neoplasms. The
young adults w~re, however. more susceptible tomyeloid leukaemia and to tumours of the harderian
gland.
317. In order to investigate the question of the highsusceptibility to radiation-induced tumours in prenatal animals. Schmahl and collaborators [S32] set up
experiments with a 2-stage carcinogenesis model. The
irradiation of the NMRI mice in utero (3 daily dosesof 0.8 Gy on pregnancy days 11-13) was regarded as apossible initiating stimulus, the promoting treatment
being represented by the skin application of thephorbol ester TPA ( 12-0-tetradecanoylphorbol-13acetate ). Mice were treated with the promoter between
12 and 26 weeks of their post-natal life and observed
for another 70 weeks for development of skin neoplasms. In spite of some differential effects in regard
to distinct diseases. the overall conclusion was that the
TPA treatment did not result in a higher tumour yield(either of the skin or of internal organs such as
marrow, lung. liver, ovary) than that observed after
pre-natal x irradiation alone. This result. which is atvariance with other observations in adult rodent skin,
was attributed to the dysplastic nature of the skinafter pre-natal irradiation, which fails to respond to
TPA as does normal feta! and newborn mouse skin. A
decrease in the synthesis of prostaglandin induced by
pre-natal irradiation would be responsible for thealtered susceptibility to TPA.
318. In another series of experiments by the samegroup [S33] pregnant NMRI mice were subjected to
fractionated x irradiation during late organogenesis(11-13 days of gestation), during the early feta! period(14-16 days) or during both periods (11-16 days).

Various fractionation schemes, with total doses ranging
from 2.4 to 7.2 Gy. were tested and the offspring were
followed for 39 months for the presence of ovarian
tumours. With daily doses of' 3 x 1.2 Gy. applied
either in late organogenesis or in the early fetal period.
a significant increase in the number of tumours wasobserved. mostly tubular adenomas, with about 20%
between luteomas and granulosa cell types. Lower
doses appeared ineffective. Autoradiographic observations on fetal ovaries I or 6 days after irradiation
during late organogenesis showed a depression of theproliferation rate which was thought to be correlatedwith the relatively low susceptibility to tumour induction during this period. There was a sharp increase in
the frequency of ovarian tumours following daily
doses of 6 X 0.8 Gy or 6 X 1.2 Gy on days 11-13.
Ovarian cysts were also found and they reached thehighest incidences in animals treated with doses of
3 X 1.0 Gy or 3 X 1.2 Gy on days 11-13. The lowest
cyst frequencies were seen in animals receiving doses
for 6 consecutive days, which had the highest incidenceof ovarian tumours.
319. Kusama and Yoshizawa [K24] reported on acomparison of the carcinogenic action of 137 Cs gamma
rays on male and female C57BL/6J mice between the
fetal (15 day p.c., I Gy), or post-natal age (30 days ofage. 1 or 4 Gy). Autopsies were performed at death;tumour incidence rate, average latent period andcomposition of the tumour spectra were the biologicalend-points scored. There was no difference in the
gross tumour incidence between control and irradiatedmice, whether irradiation took place before or after
birth. Different methods for adjustment of the datafor competing risks were applied and it was concluded
that the Kaplan-Meyer method was appropriate forthe purpose. Under such conditions of analysis. theincidences of thymic lymphoma and breast tumours in
all irradiated groups (feta! or post-natal) were foundto be similar to those of the control mice. Reticular
tissue neoplasms. on the other hand, showed a consistently decreased incidence in all irradiated groups.
with little evidence of any special trend of the pre- as
opposed to post-natal mice.
320. Pre-natal x irradiation of NMRI mice on the
12th day of gestation was reported by Schmahl toinduce both dysplastic and neoplastic disorders [S65].
Dysplastic changes (see also [S62] and [S63]) include
hypotrichosis. alopecia and skin ulcerations. appearingat 2-4 months of age in about one-quarter of the micereceiving 0.5-2.0 Gy. As to neoplastic changes. theirincidence in the irradiated mice appeared to begenerally lower than in controls, but the various
classes of tumours showed a different behaviour. For
example, the incidence of leukaemia (after correction
for competing causes) was decreased at all doses
below the normal incidence of the non-irradiatedmice. By contrast. lung tumours (both benign andmalignant), ovarian (see also [S33]) and liver tumours
were increased, with a peak incidence at I Gy. Under
the condition of the experiments, this dose was shown
to be the most effective for tumour induction. The
liver appeared to be the most sensitive organ, both forirradiation alone and in experiments using the tumourpromoting agent phorbol ester TPA.

321. Covelli et al. [C9] reported on experiments
designed to investigate the role of age and LET on thesusceptibility to radiation carcinogenesis and life
shortening. Graded single doses of x rays (250 kV,
0.5-7.0 Gy) or attenuated fission neutrons (averageenergy. 0.4 Mev, 0.09-2.14 Gy) were administered to
BC3Fl mice of both sexes in utero at 17 days p.c., or
to male animals aged 3 or 19 months. Irradiation of
3-month-old animals caused a measurable dose-relateddegree of life shortening associated with a higher
incidence and rate of appearance of radiation-induced
tumours. Irradiation during the pre-natal ages or at
19 months. did not produce significant shortening of life
after both x-ray and neutron irradiation: there was.
however. a significantly higher incidence of solid
tumours and reticulum cell sarcoma. The high variability of control values in the incidence of neoplastic
diseases. particularly of the reticular cell tumour, andthe differential carcinogenic action of x rays andneutrons in animals irradiated in utero, do not allow
precise conclusions about the overall comparative
susceptibility of mice irradiated at different ages. It isquite evident. however. that the changes in the tumour
spectrum are much clearer effects than the overall
shift of tumour incidence between animals irradiatedat various ages.
322. Baserga et al. [B36] studied tumour induction in
CxA F 1 mice after exposure to radioactively labelledthymidine. Variables analysed were: the age of theanimals at exposure (including 15-17 days p.c. fetuses,
newborn animals and adults at 2. 6 and 12-14 monthsof age): the magnitude of the thymidine dose and themanner of dose fractionation (between about 7.5 and370 kBq per gram body weight delivered as single
doses or in 6 fractions over 8 days): and the effect of
14C-thymidine, as compared to lH-thymidine. Only
malignant tumours were considered and there were noappreciable differences between mortality rate andtumour incidence between the two sexes. The data
showed that both types of thymidine produced shortening of life span and an excess of tumours: that tumour
incidence was dose-related and greater in animals
injected as young adults than later in life; and thattumour incidence was also significantly above thespontaneous level in animals treated in the feta! stage.
The small number of tumours seen, and the fact thatthe experiments reported did not extend until spontaneous death of the animals. preclude any meaningfulcomparison of tumour incidence between different
ages at irradiation in this series.
323. The leukaemogenic effects of 32 P given intraperitoneally as NaP0 4 were studied in 3-5-months-old
BALB female mice and in animals of both sexes
between 11 and 15 days p.c. [Hl7]. After contamination. animals were observed until spontaneous
death or were killed when leukaemia was diagnosed;in any case the experiments were terminated when the
animals were 2 years old. Autopsy examinations were
performed and leukaemia (including leukaemia proper.
lymphosarcoma and reticulum cell sarcoma) was
diagnosed microscopically. Lack of separation between
various types ofleukaemias diminishes the usefulness of
this study. In adult females. a significantly elevatedincidence of leukaemia was seen. but the dose depen321

dence over the range of activities of 32 P administered(about 1.5 to 4.5 MBq per animal) was not significant.
In females exposed as fetuses the incidence ofleukaemia
was also significantly increased, and the females
appeared definitely more susceptible than males.
Furthermore, there was a negative correlation with thedose in females (over the range of about 90 kBq to
3.3 MBq per animal). while in males no such correlation was documented. These differences were attributed
to the fact that 3lp at the doses given affects ovariandevelopment and disturbs an interaction with oestrogens in leukaemogenesis. Comparison of the incidenceof leukaemia showed that female animals treated as
adults were approximately as susceptible as when
injected as fetuses.
324. Between the years 1971 and 1982, Ronnback
published a series of papers [R4, Rl0-Rl4] (alsocollected and jointly commented in a thesis [R5]) in
which he described the effects of 90Sr irradiation on
the feta! ovaries of CBA mice. In none of these papers
was there information on the actual doses absorbed in
the feta! ovaries. However, it has recently been
estimated [R26] that the effects of 11.1 kBq 9oSr given to
the pregnant mother would correspond approximately
to those seen after doses to feta! ovaries of
0.010-0.013 Gy, given continuously over 4 days, fromthe 18th day of gestation to the 2nd day of extrauterine life.
325. In short, it is shown in this series of experiments[R5] that the time for administration of the radionuclide to the pregnant mothers is a very importantfactor in determining the final degree of injury to theovaries of the fetuses. When the number of germ cells
remaining in the ovaries was scored at 56 days ofextra-uterine age. it was found that the later during
pregnancy that 90Sr had been administered, the worst
was the degree of injury to be seen, the maximum ofeffect being found for contamination at 19 days p.c.
The earliest maturation stages, i.e., the "naked"
oocytes, were in all cases the most sensitive cells. An
activity-effect relationship was established in the rangeof 11-370 kBq 90Sr given to the pregnant mother, andit was found that the fraction of germ cells remaining
in the ovaries decreased linearly as a function of logactivity. The slope of the curve, however, remained
the same, whether the damage was scored at 28, 56 or
84 days of age. Considering that there is a pronounced
natural decrease of the number of oocytes as afunction of age in untreated control mice. thisobservation indicates that the rate (as a function ofdose) at which the germ cells decrease in the irradiatedovaries is independent of their initial number. A
statistically significant reduction of naked oocytes wasseen in these experiments after about 11 kBq 9oSr.
326. In parallel with the short-term morphologicallyrecognizable damage, long-term functional and carcinogenic effects were also assessed in these experiments by
testing the reproductive capacity and the incidence ofovarian tumours in the mice contaminated in utero.
Reproductive performance over a period of 7 months
was tested by standard methods over the range of
46-740 kBq 90Sr given to 19-day pregnant mothers.
Only starting from 320 kBq were there signs of
322

impaired fertility. The signs were quite obvious at
740 kBq, the rate of pregnancy being significantly
reduced. The total number of litters and of offspring
per female. and the reproductive period, started to
decline at 185 kBq and were significantly affected at
370 kBq. It should be noted, for the sake of comparison, that at 185 kBq only 25% of the oocytes andfollicles remained in the ovaries at the beginning of
the mating period and that, 7 months later, at the endof it, the ovaries were essentially depleted of germ
cells.
327. Tumour incidence was observed in the ovaries
of JO-month-old animals that had been contaminatedas fetuses with 90 Sr activities in the range of 46-740 kBq.
Eighty-three out of 104 animals showed interstitial
fibrosis, cysts and proliferation of the germinal
epithelium. Different types of ovarian tumours( 1 granulosa cell, 2 cystic adenomas, 18 tubular
adenomas) were also found in these animals. There
was a direct relationship between the frequency ofanimals showing proliferation and "down growth" ofthe germinal epithelium into the ovarian parenchyma
and the frequency of animals having tubular adenomas.
but tumours were not found until about 14% of theanimals had shown downgrowths. Stimulation of thefollicle-depleted gonads by gonadotropic hormones
might be an important factor in the proliferative
phenomena leading ultimately to the production oftumours.
328. Another paper by Walinder and Ronnback[W30] added more information on the induction of
thyroid and pulmonary tumours in CBA mice, given
pre-natally 13 11 and whole-body x irradiation at
18-19 days of gestation. This paper gave emphasis to
the role of cellular proliferation as a tumour-enhancing
factor, particularly by comparison with proliferationin the adult mouse. It confirmed that tumour frequency
was higher and latency shorter in the thyroid of
the pre-natally irradiated animals than in the adults.
Lung tumours were not more frequent in pre-natally
irradiated mice than in non-irradiated ones, although
their latent period was significantly shorter. Cellkilling was found to be more pronounced in the
thyroids of mice irradiated as adults than in animals
exposed to the same doses in their feta! life. Therefore,
the loss of cells' reproductive capacity leading to alower tumour incidence was held to be less importantfor pre- and neo-natal thyroid carcinogenesis. than for
carcinogenesis in mature glands. It was pointed out.
however, that many other factors, in addition to cellproliferation, may determine the final incidence oftumours: for example, cells in the organs of the feta!
and the adult animal could be at various stages ofdifferentiation. This condition, among others, wouldof course make comparisons across ages extremely
difficult.
2.

The rat329. Sikov and Lofstrom [S 13, S46] treated SpragueDawley rats with x rays (20 or 100 Ron day 10 and 50
or 185 Ron day 15 of gestation). Later in the animals·
life they observed decreases in growth and longevity
that were exposure-related. The incidence of mammary

tumours in female animals given the highest exposure
at 15 days was decreased. However, there was a
shortening of the latent period in tumours developing
in irradiated rats. as opposed to those appearing in thecontrols. An exposure of 50 R produced a tumour rate
almost identical with that in the controls, with agreater tendency towards the presence of multiple
tumours in tumour-bearing animals. Tumours ofother sites were not appreciably increased. After
185 R. the gonads were atrophic. and this suggested
an association between the decreased incidence of
mammary tumours and the ovarian changes at thislevel of exposure.
330. Wegner and Graul [WI 7] irradiated (180-kV
x rays, I Gy) Wistar rats in utero on the 9th day p.c.
and followed them after birth for the appearance ofmalformations and tumours. About 40% of theanimals showed brain or eye malformations and 10%
developed tumours (as compared with a spontaneous
rate of 3.4%) mostly of the malignant type, althoughbenign mammary tumours were also observed. Animals
bred through the second generation had no malformations. no significant decrease of life expectancy and noincreased tumour incidence. In other experiments[W 18]. animals of the same strain were exposed(270 R) on the 18th day p.c. There was a highincidence of microcephaly and microophthalmia andgenital hypoplasia in both sexes. In males, theincidence of tumours (total and malignant) was
increased significantly above the control level, but it
appeared to be slightly lower in these animals than inother animals exposed post-natally. In females. theincidence of mammary tumours was markedly
decreased by pre-natal exposure, while it increased inrats exposed after birth. Other tumours of the genitaltract (uterus, ovary) were reduced by pre-natal ascompared with post-natal exposure. When all genital
tumours were excluded, the incidence of othermalignancies was slightly greater in females exposed
post-natally than pre-natally.
331. The experiments by Strel'tsova and PavlenkoMikhailov [S48] were carried out on outbred whiterats irradiated during the pre-implantation period(7 days p.c., 240 animals). during late organogenesis(14 days p.c., 105 animals) and during the feta! period(19 days p.c .. 120 animals). The methods for counting
stages in development are not specified in the paper. A
single dose of 1 Gy of 60 co gamma rays was administered in all cases. The animals, together with some
controls. were followed in time to study tumour
appearance. The results reported refer to 219 test and
53 control animals dying between 200 and 600 days ofage. About 17% of the control animals developed
tumours during this time. as compared with about
65% of the animals irradiated during early organogenesis. late organogenesis (63%) and the feta! period(43%). Animals irradiated at 14 days p.c. developed
tumours up to 400 days of life, while in the other two
test groups tumours only appeared between 120 and
180 days of life. Male animals irradiated at 7 days p.c.
showed tumours of the mammary gland and bones.
while the females irradiated at the same time showedovarian and bone tumours, which were not observedin control animals.

