bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatiotemporal patterns of fibronectin distribution during embryonic development I. Chick limbs
Authors: Melnick Michael , Jaskoll Tina , Brownell Anna G. , MacDougall Mary , Bessem Conny , Slavkin Harold C.
Published: April 2021

It has been suggested that an extracellular matrix - and cell surface - associated glycoprotein, fibronectin, plays a role in the positioning of cells in morphogenesis and in the maintenance of orderly tissue organization. In the present study the appearance and distribution of fibronectin during in ovo chick limb development has been investigated by indirect immunofluorescence techniques in H.H. stages 20–30. Fibronectin is not detectable until just prior to the transition from the morphogenetic to the cytodifferentiation phase of development. Beginning at H.H. stage 25, successive nonrandom patterns of fibronectin detection and distribution, which resemble the subsequent cartilaginous elements, precede overt chondrogenesis as detected by Alcian blue staining. This corresponds to the onset of the cytodifferentiation phase of limb development. As the accumulation of acidic proteoglycan increases in the cartilage matrix and the mesenchymal cells become more round in appearance, the presence of detectable fibronectin decreases and is ultimately seen only in the perichondria and basement membrane. However, predigestion of developed cartilage tissue with testicular hyaluronidase, prior to fibronectin staining, indicated that fibronectin remains a major constituent of cartilage matrix and is apparently masked by cartilagespecific proteoglycans. This study of chick limb development is consistent with the hypothesis that fibronectin may be a molecule that facilitates the spatial organization of cartilaginous primordia cytodifferentiation.
The effect of delay on the expression of thet6/t6genotype
Authors: Nadijcka Mary , Morris Marie , Hillman Nina
Published: April 2021

In vivo, t 6 /t 6 embryos are developmentally arrested between gestation days 5·5 (short-egg-cylinder stage) and 6·75 (long-egg-cylinder stage). In the present series of studies we used both in vivo and in vitro blastocyst delay followed by in vitro outgrowth to determine whether the r°/7c lethality is time- or stage-specific. The results show that the t6/t6 genome is expressed differently in vivo and in vitro and that the in vitro expression of the homozygous t6 genome differs with different methods of effecting developmental delay. Although delay increases the life span of t6/t6 embryos it does not alter the stage of lethality. One method used to effect delay (ovariectomy) causes the t6/t6 embryos to remain as blastocysts for a significantly longer period of time than their wild-type littermates when placed into outgrowth medium. This distinction provides a unique method for obtaining a sample composed entirely of t6/t6 embryos at a stage prior to the lethal period.
X-chromosome activity in foetal germ cells of the mouse
Authors: Monk Marilyn , McLaren Anne
Published: April 2021

A cycle of inactivation and reactivation of one X chromosome in the female (XX) germ line is shown by analysis of gene dosage effects on activity of an X-linked enzyme. The ratio of activities of the X-linked enzyme HPRT and an autosomal enzyme APRT are determined in XX and XY germ cells from embryonic gonads from the 12th to the 17th day of pregnancy. Mitotic stages of XX and XY germ cells on the 12th day have similar HPRT: APRT ratios, but on the 13th day the ratios are significantly higher in XX than XY germ cells. As the XX germ cells enter meiosis they show a marked increase in HPRT:APRT ratio which is primarily due to a rise in X-linked HPRT activity. Comparisons are made with XO germ cells on the 12th and 14th day. On the 12th day, XO do not differ from XX and XY germ cells, suggesting that only one X chromosome is active in XX germ cells at this stage. On the 14th day, on the other hand, the HPRT:APRT ratios in XO and XY germ cells are similar but in XX germ cells the ratio is significantly higher. The twofold difference between the ratio in XX and XO germ cells suggests that by this stage both X chromosomes are active in XX germ cells. The subsequent large increase of the ratio in XX relative to XY germ cells is thought to reflect their differing cell states.
In vitrostudies of mouse embryos bearing mutations in the T complex: effects of culture in suboptimal medium upont6/t6and normal embryos
Authors: Pai Anna C. , Wudl Linda R. , Sherman Michael I.
Published: April 2021

We have compared the behaviour of normal and t6/ t6 embryos in PCMF, a suboptimal ‘delay’ medium which arrests normal development. Morphologically, the two types of embryos are indistinguishable in this medium. Although β -glucuronidase activity in embryos does not rise as quickly in delay medium as in cNCTC, a nutritive medium, the rate of increase is similar in t6/t6 and normal embryos. We conclude from these observations that the lethality of r®/r® embryos is not a consequence of their reaching a given absolute age. Together with previous studies, our data suggest that embryo lethality correlates more closely with metabolic state than with morphological stage.

Blastocysts maintained in PCMF are unable to give rise to trophoblast outgrowths but do so upon transfer into cNCTC medium. When a mixture of fourth-day t6/ t6 and normal embryos are transferred to cNCTC medium after lengthy pre-incubation periods in PCMF, trophoblast outgrowth is initiated from both types of embryos at approximately the same time. However, when embryos are removed from the genital tract on the second or third day of gestation, t6/ t6 embryos are slower to produce trophoblast outgrowths than are normal embryos upon transfer from PCMF to cNCTC medium. Although the reason for this differenial behaviour is not yet clear, it is hypothesized that some product(s) required for the outgrowth process is (are) more unstable in third-day t6/ t6 embryos than in normal third-day embryos or fourth-day t6/ t6 embryos.

