bluuebunny/crossref_metadata_2025 huggingface dataset:
Scanning electron microscopy of wound healing in rat embryos
Authors: Smedley Martin J. , Stanisstreet Martin
Published: April 2021

Wound healing in rat early embryos has been studied by scanning electron microscopy. Initially the wound gapes slightly and cells peripheral to the wound assume a cobble-stone appearance. Wound closure is quite rapid; some small wounds are almost closed within 10 min of incision. Wound closure is accompanied by the appearance of some elongated cells at the wound edge. These features are similar to, although less pronounced than, those which have been observed to accompany wound closure in amphibian and avian embryos. Healing of wounds made in the amnion is also accompanied by changes in the shapes of cells at the wound margins.

Wound healing in embryos cultured in Hank’s saline is similar to wound healing in embryos cultured in serum, suggesting that the macromolecular components of serum are not essential to wound healing. Cytochalasin B, which inhibits wound closure in amphibian embryos, does not inhibit wound healing in rat early embryos unless used at a concentration high enough to cause cell dissociation. Similarly chelation of the free calcium in the medium, which also prevents wound closure in amphibian embryos, does not inhibit wound closure unless the embryo is dissociating. Removal of free calcium does however cause collapse of the elevated neural folds. These observations suggest that the cellular mechanisms involved in wound healing are different in mammalian and amphibian embryos.
Patterns of cell division during visual streak formation in the frog Limnodynastes dorsalis
Authors: Coleman L-A. , Dunlop S. A. , Beazley L. D.
Published: April 2021

The site and extent of cell division were determined in midlarval tadpoles, animals at metamorphic climax and 2-month juvenile Limnodynastes dorsalis using untreated animals and those injected with colchicine or pH]thymidine shortly before sacrifice. Mitosis was restricted to the ciliary margin at all stages and there were significantly more dividing cells nasally and temporally than dorsally and ventrally. In animals injected with [3H]thymidine and killed at a subsequent stage, labelled cells were grouped at a distance from the ciliary margin and were more frequent in nasal and temporal than in dorsal and ventral retina. These results suggest that differing extents of mitosis around the ciliary margin, reflected in the number of cells entering the ganglion cell layer, may largely underlie the postmetamorphic formation of a visual streak observed using wholemounts.
The early development of mystacial vibrissae in the mouse
Authors: Wrenn Joan T. , Wessells Norman K.
Published: April 2021

The initial generation of the pattern of mystacial vibrissae (whiskers) in the mouse is described. The maxillary process is present in 10-day embryos but has a relatively flat surface. Beginning at approximately 11·5 days, the first sign of vibrissal development is the formation of ridges and grooves on the maxillary and lateral nasal processes. The first vibrissal rudiment to form subsequently appears posterior to the most ventral groove on the maxillary process. It is the most ventral whisker of the posterior, vertical row. The next few rudiments appear: (1) dorsal to the first, also in the vertical row; and (2) anterior to the first, on the ventral-most ridge and in the groove beneath it. Formation of vibrissal rudiments continues in a dorsal and anterior progression usually by an apparent partitioning of the ridges into vibrissal units.

The hypothesis that this patterning of mystacial vibrissae might be determined by the pattern of innervation in the early mouse snout was investigated. Nerve trunks and branches are present in the maxillary process well before any sign of vibrissal formation. Because innervation is so widespread there appears to be no immediate temporal correlation between the outgrowth of a nerve branch to a site and the generation of a vibrissa there. Furthermore, at the time just prior to the formation of the first follicle rudiment, there is little or no nerve branching to the presumptive site of that first follicle while branches are found more dorsally where vibrissae will not form until later. Thus, a one-to-one spatial correlation between nerve and follicle sites also appears to be lacking.

The developmental changes in ultrastructure within the neurites of the trunks and branches as well as the apparent rearrangements of the nerve trunks suggest that early innervation of the snout is a labile phenomenon and that the vibrissal pattern begins to be established before the neural pattern is completely developed. The results indicate that vibrissal pattern formation is likely to be a complex process relying on the interplay of the cells and tissues involved, rather than on unidirectional instructions from neurons to other cell types.
Initial phases of the rat testis differentiation in vitro
Authors: Agelopoulou Roxane , Magre Solange , Patsavoudi Evangélie , Jost Alfred
Published: April 2021

Rat gonadal primordia with their supporting mesonephroi were explanted in vitro at the undifferentiated stage (12 days 16 h after fertilization), at the outset of testicular differentiation (13 days 9 h) or when already containing seminiferous cords. The younger foetuses were sexed with the sex chromatin test in the amniotic membrane. The basal medium was CMRL 1066 and the culture period, 1 to 4 days. Testicular differentiation resulted from the appearance of large clear cells, the primordial Sertoli cells, and from their aggregation into seminiferous cords. Addition of 15 % foetal calf serum to the medium prevented the differentiation of seminiferous cords, but large clear cells appeared. In testes from 14- or 15-day-old foetuses, the seminiferous cords disintegrated under the influence of serum. The serum did not prevent the differentiation of Sertoli cells, but impaired organogenesis or maintenance of the early seminiferous cords. The results support previous histological observations on the initial stages of testicular differentiation.
Motor innervation of proximally rotated chick embryo wings
Authors: Laing N. G.
Published: April 2021