332. Kalter et al. published a series of papers on theinteraction of ethylnitroso-urea (ENU) and x rays
administered in utero in respect of the oncogenic
response of rats. In a first series of experiments [W7]pregnant Sprague Dawley rats received 2 Gy of x irradiation on days 15 or 16 p.c .. and 10 mg/kg ENU one
to four days later. Controls only irradiated or only
injected with ENU were also set up. By the 15th month
post-exposure, about 17% of the offspring exposed toboth agents pre-natally developed neurogenic tumours.
as opposed to 62% of those exposed to ENU alone. or
to no tumours at all in those exposed only toradiation. The first signs of tumours appeared later inanimals exposed to the combined treatment. In orderto probe deeper into this antagonistic action, in asecond experimental series [K I. K2], rats were irradiatedon day 16 with a range of doses from 0.05 to 2.5 Gy
and received ENU (10 mg/kg intraperitoneally) on
day 20 of gestation. The frequency of animals surviving
beyond 4 weeks of age that developed tumours in thecourse of their entire life span was inversely related tothe dose of x rays. Thus. ENU alone caused about
69% of animals to develop tumours; 0.25 Gy + ENU,
46%; and 2 Gy + ENU, 15%. The mean time for theappearance of neurogenic tumours and the mean
number of tumours for each affected animal wereunrelated to tumour frequency. Analysis of the rate oftumour appearance indicated that the initiation, rather
than the promotion, stages of tumour induction could
be responsible for the reduced frequency. Killing oftarget cells, probably glioblasts. was not in itself
entirely sufficient to explain the results.
333. The subject of the interaction between radiationand ENU was also taken up in another paper by
Schmahl and Kriegel [S82].. In their experiments.
single and combined treatments were administered to
Wistar rats with the chemical alone or after x-ray
doses of0.5-1.5 Gy. Animals were in the 13th day p.c.,
at which time both agents may induce brain malformations and ENU. alone or in combination. may
produce brain tumours. The possible correlations
between the teratological and the carcinogenic endpoints were investigated by a histopathological analysisof the type and degree of malformations. and the
distribution patterns of tumours in the forebrain. The
existence of a correlation was shown not only by thesimultaneous occurrence of both effects in the samebrains. but also by a higher tumour multiplicity(mostly gliomas) in the malformed brains and theirpreferential location in the subependymal layer, which
was also the most heavily malformed structure.
Another effect of irradiation, the occurrence ofheterotopic neuronal nodules called '"rosettes", which
is only apparent at doses above 1 Gy. correlated
negatively. however. with the occurrence of gliomas.
In view of the authors. this explains the substantiallydecreased incidence of gliomas following combined
treatments involving doses in excess of 1.0 Gy. andalso the consistent multiplicity of tumours, in spite of
their decreased frequency.
334. Cahill and his group studied neoplastic and lifespan effect after chronic exposure to tritium. In a first
paper [Cl], Sprague-Dawley rats were exposed toequilibrium levels of tritiated water during pregnancy
323

and up to parturition. Four different levels of theradionuclide resulted in cumulative whole-body dosesof about 0.066, 0.66, 3.3 and 6.6 Gy. The animals at
the two higher dose levels showed, throughout theirlife span. an increased incidence of mammary fibroadenomas. Data were not such as to allow thedefinition of a precise dose-response curve; they were,
however, not incompatible with linearity. Increasing
doses of radiation shortened the time for the onset of
mammary fibroadenoma. A second paper [C2] reported
that the offspring of the animals exposed in utero todoses up to 0.66 Gy showed no effect with respect tothe overall incidence of neoplasia, to incidence rate. or
to age at onset of mammary fibroadenomas. Females
exposed to 3.3 or 6.6 Gy were sterile and had lower
rates of mammary fibroadenomas than did controls.
At 6.6 Gy females showed a lower incidence of overalltumours. Regardless of dose, females had a significantly
higher incidence of neoplasia and a longer life span
than males.
335. In another set of experiments. Berry et al. [B37]
attempted to induce bone tumours in the offspring ofSprague-Dawley rats by treating pregnant mothers( 16, 18 or 20 days p.c.) with 32 P (about 37 or I 10 kBq
per gram body weight). Fetal tissues were judged tobe resistant to irradiation since no increased frequencyof tumour was found in the treated animals. However,naturally-occurring tumours showed a clear tendency
to appear earlier in post-natal life in the treatedanimals. On this basis, it was concluded that transplacental exposure of rats to 32p provided no evidence
of enhanced oncogenic effects in the fetus.
336. Other experiments were reported by Schmahl
and Kollmer [S3 l, S64] on the carcinogenic effect of
90Sr injected into pregnant rats at 18 days p.c. (3. 7 or
5.5 MBq per rat). The radionuclide accumulated in theossification centres of the skull of feta! animals todeliver a dose which was calculated to be between 0.6
and l.2 Gy (over the entire life span) within the bone
surfaces. Approximately 50% of this dose was delivered
within a week of the injection of the nuclide. The
offspring of the treated animals were examined at
autopsy and pathologically. They had an incidence of
pituitary tumours which was. as compared with nontreated control animals, about 10 times higher in themales and three times higher in the females. In about 6%of treated animals. metastasizing meningeal sarcomas
were seen, and in 2%. pituitary adenomas andmeningeal sarcomas occurred simultaneously.
337. Sikov et al. [S47. S54, M40] injected intravenously into adult, weanling, new-born and 19-daysp.c. rats 2~9Pu cytrate in different amounts. adjusted insuch a way as to deliver radiation doses of approximately
0.07, 0.23 and 0.7 Gy to these animals' femurs during
the first 10 days after injection. Doses to the various
age groups were markedly different. Most of the doseto the younger animals was absorbed within the firstfew months. Weanlings and adults received a more
prolonged and substantially higher cumulative exposure. The animals were observed until their natural
death, but the experiment was terminated after
30 months. The survival times of rats exposed postnatally became progressively shorter as the dose
increased, but no life shortening was seen in animals
324

exposed pre-natally. Bone tumour incidence increased
in all exposed groups, irrespective of age. Younger
rats appeared to be less sensitive on an administereddose basis, but were perhaps more sensitive on a
cumulative-dose basis. A biphasic cumulative doseincidence relationship with a maximum at about
0.25 Gy and a drop at 0.8 Gy was seen in animals
exposed pre-natally, but it is not clear how much ofthis phenomenon may be true or spurious (forexample, owing to confounding maternal effects). Prenatally exposed rats cross-fostered to non-injected
females, in order to ensure that they would only be
exposed while in utero, had the highest incidence ofbone tumours, even higher than that of rats remaining
with the injected mothers. This result was probably
related to the high dose given to the dams. which
affected their capacity to care for the offspring. The
site of radiation-induced tumours also varied as afunction of age: in the younger animals, most tumours
arose in the head or in the vertebrae: in the adults andweanlings, they were mostly in the extremities, with
only a few in the vertebrae or other places. It was
concluded that the younger animals were probably
more susceptible per unit dose than the adults. It isclear, however. that this susceptibility is also afunction of the rate of dose delivery and therefore is
not easily defined.
338. Two abstracts reported on the late effects of 131 1
in Fisher rats. The first [C20] regarded animals
exposed in utero to concentrations of Na 131 1 ranging
from 4 kBq to 3.7 MBq during days 16-18 of pregnancy.
Two months post-partum, the animals showed analteration of immunological reactions (cell-mediatedimmune response and antibody-dependent cellmediated cytotoxic response) to the carcinogen 1.2dimethyl-hydrazine. suggesting that the original perinatal exposure produced a persistent altered reaction
to the chemical. The second abstract [LI l] reported
on a higher immunological response of the maleanimals exposed in utero, which was l.7 times that of
the female offspring.
339. Sikov et al. [S80] exposed groups of 50 to80 pre-natal, new-born, weanling and adult rats tovariotts levels of 131I and scored their thyroids
microscopically at death or at 30 months of age forthe presence of tumours. These were commonly C-cell
type in control animals and follicular in the exposed
ones. Doses were estimated by radioanalysis in the
thyroids of the various groups. When given postnatally the 131 1 increased tumour incidence significantlyin the range of I Gy. but led to a further decrease
down to control levels after doses of 20 Gy or greater.
When given pre-natally (from 17 days on). the 131 I
produced a higher incidence of tumours at all doselevels of 0.4, 4.25 and 34 Gy. The form of the doseresponse relationships was characterized by a peak atintermediate doses in the two older groups, but was
progressively increasing with dose in the two groups
exposed perinatally.
3.

The dog

340. Preliminary data are also gradually becoming
available for the dog irradiated in utero. In one report'

[P7]. 4 groups of 120 beagles were irradiated withgamma rays from 60Co in the late feta! period (55 daysp.c.) or soon after birth (2 days of age) with 20 or
100 R delivered over a constant time of 10 minutes.
The morbidity and mortality of these animals wasfollowed over 2 years and compared with that of 450
control dogs and of other dogs exposed to the same
dose levels at other ages. Three of the 240 dogs at the
100 R level and I of the 240 at the 20 R level died withtumours within the observation time. while in controls
and animals exposed at other ages. no tumours wereobserved . No tumour would be expected to appear bychance in irradiated dogs.
341. An abstract published in 1978 [84] updated theexperience to 5 years post-exposure and concluded
that of 20 dogs developing tum ours. 19 had been
irradiated. The experiments were not sufficiently
advanced to draw significant conclusions as to the roleof age as a factor in radiation injury. Another report[828] gave evidence that the incidence of lymphoma inthe irradiated dogs. especially those exposed at 55 days
p.c .. was considerably higher than that in the controls
or than that expected in a random canine population.
Final results of these series, as well a:, of another series
on beagle dogs receiving continuous whole-body
irradiation [S29. S30], will not be available for a few
years.
342. Experiments by Momeni [M41] were aimed atstudying the incidence of bone sarcoma and myeloproliferative disorders as a function of dose rate andtime in beagle dogs fed diets containing 90SrCI (inequilibrium with 9oy) from mid-gestation to 1.5 yearsof age. Median incidence time and standard deviationof the distribution were calculated for the tumours at
each level of administered activity. For primaryosteosarcoma, this time increased from 2.8 years ofage for animals receiving 1.3 MBq 90Sr/day to 12.6 years
for those receiving about 1.50 kBq 9oSr/day. Althoughvery valuable for extrapolation of the carcinogenic
effect to low doses and dose rates, these data do notlend themselves to a comparison of susceptibility totumour induction in the pre- as opposed to post-natal
ages.

B.

DATA FROM MAN

343. One of the most important sources of information about the induction of tumours in children
irradiated pre-natally is the so-called Oxford Survey ofChildhood Cancer. Data from this series were firstreported in 1956 [S44] by Stewart and her colleagues.
These authors confirmed in 1958 (S23] that themothers of children who died from leukaemia or othermalignant diseases remembered having been subjected
to abdominal or pelvic x rays during the related
pregnancy with a frequency that was higher than that of
the mothers of control children who had not developed
cancer. This finding stimulated much interest. For a
description and history of the Oxford Survey see
Steward [S22]. Soon other reports followed and theexperience accumulated up to 1961 [F6, K21. K28.
M39, P9, S23. S44, S45] was summarized by UNSCEAR[U3] as in Table 12.

344. In its 1964 report [U3] UNSCEAR reviewed
other data that had appeared up to that time [L5,Cl.7, M22. M25, Wl6]. It pointed out that, asestablished by MacMahon [M22], the joint maximum
likelihood estimate of the relative risk factors in all thesurveys reported was 1.4, with 95% confidence limitsof 1.2 to 1.6. On that basis, UNSCEAR concluded
that. although accurate dose estimates were notavailable, irradiation of feta! tissues could give rise toa greater risk per unit dose than post-natal irradiation.
possibly by a factor as high as 5. It pointed out at thesame time. however, that it was difficult to separate
such Ieukaemogenic effect of radiation from otherpossible etiologic factors connected with the reason
that had prompted the radiological examination of theconceptus in utero.
345. Further data on the carcinogenic risk of irradiation in utero were reviewed again in the 1972
UNSCEAR report [U4]. The papers by Graham et al.
[G 11]. Jablon and Kato [J4] and Stewart and Kneale[S24, S26] were specifically examined. The report drewattention to the discrepancy between the negative datafrom Hiroshima and Nagasaki and those from irradiations performed for medical reasons. It also discussed
the difficulties of deriving precise estimates of risk fromcohort (rather than from case control) studies. because
the cancer risk in children less than I Oyears of age is rare(of the order of I 4 ) and cohort studies of sufficient size
are therefore difficult to carry out. The report concluded
that children born of mothers irradiated during
pregnancy seemed to have an increased risk of cancer intheir early age. The estimate put forward at the time.
most likely in excess. was of the order of 2 10- 3 Gy- 1
a-l over a 10-year period. in the range of 0.002 to
0.2 Gy. However. the possibility could not be ruledout that the associations found (or at least part ofthem) could be related to factors other than irradiation
itself.

o-346. In 1977, UNSCEAR again examined the problemof the carcinogenic risk of irradiation in utero. on thebasis of other data by Mole [M23]. Newcombe andMacGregor [N3]. Holford [H 16], Stewart and Kneale[S24], Jablon and Kato [J4). and Kato [K20]. The
analysis led to the conclusion that the risk, per unit
absorbed dose. of fatal malignancies induced by feta!
irradiation could be in the region of 2-2.5 10- 2 sv- 1•
Half of these malignancies could be attributed toleukaemia and one quarter to tumours of the nervous
system.
347. Subsequently. the Committee on the Biological
Effects of Ionizing Radiation (BEIR) [CI3] examined
the literature available up to 1980 on the subject ofradiation carcinogenesis in utero. In essence, it concluded that an association existed between intrauterine irradiation and childhood cancer, but it
recognized that such an association could be attributed,
to some extent. to factors involved in the selection
process for submitting the pregnant mothers toradiodiagnostic examinations. Strong support for acausal relationship came from twin data from theOxford series. while weak support for a selection
effect came from the ascertained relationship between
pre-natal irradiation and neo-natal health and from325

the relationship between neo-natal health and later,
clinically detectable cancer. The BEIR Committee, on
the basis of adjusted data from the Oxford Survey
estimated the relative risk of cancer after exposure inutero relative to irradiation after birth to be 5.0 for
the first-trimester exposure and about 1.5 for theremaining intra-uterine life. The period at risk appeared
to be within the first 12 years of post-natal life forhaemopoietic tumours and within 10 years for othersolid tumours. The risk coefficients were estimated to
be at 2500 excess fatal leukaemias and 2800 excess
fatal cancers of other types per million children, yearand gray.
348. MacMahon [M52] later reviewed the conclusions of the BEIR Committee report on this subject,adding some useful considerations. According to thisauthor, there is no doubt about the existence of a
relationship between exposure in utero and subsequent
likelihood of developing a malignancy within the first
10 years of life. The relative risk associated with prenatal exposure is of the order of 40 to 50% over nonexposed children. A number of variables (medical,
sociological, methodological) have clearly been identified which tend to confound the association, but evencorrecting the data on their account would not be
sufficient to justify the association quantitatively. The
most difficult problem is that of establishing whether
the association is causal or by chance. On the one
hand, it is difficult to prove that this may not havebeen due to characteristics of the mothers, thechildren, or the pregnancy, that may also have been
associated with a tendency of the children to develop amalignancy. On the other hand. no such characteristics
have been found. There is. in favour of a directcausality, the observation of Mole [M23] on single andtwin pregnancies, but this is based on a re-interpretation of the same series. Against a causal relationshipthere are. however, cogent data from Hiroshima andNagasaki and from experimental animals. According
to MacMahon [M52] it is biologically implausible thatfeta! doses in the region of a small fraction of a Gy
should increase the relative cancer risk by 40 or 50%
within I O years of age while doses in excess of I Gy
should fail to do so over longer periods of time, as
pointed out by Jablon and Kato [J4]. There is,further, [M52], the constancy of the relative risk overall categories of malignancy. which is not in agreement with evidence on radiation carcinogenesis [U2]indicating a remarkable degree of variability between
tissues and ages in their response to radiation. Finally,on the question of the alleged dose-response relationship, MacMahon [M52] argues that data analysis as afunction of dose is based on the acceptance of thecausal nature of the association, which he is inclined
to doubt in the case at hand. Given the difficultyof finding new material for independent analyses,
MacMahon is also inclined to regard this issue.
important as it may be, as an unresolved curiosity
which will gradually fade into medical history. giving
rise. in the process, to more confusion than understanding.
349. The references given thus far. on which previous
conclusions have in large part been founded. are themost relevant to the point at issue but they by no326

means exhaust the long list of papers published on thissubject. even before the last UNSCEAR review. Other
references should also be kept in mind to complete thepublications available up to about 1977 [A9, B 11.
B22. B32, D14, D15, G2. GIO, GI2, J8, L6, M26. 05,
S22. S25, S39. S41. S42, W15]. What follows updates
information published since then.

1.