Our ability to separate t6/ t6 from normal embryos by their delayed initiation of trophoblast outgrowth provides us with a convenient way to identify, and to isolate for analysis, enriched populations of homozygous mutant embryos prior to the time at which they show gross morphological abnormalities.
Development of a sensory afferent projection in the grasshopper embryo:I. Growth of peripheral pioneer axons within the central nervous system
Authors: Shankland Marty
Published: April 2021

The grasshopper’s cereal nerve is established early in embryogenesis by an identified pair of peripheral neurons called the cereal pioneers. Like the peripheral pioneer neurons in other insect appendages, these two cells send their axons from the periphery to the rudimentary CNS and thus lay the foundation for a nerve that will later be followed by a large number of sensory axons. In this paper, cobalt fills of the primordial cereal nerve were used to characterize the disposition of these peripheral pioneer axons within the embryonic CNS. The pioneer axons stained by this technique terminate in ellipsoidal growth cones which have filopodia radiating from the leading edge and a single long terminal filament pointing along the path the axon is taking. The growing axons also bear filopodia along their sides, but these structures disappear as the cells mature. The pioneer axons of the cereal nerve make an abrupt turn where they first enter the ganglion rudiment and join the axons of the primary longitudinal tract. The pioneers then grow along this tract for several hundred microns without forming secondary growth cones or branches. This prolonged absence of central arborization distinguishes the peripheral pioneer axons from the axons of later-arising epidermal sensory neurons.
Development of a sensory afferent projection in the grasshopper embryo:II. Growth and branching of peripheral sensory axons within the central nervous system
Authors: Shankland Marty
Published: April 2021

The morphogenesis of several types of sensory axon branching patterns has been described by cobalt filling the cereal nerve of the grasshopper embryo at a series of different stages in development, thus staining the earliest sensory axons as they grow through the CNS. This embryonic sensory projection contains all five types of cereal afferents seen in the adult, and no new sensory tracts are added during postembryonic life. When the embryonic sensory axons first follow their pioneer axons into the neuropil they choose pathways which are characteristic of the adult sensory tracts. Since the afferents follow these paths without sending collaterals into the other tracts, it appears that the growing axon chooses its specific pathway without extensive exploration of alternative routes. Likewise, nearly all of the branches which arise from the embryonic sensory axons remain within the eventual domain characteristic of each cell type. This precise, determinate pattern of initial growth implies that the sensory axons are guided through the neuropil and achieve their final branching patterns with a minimum of overgrowth and pruning. The fact that initial growth is so precise also suggests that the parameters which guide the growing axon may help to determine its eventual pattern of synaptic connectivity by limiting its physical access to large portions of the neuropil which contain potentially compatible synaptic partner cells. Two different types of neurons may be supplying the sensory afferents with guidance cues: (i) Although most of the cereal sensory axons diverge from the cereal pioneer axons within the CNS, some sensory afferents continue to follow the pioneers through several ganglia, (ii) In the adult, a large number of the cereal sensory axons form a hollow shell of arborization around the main dendrite of an identified synaptic target cell, the Medial Giant Interneuron (MGI). This structure, the interneuron dendrite and the shell of sensory arbor, is called the cereal glomerulus. Since the MGI’s dendrite is already present at the stage when the first sensory axons enter the CNS, interactions between these cells could serve to guide the glomerular sensory axons away from the pioneers into their future tracts.
Early erythropoiesis in foetal rat bone marrow: evidence for a liver-to-bone marrow relay
Authors: Nagel M. D. , Nagel J. , Jacquot R.
Published: April 2021

Erythropoietic activity of foetal rat femoral marrow was examined during the last four days of intra-uterine life. Insignificant at day 18, it develops slowly thereafter until birth. In the non-suckled neonate (not older than two hours), it appears notably enhanced. In order to test the potential of the foetal marrow to develop precocious or increased erythropoiesis, the activity of the erythropoietic organ predominant at this time, the liver, was altered by modifying the level of circulating corticosteroids which govern its function. Maturation and involution of the hepatic erythron were prevented by corticosteroid deprivation of the foetus (maternal adrenalectomy and foetal hypophysectomy). Precocious maturation and exhaustion of the hepatic erythron was induced by submitting foetuses to corticosteroids excess from day 14. Both corticosteroid deprivation and excess increase the erythropoietic activity of the femoral marrow. This activity can reach and even exceed by day 20 of intrauterine life that in neonatal marrow. Foetal hepatic erythron misfunction can therefore initiate and stimulate bone marrow erythropoiesis. The study of circulating red blood cells demonstrates that: (1) anaemia initiates medullary erythropoietic activity; (2) this anaemia is largely corrected by the bone marrow. The regulatory mechanism is presumably erythropoietin mediated.
Cytoskeletal coordination and intercellular signalling during metazoan embryogenesis
Authors: Tucker J. B.
Published: April 2021

This article draws attention to certain recently discovered features of cell surface organization and cytoskeletal deployment that may be revealing a new basis for intercellular signalling during metazoan embryogenesis. It is a signal mode that could coordinate many aspects of ‘Entwicklungsmechanik’ by spatiotemporal integration of the cytoskeletal/motor network throughout developing tissues. Evidence that this is achieved by ‘intercellular cytoskeletal/ plasma membrane connecting systems’ which coordinate the spatial organization of microtubules, microfilaments, and intermediate filaments in developing animal tissues is critically examined. It is argued that this system does operate but that it is not used to transmit positional information in embryonic fields. However, it probably responds to such information and might play an important part in establishing field boundaries during the very earliest stages of embryogenesis.