Chick embryo wing buds were rotated close to the lateral edge of the somites at stage 19, prior to limb innervation. Despite the abnormal orientation of the resulting limb, the motor pools to biceps and triceps were largely normal, as judged by electrical stimulation and horseradish peroxidase labelling just prior to hatching. The only abnormalities were a few caudal motoneurons innervating biceps and a few rostral motoneurons innervating triceps. This distribution is similar to that seen normally in young embryos before the completion of motoneuron death and it is suggested that the rotation may be keeping alive motoneurons which otherwise would die. The morphology of the brachial plexus supplying rotated wings was abnormal. It is concluded that axons growing into the limb bud from the spinal cord can compensate for reversal of both the limb axes and selectively innervate appropriate muscles. The result is consistent with others in which proximal reversal of one limb axis alone produced normal innervation.
Changes in hair growth characteristics following the wounding of vibrissa follicles in the hooded rat
Authors: Jahoda Colin A. B. , Oliver Roy F.
Published: April 2021

The effect on hair growth of wounding the lower region of whisker follicles, and in particular the dermal papilla, with sharply pointed tungsten needles was studied in adult hooded rats. Following injury hair growth ceased, but was subsequently resumed. While it might have been anticipated that follicle wounding would have a negative effect on whisker length, regular postoperative length measurements revealed that in follicles where cellular material was not displaced from the follicle by the original manipulation, 50 % of the subsequent hairs produced were longer than their counterparts on the opposite side of the face, with 25 % shorter and 25 % with their length unchanged. In every case increased hair length was achieved by a prolongation of the growing period of the hair. Growth rate, when altered, was reduced. These results suggest that the factors which control the duration of the hair cycle and fibre growth rate are independent in vibrissa follicles. Since removal of most of the epidermal component by plucking of the hair just prior to injury produced equivalent hair length increases, this implicated the proximal dermal components as being mainly responsible for the observed changes.
Haemoglobin and globin synthesis in the isolated primitive and definitive erythroid cells of chicken embryos. Evidence for a non-clonal mechanism at the haemoglobin switch
Authors: Schalekamp Margreet , Goor Diety Van
Published: April 2021

Primitive and definitive erythroid cells of chicken embryos aged 4–8 days, were separated by unit gravity sedimentation and pulse labelled with [3H]- and [14C]leucine. The haemo-globin and globin synthesis in the cell populations was analysed by chromatofocussing, isoelectric focussing, urea starch gel electrophoresis, and immunofluorescence or radioimmunoassay, using globin specific antibodies.

We found that both embryonic and adult agiobins are present in primitive erythroid cells, but relatively more of the adult α-type globins are synthesized in the late primitive erythroid cells. In young definitive erythroid cells exclusively adult a-type globins are synthesized. From these findings we conclude that a command to synthesize adult agiobin is perceived in both cell types at the time of the switch. This supports an environmental model rather than a clonal model of haemoglobin switching.
Dermal cells form strong adhesions to the basement membrane during the development of feather primordia in chick skin
Authors: Davidson Duncan
Published: April 2021

Experiments are described which provide a direct measure of the adhesion between dermis and epidermis during the development of feather primordia in chick dorsal skin in culture. The epidermis was peeled from the dermis and the surfaces so exposed were examined under the scanning electron microscope: regions of strong adhesion between the tissues were revealed as areas where their separation was incomplete. The results show that soon after primorida become morphologically distinct, cells from the surface of dermal condensations form adhesions to the basement membrane which are stronger than those between dermis and epidermis interplumar skin. These adhesions may help to hold the epidermis and dermis together during the outgrowth of the primordium.
The cellular contributions of blastema and stump to 180 ° supernumerary limbs in the axolotl
Authors: Maden M. , Mustafa K.
Published: April 2021

By inverting triploid blastemas onto diploid stumps (and vice versa) the cellular contributions to supernumerary limbs so generated have been assessed. The four classes of 180° supernumerary limbs each had a different mixture of stump and blastemal cells. The mesoderm of normal supernumeraries was composed entirely or almost entirely of stump cells and were always of stump handedness. The mesoderm of symmetrical supernumeraries was of variable composition, it could be mostly stump, mostly blastema or half and half. In part normal/part symmetrical supernumeraries the normal part was usually of stump origin and the abberant symmetrical part of blastemal origin. In part normal/part inverted supernumeraries the normal part came from the stump and the inverted part from the inverted blastema. The handedness of each part of these supernumeraries corresponded with its cellular origin. The epidermis of the supernumeraries was not of the same relative composition as the mesoderm, it tended to have a larger stump component. The black/white marker was also used and this too tended not to conform to the mesodermal contribution patterns. These results are discussed in terms of rules for generating supernumeraries and it is concluded that with the exception of symmetrical supernumeraries the cellular contributions of stump and blastema determine their structure.
Butterfly wing patterns: how good a determining mechanism is the simple diffusion of a single morphogen?
Authors: Bard Jonathan B. L. , French Vernon
Published: April 2021