Atomic bomb sunirnrs

350. The most recent report on the mortality experience of children exposed in utero to the atomic
bombs refers to the period 1945-1976 [K4]. The study
sample included 1293 children (1074 exposed at
Hiroshima and 219 at Nagasaki) whose status andcauses of death were obtained from Japanese family
records and death certificates. Doses in utero were
estimated by two different groups: one at the Oak
Ridge National Laboratory in the United States, theother at the National Institute of Radiological Sciencesin Chiba. Japan. The methods used by the two groups
were slightly different, but the tissue doses obtained
were almost identical and correlated well with the T65
dose, although they were lower by about 40%. (These
doses will, however. be subject to revision at somelater stage.) Total deaths up to 1976 numbered 203,and the mortality ratio increased linearly with T65
and tissue dose in both cities. The dose-related
mortality ratio varied. however. with age at death: it
increased with dose for those dying under 1 year ofage, particularly for those dying within 1 month ofbirth; it showed no increase at 1-9 years of age: and anincrease was again suggested after 10 years of age.
Analysis of the data as a function of trimester of
exposure showed a significant increase of mortality
ratio with dose only for children exposed in the third
trimester.
351. Mechanical injury received by the mother was
insufficient to provide an explanation for the excess inearly infant death. Birth order and birth weight could
not have seriously affected the relationship between
mortality and exposure. Precise information about thecauses of death was not available for 55 of the 203
deaths, and these 55 occurred almost exclusively
within a year of birth. This circumstance did not allow
a fine breakdown of causes of death. The following,
however, were analysed: perinatal death. which was
found to increase with dose: and cancer death(2 leukaemias and 3 solid tumours of the digestiveorgans), which was not found to be related to dose,even when additional samples defined only in 1960
were analysed. There was no statistically significant
evidence of an excess in cancer death among individuals irradiated in utero. However, the sample size
and the number of deaths were too small to reach afirm conclusion.
352. A further report by Ishi maru et al. [13] is of
relevance. These authors studied the incidence ofleukaemia in a sample of 3636 children (including
controls and exposed in utero) in the two cities.
Exposed were those whose fetal doses were between
0.01 and 0.49. and in excess of 0.5 Gy. The crude
annual incidence rates for exposed and controls inIIboth cities during 1945-1979 are shown in Table 13.
During the 34-year follow-up period, 3 leukaemia
cases were identified. 2 in the control groups, and l in
the group exposed between 0.01 and 0.49 Gy. It wascalculated that, in order to show any statisticallysignificant effect of radiation. 7 cases of leukaemia
would have to occur among the 705 irradiated subjects
included in the sample. Thus. there appears to be no
excess risk for the exposed children. This is inagreement with previous findings [J4. K4] in thisseries.
353. In 1974, Mole [M23] used a radiobiologicalargument to show that the discrepancy between theobservations from Japanese children exposed to atomic
radiation in utero and medically irradiated British
children could be the result of an artefact, since thekilling of potentially transformed cells. particularly at
the highest doses, might be responsible for the low
rate of tumour induction observed in the Japaneseseries. Using reports from the early 1970s [J4. K20].
and correcting the tumour induction data according to
an exponential cell killing function, Mole showed(under a number of qualifying assumptions) that thetrue induction rate per unit dose might have been
three to four times higher than that actually found,thus bringing the results of the Japanese series muchcloser to those of the Oxford series. In the meantime,
new information has been produced [K4] which
extends the observation period up to 1976 in Japan,
always with negative conclusions. Mole's reasoning
was also based on old dosimetric data that are
currently under revision. His conclusions, therefore,
will have to be verified against the new dosimetry(particularly the doses to the fetus) to sec whether theyretain their validity.

2.

Medical exposures

354. The present study reviews only the contributions
that have appeared since the last UNSCEAR reporton the subject [U2]. Kneale and Stewart returned tothe analysis of the Oxford Survey with two more
papers published in 1976. The object of the first paper[K9] was to discover whether the excess of fetal
irradiation histories associated with development ofearly neoplasms could be an artefact due to theassociation between obstetric radiography and otherbirth factors of etiologic importance. To this end theauthors applied the Mantel-Haenszel procedure, which
permits identification of separate effects of associatedfactors in retrospective surveys, to analyse the effects
on childhood cancer of social class, maternal age.
birth order, and fetal irradiation. A further objective
was to test whether these conditions might also have
similar effects on other childhood cancers than reticuloendothelial-system (RES) neoplasms. The new analysis
showed that all these factors did exert independent
effects on RES neoplasms and all, except sibship
position. independently influenced cancers other than
leukaemia. However. the degree of these joint associations could not account for the fact that radiation
exposure in utero. under the conditions tested, isassociated with a higher risk of developing juvenile
cancer.

355. The second paper [K 19] addressed several
different points and led to a number of conclusions.
There was no statistically significant proof of a doseeffect relationship. but only suggestions. that the riskof developing juvenile cancer did increase in proportion to the number of radiographic films used. In viewof the retrospective character of the survey, of thevariability in the number of films used and their
exposure in different places and over a long period.
this evidence may not be very reliable. There was alsoa suggestion of a higher susceptibility to cancer
induction for children irradiated in the first trimester.
There were relatively fewer cancer cases amongchildren with abnormal radiological findings thanamong other x-rayed children. owing, presumably, toa positive association between several ante-natal conditions and early non-cancer death. There were also
relatively fewer cancer cases for recent than for remote
exposures. probably due to a progressive lowering offilm doses between 1940 and the time of reporting.
356. In 198 l, Totter and MacPherson [T2] publisheda critical review of the data in the Oxford Survey, inorder to re-examine the evidence in favour of a causal
relationship between feta! irradiation and cancer
development. This review was based on a number ofarguments. First. the authors showed that the greater
frequency of x-ray examinations reported among thecancer cases than among the non-cancer cases decreased
with time between I 946 and 1963. This observation
had been made before by others [B22, S24. S26], but.
according to Totter and MacPherson [T2], had notbeen given the appropriate weight. In particular, thealleged risk had not been placed in a context relative
to the natural risk of cancer induction. Secondly, theauthors pointed out the striking discrepancy between
the Oxford experience and that of Hiroshima andNagasaki, also repeatedly noted in the past [J4, K4]
and confirmed by the most recent findings [I3. K4].
357. Thirdly, Totter and MacPherson [T2] criticized
the method used in Stewart's work of separating the"radiogenic'' and "idiopathic" cases of cancer fromthe overall cancer cases. In their view. this method
assumes that the idiopathic cases have the same
likelihood of being irradiated as the general population
and, therefore, as the control. Such an assumption
would be acceptable if it could be shown that: (a) theirradiation of the mothers took place at random; and(b) the characteristics of the idiopathic cases were
sufficiently close to those of the control population. In
order to test for (a). irradiated and non-irradiatedcontrols were compared for a number of traits(described in [B22]), adjusted for the purpose ofidentifying the mothers most likely to have problems
during pregnancy or selecting the most affluent
families. The analysis showed consistent differencesamong the irradiated and non-irradiated controls
treated as two populations. From these. Totter and
MacPherson concluded that the choice of irradiating
the pregnant mothers was not made at random. To
test for (b ). the cases developing tumours werecompared with tumour-free controls and then shown
to be significantly different from the controls in
respect of a variety of population characteristics(social class, urban or rural residence, sibship, age and327

medical history of the mother, etc.). The differences
to be sienificant
and taken as evidence
were thouol1t:::,
against the assumption that the idiopathic cases would
be irradiated with the same frequency as control cases.
358. Fourthly. Totter and Mac Pherson [T2] criticizedthe dose-response curves of Stewart [B22, S24] thatshow an excess risk of childhood malignancy. increasing
as a function of the number of films used in the prenatal x-ray examinations. They pointed out that the
Oxford series only demonstrated a difference infrequency of pre-natal x rays between the groups
developing and not developing cancer. Under suchcircumstances, the greater risk caused by the irradiation
procedure may only be inferred on the unprovenassumption that the normal x-ray frequency in thecancer group is the same as is the non-cancer one.
Furthermore, the notion of excess risk as defined by
Stewart (the relative risk minus l ). presupposes zerorisk for zero dose and therefore assumes that any
value greater than zero for each number of x-ray films
may be attributed to the x-ray exposure. Finally, theexcess risk in Stewart's work is not related to dose,
but co the number of films used in the x-ray
examination, which is not a good measure of dose
because this number was never known with any
accuracy. Furthermore, the dose per film varied
consistently with time and. therefore, by poolingexposures received at different dates, different doses
per film would be indiscriminately mixed.
359. In an expanded treatment of the Oxford data
published in another paper [T6], Totter showed thatthe dose-response relationship claimed by Stewart andKneale [S24] could be duplicated or improved uponby substituting random numbers in place of the actual
numbers found in the survey. According to Totter[T6], the claimed dose-response relationship would be
the result of a sratistical fault in the use of "excessrisk", due to the fact that the highest exposures arereceived by the smallest number of persons. Therefore,_data points with the greatest relative error are
multiplied by the reciprocal of the lowest frequency
which is itself expected to have a large error. The
numbers resulting would thus carry the greatest. andnot the least, weight in an ordinary regression
analysis.
360. Commenting on the correlation-as opposed tocausation-between in utero exposure and subsequent
increased risk of malignancy, Burch [B 13] pointed out
the impossibility of excluding that the medical conditions prompting the selection of cases to undergo
obstetric radiology might themselves be associatedwith an enhanced occurrence of childhood malignancy.
He stressed the lack of supporting evidence from theHiroshima and Nagasaki studies, and pointed out thatan independent analysis of the age of onset ofchildhood malignancies. as found in the Oxford series[B34], cast doubt on the validity of the causal
association.
361. Brass's hypothesis, that leukaemia risk fromobstetric radiology ::ould be increased by the presenceof genetic factors that also enhanced the risk ofcertain indicator diseases. was tested on the Oxford
childhood cancer cases by Kneale and Stewart [K JO]
328

who analysed the interaction of pre-natal irradiationand posL-narnl diseases in children developing tumours.
The authors studied about 6000 cases born between
19 53 and I 962 who died before 12 years of age,
together with matched controls. Reticular system neoplasms (3199 cases) and other solid tumours (2803
cases) were studied. together with matched controls(6002). Those that had developed a serious infection
prior to death were evaluated separately. The data
showed that there was a significant association (at the
5% level) between the two variables (cancer andinfection) in those which were x-rayed, but not in
the non-irradiated controls. Respiratory infections(pneumonia. bronchitis, tonsillitis) were the diseases
mainly responsible for the effect. The data arecompatible with both the hypothesis of an earlydamage to the immune system caused by radiation, orthat of an earlier origin and termination of the nonirradiated than irradiated cancer cases. Comrary to
Bross· interpretation, it is suggested that the relatively
low frequency of infections among the non-x-rayedcases might be an artefacr caused by an earlier
development of immune system faults in the xirradiated cases.
362. In a short letter [B35], Burch commented on
Mole's findings [M23] that rhere is a similar excess ofleukaemia and solid cancers in singleton and twinbirths, in spite of their different rates of radiologicalexamination. He pointed out that calculations ofdeath rates for leukaemia and solid tumours during
the first 10 years of life. combining irradiated andnon-irradiated singletons. on the one hand, andirradiated and non-irradiated twins, on the other,result in a slightly, but not significantly, lower rate in
the latter. Considering that 50% of twins were
irradiated in utero (as opposed to only 10% of
singletons). which would lead one to expect a 20%
higher rate in twins, the finding cannot easily bereconciled •.vith the view that tumour incidence iscausally related to radiation exposure. Burch further
pointed out that Mole's estimates [M23] are affectedby such wide error limits that they cannot legitimately
be used to support the idea of the causal relationship.
He concluded, therefore, that, although it may be
prudent to act as though ante-natal exposure may
increase the carcinogenic risk, the validity of thephenomenon has not been scientifically proved.
363. Einhorn [E6] observed that cell proliferationand differentiation are most active during embryoniclife. but cancer is rarely seen 10 occur after irradiation
at these stages. On the other hand. human embryos
become susceptible to rumour induction after theperiod of major organogenesis, particularly in thosetissues that have a late development and are still
immature at birth. The author suggested that these
phenomena might be explained on the assumptionthat the regulators influencing development, which aremost active during the embryonic life, might be thesame ones that control cancer induction during thatdevelopmental period.
364. In its 1964 report [U3] UNSCEAR alreadyreviewed a study by MacMahon [M25] on 734,243
children born and discharged alive in 1947-1954 in 37

maternity hospitals in the north-east areas of theUnited States. The frequency of intra-uterine exposureof the entire child population was estimated byreviewing the records of a I% systematic sample (7242
singleton children). Abdominal or pelvic irradiation
was found to have taken place in 770 of thesechildren's mothers, corresponding to 10.6% of thereviewed sample. It was ascertained, by review ofdeath and birth certificates, that 584 children born inthe study population had subsequently died of cancer.
The pregnancy and delivery records of 569 cases
were inspected. Out of 556 children dying of cancer
that had been born of single pregnancies, 85 (15.3%)
had been exposed in utero to diagnostic x rays. The
higher frequency of pre-natal x rays in these cancer
cases, compared with the rest of the sample ( 10.6%),
was found to be statistically significant. Data werecorrected for indirect associations with birth orderand other complicating variables. and the final adjusted
cancer mortality risk was found to be about 40%
higher in the in-utero-irradiated than in the nonirradiated members of the test population. The excess
of neoplasms found applied equally to leukaemia,
tumours of the CNS and other neoplasms. The excess
cancer mortality of the exposed children was mostmarked at the ages of five to seven years (with arelative risk of about 2.0) and apparently exhausted by
the age of 8 years. There was a small non-significant
trend toward a higher mortality in the most heavily
irradiated children. but no significant variation of therisk was found with the intra-uterine age of children.
365. Monson and MacMahon [M53] reported recently
on an extension of their previous study [M25]. The
new paper includes data for five additional hospitals
and extends the period of coverage of birth and death.
It is based on cancer deaths occurring between 1947
and 1967 among 1.429,400 children born between
1947 and 1960 in 42 maternity hospitals. It excludes
children born of multiple pregnancies or dead in thedelivery hospitals. Out of this study sample, through asearch of death certificates. 1342 children were identified who died from cancer before the age of 20. The
birth records of these children. and of a I% sample ofthe total population. were reviewed in order todetermine which of the children had received irradiation in utero through irradiation of the mother's
abdominal region. Since pre-natal x-ray exposure may
be associated with many causes of childhood death. in
addition to cancer death. the authors followed the
x-ray experience of other children who died fromcauses other than cancer. This control sample included
children born in 17 hospitals during the study period,
who died after 3 months of age in 1950. 1955. 1960 or
1965. The risk ratio for leukaemia between the I%
sample and the study sample was 1.48, that is, very
similar to the 1.5 found in the earlier analysis [M25].
On the other hand, the risk ratios for cancer of the
CNS and other cancers were appreciably lower ( 1.16
and 1.14) by comparison with those found in theprevious study (respectively, 1.6 and 1.4). In view ofthe similarity of the risk ratios, all solid tumours werecombined in the subsequent analysis.
366. The excess of leukaemia cases in children with ahistory of x-ray exposure (over the selected cases

based on the 1% sample) was only evident between
2 and 7 years from birth, whereas the excess of solid
tumours occurred between 2 and 9 years. Thus.
further analysis was limited only to children who died
before their 10th birthday. The maximum likelihood
risk ratio (MLR) for leukaemia was essentially thesame in the initial study ( 1.48) [M25] and in theextension ( 1.58) [M53]. For solid tumours. the MLR
was 1.45 in the initial study and 1.06 in the extension.
The overall total MLR for leukaemia of 1.52 and forsolid tumours of 1.27 are not significantly different
when directly compared (P = 0.4). In order to evaluate
other possible confounding factors. the frequency of
x-ray exposure of children in the I% sample was
examined according to selected characteristics, together
with the incidence rate of cancer for the samecharacteristics. This analysis showed that factors suchas birth order, sex. birth weight, year of birth andabnormality of pregnancy (acting alone or in association) could confound the correlation between x rays
and cancer, being associated with both in the same
direction. On the other hand, race. pay status andprevious fetal loss were potential negative confounding
variables. in the sense that children with more
exposure had developed less cancer. Maternal age andreligion were not consistently associated with x-ray
exposure or cancer risk and could not. therefore. haveconfounded the data.
367. Correction for each of the potential confounders
brought about little change in the risk ratio. Therewas, at the same time, no indication that the overall
association between cancer and x irradiation ,vas present
only in a limited sub-population of the whole sample.
When the number of x-ray examinations observed
among children with cancer was compared with those
expected according to the year of birth, an excess of
x rays was observed from 1947 through 1957 forleukaemia, but only for the period 195Fl954 for solidcancer. However. no reason for this clustering of theexcess was identified. Children developing leukaemia
did not appear to have received a higher rate of
examination with x rays until the third trimester. butchildren developing other solid tumours had a higher
rate of examination also during the first and second
trimesters. Although no information was available on
the actual doses received. there was no suggestion of acrude dose-response relationship, based on the type of
examination.
368. From a discussion of possible sources of ascertainment bias, related to the facts that the record
search was not blind and that there was some loss of
information between the first and the second assessment, no evidence appeared that these conditions
might have introduced any significant bias in theassociation between x-irradiation and cancer. It was
concluded, therefore. that these new data were in
overall agreement with other data pointing to a risk
ratio of malignancy associated with pre-natal exposure
to x rays of about 1.5. There is, therefore, no questionin the mind of the authors as to the significance of
their findings, but they believe that the relationshipshown is not causal. This view is argued on the basisof the absence of confirmatory evidence from theexperience of Hiroshima and Nagasaki, and from329

experience in experimental animals. The authors pointout that searches for variables that may have confounded the association have failed to identify any
factor that might be of sufficient weight to explain thedata. There may be. in principle, as suggested by
Totter and MacPherson [T2], some kind of generalized
greater access to medical care among fetuses that will
eventually develop cancer. but such a hypothesis isdifficult to test experimentally.
369. In a review article [D23], Doll outlined themain features of the Oxford Survey and discussed
some of its difficulties. These included: (a) the possible
bias introduced by incorrect reporting of previous
x-ray history by the mother of a child who later
developed cancer: and (b) the possibility that theassociation shown in the Survey might be a secondary
one, due to the fact that some unidentified factor
might be the cause of both the higher tendency todevelop a tumour and of the higher probability of prenatal x-ray exposure. Doll was satisfied that the firstissue was not likely to have influenced the results toany appreciable degree. As to the second one, although
recognizing that it may be impossible to overcome itin any observational study. Doll believes that it isunlikely for three reasons: first, the failure to identify
such a factor, in spite of intensive searching; second,
the allegedly good relationship between number of
x-ray exposures and the increase in excess risk (anargument which has, however, been challenged by
Totter [T2, T6]); and, third. Mole's evidence on twins[M23) (although this, while in agreement with the later
data of Harvey et al. [H23], is not accepted by
MacMahon [M55] as a definitive proof).
370. Shiono et al. [S 11] reported on a prospective
study of 55.908 women participating in a collaborative
perinatal project in the United States. They examined
the x-ray exposure history of 145 mothers whosechildren developed neoplasms up to 7 years of age,and of 290 matched control mothers whose children
developed no tumours. Of the 145 tumours understudy. 40 were malignant and 105 benign. For x-ray
exposure during pregnancy the relative risk values
were 1.09 (P = 0.526) and 0.94 (P = 0.635) for malignant and benign tumours, respectively. Corresponding
values for exposure before pregnancy were 2.61
(P = 0.021) and 1.07 (P = 0.475). For pre-conception
exposure. there was no dose-response relationship ofstatistical significance in the groups of mothers whosechildren developed either benign or malignant tumours.
For pre- and post-conception exposure combined,
there was a suggestion that the relative risk could behigher for malignant neoplasms, but the significanceof probability value was borderline. For benign
tumours, the values of relative risk were not significantly
different from 1.
371. Taken together, the data [SI I] were thought tobe consistent with an increased risk of malignant
neoplasms among children whose mothers had been
exposed to x rays before and during pregnancy, with asomewhat higher relative risk estimate for pre-conception exposure. There was no significant association
between exposure and development of benign tumours.
The data were interpreted according to the model of330