Certain aspects of cell surface organization in contemporary protozoans reveal ways in which the Protozoa could have been pre-adapted for the employment of cytoskeletal/cell surface signalling during the advent of multicellularity. In marked contrast, such signalling does not appear to be exploited during plant morphogenesis. The extent to which cytoskeletal organization might be coordinated in sister cells by transmission of spatial instructions during cell division in both animal and plant tissues is also considered.
Sex differentiation in bilaterally allophenic animals produced by cloning of two bipartite male/female chimaeras ofLineus sanguineus
Authors: Sivaradjam S. , Bierne J.
Published: April 2021

Two bipartite chimaeras were constructed in Lineus sanguineus by grafting the lateral halves from a phenotypically dark-brown male onto the anatomically complementary halves from a phenotypically light-brown female. Regeneration of a large number of pieces transected from these two bilaterally allogeneic chimaeras produced two clones of bilaterally allophenic nemertines (♂/ ♀ and ♀/ ♂).

Sex differentiation in the cloned worms started with a transitory stage of gonad developmental autonomy, termed the primary gynandromorphous state; at this stage there were young testes in the originally male lateral halves and juvenile ovaries in the originally female ones, the only abnormality then was that the ovarian development was more advanced than the testicular development relative to those in male and female controls. Then, unilateral sex reversal occurred, with feminization of the testes, i.e. oogenesis took the place of spermatogenesis in the many male gonads located in either the right or the left side of allophenic worms according to the symmetry patterns of the two clones. Finally, when the gonads reached maturity, both sides of allophenic L. sanguineus contained only ovaries with ripe oocytes.

The complete feminization of these allophenic worms and the previously observed masculinization of ‘heterosexual’ chimaeras in L. ruber suggest that a diffusible factor controls gonadal differentiation in worms of the prevailing sex, which is the female sex in L. sanguineus and the male sex in L. ruber.
Transfilter evidence for a zone of polarizing activity participating in limb morphogenesis
Authors: Kaprio Eero A.
Published: April 2021

Barriers were inserted into stage-20 HH chick embryo wing buds to separate the zone of polarizing activity from the anterior two-thirds of the wing bud with its overlying apical ectodermal ridge. Half of the barrier length projected out of the wing bud at insertion. Sham-operated wing buds developed only occasionally into wings with cartilage deletions. After insertion of an impermeable membrane (Cellophane), the typical wing skeleton contained only a humerus and a radius. In order to differentiate between diffusion, cell contact and cell penetration, Nuclepore filters with pore sizes of 0·05μm, 1·0 μm and 8 ·0 μm, respectively, were inserted. The typical wing skeleton after Nuclepore filter insertion was one with post-axial deletions. None, however, developed with complete distal deletions as after Cellophane. Deletions in the wing skeletons after Nuclepore insertion were the least with 1·0μm filters and the most with 8·0 μm filteis. Elevation of the apical ectodermal ridge was noted until 18 h after the insertions. In none of the groups did the ridge flatten. The results suggest that the zone of polarizing activity does have a role in normal limb morphogenesis. The mechanism by which its morphogen spreads is diffusion rather than being mediated via cell contacts.
How axons grow down theXenopusoptic nerve
Authors: Fawcett J. W.
Published: April 2021

Retinotectal fibres from different parts of the retina have been filled with Horse Radish Peroxidase (HRP). During their course down the nerve their ordering decays, but it is reestablished at the chiasma, except for nasal fibres which remain relatively disordered. The consequences of this for development are discussed.
An experimental study of the relation of cardiac jelly to the shape of the early chick embryonic heart
Authors: Nakamura Atsuyo , Manasek Francis J.
Published: April 2021

The structural roles of cardiac jelly components were examined in the early developing chick embryonic heart. Cardiac jelly matrix components were enzymically removed in situ by injecting nanogram quantities of enzymes directly into the cardiac jelly. Injection of ovine testicular hyaluronidase caused shrinkage and the heart became flaccid, but overall heart shape did not change. These responses were the result of enzymatic removal of glycosaminoglycan sugar moieties and were not due to lumenal collapse. Although purified collagenase did not cause any noticeable change, enzymes with non-specific proteolytic activity induced marked cardiac shape changes. In such hearts the dorsal mesocardium opened completely, and the myocardium as well as splanchnic mesoderm of foregut detached from their substrata and the entire heart region swelled. Consequently the shape of the heart was altered completely. These results suggested that in the normal condition the myocardial envelope was under an internal pressure due to the presence of glycosaminoglycans in the cardiac jelly space, and that some matrical non-collagenous protein components were essential to control the internal pressure. Therefore it is suggested that the internal pressure of cardiac jelly may be the direct driving force for the looping process and protein components of cardiac jelly may be important in directing the force for the morphogenetic process.
Effects of concanavalin A on developing ganglion cells in the retina of chick embryos
Authors: Meller K.
Published: April 2021

The administration of concanavalin A (Con A) (50−200 μg/egg) to chick embryos between the third and the seventh day of incubation has the following effects on the retina:

Con A causes the degeneration of a large number of ganglion cells and consequently the layer that should be formed by these cells is not present or is constituted only by a small number of ganglion cells. The lectin seems to be effective only when it is administered during the postmitotic phase of the ganglion cells. The degenerated cells are phagocytosed by the Müller cells in a manner similar to that occurring during the natural cell death in normal retinal development. The differentiation of other retinal elements (photoreceptors, bipolar, amacrine and Muller cells) is not affected by the lectin administration. The administration of Con A in later stages of development, even at ten times higher dosages (2000 μg/egg), fails to affect retinal neurogenesis.

It is suggested that Con A binding to receptor sites of the cell membrane affects the distribution or mobility of surface components producing an alteration in the mechanism by which the developing cells regulate positional information during retinal neurogenesis.
Polymorphism and specificity of positioning of contractile vacuole pores in a ciliate,Chilodonella steini
Authors: Kaczanowska Janina
Published: April 2021

The unicellular ciliate Chilodonella steini has a well-defined flat and ciliated ventral field. During divisional morphogenesis two sets of new contractile vacuole pores (CVPs) are formed on this field. During final pattern formation some of these CVP primordia and the old parental set of CVPs are completely resorbed. Primary pattern of distribution of the CVP primoidia and final pattern of distribution of the matured CVPs manifest an intraclonal polymorphism.

From analysis of this polymorphism some features of mechanism(s) of CVP pattern determination are deduced:

There is a strict, short-distance negative control of appearance of CVP primordia at sites of oral morphogenesis and around the ventral field. Certain indeterminacy of large-scale patterning of CVP primordia is observed over the area competent to yield CVP formation. However, within this area three longitudinal sectors with a high probability of occurrence of CVP primordia are alternated with sectors nearly deprived of their occurrence. Positive control of probability of occurrence and of specificity of location is found for certain CVP primordia. An interaction of mechanism of positioning on cellular longitudes and latitudes is proposed to account for these facts. The resorption of supernumerary CVP primordia does not alter the character of the global map of distribution of CVP primordia achieved during primary pattern formation. The primordia located at some latitudes persist, whereas others are resorbed at random. It is suggested that all CVP primordia which do not mature at the time of stabilization of divisional morphogenesis are resorbed. Thus the global map of CVPs distribution would result from the sum of the individual determinations of the fates of each CVP primordium, superimposed on an initial spatially non-uniform distribution of CVP primordia.
Morphallaxis in an epimorphic system: size, growth control and pattern formation during amphibian limb regeneration
Authors: Maden M.
Published: April 2021

An essential component of most theories of pattern formation in epimorphic systems is that growth and pattern formation are strictly linked. Furthermore it has been assumed that epimorphic systems display size dependence, i.e. the pattern elements are always of a fixed size. These assumptions are challenged in the work described here on amphibian limb regeneration. The sizes of elements regenerating from small and large limbs and from normal and partially denervated limbs have been measured along with a detailed study of their subsequent growth stages. It is shown that the size of elements within one group of animals is remarkably constant even though their final, adult size is very different. But between groups of animals (large versus small or normal versus partially denervated) their sizes vary considerably. Therefore this classical epimorphic system is not size dependent, calling for a revision of current theoretical concepts. The similarities between this type of behaviour and that in morphallactic systems is discussed as well as similarities with growth control in limb development.
Control of growth related to pattern specification in chick wing-bud mesenchyme
Authors: Cooke Jonathan , Summerbell Dennis
Published: April 2021

The distribution of raised mitotic index, and the co-incidence of this with lowered cell packing density, has been studied across the anteroposterior dimension of the terminal 500μm of chick wing buds following various numbers of hours signalling from an anteriorly grafted extra Zone of Polarizing Activity (ZPA). The results show propagation of the situation that causes these correlated phenomena, from graft-host interface essentially right across the limb mesenchyme, frequently within 8 h. This contrasts with the much slower and more local succession of changes in position memory, for differentiation of a duplicated limb pattern, that also occurs in mesenchyme relatively close to the graft after this operation. The results are discussed, in relation to current ideas about the control of pattern during limb development.
Pattern formation and growth in the regenerating limbs of urodelean amphibians
Authors: Holder Nigel
Published: April 2021

The results of numerous types of grafting experiments involving the amputation of symmetrical limbs are described. These experiments were designed to test the tenets of the polar coordinate model. The analysis of the results of these grafts coupled with a quantitative analysis of blastemal shape strongly indicates that pattern regulation during amphibian limb regeneration can be understood in terms of the model.
Morphogenetic behaviour of the rat embryonic ectoderm as a renal homograft
Authors: Švajger Anton , Levak-Švajger Božica , Kostović-Knežević Ljiljana , Bradamante Želimir
Published: April 2021

Halves of transversely or longitudinally cut primary ectoderm of the pre-primitive streak and the early primitive streak rat embryonic shield developed after 15–30 days in renal homografts into benign teratomas composed of various adult tissues, often in perfect organspecific associations. No clear difference exists in histological composition of grafted halves of the same embryonic ectoderm.