Knudson [K26]. who postulated that certain tumours
might be the results of two mutational events which
could be in the form of either two somatic mutations
or of one transmitted germinal mutation followed by
one somatic mutation. According to this hypothesis,
any agent that would increase the rate of mutation ingerminal cells would be expected to enhance the riskof tumours. In addition, owing to the time factor
involved in the accumulation of mutations. tumours(such as retinoblastoma) caused by a germinal plus
somatic mutations would be expected to arise in earlyages.
372. Salonen [S56) carried out a study on pre- andperinatal factors in childhood cancer, based on tumour
registry data in Finland during the years 1959-1968.
The immediately preceding baby born in the samematernal health centres from which the test cases had
been drawn provided the control data. The number oftest-control pairs of children included in the final
analysis was 972. For all these pairs of children,
information was sought concerning the parents. thepregnancy, the events at birth and the post-natal
health. in order to compare the outcome of control
and test children histories. Comparisons were carriedout in terms of the total number of tumours and of
leukaemias, brain and other tumours. There was no
significant correlation between cancer and a numberof potential etiological factors such as parental factors(ages of parents, mother's blood group, duration of
marriage, social class of the family), pregnancy factors(numbers of deliveries or of pregnancies, pre- or postmature delivery, use of drugs) or child factors (date ofdelivery, birth weight, vaccinations). As to leukaemia
induction, the risk ratio in the group that had been
irradiated in utero with pelvic radiology was 1.9, withan upper confidence limit of 6.7 at the 2% level.
However. 5% of the mothers carrying children thatdeveloped leukaemia underwent pelvic radiography,
as opposed to 3. 7% of the mothers of non-leukaemic
children. Therefore, owing to the rare occurrence of
intra-uterine irradiation. the group was too small torender the increase of the risk ratio statisticallysignificant.
373. A new study on childhood cancer in twins
exposed pre-natally has recently been published [H23].
It is a case-control study. involving over 32.000 twinsborn in Connecticut, United States. from 1930 to 1969
and followed to 15 years of age. The twins were
investigated for a possible relationship between prenatal x-ray exposure and childhood cancer andleukaemia. By concentrating on twins, rather thansingleton births, it was hoped to reduce the chance ofa medical selection bias, since twins were often
exposed to x rays for diagnostic purposes, more so, in
any case, than for any other medical condition. By
matching the records of a twin and a tumour registry,
31 childhood cancers were selected for study (11 inboys and 20 in girls) and each of these cases wasmatched with 4 twin controls .. Evidence for x-ray
exposure and for other factors affecting pregnancy,
delivery and maternal health was collected from anumber of other sources. Abdominal exposure of themother was particularly sought. It was estimated thatthe dose to the fetus from such procedures ranged

from 0.0016 to 0.04 Gy. with an average of 0.01 Gy
per examination. All but one exposure occurred in thethird trimester. The relative risk was taken as ameasure of the association between radiation exposureand childhood cancer. Twelve out of 31 (39%) of thecases and 28 out of !09 controls (26%) were found tohave been exposed pre-natally. The crude relative risk
associated with pre-natal exposure was calculated tobe 1.8 (0.8-4.2 were the 95% confidence limits). Whenadjusted for all the potentially confounding factors.
the relative risk was 1.4-1.9. The adjusted risk forleukaemia was 1.6 (0.4-6.8) and for other childhood
cancer 3.2 (0.9-10.7). In addition to pre-natal exposure.
low birth weight was the only variable that appeared
to increase the cancer risk. It has been suggested[D24]. and accepted as a likely explanation [H26].
that post-natal exposure of low-weight children might
be responsible for such an effect. These data [H23],
showing an overall 2.4 risk of childhood cancer
associated with pre-natal exposure, are in agreementwith the results from the Oxford series [S23] and the
U.S. study [M53] and with the twin data as examinedby Mole [M23]. In the view of the authors, they
provide further evidence. though based on small
numbers, that low-dose pre-natal irradiation may
indeed increase the risk of childhood cancer.
374. MacMahon, however, in an editorial publishedat the same time [M55) questioned this conclusion on
three different accounts. First, the small number ofcases involved (31 twins with cancer). which is barely
enough to gi\·e statistical significance to the excess riskof 2.4 found in the study. Second, the assumption thattwins were only exposed on account of their being
twins. and ·not also for other reasons, among whichmight have been the elusive factor operating insingletons (as well as. perhaps, in twins) which makes
irradiated children more prone to develop cancer inchildhood .. Third, the possibility. mentioned in thepaper, that natural cancer incidence may be lower intwins than in singleton babies. If this is true. and if
twins are irradiated more often than children born
single having a higher cancer risk. twins should, infact. have a higher cancer incidence. The series may.
however. have been too small to document any
difference. It should also be mentioned that this latterargument may lose much of its validity because in the
study by Harvey et al. [H23]. mono- and di-zygotic
twins were pooled and there is no a priori reason tothink that these two classes may have the same
natural incidence of cancer. an issue also taken up
by Waight [W3 l]. In conclusion the new study [H23]does not seem to provide any firmer evidence infavour of a causal association between pre-natal
exposure and childhood cancer.
375. Preliminary data showing a non-significantly
elevated risk of malignancy in children exposed toante-natal diagnostic radiation among 555 childhood
cancer cases in Great Britain have also been reported
recently [H30]. Although the relative risk of developing leukaemia or lymphoma over the first 14 yearsof life was 1.33 (C.I. 0.85-2.08) among exposed (ascompared to non-exposed) children, it was 5.57 (C.I.
1.72-18.09) for children developing such malignancies
within 2 years of age. It has already been pointed out[ES] that with the small number of cases involved
when statistics arc broken down. the possiblity arisesof calculating a large number of relative risks andfinding one "significant" purely by chance. Further
analysis of this series will have to await a fuller
account of the findings.
376. A summary of the relative risks of leukaemia
found in studies of pre-natally exposed children sincethe 1964 report of UNSCEAR is given in Table 14.

C.

CONCLUSIONSI.

Data from animals

377. Even though more relevant data have appearedsince the 1977 UNSCEAR review, it remains difficult
to provide overall conclusions on the subject oftumour induction in pre-natally irradiated animals, incomparison with irradiation of post-natal ages. It is,perhaps, appropriate to underline the difficulties
hindering an overall judgement. because their discussion may help to point out the reasons why conclusions
should be prudent.
378. First, there is the natural variability between
species and strains with respect to the number andtypes of tumours arising in intact animals and thedifferent susceptibility of various tumour types toradiation. Given this background variation, it isdifficult to project information across species for theoverall tumour induction or for the susceptibility togiven tumour types. This objective is rendered evenmore difficult by the fact that information is rarely
available over a large range of doses and an adequate
number of animals. It should be stressed that generalizations based on only a few dose groups may be
misleading. because information collected so far points
to the existence of dose-induction relationships ofcomplex forms. Just as in adult systems. curves going
through a maximum, and then decreasing with increasing doses. have been documented in animals
irradiated in utero. There is every reason to suspectthat the placement of such maxima in respect to thedose axis and their relative height may vary withdifferent animal species and tumour types, and to
expect that. as a first approximation. wide extrapolations between species may be unwarranted.
379. Some experiments have convincingly documenteddifferences between sexes in the susceptibility totumours by irradiation in utero. not only in respect oftumours of the reproductive organs (or known to be
dependent on the action of sexual hormones) but also
in respect of neoplastic diseases. such as those of thehaemolymphopoietic system. for which such dependencies are not very strong in the adult. The concomitant action of radiation on the developing reproductive and hormonal systems may to some extent be
related to such differential actions. There are also
indications that differential effects may be present forradiation of low- and high-LET, the latter being
considerably more effective for carcinogenesis ofanimals in utero. Data on such matters are few andpreliminary, however.
331380. The notion that radiation may be consistently
more carcinogenic when delivered to animals in utero
than to adult animals is not easy to document on theevidence currently available: but, here again, comparisons should only be made with great circumspection.
It is not a straightforward procedure to compare theresults obtained from animals early in developmentwith those obtained from animals in full maturity,
because the target cell responsible for the induction ofsome tumour types may not yet be present inembryonic or fetal animals. Under such conditions.
comparisons of induction rates may be of little value.
Furthermore, for given histotypes. the target cells,
even if already present, may be in various stages ofdifferentiation. a condition which could reflect, to an
unknown extent, on the final incidence of tumours
produced. Finally, in view of the other radiation
effects induced in developing animals (growth disturbances, malformations, killing), the dose "window"
over which the carcinogenic action can be analysed israther narrow. This adversely affects the significanceof the results, because the number of animals available
for analysis of tumour induction may be reduced by
the concomitant presence of such effects.
38 I. All the above considerations apply both toexternal and internal irradiation. In the latter case.
however, comparative estimates of tumour susceptibility
between in utero and adult animals are further
affected by the fact that the rate of dose delivery to
the sensitive structures may be substantially different
and the dose delivered to the target cells (particularly
in case of tissues such as the bone) may have abiologically different meaning when delivered at various
developmental stages. The structures into which thedose is absorbed. the rate of delivery and the total
integration times are. in the case of internal irradiation, of overwhelming importance for meaningful
assessments.
382. Under these conditions. conclusions applying to
one tumour type could not easily be extended to othertumours or to all tumours. This further reinforces thenotion that extrapolations in such matters should be
extremely guarded and generalizations very cautious.
Taking into account these numerous pitfalls, theevidence reviewed does not convey an overall impression of a significantly higher susceptibility to the
induction of tumours of animals irradiated in utero.
This conclusion, based on a review of all or specifictumour types, is also in accordance with the conclusions of a previous UNSCEAR review [U5]. This
showed that irradiation in utero of mice produces lessmarked life-shortening than irradiation during postgestational ages, or even no life-shortening at all,particularly for irradiation of the early embryo.
Similarly. experience with the rat showed a doubtful
effect of radiation in inducing life-shortening, and, inany case, an effect not substantially different fromthat produced by similar doses given soon after birth.
This conclusion is also based on the assumption,
exhaustively discussed and accepted [U5], that radiation-induced life shortening, at doses below the lethal
range, is mainly attributable to the appearance of
extra tumours.
332

383. Given the above reservations, it appears thatthe majority of cases analysed in terms of total
tumour rates points to a lower, rather than a higher.
susceptibility to tumour induction of the developing
animals. What is most convincingly borne out fromthe data is that irradiation in utero produces adifferent spectrum of tumours than does post-natal
irradiation of young or old animals. This observation
is in line with the general belief that the histogenesis of
the cell lines, which arc the probable targets fortumour induction, proceeds at different times andrates in the developing mammal. Accordingly, one
should not expect to find the same types and rates
of neoplastic transformation for irradiation of theembryonic. fetal and adult stages. Furthermore. changes
in tumour spectra between animals irradiated pre- andpost-natally differ for different species. This is inagreement with the notions that the rate of organ andtissue formation is very dependent on the species andthat each has a characteristic pattern of naturally
occurring tumours.

2.

Data from man

384. Data on the induction of tumours by pre-natal
irradiation in man may be divided into two groups:
those obtained from the survivors of the atomic
bombs at Hiroshima and Nagasaki, and those derived
from medical exposures. The most recent reports on
the Japanese survivors have continued to show noevidence of excess cancer death among individuals
irradiated in utero at the time of the explosions.
Although the sample size and the number of deaths
involved in this survey up to now are too small toallow firm statements, the doses received are sufficiently
large, particularly in comparison with those deliveredin the course of obstetric examinations, to give weightto the general conclusion. For leukaemia. in particular,a second independent report confirmed the absence ofan excess risk in a sample of over 700 irradiatedsubjects. It is of course possible that when the atomic
bomb survivors irradiated in utero will reach the older
ages at which cancer normally appears, they mightshow higher incidence rates, as compared to nonexposed controls.
385. The Oxford Survey. the largest survey onchildren exposed for medical reasons while still inutero, has continued to stimulate controversial discussion. On the one hand. the group involved in thesurvey from its very origin have refined the analysis of
the data and shown the existence of a number offactors of social and medical relevance that could
affect the association between exposure in utero andproneness to develop cancer in childhood. These
factors could not have been of such weight to render
the association statistically non-significant.
386. On the other hand, critics of the Oxford series
tend to believe that there may have been someselection of the mothers undergoing radiologicalexamination during pregnancy. They point to differences between the social and medical conditions ofchildren developing tumours and those not developing
them, the differences being such that. for the traits

examined, the children could be considered to belong
to different populations. Criticism has also been
expressed of the analysis of the dose-response relationship and the way the doses (or their equivalent
expressions) were defined for the purpose. Other
analyses of the Oxford series data tended to disprovethe causality of the association between x-ray exposureand tumour induction. According to these analyses. it
is impossible to exclude that medical or other conditions leading to the selection of cases undergoing prenatal radiology might themselves be associated with
the increased occurrence of malignancy in childhood.
387. One strong argument in favour of a causal
relationship between pre-natal irradiation and childhood cancer was the finding of a similar excess ofleukaemia and solid tumours in children born of
single or twin pregnancies, in spite of the fact that thechildren in the latter category had a much higher rateof radiological examinations. This argument has.
however. been challenged on statistical grounds because
the numerical estimates on which it had been based
could be affected by such wide error limits as torender the comparisons meaningless.