The primary ectoderm of the pre-primitive streak rat embryonic shield grafted under the kidney capsule for 2 days displayed an atypical morphogenetic behaviour, characterized by diffuse breaking up of the original epithelial layer into mesenchyme. Some of these cells associated into cystic or tubular epithelial structures.

The definitive ectoderm of the head-fold-stage rat embryo grown as renal homograft for 1–3 days gave rise to groups of mesenchymal cells. These migrated from the basal side of the ectoderm in a manner which mimicked either the formation of the embryonic mesoderm or the initial migration of neural crest cells. This latter morphogenetic activity was retained in the entire neural epithelium of the early somite embryo but was only seen in the caudal open portion of the neural groove at the 10- to 12-somite stage.

The efficient histogenesis in grafts of dissected primary ectoderm and the atypical morphogenetic behaviour of grafted primary and definitive rat embryonic ectoderm were discussed in the light of current concepts on mosaic and regulative development, interactive events during embryogenesis and positioning and patterning of cells by controlled morphogenetic cell displacement.
Catch-up growth
Authors: Williams J. P. G.
Published: April 2021

Catch-up growth, defined as growth velocity above the statistical limits of normality for age or maturity during a defined period of time, is distinguished from compensatory growth since it makes up for a potential loss rather than an actual loss and is seen in the whole body as opposed to specific organs. The cellular explanation for catch-up based on the work of Winick is described and a recent challenge to this explanation is briefly discussed. The mechanism of mismatch between actual size and ‘planned’ size suggested by Tanner is described and tested. In a series of experiments conducted in rats of different ages the degree of mismatch and the role of catch-up are compared for two different parameters, body weight and nose-rump length. It was found that the two parameters behaved differently and it is suggested that while the concept of mismatch is still acceptable the idea of a single central mechanism is not supported. It is suggested that the mismatch mechanism is a cellular phenomenon.
Abnormal neural fold development in trisomy 12 and trisomy 14 mouse embryos
Authors: Putz Barbara , Morriss-Kay Gillian
Published: April 2021

The early development of the exencephalic malformation in trisomy 12 (Tsl2) and trisomy 14 (Tsl4) mouse embryos was examined by means of scanning electron microscopy and compared with cranial neural tube formation in euploid litter-mates. Embryos from normal laboratory mice were used as additional controls.

The euploid control embryos of the trisomy-inducing breeding system showed a slight delay and some variation in the timing of cranial neurulation. The pre-exencephalic trisomic embryos showed hypoplasia, and lower somite number when compared with euploid littermates; there was also a retardation of development of the whole neural tube, when related to the somite stage. External differences from the control embryos were observed at the late pre-somite stage, when the anterior part of the neural plate showed a crumpled appearance. At 6 somites the lateral edges of the forebrain were everted instead of elevated in Tsl2 and Tsl4 embryos. At later stages, however, the forebrain showed a tendency towards the normal morphogenetic pattern, so that the optic vesicles were eventually formed and the most anterior part fused. The caudal forebrain and the midbrain were more permanently affected by the disturbance of trisomic conditions and grew laterally, failing to appose or fuse in the midline in both Tsl2 and Tsl4 embryos. Hindbrain morphogenesis was different in Tsl2 and Tsl4 excencephaly : in Tsl2 embryos it did not close rostral to the otic pits, whereas in Tsl4 embryos it showed a normal closure up to the hindbrain/midbrain junction.

These observations support the hypothesis that in mammalian embryos the mechanism of neural tube formation of the future brain region is more complex than that of the spinal neural tube and therefore may be more likely to react to a general delay of neurulation with a gross malformation.

Tsl2 and Tsl4 exencephaly are due to a primary non-closure of the neural tube.
Interactions between compound and normal eye projections in dually innervated tectum: a study of optic nerve regeneration inXenopus
Authors: Straznicky Charles , Tay David
Published: April 2021

Right compound eyes were formed in Xenopus embryos at tailbud stages by the fusion of two nasal (NN), two temporal (TT) or two ventral (VV) halves. The left eye was kept intact. Two to four weeks after metamorphosis the optic nerve from the intact eye was severed to induce bilateral optic nerve regeneration. The contralateral retinotectal projections from the compound eye and the induced ipsilateral projections from the intact eye to the same (dually innervated) tectum were studied by [3H]proline autoradiography and visuotectal mapping from 3 to 6 months after the postmetamorphic surgery. The results showed that the NN, TT and VV projections, in the presence of optic fibres from the intact eye failed to spread across the whole extent of the dually innervated tectum. Unexpectedly the bulk of the regenerating projection from the intact eye was confined to the previously uninnervated parts of the dually innervated tecta, the caudomedial region in TT, the rostrolateral region in NN and the lateral region in VV eye animals. The partial segregation of the two populations of optic fibres in the dually innervated tectum has been taken as a further indication of the role of fibre-fibre and fibre-tectum interactions in retinotectal map formation.
Postembryonic development of the visual system ofPeriplaneta americana
Authors: Stark Robert J. , Mote Michael I.
Published: April 2021