388. Additional independent evidence for an association has come from the extension of a survey alreadyreported in 1962, based on about 1.5 million childrenof whom more than 1300 developed cancer before theage of 20. The new study confirmed in part theprevious findings for leukaemia, pointing out a 50%
higher risk of developing this disease among children
irradiated in utero than among non-irradiated children.
In part, however. it modified previous conclusions
regarding incidence of solid tumours, lowering therelative risk in the exposed population to about 15%
higher than the controls from a previous value of 50%.
The study also analysed and confirmed the existence ofconfounding variables that could potentially enhanceor reduce the association between in utero irradiationand the likelihood of developing cancer. At the sametime, it pointed out that correcting for such factors
would be insufficient to explain the excess risk in
the irradiated children. Overall. therefore, these conclusions are in reasonable agreement with those fromthe Oxford series. Other, smaller series have given
essentially non-significant answers.
389. The experience concerning the carcinogenic
action in man of irradiation in utero thus appears
paradoxical in many respects. On the one hand. aprospective study on children exposed to relatively
high doses from the atomic explosions has consistently
failed, over the years, to reveal any increased risk [J4,K4. 13). On the other hand, two larger, independent,
retrospective studies carried out in Great Britain [S44]and in the United States [M25, M53] have consistently
shown an increased risk of developing leukaemia andsolid tumours in children exposed for medical reasons
to much lower doses. The magnitude of the leukaemia
risk is evaluated at about 50% higher than in nonexposed children, and that for solid tumours possibly
somewhat less. None of the critics of these findings
have denied the existence of such an apparently
increased risk: they have rather been inclined to
explain the finding on the ground of faulty methodo-

logy, or to deny, on various grounds. the causality of
the association between exposure and malignancy.
390. The methodological defect of the Oxford series
would consist, according to Totter (T6]. in the use ofthe "excess risk''. This notion is faulty. because: (a) it
is based on the assumption that the frequency of x-ray
examinations is similar in both the leukaemic and thenon-leukaemic children: (b) it presupposes that therisk is zero for zero dose; and (c) it is based onimperfect "dose'' estimates. There is also a statisticaldifficulty in the use of ··excess risk". owing to the factthat the data points with the largest errors are given
the greatest weight in the analysis. These criticisms,
however, could not apply to the United States series,
which uses the methodology of the maximum likelihood risk ratio as a summary statistic. and yet comesquite independently to very similar conclusions.
391. Other critics have tended to deny the causalityof the association shown in the Oxford and New
England series by providing evidence that the populations of children who will or will not develop
malignancies are different in respect of a number of
medical and socio-economic characteristics [T2]. The
existence of confounding factors that could affect theassociation between exposure and increased risk is awell established fact, one to which the researchers
themselves in the Oxford [K9] and New England[M53] series have given much attention. In both cases,
the conclusion is that correcting for such factors is
insufficient to disprove the significance of the association, which must therefore be accepted as soundobservational evidence. Differences between the two
sets of children who may or may not eventuallydevelop a malignant disease imply that some characteristics of the mothers, of the children themselves, orof their relationship during pregnancy may haveprompted a different rate of radiological examinationof the two populations. These characteristics. rather
than the exposure itself, could be associated with theinduction of tumours. It is, of course. impossible toprove that this may not have happened, but it mustalso be recognized that none of the factors examinedso far has been shown to be responsible for the
association and it is difficult to suggest other factors
for further examination [M52].
392. Other arguments in favour of a causal relationship have been proposed by Mole [M23] who examinedthe excess of leukaemia and cancer in children born of
single or twin pregnancies and observed that. in spiteof the higher rate of radiological examination of thetwins. they had a similar excess of leukaemia andcancer. If valid. this would be an internally consistent
proof of the causality, but Mole's calculations havebeen challenged [B35] on the ground that the risk in
twins could actually be lower than in singletons and,in any case, the errors affecting the estimates are toolarge to be conclusive in either sense. Proportionality
between risk and exposure might be another indirect
proof of the causality, but the difficulties of assessing
the doses precisely have been long recognized [S24,S26, 822, T2] and these cast doubt on the foundations
themselves of the dose-response analysis.
333

393.

In summary, therefore:(a)

The finding of a positive assoc1auon between
exposure in utero and subsequent developmentof leukaemia and cancer during childhood. in the
Oxford and New England series is in contrast
with the absence of any such correlation in theJapanese series:(b)

The fact that higher doses are involved in theJapanese than in the other two series is a further
cause for concern, as radiobiological effects areusually expected to follow some positive functionof dose. The different nature of the exposures.
and the fact that the Japanese series is prospective, and the other two retrospective. do not help
in resolving the difference;(c)

It is also difficult to explain why high doses given
at Hiroshima and Nagasaki soon after birthshow their carcinogenic potential only many
years after exposure. while the relatively smaller
doses involved in medical exposure in utero seemto exhaust their potential within the first 10 years
after exposure;(d)

The Oxford and the New England series are atvariance with data from experimental animals,
which are, in the vast majority. essentiallynegative in terms of a higher carcinogenic risk of
pre-natal exposure. Nothing can be said as
to whether the change in the overall tumour
spectrum. often found in experimental animals
irradiated pre- or post-natally. may also apply toman, because the epidemiological series have notbeen followed long enough. It may. of course, bepossible that, considering all malignancies thatwill be expressed up to the death of the humancohorts irradiated in utero and the attendantdetriment. pre-natal irradiation might result inless overall detriment for the same dose thanpost-natal exposure;(e)

On radiobiological grounds. the constancy of therelative risk of irradiation in utero over thewhole spectrum of malignancies appears to be incontrast with the well known variability ofdifferent tumour histotypes to the carcinogenic
ac~ion of radiation. This observation could be
due to chance. or to the short observation periodof the medical irradiation series, and could still
be modified with longer follow-up, but it remains,at the present state of knowledge. unexplained;(f)

The constancy of the relative risk also implies
proportionality between natural and radiationinduced malignancies over all developing tissues(at least up to the present stage of follow-up)
which is by no means a common experience forirradiation of adult systems;(g)

Similarly implausible on embryological and radiobiological grounds is the observation of a higher
relative risk of leukaemia for exposure during thefirst trimester (than during the last two) when it is
known that adult haemopoiesis has not yet begun
and target cells to be transformed into leukaemic
cells have not presumably differentiated.

334

394. It is, in the last analysis, the joint considerationof the findings described, and their evaluation in thelight of all the above-mentioned inconsistencies, thatmay determine acceptance or rejection of the causalityof the relationship. because data. to be credible,
should fit into an overall logical framework accounting for all of them. and not for only a few. It isinteresting to note, in this respect, that the authors
involved in the Oxford series favour the causal natureof the association. while those involved in the New
England series reject such a notion. In the view of
UNSCEAR, the important consideration is the existenceof the association. For the Committee's own purposes
to neglect the matter until (and if) it is given someplausible explanation would be unacceptable because.
if the association exists. it could, to some extent. affectthe estimates of radiation burden in man. Under thecircumstances, the only possible course of action is torecognize the asso.ciation and to assume its causal
nature. keeping in mind, howe\'er. that its true natureremains unresolved because the data do not fit into an
overall logical picture of radiation carcinogenesis as it
is currently understood. It is to be hoped that further.
independent data will help to resolve the existing
discrepancies, although it appears unlikely that direct
evidence will ever be produced for lack of suitablematerial to study. In choosing this course of action.
UNSCEAR wishes to stress that acceptance of thecausality of the association is assumed simply on
account of prudence for any relevant practical consequences. On purely scientific grounds, in fact. theissue should either be dismissed for lack of scientificcoherence or should be left undecided, pending further
new evidence. UNSCEAR. although its functions are
only those of a body assessing data on their scientificmerit, is not insensitive to the practical consequencesof its deliberations. Thus. it believes that not toinclude the potential harm of exposures in utero(whatever it may be) in the total balance of radiation
risks would unduly weight scientific orthodoxy against
the practical needs of safety. The Committee will, ofcourse. be ready to modify this position on thestrength of any new scientific evidence that might beforthcoming.

VIII.

RISK ESTIMATESA.

GENERAL

395. Quantitative risk estimates for radiation damage
after exposure in utero are of considerable importancefor their possible practical implications. An early
expression of the absolute amount of risk likely tooccur under these circumstances is to be found in the1969 UNSCEAR report [UJ]. This stated that, inrespect of severe mental retardation associated with areduced circumference of the skull. an estimate of therisk coefficient derived from observations in Japan,
with high acute doses in excess of 0.5 Gy, might be set
at about 10- 1 Gy- 1, averaged over the gestation
period. The report stressed the uncertainties involvedin extrapolating such a value to smaller doses, in theabsence of suitable human data and with insufficientknowledge of the underlying mechanisms. The 1977

UNSCEAR report [U2] endorsed this provisional
value. again underlining the limitations under which it
had been derived and the difficulties involved in
extrapolating it to other exppsure conditions. The
report also pointed out that a survey of the available
human experience. with doses in the region of
0.0 l-0.2 Gy. had given negative or inconclusive answers.
They could only be of indirect value in excluding thepossibility that, at such doses, the human embryo
might be 10 times more sensitive than implied by theincidence of malformations at higher doses.

experience. This observation (which had been previously
overlooked by workers in radiation biology and by the
1977 UNSCEAR report) is indeed very valuable because
it correlates precisely cellular events in the humanbrain with radiation-induced functional damage. It is,however. in line with the general notion that thesensitivity of a given structure is highest at the time ofits maximum growth, an observation repeatedly documented for many structures in experimental animals
and to which the human experience would be expected
to conform.

396. The 1977 report [U2] was criticized in a paper
by Mole [M36] as being an inaccurate account of facts
which could convey the impression that radiation
exposure of the human embryo in its early stages
could be positively harmful and thus might engenderapprehension about the relevant risk. The paper inquestion is a complex one. It comprises a text aimed
at setting risk in perspective (particularly with respect
to the normal risk of handicaps at the end of a normal
pregnancy), in view of a reconsideration of the socalled 10-day rule, and an appendix setting out indetail the criticisms of the UNSCEAR report and thearguments on which the main conclusions rest.

400. Mole further discussed [M9. M 10] the significance of malnutrition in connection with the Japaneseexperience, pointing out that food shortage before.
and particularly after, the bombing might have been
responsible for an unusually high control level ofmental retardation and might, if so, have led to an
over-estimate of the effect of irradiation (under the
hypothesis of a combined action). Mole also discussed
points such as: the effect of excluding 2 cases of
Down's syndrome in the irradiated subjects and l caseof encephalitis in infancy in the control subjects: thedifficulties with the definition of microcephaly as
diagnosed at Hiroshima and Nagasaki; the Jack ofcorrelation between small head size and mental
retardation; the effect of perinatal mortality: and thelack of other data on morphological abnormalities in
the Japanese series. The author also reviewed otherseries, to show the relative infrequency of major
developmental malformations as commonly understood.

397. While recognizing that the discussion of experimental findings could in each instance be extremely
detailed. UNSCEAR believes that this should not be
the specific purpose of its assessments, which aim at
overall evaluations of specific fields. It believes thatthe assessment of developmental effects of irradiationin utero. on the basis of the evidence available in 1977
[U2], should not be modified on account of thecriticisms raised. It also believes, however. that themost recent findings in this field, and the widespreadattention they have commanded, have made the field aparticularly worthy subject for further study andreview.
398. In other papers [M9, M 10], Mole discussed. andrejected. the notion that the very early humanconceptus (0-14 days) is an especially radiosensitive
organism. This notion. he found, was derived from toofacile generalizations based on laboratory animal experiments. without due consideration for the specificity ofhuman development. First. he examined the mostrecent evidence on small head size and mental
retardation from Hiroshima and Nagasaki [03] toshow that the distribution of cases. as a function of
the week of gestation at the time of exposure, isincompatible with the hypothesis that the earliest
stages of pregnancy are the most sensitive in regard to
impairment of mental function. This should, ofcourse. be expected on the ground that embryo killing(either pre- or post-implantation) and malformations.
and not mental retardation, are the types of damage
expressed by irradiation at the early stages.
399. Mole then underlined [M9, MIO] the goodcorrelation, originally pointed out by Dobbing andSands [D 12], between the period of rapid proliferationof neuroblasts in the developing human CNS between
about 10 and 18 weeks of pregnancy (8-16 weeks after
fertilization) and the clustering of mental retardation
cases at around the same time in the Japanese

401. In that part of the paper (M9] which refers to
observations on laboratory animals, Mole stressedthat species intercomparison should only be carriedout with due regard for the timing and duration ofdevelopmental events. This conclusion is quite acceptable when actual times or rates of induction are
discussed (which was never the case in the 1977
UNSCEAR report [U2]) but not when projections
across species are so general as simply to point out thegood correlation, found in various species. between
stages of organogenesis of structures and their sensitivity to irradiation.
402. Finally. Mole discussed [M9] the mechanisms ofmorphological and functional damage following irradiation in utero. pointing out the. distinction thatshould be made between "malformations" resulting
from failure of embryonic organization and having
clear teratogenic connotations and "maldevelopment"
resulting from cell depletion distributed randomly
throughout an irradiated tissue. Mental retardation
would belong to this second class of damage. Difficulties in the definition of malformations and growth
disturbances, and the frequent association of various
types of damage, were discussed at length in the 1977
UNSCEAR report [U2]. In the light of the above
definitions, Mole believes there is no evidence in thehuman species to show that radiation is an efficientteratogen.
403. When teratogenic effects are described, adistinction is often made between malformations andgrowth disturbances, even though it is recognized that335

the two effects are often associated. In a paper bySpiers [S 15], the hypothesis is developed that growth
retardation and teratogenic effects may not be independent end-points of common causes. Evidence forthis is drawn from epidemiological observations onsome human congenital malformations in twins: frommultiple as opposed to single malformations: andfrom the association between malformations andgrowth retardations in patients with Down's syndrome
and in children from norrr>al and diabetic mothers. On
the basis of analyses carried out on such data, it isargued that growth retardation may represent a stateof increased susceptibility to congenital malformationsin the sense that they will be more likely to occur
when the overall growth rate of the fetus is grossly
disturbed. A more recent paper by Tchobroutsky et al.
[TJ3] has pointed to a high correlation between fetal
defects and early growth delay observed by ultrasound, thus confirming that growth retardation may
constitute a state of increased susceptibility to theoccurrence of congenital malformations.
404. In another paper [M2 I]. Mole discussed thelatest evidence from Japan, emphasizing the good
agreement between embryological and radiologicalfindings in man and the absence of any record ofmaldevelopment other than mental retardation. This
paper also contained a valuable review of case reports
in the literature, pointing to mental retardation as themost significant category of damage in the humanspecies. As to experimental observations, the paper
accepted that pre-implantation irradiation of theconceptus leads to a dose-dependent reduction ofsurviving normal individuals, while irradiation during
the period of major organogenesis causes a great
variety of visible malformations. In pointing out thedifferences between findings in man (where true
malformations are uncommon) and in other animals(where teratological damage is prevalent), the paper
suggests that a slower rate of cell division of the humanembryo might be the reason for such differences.
405. The fact that in man growth disturbances, smallhead size and mental retardation are the predominant
effects of acute exposure during pregnancy was also
noted by Brent [B25-B27]. This is at variance withdata in experimental animals [U2] in which malformations of different organs and structures are acommon finding after comparable doses. According to
Brent. there may be various explanations for suchdifferences. First, development of the CNS in man
proceeds throughout gestation and well into the neonatal period [D 12. H 14], while other organ systems
develop over a much shorter time. For example, in
man organogenesis may last from the 2nd to the
8th week of gestation, corresponding to about 5% of
the whole pregnancy, while in the rat it may last fromday 9 to 13. corresponding to 20% of pregnancy [PI I].
Secondly, the manner of exposure is usually different
in animal and human populations. In the first case.
exposure is often aimed at inducing specific malformation by selective irradiation at certain times. while inman exposure has usually occurred at random and has
therefore been relatively more likely during the feta!
stages when sensitivity of the developing brain remains
high.
336