The compound eyes of Periplaneta americana are connected by optic fibre tracts to an optic lobe composed of three sequential ganglia, the lamina, the medulla and the lobula respectively. The eyes and optic ganglia are organized into repeating sub-units arranged in a regular pattern. During postembryonic development, the number of subunits in the eye (ommatidia) increases from between 50 and 60 to over 2000, with a concomitant increase in the size of the optic lobe ganglia. The patterns of cell growth and proliferation were examined in serial section autoradiagraphs prepared following long and short exposures to [3H]thymidine during each developmental stage. Aspects of structural differentiation were examined in reduced silver-stained sections of nymphs at each developmental stage. Growth of the eye and optic ganglia resulted from the continuous proliferation of new cells throughout postembryonic development. Unlike other body tissues, growth of this system was independent of the moulting cycle. The pattern of growth observed in the optic ganglia directly reflected the growth of the eye. Growth of the compound eye occurs from a special zone of proliferation and differentiation located along all but its posterior margin. The lamina and medulla both grow by cell proliferation from a single neuroblast region located at the apex of the angle subtended by them. Cells which proliferate distally from this region differentiate into lamina neurons, while those that proliferate proximally differentiate into medulla neurons. Axons growing from these two adjacent regions meet at and add new new fibres to the distal end of the medulla neuropil. Specificity of the interneuronal connexions appears to result from a precise temporospatial sequencing of growth with the formation of the optic ganglia dependent on retinal development.
The proportion and distribution of polarizing zone cells causing morphogenetic inhibition when coaggregated with anterior half wing mesoderm in recombinant limbs
Authors: Frederick Jeanne M. , Fallon John F.
Published: April 2021

Previous work demonstrated that dissociated polarizing zone cells inhibit morphogenesis when dispersed among dissociated anterior wing mesoderm cells in recombinant limbs. In the present study the proportions and distribution of polarizing zone mesoderm which inhibit morphogenetic expression in recombinant limb buds were determined. Completely dissociated and pelleted anterior half wing mesoderm packed into leg ectodermal jackets produced slender, digit-like outgrowths of bilateral symmetry in 60 % of the cases. This represented the baseline with which recombinants containing polarizing zone cells were compared. The inhibitory influence was first detected when polarizing zone cells constituted 16 – 18 % of the mesodermal component of recombinant limb buds; at this percentage the incidence of distally complete outgrowths dropped to 23 % of the cases. The addition of 20 – 36 % polarizing mesoderm to dissociated anterior half wing gave reduced incidence (9 %) of distally complete outgrowths. Percentages greater than 36 % polarizing zone cells led to the total failure of distally complete limb-like morphogenesis in all cases. Further, when more than 60 % polarizing mesoderm was dispersed with anterior half mesoderm, the outgrowths obtained were small, fleshy, and completely deficient in limb character. Finally, the distribution of polarizing mesoderm in recombinant grafts was demonstrated to be random at all percentages examined through 8 days, using chick/quail and autoradiographic methods. In a separate set of experiments, flank mesoderm was not found to have the same inhibitory effect on recombinant limb morphogenesis as described for polarizing mesoderm.
Studies of early stages of differentiation of the cellular slime mouldDictyostelium discoideum
Authors: Sharpe P. T. , Treffry T. E. , Watts D. J.
Published: April 2021

Countercurrent distribution in a polymer, two-phase system has been used to study changes in the cell surface properties of amoebae of Dictyostelium discoideum. Amoebae harvested during exponential growth in axenic culture and during the subsequent first six hours of development on Millipore filters were distributed as a single peak. However, the position of the peak changed during the period of early development which showed that changes in cell surface properties were occurring. At aggregation (8 h), the peak markedly broadened, indicating considerable increase in cell surface heterogeneity amongst the amoebae, and heterogeneity was so great by 9 –10 h that the amoebae distributed as two peaks. Amoebae from one peak were shown to be precursors of spores while amoebae from the other peak appeared to be precursors of stalk cells. Similarly, amoebae from the trailing and leading edges of the broad peak, formed from amoebae beginning to aggregate (8 h), were found to have different fates. Thus cell differentiation had been found at times of development prior to formation of aggregates having apical tips or anterior-posterior polarity and neither of these features of aggregates can be essential for initiation of cell differentiation. It is therefore concluded that differentiation is not initiated in D. discoideum in response to ‘positional information’.
Calcium and wound healing in Xenopus early embryos
Authors: Stanisstreet Martin
Published: April 2021