406. ln 1977, the International Commission on
Radiological Protection (JCRP) [14], in the context ofa general attempt to develop an index of harm
applying to a working population for radiation
protection purposes, provided some estimates of risk
during various stages of pregnancy and for various
types of effects. In summary, the risk of lethality(before implantation or in Ulero) for irradiation
during the pre-implantation stages was estimated, byextrapolation from data in rodents. to be of the orderof 10-3 a-1 for exposure of a female working
population at the rate of I Sv a- 1, weighted over the 8days pre-implantation during which this risk was
assumed to operate. (This rate of exposure was taken
simply for comparative purposes.) For all malformations occurring after exposure at the same rate during
the period of major organogenesis (assumed to last for
17 weeks during pregnancy) the risk was set at 10-3
a- 1 sv- 1. The risk of inducing a fatal childhood
malignancy was set at 2.3 I 0-2 sv- 1. On the assumptionthat the risk of fatal malignancy in females (allowing
for breast cancer) is 1.5 10-2 sv-1, that the fetus is
exposed for an average period of 7 months during
which the mother remains at work at the rate of I Sv
a- 1, and that the average number of births per year
per woman is 0.065, it was further calculated that the
average risk of inducing a fatal childhood malignancy
would be about 6% of that in the adult member of afemale working population.
407. In 1979. Mole [M36] addressed the problem ofquantitative risk estimates and produced the following
figures. For mortality after exposure to x or gamma
irradiation in the 1-4 months embryo or fetus,
0-1 additional cases per I OOO births after a tissue doseof 0.05 Gy low-LET radiation: for mental retardationin adolescence. 0-25 additional cases per IOOO liveborn after 0.5 Gy tissue dose (0-1 case after 0.05 Gy);and for malignant diseases before the 10th birthday,
about 50 excess cases per 1000 of all live births after
0.5 Gy tissue dose (about 5 cases after 0.05 Gy). Suchestimates, derived from evidence concerning brief
exposures, would not be expected to be strongly
influenced by dose protraction because most of therisk is attributable to cancer production. Scaled down
to 0.0 I Gy of low-LET radiation on a linear nonthreshold hypothesis, the overall risk would lie in therange of zero to I 0- 3 cases, which should be comparedwith the natural probability of occurrence of serious
handicaps in average pregnancies, which was estimatedto be upwards of 3 10-2.
408. Brent [B25. B27] discussed the risk estimatesfrom irradiation in early life. He stated that, since.
contrary lo most other teratogens or embryopathic
agents, radiation has an effect on the developing
embryo which is direct and not mediated throughmaternal metabolism and placental transfer, one may
extrapolate the results of irradiation experiments withpregnant mammals to the human more readily thanwith any other teratogen. However, in view of thenumerous differences pointed out in the developmental
pattern and in the response of man and othermammals, UNSCEAR believes that great caution
should be exercised in such extrapolations. especiallyfor the more quantitative projections. Moreover. risk
estimates ultimately to be applied to humans forpractical purposes should essentially be derived fromhuman experience. carefully adapted to the special
situation to which exposure applies.
409. In order to estimate risk, Brent summarized, intables. the effects of acute exposures at I Gy and at
0.1 Gy or less, on various developmental stages of the
rat and mouse (displayed in relation to corresponding
human gestation periods). He then extrapolated fromthem the minimum doses to produce lethality. malformations. growth retardation and other effects in
the human embryo. as in Table 15. Brent's conclusionfrom these data is that the hazard of exposure in therange of diagnostic roentgenology is extremely low
when compared with spontaneous risks in humanpregnancy. Tubiana [T4] estimated that a dose of
0.02 Gy to a fetus could induce, at the most, the riskof I case of cancer in 2000 children.
410. Kameyama [K22] addressed the problem of risk
to the human embryo and fetus by simple extrapolation to humans from observations on the lowest
effective doses in experiments with rats and mice.
warning. however. about the danger of such straightforward projections. In a general review of the field.
he estimated the lowest doses for non-stochastic
effects in the mammalian embryo and fetus to be as
follows (figures given in Gy to the relevant tissues):
for resorption of pre-implantation embryo. 0.05: foracute cytological changes, 0.05-0.1: for minor skeletal
anomalies, 0.05: for malformations. 0.15-0.20; forhistogenetic and functional disorders of the CNS,
0.20-0.25; and for impaired fertility, 0.20-0.25. Fromthese data. Kameyama concluded that the risk of nonstochastic effects of diagnostic irradiation of children
in utero. for doses which are normally less than
0.01 Gy and up to 0.02 Gy as a maximum, would be
extremely low. and probably negligible, given theappropriate control measures. It should be noted thatcarcinogenic and mutagenic effects are specifically
excluded from these estimates, although these effects,
according to other evaluations. constitute the major
portion of the overall risk.
41 I. Russell [R8] discussed the problem of critical
periods in the course of development. Although her
contribution has mainly methodological value, it does
contain observations of very general importance,
particularly regarding the role of sensitive stages in theestimate of the attending risks. It points out. forexample, that to consider the embryo as a whole.
without defining the sensitivity of its cell subpopulations in the course of time, is a simplification thatmay produce a loss of sensitivity in the detection ofrisks, i.e .. in an under-estimation of it. Conversely.
extrapolation of the risk from single short-term
exposures, carried out during high susceptibility stages,
to protracted exposures, may lead to over-estimationof the risk because a given effect which can readily be
induced with an optimum dose at a well defined stageof development may not be visible at all at otherstages, even those occurring in close proximity to thecritical periods.

B.

EFFECTS AND PERIODS OF MAXIMUMSENSITIVITY

412. Before attempting any quantitative estimate, itis necessary to examine the nature of the risk underdiscussion and the time periods in human development over which such risk might apply. The effects
that have been identified in the preceding discussion
as relevant to human irradiation are reviewed below.
For exposure during the pre-implantation phases,
animal data (particularly in rodents) have documented
pre-implantation death. As discussed in the 1977
UNSCEAR report [U2] this is more readily apparent
in polytocous animals and may appear as death of
single individuals (inferred from a dose-related decreaseof the average number of implants) or as death ofwhole litters (documented by a dose-related increaseof sterile matings). There are no data that mightsuggest the presence of such an effect in man. This isprobably due to the fact that it is difficult to observe.
Pre-implantation loss would actually be likely to result
only in a missed menstruation, an event of such minor
clinical consequence that it is hardly necessary to
account for it. However, there is every reason to
expect that a sufficiently high dose of radiation
received by a pre-implanted human embryo would
entail a definite probability of death: the question is,what magnitude should be assigned to this risk? In
vivo data from the mouse, which were discussed in the
1977 report [U2] (and which have not been contradicted by the latest evidence on embryos irradiatedand cultured in vitro. discussed in chapter II) point to
a net increment of pre-implantation embryonic loss
coefficient of the order of I Gy- 1• (For the purpose of
the present study. the exposure unit R will be taken to
give an absorbed dose of 0.01 Gy. a procedure that is
not likely to underestimate the risk.) Evidence inrodents reviewed in the 1977 report [U2] is notincompatible with non-threshold dose relationships
for irradiation during the pre-implantation stages. The
same could be said for the in vitro data reviewed inchapter II. As to the length of the period over whichsuch a risk could remain in effect, it might extend as a
maximum over the first two weeks from fertilization.
413. In man. the phase of major organogenesis may
be taken to last approximately from the 2nd to the 8th
week after fertilization. Earlier in the present study,
attention was called to the fact that, contrary to what
happens for other mammalian species (e.g., rodents),
where the relative duration of organogenesis over thewhole period of pregnancy is longer. there is little orno evidence in the human that malformations in the
teratological sense are to be seen. This may mean that:(a) the observations available for irradiation duringorganogenesis are too few: or (b) that exposures were
too low: or (c) that grossly malformed fetuses are
eliminated in the course of further development; or(d) that the human species behaves differently fromother mammalian species in the sense that radiation
during organogenesis may not result in gross teratological damage. The first of these alternatives iscertainly true: the shortage of human data concerning
irradiation of major organogenesis stages was pointed
out in chapter III. The probability may not be
excluded that a wider data base, or higher doses,
337

might have provided evidence in the positive sense.
The other alternative, that grossly malformed fetuses
are lost, is actually a well ascertained fact: as pointed
out in chapter I, the incidence of malformed embryos
decreases as a function of their increasing age. The
phenomenon is to be seen as a result of a progressive
elimination of embryos and fetuses carrying malformations that are too severe to be compatible withfurther development in utcro or extra-uterine life. As
to the human species being an exception to the generalbehaviour of other mammalian species, in the sensethat it does not respond to irradiation during thephases of major organogenesis with induction ofteratological defects. this alternative is hardly credible.
As pointed out before, the lack of teratological effects,
compared with maldevelopment of the CNS, may bejustified on different grounds: for example, the much
longer duration of CNS development in man over thetime for organogenesis of other body structures.
414. It may be asked whether, for the purpose of risk
estimation, the lack of data on teratological damage inman (compared with the relatively more common
occurrence of CNS lesions) is sufficient reason todiscard the possibility that such damage might occur
at all. UNSCEAR believes that it is not, and willassume accordingly that such damage might occur
during the 3rd to 8th week post-conception. Since thenumber of body structures that could be damaged isroughly similar for all mammals, irrespective of theirbody size, one could assume. if one considers together
malformations of all types, that the ratio of malformed
to normal fetuses per unit dose seen in animals may betaken as an expression of the risk of malformationinduction per unit dose over the period in question forman. Therefore, for UNSCEAR's purposes, it will beassumed that the risk of an absolute increase of
malformed fetuses of the order of 5 10- 1 Gy- 1 (seeparagraph 95) in animals might apply to the humanspecies as well. over the period from 2 to 8 weeks postconception. This must be interpreted as an upper value.
(Here again the exposure unit R is taken to give anabsorbed dose of 0.0 I Gy on the average over all
embryonic structures.) It is actually possible that themost severe of these conditions may never be seen inman, because they are aborted in early stages ofdevelopment. It must be emphasized that using data
derived from experimental animals on the assumptionof linearity. and projecting these data to man, is likely toresult in an upper estimate of risk. not only becauseradiation-induced teratological defects are not seen at
low doses in humans, but also because the existence of athreshold may not be excluded in any species.
415. Data in animals have shown [U2) that as
development of the embryo proceeds, the dose needed
to kill the embryo becomes greater and the form of
the dose-response relationships becomes increasingly
sigmoid. For the purpose of UNSCEAR's estimates.
doses resulting in death of the embryo post-implantation are too high to be considered in the present
context. They would not be relevant to the exposureof the population, but only to exposure of pregnant
women under accidental conditions. The evaluation ofsuch cases is clearly beyond the scope of the present
exercise, which aims at providing estimates of general338

validity for the exposure of the whole population, or
large population groups, under controlled exposures.
Death of the embryo in utero during embryonic orfeta! stages (or death during post-natal life as aconsequence of exposure in utero), therefore, is notconsidered further here.
416. The number of children classified as severely
mentally retarded is increased by exposure in utero.
From the data discussed in III. B. it appears reasonable
to assume that the risk coefficient for low-LET radiation
over the period 8-15 weeks is 4 I 0- 1 Gy- I and over theperiod 16-25 weeks, 10- 1 Gy- 1• Before 8 weeks the risk
is apparently zero and after 26 weeks the risk is very low.
417. Growth disturbances and other effects have also
been shown to occur for exposure during the feta!
period. Quite aside from the clinical significance of areduction in body size in the absence of other clinicalsigns (which is indeed doubtful), it is difficult to express
growth disturbances per unit dose and it is therefore
impossible to provide a value of the risk for such graded
effects. As for those effects on the developing gonads,
which result in reduction of the reproductive capacity. it
has been shown that distinctly sigmoid relationships
prevail for these effects. so that it may be assumed thatthey may not be observed at doses of one tenth of a Gyor less. Accordingly. no attempt will be made to estimate
the risk for these effects.
418. The last type of effects to be discussed is theinduction of leukaemia and other solid tumours.
Reasons have been given in chapter VII for assuming theexistence of a causal relationship between pre-natal
irradiation and induction of malignancy. in spite of thefact that, on purely scientific grounds. the supporting
information is not entirely convincing. At face value.
data show that the risk of developing a malignancy after
irradiation in utero at doses of the order of 0.0 I Gy is
fully expressed within the first 10 years of life and isincreased by about 50% over the normal rate. The risk
may be estimated, according to a previous UNSCEAR
analysis [U2] to be of the order of 2 10- 2 sv- 1• Some
evidence indicates [CI3. M36] that the risk during thefirst trimester might be substantially higher than thatfor the last two trimesters. The relevant data appear
unconvincing, however: constancy of the risk throughout pregnancy is therefore assumed in the following
discussion.

C.

RISK ESTIMATES

419. The risk estimates given in the preceding section
relate to effects of different nature and severity.
Therefore, they cannot be easily compared or
aggregated. They should be viewed in the light of anumber of qualifications, as follows:(a)

The risk estimates have been specified as if thedose relationships for the relevant effects werelinear, although, as previously discussed, many
dose-effect relationships for the most complex
functional non-stochastic effects have actually
been shown to be curvilinear (concave upwards).
The estimates are therefore likely to be in excess

by some unknown factor, particularly at the very
low doses.
(b) The risk estimates have been derived by using
data on acute irradiation at high, rather thanlow, dose rates and apply therefo're to this
irradiation condition. Basic radiobiological considerations suggest that projection of these estimates to low-dose rate low-LET irradiation
could cause these estimates to be lowered bysome factor. at least for those effects that arecharacterized by curvilinear dose-response relationships. It has, however, been argued [M36]that no modification of the risk would occur inthe case of tumour induction, which is regarded
as a process initiated in a single cell. If this were
true. the overall reduction of incidence due tolow-dose rate would thus depend on the relative
importance of cancer induction and other harmful conditions.
(c) Overall. for small doses, the detriment calculated
from the risk estimates given in the previous
section appears rather small. in comparison with
the natural prevalence of malformations at birth
which has been taken in chapter I section C to
be of the order of 6 10- 2• For example. a dose to
the conceptus of 0.01 Gy delivered over thewhole pregnancy would add a probability ofhealth effects (mortality and induction of malformations, severe mental retardation, leukaemia
and other solid tumours) in the liveborn of lessthan 0.002 to the natural probability of 0.06 of a
new-born child being seriously handicapped. In
addition. it should be kept in mind that whenradiation is delivered acutely. not all the aboverisks will operate jointly: on the other hand. forradiation delivered chronically, for reasons developed in point (b) above, the risk would be
expected to be lower.

IX.

CONCLUSIONS

420. As a follow-up to its 1977 study [U2], UNSCEAR
has undertaken a revision of data pertaining to theeffects and risks of pre-natal irradiation. The revision
has taken into account data in human embryology,
particularly of the CNS, which were either unavailable
or not sufficiently well established at the time of theprevious study: new information derived from experimental animal systems: and re-evaluated dosimetricand medical data on children exposed before birthduring the explosions at Hiroshima and Nagasaki. All
this called for a new assessment of a field that has
attracted much attention over the last few years. In
addition to covering such effects as tumour induction
by pre-natal irradiation. this new revision aims toprovide a consideration in depth of the human data
and attempts to quantify the risk to man by irradiation in utero.
421. Data derived from human material have been
helpful in mapping out, with increasing precision.
developmental events that are of importance in thecontext of radiobiology. Research on morphologicalembryology is yielding an accurate description ofhuman developmental stages at the same time thatnew techiques of observation in vivo are establishing agood correlation between descriptive morphology andbody size in embryos and fetuses. The histogenesis
and development of certain structures, such as the
CNS, which are particularly susceptible to radiation
damage at pre-natal stages. are also becoming better
known at the biochemical level. Through biochemical.
histological and cell kinetic analysis, the main eventsin the development of the brain structures are being
followed: and the overall process of brain maturation
is emerging as a long sequence of phenomena lasting
well into the post-natal stages of development.
422. In primates, new techniques of cell labelling
have allowed the reconstruction. to a remarkable
degree of precision, of the kinetics leading to theformation of the cerebral cortex. It has been establishedthat cell division, cell migration and cell maturation ina tightly arranged sequence of time and events are
involved in the formation of the brain cortex. The
time of most active division of undifferentiated neuronalcells is the most sensitive to the radiation insult in man.
This is in good agreement with what had previously been
ascertained in experimental animals. Radiation-induced
disturbances of cell division, migration and maturation
invariably result, through various mechanisms. in aloss of neuronal function. This loss may not be
repaired by other neuronal cells, which are unable todivide, or by glial cells. which have no neuronal
function.
423. After irradiation, congenital anomalies of otherbody structures, besides those of the CNS, areuncommon in man, but very common in experimental
animals. The apparent discrepancies have been discussed, and reasons and explanations given. The
relevant findings must be taken as warnings against
any indiscriminate attempt to project to man results
obtained in animals. Any such projection must take
into account the embryological peculiarities of eachspecies. Extrapolations. particularly of the more quantitative findings, are. in principle. unjustified.
424. Congenital malformations in the human species
may be classified, according to their etiology. into
those of simple genie origin (about 6% of all malformations scored at birth). those having a complex multifactorial origin (about 50%), those due to chromosomal
defects (about 5%) and those known to be associatedwith a variety of environmental factors (about 6%).
For about one-third of all malformations seen atbirth, therefore, no apparent cause can at present be
identified. Children malformed at birth are the survivors among a greater number of malformed embryos
and fetuses who have died at some stage of intrauterine development. If malformations are scored ingrown-up children, the incidence is, again, higher thanat birth. Total incidence figures are highly dependent
on the method of scoring; the size of sample;
diagnostic criteria; ethnic and genetic background; age
and parity of the parents: and seasonal, geographicaland socio-economic conditions. The same is true forthe various classes of malformations. As a convenient
average for its assessments. UNSCEAR has adopted a339

natural figure of about 6% of newborn children being
affected by anomalies that may seriously reflect on
their viability and physical well being.
425. No new data have appeared on the effects ofirradiation in man during the pre-implantation stages.
There have been, however. many new findings on invitro irradiation and culture of rodent embryos. A
generally decreasing sensitivity to killing of preimplanted embryos, as a function of time and complexity of development, has been -:-onfirmed by thesenew data. There are also ample oscillations ofsensitivity in relation to the various phases of the cell
cycle during the earliest segmentation divisions. From
the whole of these data. killing of single embryos. orof entire litters in polytocous animals, emerges as themost important effect of pre-implantation irradiation.
For rodent irradiation in vitro and in vivo, doses ofthe order of 0.1 Gy or less have been reported to havea significant effect on embryonic and feta! mortality.
426. Recent data on the irradiation of experimental
animals during the stages of major organogenesis haveadded significant details to previous conclusions of
UNSCEAR [U2]. but have not changed them drastically. It remains true that the effects resulting from
irradiation of experimental animals during this phaseare mostly the teratological ones, accompanied by
growth disturbances of the various body structures orof the whole body. There is a pronounced timedependence for each class of malformation, because
the period of maximum sensitivity of any one structure
coincides with the time of its most active proliferationand differentiation. For each specific type of malformation, particularly those of the skeleton and of the
CNS for which new data have been gathered. sigmoid
dose-response relationships are usually found. A
wealth of data on the pathogenesis of malformationsin the CNS has been of help in understanding bow theradiation damage to the developing neuronal cells
might be translated into functional CNS damage.
through the disturbance of division, migration andintercellular connection of the irradiated neurons.
427. Sensitivity to radiation damage in the human
CNS starts at the borderline between the end oforganogenesis (8 weeks after conception) and proceeds
well into the conventional feta! stages, up to 25 weeks.
An important step forward in the analysis of thiseffect and the attendant risk has been made possible
through a review of dosimetric and clinical data onchildren irradiated in utero during the atomic explosions at Hiroshima and Nagasaki. It has been
confirmed that some of these children have developed
clinically severe mental retardation and the incidenceof this condition has been analysed as a function of
post-conceptional time at irradiation and as a functionof the dose received by the embryos, according to themost recently available dosimetric evaluations. Mental
handicap is not seen in children irradiated up to 8
weeks from conception. is at a maximum between 8
and 15 weeks when neuronal proliferation is mostactive and decreases to lower values between 16 and
25 weeks, when glial cell proliferation and neuronal
synaptogenesis occur. The number of children classified as severely mentally retarded is increased by340