The role of calcium in the healing of wounds made in the ectoderm of Xenopus neurulae has been studied. Embryos have been wounded in the presence of calcium inhibitors, and the effects on wound healing observed by scanning electron microscopy. In addition, unwounded embryos have been exposed to a local application of ionophore A23187 to simulate the possible calcium fluxes following wounding. Lanthanum, which competes for calcium channels, inhibits wound healing. EDTA, which binds divalent cations, also inhibits wound healing, but its effect can be reversed by the addition of excess calcium. Local application of ionophore A23187, which promotes transport of calcium across biological membranes, results in a local change in cell shapes. These observations lend support to the hypothesis that wound healing in amphibian early embryos, which is effected by changes in cell shapes similar to those seen in certain examples of normal morphogenesis, is initiated by a local influx of calcium into cells.
Turnover of embryonic messenger RNA in preimplantation mouse embryos
Authors: Kidder Gerald M. , Pedersen Roger A.
Published: April 2021

We have estimated the average half-life of embryonic messenger RNA in mouse embryos at two stages of preimplantation development. Embryos were collected at 48 and 75 h post-hCG and cultured overnight in the presence of [3H] uridine. Beginning at 65 –68 h (morulae) or 92 –94 h (early blastocysts), the label was withdrawn and replaced with unlabelled uridine, and samples were taken at intervals thereafter for RNA isolation. Label in cytoplasmic, poly(A)-containing RNA was measured after binding to oligo(dT)-cellulose, and was normalized to label in 28S and 18S ribosomal RNA, separated on sucrose gradients. The stability of rRNA in both stages was verified directly, as was the integrity and purity of the isolated mRNA. With morulae, the mRNA decay curve was monophasic, with an average half-life of 9 ·5 ±0 ·9 h. In three experiments with early blastocysts the decay curve appeared to be biphasic, consisting of short-lived (less than 6 h) and long-lived (30 –50 h) components; in two other experiments a short-lived component was not evident. In all cases, however, the overall average half-life of mRNA in early blastocysts, determined by linear regression assuming monophasic kinetics, was greater than that in morulae. Our data indicate that the stability of embryonic mRNA increases by at least twofold during the morula-to-blastocyst transition. The results are considered in terms of the transcriptional dependency of early mouse embryos and the regulation of maternal and embryonic mRNA.
Interaction of granule, Purkinje and inferior olivary neurons in Lurcher chimaeric mice : I.Qualitative studies
Authors: Wetts Richard , Herrup Karl
Published: April 2021

Heterozygous lurcher (+/Lc) mutant mice lose 100% of their Purkinje cells (PCs), 90% of their granule cells, and 75 % of their inferior olivary neurons. In order to determine the primary site of Lc gene action, lurcher ↔ wild-type aggregation chimaeras were produced. The cerebella of the three chimaeras examined were intermediate or normal in size compared to + /Lc and wild-type cerebella. The PCs were reduced in number. Using the β-glucuronidase locus (Gus) as a cell marker, all of the PCs present were identified as having descended from the wild-type embryo. It appears that all of the + /Lc PCs degenerated. Hence, the Lc gene acts directly on PCs to cause their degeneration.

The inferior olivary nuclei of the chimaeras seemed to have fewer neurons than wild-type but more than + /Lc animals. As revealed by β-glucuronidase histochemistry, both + /+ and + /Lc cells were present, and the ratio of genotypes was similar to the ratio seen in other regions of the brain. The evidence suggests that the death of olivary neurons in lurcher is secondary to another defect, probably the loss of PCs. β-glucuronidase is not an accurate cell marker for granule cells, and so no conclusion concerning the action of the Lc gene on granule cells could be made with these chimaeras.
Normal anterior pattern formation after barrier placement in the chick leg: further evidence on the action of polarizing zone
Authors: Rowe Donene A. , Fallon John F.
Published: April 2021

Impermeable barriers were inserted into the stage-20 to -21 leg bud to test whether or not such an interruption of diffusion of the proposed morphogen from the polarizing zone would result in failure of leg elements to develop anterior to the barrier. Tantalum foil was placed at somite levels 30/31 or mid-31 through the dorsoventral extent of the leg bud separating anterior from posterior mesoderm and ectodermal ridge. In the resulting legs, structures developed anterior to the level of the barrier. For example, legs with foil at the 30-/31-somite level developed digits 1 and 2. We conclude that either the barrier is not an effective block of diffusion of polarizing zone morphogen or that the influence of the polarizing zone is not required for determination of leg structures at these stages.
Importance of trophoblast genotype for survival of interspecific murine chimaeras
Authors: Rossant J. , Mauro V. M. , Croy B. A.
Published: April 2021

Interspecific chimaeras between Mus musculus and Mus caroli were made by injection of M. musculus inner cell masses (ICMs) into M. caroli blastocysts, and by aggregation of 8-cell embryos. Both types of chimaera were transferred to M. musculus recipients: the former did not survive to term, but viable chimaeras were produced following embryo aggregation. Previous experiments have shown that chimaeras produced by injection of M. caroli ICMs into M. musculus blastocysts are viable, whereas M. caroli blastocysts cannot survive in the M. musculus uterus. These results indicate that the presence of tropho-blast cells of maternal uterine genotype allows M. caroli ↔ M. musculus chimaeras to survive in the M. musculus uterus. It is suggested that M. musculus trophoblast components may protect the M. caroli embryonic cells from maternal immune rejection.
The development of monosomy 19 mouse embryos
Authors: Magnuson Terry , Smith Sandra , Epstein Charles J.
Published: April 2021