radiation exposure in utero. The excess number ofmentally retarded in the dose range (0.04-2 + Gy) isapparently linear with dose at 8-15 weeks, with anaverage risk coefficient of 4 10- 1 Gy- 1; it is. on theother hand, concave upwards at 16-25 weeks with acoefficient of the order of 10- 1 Gy- 1• There is anindication that, in addition to these extreme mental
handicaps, other less prominent functional brain
deficits might be present in children irradiated inutero. While not all aspects of these findings may be
explained on the basis of present radiobiologicalknowledge, their significance is thought to be high,particularly in respect of the quantitation of theattendant risk.
428. For irradiation of animals during the feta!
stages, a variety of effects have been described. but
those on the developing gonads have been particularly
well documented, both at the anatomical level andalso in respect of the ensuing loss of reproductive
capacity when animals irradiated in utero reach thefertile age. Although the form of the dose-effectrelationships for loss of germ cells may (at least undersome conditions) appear to be exponential, pronounced
sigmoid relationships with dose appear to prevail for
impairment of reproductive capacity, with a fairdegree of inter-strain and inter-species variability.
Doses of a few tenths of a gray as a minimum, arenecessary to elicit observable fertility changes invarious animal species. Other effects induced by feta!
irradiation have been described for the haemopoietic
system, the liver and the kidney. Such effects all occur
at fairly high radiation doses.
429. The effects of internal irradiation of the mammalian conceptus are greatly influenced by the nuclides
tested. their chemical form. the route and schedule ofadministration. and the kinetics of transfer andmetabolism of the radioactive substances from themother to the fetus through the placenta. Informationon all these aspects. for all nuclides. is far fromcomplete and only a few nuclides appear to havereceived some attention. Tritium has been investigatedin relation to its different chemical forms, its metabolism. the type of effects induced as a function ofdose, and its relative biological effectiveness bycomparison with external gamma exposure. Radioiodine has been studied in respect of its kinetic
behaviour following acute or protracted exposures.
through a comparison of the uptake in the mothers' or
fetus' thyroid. Plutonium distribution and incorporation in various organs of the mother and the fetus
have also been studied in three different mammalian
species in an attempt to compare the findings among
species. The available evidence does not show apreferential tendency of actinides to concentrate in theembryo and fetus. There appears to be a need toenlarge the data base for the nuclides of importance.
particularly in regard to their uptake, distribution andeffects over an adequate range of concentrations andtissue doses.
430. Radiation effects in the mammalian conceptus
may be modified to different degrees by a number offactors. Among these factors, the type and energy ofradiation have been investigated. Relative biologicaleffectiveness (RBE) values in the region of about 5 forfast neutrons and helium ions (as compared withgamma radiation) have been reported, with a clear
indication that the actual values could vary according
to the types of effects scored and the dose level. An
increase of RBE with decreasing radiation dose has
not been convincingly shown, but may be expectedfrom the shape of the dose-response relationships
reported so far. Information on the interplay of LET.
the oxygen effect, and dose-rate or fractionation has
not been found to be very abundant for effects inutero, although there is no reason to suspect that thedeveloping conceptus may behave differently fromadult mammalian systems in respect of these major
radiobiological parameters.
431. In the field of combined actions, ionizing
radiation has been found, in isolated instances. tobehave synergistically with some chemical agents,
ultrasound or high-frequency radiation. More thoroughand systematic studies would be required, however, tosubstantiate these reports over a wider range of doses
and effects. Radioprotective and radiosensitizing drugs
have also been tested in combination with radiation;
the results indicate that the developing tissues areroughly similar to adult ones in their response to thesesubstances.
432. UNSCEAR has analysed and assessed a large
body of literature on tumour induction following inutero irradiation of experimental animals. It has
identified a number of weak points in these contributions which make it difficult to draw overall
conclusions. particularly for a comparison of the
carcinogenic action of radiation in pre- as opposed topost-natal stages. The species, strain and sex variability,
the lack of extended dose-response analyses. thedifferent stages of organ histogenesis and differentiation, the variable types and dosages of radiation, as
well as objective methodological and technical difficulties, may affect any such straightforward comparison. However, there is no good evidence of asignificantly higher susceptibility to tumour inductionof pre-natal animals. On the contrary, the data point
to a lower susceptibility upon irradiation in utero. In
any case. the most striking effects are on the spectrumof the types of tumours developed after irradiation ofpre- or post-natal ages. rather than on the incidence
rate of all tumours.
433. Data on tumour induction by pre-natal irradiation in man come from two different major sources.
On the one hand, the survivors of the atomic
explosions at Hiroshima and Nagasaki, who were
exposed while still in utero. have continued to show,upon successive reviews, no evidence of excess cancer.
On the other hand. two much larger retrospective
surveys in children exposed in utero on account of
medical indications have consistently shown an excess
of tumour and leukaemia cases over the first 10 yearsof post-natal life, which may be estimated to beroughly 50% above the natural incidence for the doses
involved in those studies. A number of social andmedical factors have been identified that may affectthe association between irradiation in utero andproneness to develop cancer in childhood. However,these factors are not of sufficient weight to explain theassociation entirely. Other reasons have been discussed
which render the results of the different series difficult
to reconcile.
434. The authors involved in the analysis of the two
retrospective surveys described disagree about thecausal association between irradiation in utero andthe likelihood of developing leukaemia and cancer.
UNSCEAR believes that the important considerationin these matters is the existence of the association.
which has been firmly established. As to the causal
relationship. to deny it on the ground that such
positive evidence does not agree with the experience inJapan or the data in animals would mean putting
undue weight on scientific considerations over thepractical need of allowing for any possible risk.
Therefore UNSCEAR accepts. provisionally, the causal
nature of the association, for practical purposes. butemphasizes that this is simply on account of prudence
and not on any scientific grounds. UNSCEAR will
follow these matters and be ready to modify its
position on the strength of any new evidence.
435. UNSCEAR has attempted to work out quantitative risk estimates for a number of radiation-induced
effects in utero (mortality and induction of malformations. mental retardation, tumours and leukaemia)
and to attribute the risk to the period of pregnancy
over which it applies. Under a number of qualifying
assumptions, it is possible to conclude that, for thesmall doses likely to be encountered in practice. theoverall risk is relatively small (no more than 0.002 forthe liveborn at 0.01 Gy) in relation to the natural
incidence of malformations in non-irradiated individuals, which is of the order of 0.06 in the humanspecies.

X.

RESEARCH NEEDS

436. At various points in the preceding text attention
has specifically been drawn to areas where there is alack or substantial limitation of information thatmight enable full understanding and assessment of thedata in man on which risk estimates should be based.
In other cases such limitations have been implied in
discussing the difficulties of deriving sufficiently firm
generalizations. It appears useful to summarize suchresearch needs. in the hope that they might beconsidered when planning future work.
437. Any occasion to collect information directlyfrom the human species should of course be pursued
with the highest priority. This applies first of all toobservations on radiation effects on embryos or
fetuses irradiated in utero. although it appears that thematerial available for further studies is rather limited.
In this respect. re-examination and updating at suitableintervals of children exposed for medical reasons would
still be useful to search for possible long-term effectsof radiation. Similarly useful and very important is theanalysis of mental and physical development. school
performance and other functional tests on the survivors
341

induced phenomena as they correlate with theoccurrence and the duration of developmental eventsin each particular species.

of the atomic explosions irradiated in utero, in order
to complete the picture with the more subtle and lessconspicuous effects of irradiation that may haveaffected these subjects.

441. It should be realized that. particularly in thecase of the development of nervous system andradiation effects thereon, the size of the structures tobe analysed and the duration of the phenomena
involved make it preferable to use primate species formore meaningful extrapolations to man. The same
may be said for the study of somatometric growth of
the body and brain and for the analysis of higher
nervous functions in such animals. There is also aneed to correlate in these animals morphological withfunctional effects by making use of modern techniquesof behavioural analysis. However, financial and otherlimitations involved in this type of research should berecognized.

438. The need for more detailed quantitative information applies equally to normal human development,
as a necessary background for the study of radiationinduced effects. There is a need for a fine description
and timing of the landmarks of human development
during the embryonic and feta! stages, the variability
in the occurrence of such landmarks as a function oftime, and of ethnic and other characteristics. Attention
and support to institutions whose scope is to describesuch developmental events is important. because thisknowledge is fundamental to many branches of the
medical and biological research.
439. In view of the limited amount of informationthat may be obtained directly on human subjects. theuse of models of radiation effects in utero willcontinue to be of paramount importance for theunderstanding of mechanisms and the projection ofeffects and risks. Although all models may be useful indifferent respects, depending on the answers required
from them, it is to be expected that research on small
mammals (rodents in particular) may be more suitable
for the study of mechanisms. while research on higher
mammals (primates) may be more interesting for
extrapolation of the effects to man. In both cases, it is
the consequences of small doses (below about 0.1 Gyof low-LET and correspondingly lower doses of highLET radiation) that should be particularly studied.
This implies designing and testing of techniques which
may be sensitive to such small doses, while retaining
significance and value for predictive purposes.
440. In very general terms, considering the large
sample size needed, models in rodents are probably
more useful for the analysis of dose- and time-effectrelationships. the study of repair effects. the analysis
of variability inter- and intra-species. the analysis ofcombined modalities of treatment. the comparative
effects of radiation of different types and energy anddifferent patterns of radiation exposure. Comparative
studies on different species should also be useful in theanalysis of the time sequence of various radiation-

442. It is also important to realize that it is nolonger possible to view the brain as a single organ.
but to analyse the developmental events and radiationinduced changes systematically in the different structures of the brain, to apply the newest neurobiologicaland neurochemical techniques, to move from thestudy of effects such as cell death to the migration ofcells, synaptogenesis, and the fine tuning of cellularand histological interactions.
443. In order to obtain much needed information ofpotential value under normal and accidental conditionsof exposure to internal radiation sources. there is anurgent need to enlarge the data base on the metabolism and effects in utero of radionuclides, especiallyof those having practical importance. over a wide
range of chemical forms and activity concentrations.
Such studies should be carried out in relation to tissue
dose. rather than intake concentrations. in order tofacilitate generalized conclusions on dose-response
relationships. More generally. better estimates of theactual doses absorbed by the fetus at various representative pre-natal ages could be very useful.
444. Finally. there is a need for inter-species comparisons of the radiation response of developing germ
cells in various animals and both sexes for acute andlong-term exposure.

T a b 1 e

Approximate t1me of the beginning and endof the ma!or developmental periods 1n some manmallan species(days p.c.)[U2]

Species
Hamster
Mouse
Rat
RabbltGuinea-pig
Dog
Kan

342

Pre1mplantat1on

Major
organogenes Is

fetal period

0-5
0-5
0-7
0-5
0-8
0-17
0-8

6-12
6-13
8-15
6-15
9-25
18-30
9-60

13-16.5
14-19.5
16-21.5
16-31.5
26-63
31-63
60-270Table2

Correlations between menstrual age and crown-rump length
according to [SSS] or (J9]

Menstrual age

Crown-rump length
accordlng to [SSS]

Crown-rump length
according to [J9]

<nn>

a/

weeks

Days

(nm)

9
10

63
10

11

11

12
13
14
15
16

84
91
98
105
112
119
126

31
40
50
61

17

18

24

33
43
53.5
66
82.5
99.5
117
135
151

74

87
101
116
130
142

~I This study was based exclusively on fetuses obta lned
from artificially Induced abortions.

Ta b l e

3

Regression coefficients for fetal organ weights[ 814)

Coefficients ~I
a

b

Organ

N

( 10- 2)(10- 4)

Brain
Liver
Lung
KidneyHeart
ThymusThyroid
Pancreas
Adrena 1
Spleen

71
75
76
78
76
76
70
71
76
75

14.828
6.097
3. 777
0.857
0.592
0.073
0.035
0.085
0.478
-0.001-0. 126-0.421-0.286-0.0lli
0.029
0.042
0.074
-0.002-0.033
0.046

~/

C

Standarderrorof
est !mate(l 0-li)

0.032
0.016
0.003
-0.002-0.0020.002-0.003

6.127
4.919
3.475
0.842
0.498
0.417
0.048
0.118
0.384
0.244

Coefficients for the polynomial regression formulay. ax + bx2 + cx3 where x Is body weight andy Is organ weight.

343

Ta b l e

4

Develoj;!mental stages 1n human embr:tos(DB]

Carnegie Pairs ofsom\tesstage

Length( ITl11)Age~/(days)

1

1. 5-3

2
3
4
5
5(a)
5(b)
5(c)
6

4

5-6
7-12
7-8

0.1-0.2
0. lg
D. 1
0.15-0.2 11-12
0.2
13

Age~/
(days)

features

1
2- 3
4- 5
5- 6
7- 12

fert I l\ zat I on
from 2 to about 16 cells
free blastocyst
Attaching blastocyst
Implanted although prevlllous
Solid trophoblast
Trophoblastlc lacunae
Lacunar vascular circle
Chorlonlc villi; pr1m1t1ve streak
may appear
Chorlonlc villi
Primitive streak
Notochordal process
Primitive pit; notochordal andneurenterlc canals
Somltes first appear
Neural folds begin to fuse; twopharyngeal bars; optic sulcus
Rostral neuropore closes;
optic vesicle
Caudal neuropore closes; threepharyngeal bars; upper limb
buds appearing
four limb buds; lens disc;otlc vesicle
Lens pit and optic cup;
endolymphatlc appendage distinct
Lens vesicle; nasal pit; ant\tragus beginning; hand plate;trunk relatively wider;cerebral vesicles distinct
Nasal p1t faces ventrally; retinal
pigment visible 1n Intact embryo;auricular hillocks beginning;foot plate
Head relatively larger; trunk
straighter; nasofrontal groove
d1stlnct; auricular hillocks
distinct; finger rays
Body more cuboldal;elbow region andtoe rays appearing; eyelids beginIng; tip of nose distinct; nipples
appear; osslf1cat1on may begin
Trunk elongating and straightening
Upper limbs longer at bent elbows
fingers longer; hands approacheach other, feet likewise
Eyelids and external ear moredeveloped
Head more rounded; limbs longer
and more developed

13-15

6(a)
6(b)
7

0.4

B

1 .0-1. 5

16
18

15-17
17-19

9
10

1- 3
4-12

1. 5-2. 5
2 -3.5

20
22

19-21
22-23

11

13-20

2.5-4.5

24

23-26

12

21-29

3-5

26

26-30

13

30-?

4-6

28

28-32

14

5-7

32

31-35

15

7-9

33

35-38

16

8-11

37

37-42

17

11-14

41

42-44

18

13-17

44

44-48

19
20
21

16-18
18-22

47.5

48-51
51-53

22-24

50.5
52

22

23-2B

54

54-56

23

27-31

56.S

56-60

53-54

al According to (012) for stages 11-23; miscellaneous sources for stages 1-10.
~I According to [Jl4].