In general, autosomal monosomy is lethal much earlier in mammalian development than autosomal trisomy. In an attempt to understand why monosomy is so deleterious, we have begun to characterize the development of mouse embryos monosomic for chromosome 19. A dramatic loss of monosomy 19 embryos was found to occur between days 3 and 4 of development. This loss occurred both in vivo and in vitro and with intact blastocysts or isolated inner cell masses. Experiments with inbred strains showed that this loss was not; due to the expression of recessive lethal genes. While monosomic embryos were found to have fewer cells than normal and trisomic litter-mates beginning at the early morula stage, the ability to form blastocysts is not interfered with. Electron microscopy revealed no difference in the cellular ultrastructure of monosomic when compared with diploid embryos” Furthermore, two-dimensional gel electrophoresis did not reveal any differences in the proteins synthesized by monosomie, trisomic or diploid litter-mates when examined at day 3 of development. These results indicate a lack of gross genomic disturbances in monosomic embryos. When monosomy<-> diploid chimaeras were made, viable monosomic cells were found in day-9 post-implantation embryos, well past the lethal period. Thus, in chimaerip embryos, the normal cells appear to be able to provide whatever is lacking, suggesting that monosomy 19 is not a cell lethal. Instead, death may be due to a dosage alteration in specific gene products needed during early development.
A scanning electron microscope study of the development of a peripheral sensory neurite network
Authors: Roberts Alan , Taylor J. S. H.
Published: April 2021

The formation of the sensory neurite plexus on the basal lamina of trunk skin in Xenopus embryos has been examined using the scanning electron microscope. It is formed by Rohon-Beard and extramedullary neurons which provide the first sensory innervation of the skin. By observing the distribution of growth cones on the inside surface of the skin of embryos at different ages, the development of the plexus has been followed and related to the development of sensitivity to sensory stimulation. The general features of the plexus are illustrated using a photomontage taken at x 1100. Measurements on neurites from this, and of growth cone orientations demonstrate a general ventral growth pattern with some small regional variations. Interactions of neurites within the plexus are examined. Neurites meeting at shallow angles tend to fasciculate, while those meeting at close to 90° tend to cross each other. Angles of incidence and separation of neurites show few angles less than 30°, which suggests that active adjustments occur after a growth cone meets or leaves another neurite. The observations allow comparison of behaviour of growing neurites in vivo and in vitro. Our evidence suggests that adhesion between growth cones and neurites is stronger than that between growth cones and the basal lamina of the skin.
Control of pattern formation in urodele limb ontogeny: a review and a hypothesis
Authors: Stocum David L. , Fallon John F.
Published: April 2021

From a review of the literature, the hypothesis is advanced that the forelimb region of the urodele embryo acquires its transverse axial polarity and pattern by the action of posterior and dorsal polarizing regions. The anterior-posterior and dorsal-ventral axes are determined simultaneously and their determination is a prerequisite for proximal-distal outgrowth. Outgrowth of the limb bud is accompanied by the generation, between proximal and distal boundaries, of a set of positional values specifying proximal-distal axial polarity and pattern. The proximal boundary is the initial positional value carried by the cells of the limb area. The distal boundary is imposed upon the outermost layer of limb disc cells by the overlying ectoderm.
Studies in Embryonic and Larval Development in Amphibia : I. The Embryology ofEleutherodactylus ricordii, with Special Reference to the Spinal Cord
Authors: Hughes Arthur
Published: April 2021

In the West Indian Anuran genus Eleutherodactylus the larval stage is wholly suppressed. As in other systems of organs there are features in the spinal cord which are correlated with this embryonic type of development. The cord in the trunk region resembles more that of a chick embryo than that of a larval Amphibian. Within the thick neuroepithelium mitoses in the ependymal zone persist much longer than in larval forms. There is a relatively thin layer of white matter. The Rohon-Beard system of sensory neurones persists, but does not precede the development of the dorsal root ganglia. In the trunk there are no conspicuous primary motor-cells as in larval Amphibia. The numbers of cells within the dorsal root ganglia and ventral motor horns have been counted both in the embryo and in the adult. From the first, the numbers of cells present in both are similar to those in the adult. During development, increase by mitosis is opposed by cell degeneration. Pycnotic nuclei are conspicuous in both spinal ganglia and ventral horns in the embryo. As soon as the spinal ganglia are recognizable, their relative sizes are similar to those of the adult. The ganglia at limb levels are already larger than those elsewhere a day before nerve fibres first enter the limb-buds.
Protein Accumulation in Early Chick Embryos Grown under Different Conditions of Explantation
Authors: Britt L. Gwen , Herrmann Heinz
Published: April 2021

Chick embryos were explanted at the 11 – 13 somite stage to an agar medium and the protein content of the embryos was measured after 8, 18, and 24 hours of explantation. An increase in the concentration of yolk-albumen in the medium as well as an increased extra-embryonic area was found to increase the protein contents of the explants. In the presence of a large extra-embryonic area and a high concentration of yolk-albumen in the medium, the protein content of the explant was found to approach, during the first 18 hours of explantation, the protein content of the embryo developing in the egg.