344

TableS

Birth prevalences of all Isolated conmon congenital abnormalities
per 1000 total births. based on a WHO study[S3B)

ICO code Congenital anomaly[W28)

Average

SourceSourceKini- of
Maxiofmumdata mum
data

~,

~_/

740
741.0
742.0
745.0747 .9•
745.3-4*

749.1-2

Anencephalus andsimilar anomalies
Spina blflda
Encephalocele
Neural tube defects
Congenital cardlovascularma lformat I ons

1.1

0.1

LL

4.5

BE

0.8
0.1
2.0
6.1

0.1
0.0
0.6
3.2

K
X

4.2
0. 7

C

BI

9.0
18.5

BE
AL
BE

Ventricular septal
defectCleft lip and/or
cleft palate

1. 7
1.0

0.4
0.0

BJ
KE

3.3
1.6

LE
J

3.3

A

er

G

750.5•

Congenital
hypertrophlc
pyloric stenosls

2.0

0.1

SI

752.5

Undescended testicle

1.2

0.0

X 12.2

BO

752.6*

Hypospadlas

2.8

0.4

S

4.3

R

754.3

Congenital
dislocation of hip

0.3

0.0

X

3.2

BO

754.5754.6

Varus, valgus andother deformities

2.7

0.4

C 20.9PA

754.53*Structural tallpes
different types
equlnovarus
Polydactyly
Down's syndrOIDe

1.3

1.0

S

3.0

L

1.1
0.8

0.0

KE

0.0

AL,
CT

6.2
3.9

LL

Congenital Inguinal
hernia

18.l

10.2

H

130.0

E

755.0
758.0
550.1*

A

AL
BE
BI
BO
C

er
E
G
H

J
K
L
LL

Aberdeen, United Kingdom
Alexandria, Egypt
Belfast, United Kingdom
Birmingham, United Kingdom
Bogota, Colombia
Calcutta, India
Cape Town, South Africa
Edinburgh, United Kingdom
GreenlandHong Kong
Johannesburg, South Africa
Kuala Lumpur, Malaysia
London, United Kingdom
Llubljana, YugoslaviapConments

Obvious territorial
cluster In the
British Isles
Very heterogeneous
category. Various
types should be
evaluated separately
Ethnic differences:
African:
0.4
European: - 1. O
Oriental: - 1.9
Ethnic differences:
African: - 0.5
European: - 2.0
Oriental: - 0.4
Diagnosis depends ontime of birth andpost-natal age
There are conrnon
minor variants
Increasing trend owing
to neo-natal
screening
Very heterogeneous
category. Various
types should be
evaluated separately

Obvious ethnic cluster
Decreas Ing
trend
Hot Included In
chapter XIVof the ICD

SI
SPLeiden, The Netherlands
Manila, Philippines
Madrid, Spain
Mexico City, Mexico
Melbourne, Australia
Newcastle, United Kingdom
Pretoria, South Africa
PanamaRochester, United States of America
SwedenShiraz, Islamic Republic of Iran
Sao Paolo, Brazil

X

Several centres

LE
M
MA

MC
IIE
H
p

PA
R

s

345

Table6

B1rth prevalences of all isolated co1T111on congenital abnormal1t1es
per 1000 total births based on a Hungar1an study(ClB)

ICD Code[W28]

Congen1tal anomaly

Average740

Anencephalus ands1m11ar anomal1es
Encephalocele
Neural tube defects
Congenital cardiovascular malformations
Ventr1cular septal
defect1 • 1 ( 0 . 8)

MTH

0.2 (0.2)
2. 9 ( 1 •9)
10. 6

MTM
MTH
H

2.1

MTM

1. 0

MTM

1. 5

MTM

Only operated cases

3.6

MTH

2 .2

MTM
MTM

After thlrd month. Thls
f1gure ls - 35 1nmales at blrth
Trend ls 1ncreas1ng
Only treated cases,
1.e., without true
abnormal lty
Ma1nly posturaldeform1t 1es

742.D
745.0747. 9745.3-4
749 .1-2

750.5
752.5

Cleft 11p and/or
cleft palate
Congenital hypertrophlc pylor1c
stenos1s
Undescended testicle

752.6
754.3

Hypospadlas
Congenitald1slocat1on of h1p

754.5-

755.0

Varus, valgus andother deform1t1es
of feet
Structural tallpes
equlnovarus
Polydactyly

758.0

Down's syndrome

550. lCongenital 1ngu1nal
hem la754.6
754.5328.0

Etlology~/

Conments

A decreasing trendIn
recent years (shown
1n parentheses)

Result of population
screen1ng
In a proport1on of cases,
spontaneous closure
occurs

3.0

H

1 .3

MTM

A true abnormal1ty

0.3

G

1.2

C

Not a co1T111on abnormality
1n Hungary
Ver1f1ed by chromosomal
analysisOnly operated cases

11 .4

MTM

~/ C = chromosomal; G = genlc, at least some part; H = heterogeneous:
H = maternal; MTH = mult1factor1al threshold model; 1.e., polygenlc
d1spos1t1on 1nteract1ng wlth a variety of env1ronmental factors.

346

Ta b l e7Birth 11revalences of some Isolated moderately freguent congenital abnormalities
11er 1000 total births

WHO Study [ S38)

Aver- Hlnlage
mum

ICD Code Congenital anomaly
[W2BJ

742.3
749.0
750.3

751.2
753.1
755. l755.3-5
756.6
756.7

Congenitalhydrocephalus
Cleft palate
Oesophageal atreslaand stenosls and/or
tracheo-oesophageal
fistulaAtresla and stenoslsof large Intestine,
rectum and anal canal
Cystic kidney
Syndactyly
Reduction deficiencies
of limb
Anomalies of diaphragmExomphalos

Sourceof Maxidata mum
M

Hungarian Etlology
study
Source
of
data

~/

[Cl8]

~I

0.6

0.2

MC

2.0

AL

0.8

H

0.2
0.1

0.0
0.0

X
X

0.5
0.5

HE
SP

0.4
0.2

MTM ( ?)

0.2

0.2

ME
CT

0.6

I(

0.2

u

0.03
0.2
0.2

o.o

X

J

H

0.7
1.8

SP

0.1
0.3
0.4

61
G2
H

0.1
0.1

0.0

X

0.5

HA

0.2
0.2

IHH
HTM

0.0

p

u

a/ Abbreviations explained In the footnote !I to Table 5.
G1 • genie, at least type I and II; 62 • genie, at least s OGie pa rt;
H. heterogeneous; HTM • multlfactorlal threshold model.

~I

Table

8

Percentage distribution of congenital anomaly groups(modified frDlll [VB])

Authors !I
A

B

Group of congenital
anomal lesCentral nervous system
Cardiovascular system
Cleft lip and palate
Alimentary system
Genito-urinary system
Ll!M>S
Down's syndrome

C

0

E

r

G

H

Total birth prevalence
1.4

1.5

1.3

2.8

3.8

2.2

8.5

7.4

26.3
7.2
11.6
5.4
18.2
27.2
3.5

13.3
7.o
10.8
4.9
7.2
53.0
3.6

21.0
5.9
9.5
4.1
2.0
38.0
6.5

11.6
8.7
8.8
8.7
7.8
35.9

5.7
10.9
6.3
7.4
15.7
36.8
5.7

7.7
8.5
19.2 10.2
7.9
3.2
7.9
7.3
12.4 13.6
28.1
39.2
6.3 ~/ 1.4

4.2
11.0
2.0
3.8
12.7
41.2
1.6

Buehl, 1950 (reference given In [VB];
B. Stevenson et al., 1958 (reference given In [VB];
C • Stevenson, 1966 (reference given In [VB];
D • Klemettl, 1966 (reference given 1n [VB];
E. Vlllumsen, 1970 [WS];r • Trimble and Doughty, 1974 [TlOJ;
G • Hyrlantropoulos and Chung, 1974 [M45];
H. Czelzel and Sankaranarayanan, 1984 [C19].
~/ All chromosomal aberrations.

!I

A a

347

Table9

Inc1dence of severe mental retardat1on 1n 1nd1v1duals
exposed pre-natally to the atom1c bomb1ng of H1rosh1ma and Nagasak1 !I

Data for the two c1t1es have been comb1ned and the cases d1strlbutedby gestat1onal age at exposure and fetal absorbed dose,
based on the T65 rev1sed dosimetry !1_/
(Source of data: [04))[Th1s table 1s taken from Ill)

Dose categor1es (Gy) £/Ages> 0.010.01-0.09

0.10-0.49

0.50-0.99

1.00+

C1t1es comb1ned
All gestat1onal ages
Subjects
Retarded
Percent
0-7 weeks
Subjects
Retarded
Percent
8-15 weeks
Subjects
Retarded
Percent
16-25 weeks
Subjects
Retarded
Percent
26+ weeks
Subjects
Retarded
Percent

1085
9
0.8

292
4
1. 4

169
4
2.4

34
6
17 .6

19
7
36.8

210
l
0.5

55
0
0.0

26
0
0.0

2
0
0.0

2
0
0.0

257
2
0.8

69
3

50

13

9

4

4

4.3

8.0

30.8

6
66.7

86
l
1.2

45

15

5

2

l
20.0

312
2

0.6
306
4

1.3

82
0
0.0

0
0.0

13.3

48

4

0
0.0

0
0.0

3
0
0.0

Note that the frequency d1str1but1on by gestat1ona1 age 1n this Table 1sslightly 1ncons1stent with that g1ven 1n [03) due to dHferences 1ngroup\ng; these d1fferences have 11ttle effect on the r\sk estimates. The
data for the two c1t1es are separately presented elsewhere [03, 04).
!1_/ The t1ssue garrma dose 1s taken to be equal to 0.42 t\mes the ganrna kerma
plus 0.077 t1mes the neutron kerma, the t1ssue neutron dose to be equal to0.14 t1mes the neutron kerma, and the total t1ssue dose to be the sum of
the two 1nd1v1dual t1ssue doses.
£/ The mean doses w1th1n these dose categor\es over all gestat\onal ages are0, 0.04, 0.23, 0.72, and 1.61 Gy, respect\vely.
!I

348

Ta b l e

10

The relat1onsh1p of mental retardatlon to absorbed fetal dose
(Source of data: [04])[This table ls taken from Ill]

Cltles comb1ned
Gestatlonal age
Oevlance

p

0.29

0.059

3.78
l. 11
3.25

Relationship of mental retardation to dose:'controls' excluded
0.378 a/2. 1892.242
8-15 weeks
0.106 l .055
0.409
16-25 weeks

0.089
0.068

0.61
3.20

0. 74

Relat1onshlp of mental retardation to dose:'controls' combined
Pooled control
0.863
0.278
0.406 ~/

0.077

1. 11

0.77

a

All gestational ages
8-15 weeks
16-25 weeks

0.768
0.917
0. 601b( Gy-1)
0.253
0.567
0.415

0.174 a/o.404
0.101 -

0.041

a, 0.010

o. 77

0.3&

0.20

The deviance, which ls a measure of the goodness-of-flt of the model to thehas three or two degrees of freedom depending upon whether the zerodose group ls or ls not Included In the modellng; the P value ls theprobablllty (two tailed) of exceeding the deviance by chance under the null
hypothesis. als the estimated number (Intercept) of cases of mental
retardation (per 100 1ndlvlduals) ln the zero dose group and sa lts standard
error; bD ls the Increase 1n the frequency of mental retardatlon, with doseD, expressed ln grays ( 100 rad); and sb ls Its standard error. Note thatwith the binomial distribution the deviance takes the form 2{Sum z ln(z/0) +Sum (n-z) ln [(n-z)/(n-z)]} where the z's are the observed values, 0 theestimates of the p's under the complete model, that ls when the p's are alldifferent and match the data completely, and the z's are the estimates of thep's under the fitted model.
data,~/ S1gnlf1cant at the 0.001 level.

Table

11

Approximate length of the oogenetlc per1od In the female!/
and approximate life span of gonocytes ,n the male~/for selected malffllallan spec1es(Data from (E2])

SpeciesSex

Days

References as quoted In theorlg1nal publication [E2]

Mouse

male
female14

5Erickson [E2] (1978)
Peters (1970)

Rat

male
female17
6

Huckins and Clenaont (1968)
Beaumong and Mandl (1962)

Guinea pig

malefemale50
25Erickson (£2) (1978)
I cannon (1964)

Pig

male

158
77

Erickson (1964)
Black and Erickson (1968)

340
100

Erickson and Mart1n (1972)
Erickson (1966)

femaleBovinemale
femaleMonkeymale-1000
female 100

Van Wanegen and Simpson (1954)
Baker (1966)

Man

male
-3500
female 120

Charney et al. (1952)
Baker (1963)

Tl111e In days from the formation of the germinal ridge tothe end of s1gn1f1cant mitotic activity of the oogonla.
!!/ Time In days frOll the formation of the germinal ridge tothe appearance of the first differentiated spermatogonla.
!I349

Ta b 1 e

12

Relatlve leukaemia rlsk 1n retrospectlve studies
of chlldren d~ing of leukaemia
after diagnostic irradiation in utero
[U3]

Age
of
leukaemlcs(years)Years ofdeaths forleukaemlcs< 101953-1955< 10

1951-1955?

1955-1956?

1955-1956

7

1950-1957?

1946-1956< 20

1940-1957< 20

1940-1957< 20. 1940-1957

Percentage of mothers
receiving abdominal
irradiation during pregnancy
Leukaemics.controls

96/780(12.3")
20/70(28.6")
37/150(24. 7")
34/125(27 .2")
72/251(28. 7")5/55{9.1%)
3/65( 4.6")
3/65( 4.6")
3/65( 4.6")

117/1.638( 7. l")48/247(19.4")
24/150{16. 0%)27/125(21.6")
58/251(23. l")8/55( 14. 5")
3/65
{ 4.6")
7/93{ 7. 5")
2/82( 2. 4%)

Table

Relative risk(95" llmlts
in parentheses)

Ref.

l. 8

2.4-1.4)

[S23]

l. 7

2.9-0.8)

[F6]

1. 7

3. 7-1.0)

[K21]

1.4

2.5-0.7)

[K21]

1.3

2.0-0.9)

[K21]

0.6

2.0-0.2)[K28]

1.0 (12.0-0.6)[M39]

0.6 { 2.4-0.1)[M39]

1.9 {40.0-1. l)

[M39]

13

Crude annual 1nc1dence rate of leukaemia
among chlldren exposed 1n utero and controls, 1945-1979(13]

Fetus total dose ( Gy)
Total

Subjects
Person-years
Leukaemla cases
Rate (l0-5)
90% confldence
l 1m1t

350

Upper
Lower

Not 1n city

00.01-0.4

> 0.5

2306
72444
l
1.38

625
19403
l
5. 15

620
19078
l
5.24

85
2255
0
0.00

6.55
0.07

24.46
0.27

24.87
0.27

3636
113180
0
2.65
6.85
0.72

Table14

Relat1ve r1sk of leukaemla 1n pre-natally exoosed chlldren:
surrmary of most recent stud1es !/Study source

Years oflncldence

Relatlve
r1sk

Reference

Unlted States
Un1ted States, whlte
black
Un lted KI ngdom
f1nlandHlrosh1ma/Nagasak1
Unlted K1ngdom, tw1ns
Unlted States, twlns
Unlted Klngdom

1947-19&0
1947-1967
1947-1967
1943-1965
1959-1968
1945-79
1943-1965
l 930-19&9
1980-83l. 52 !1_/
2.88
0.00
1.48 !1./
1.90
2.15 c/2.20 b/1.60 ~/
1.33[M53)[ Dl 5)[ Dl 5][S24)[S56)[K4, !13)
[M23]
[H23)
[H30)

a/ Host cases and excess r1sks reported here occurred before the age of 10.
~/ S1gn1f1cant at 0.5% level or better.
£/ Relative rlsk from Japan was calculated from l case of leukaem1a 1n
21,333 person-years at r1sk 1n lrradlated groups compared wlth 2 cases 1n
91,847 person-years In control groups (Table 13) [I3); the difference 1s
not stat1st1cally slgnlf1cant.
~/ In this ser1es, the relat1ve r1sk of solld tumours was a non-slgn1f1cant
3.2; other ser1es have found values less than the relative risk forleukaemla.

Table

15

[st1mat1on of the acute LD/50 dose, m1n1mal malformlng doses,
cell depletlng dose, and doses for the human embryo
based on comp1lat1on of mouse, rat, and human data
[827]

Approx1mate
m1n1mallethal dose

Age

Day l
Day 14Day 18Day 28
Day 50Late fetus
to term!I

Approx\mate
LD/50

(Gy)

( Gy)

0.10
0.25
D.50
> 0.50> 1.000.7-1.0

M\n1mum
Hlnlmum
dose fordose forrecuperable recognizable
growthgross
retardation malformation
1n adult( Gy)( Gy) !/

Hln\mum dose
for lnductlon ofgenet1c,
carclnogenlc
and mlnlmal
cell deplet1on
phenomena

No effect

2.2
2.6

No effect
0.25
0.25-0.50
0.50
0.50

0.25
0.25
0.50

Unknown
Unknown
Unknown
Unknown
Unknown

3.0-4.0

0.50> 0.50Unknown

1. 4
l. 5[st1mates for maxlmum dose effective 1n reduclng body weight. Spectflc
organs or measurements may be more or less sens1t1ve.

351

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A2

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BI

Balla, I., C. Michel and H. Fritz-Niggli. Cellular
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B2

Balla, I., C. Michel and H. Fritz-Niggli. Cellular
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A3

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B3

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B4

A5

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A6

A7

A8

Andrew, F.D., R.L. Bemstine, D.D. Mahlum et al.
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