The formation of the wing pigmentation patterns of three species of butterflies has been modelled using a mechanism based on a tripod of assumptions. First, that there may be morphogen sources in the foci of eyespots and morphogen sinks at some parts of the wing margin, all other cells being passive. Second, that the morphogen has a finite half life and diffuses simply and freely away from the sources throughout a wing of hexagonally packed cells. Third, that the overt pattern derives from cells interpreting the local morphogen concentration with respect to thresholds which determine scale colours. The final pattern thus follows lines of constant morphogen concentration and may, depending on the distribution of sources, comprise rings, curves or bands. With such a model, we have been able to compute stable patterns having the essential topology of the compound spots of Tenaris domitilla, the large rings of Diaethria marchalii and the pattern of eyespots, rings and asymmetric bands of Ragadia minoa. Quantitative analysis of the pattern-forming process shows that, with a biologically realistic diffusion constant (∽ 540−7cm2 sec−1) and a morphogen half life less than 6h, the patterns form within ∽ 12h over a wing of ∽ 1000 cells in length. The limitations of the model are that the exact morphology of the eyespots and bands do not match precisely those of the original wings, that there are edge distortions and that optimal patterns may be critically dependent on the exact positions of sources and sinks. An explanation for part of the discrepancy is that we have assumed an adult wing shape and foci coordinates in modelling a process that took place earlier in development. Nevertheless, the limitations of the model argue against a mechanism based on a single morphogen operating in vivo. However, as the model can generate many features of butterfly wing patterns, it may be considered as a degenerate case of that mechanism.
The response of denervated axolotl arms to delayed amputation
Authors: Wallace H.
Published: April 2021

Forearms of juvenile axolotls can be kept denervated for up to 4 weeks by deflecting brachial nerves to the flank. A more orthodox second denervation prolongs this state up to a total of six weeks. The denervated arms are unable to regenerate for the whole period, but eventually become reinnervated and then regenerate normally. These results and analogous experiments on adult newts prompt a partial retraction and reinterpretation of my previous report on regeneration after prolonged denervation.
Cell replication and terminal differentiation in the embryonic chicken lens: normal and forced initiation of lens fibre formation
Authors: Zwaan Johan , Kenyon Robert E.
Published: April 2021

The relation between cell replication and lens fibre formation was studied in normal 5-day chicken embryos and in embryos in which the lens was reversed. After this microsurgery the epithelial cells were facing the retina and were forced to start fibre differentiation. Embryos were treated with [3H]thymidine followed by autoradiography or with Colcemid to induce metaphase arrest.

About half of the control epithelial cells were actively replicating (in contrast, in the earliest stages of lens formation all cells are multiplying). Half of the non-dividing population was accounted for by the annual pad cells. The other half, probably in a resting phase (GO) of the cell cycle, was perhaps slightly more common in the central epithelium. This suggests that a germinative zone may be forming in the lens epithelium as early as 5 days of embryonic life.

Normal epithelium had a cell cycle time of about 11 h, with S-phase, M, G1 and G2 lasting 5·5, 0·5, 2, and 3 h. After reversal the labelling index remained at control levels for about 4 h and then dropped to zero at 9 h. Similarly, the mitotic index gradually decreased to zero in 15.

It appears that reversal forces the epithelial cells to cease replication when they reach Gl. Cells already in GO may form fibres without first going through DNA synthesis. The very sharp boundary between dividing and non-dividing cells at the border of the optic cup suggests that the retina may regulate the irreversible cessation of cell replication associated with lens fibre differentiation.
Trophectodermal processes regulate the expression of totipotency within the inner cell mass of the mouse expanding blastocyst
Authors: Fleming Tom P. , Warren Paul D. , Chisholm Julia C. , Johnson Martin H.
Published: April 2021

Mouse blastocysts, aged 0, 2, 6 and 12 h from the onset of cavitation, were examined by transmission (TEM) and scanning (SEM) electron microscopy. In TEM sections, trophectoderm cells (TE) differed morphologically from those of the inner cell mass (ICM) by their flattened shape, paler cytosol staining and polarized disposition of both junctional complexes (apicolateral) and intracellular secondary lysosomes (SL; basal). Throughout this period of development, cytoplasmic processes, characterized by abundant SLs, cover approximately 80 % of the juxtacoelic face of the ICM. These processes are shown to be derived from the basal surface of TE cells intermediately placed between polar and mural regions. In SEM preparations of the juxtacoelic ICM surface, revealed by ‘cracking open’ blastocysts, the processes appear as tongue-shaped, centripetally oriented structures which terminate collectively at a central area on the ICM surface.

The potential of cultured ICMs to generate TE was demonstrated following their immunosurgical isolation from blastocysts aged up to 12 h post cavitation and by examining the sequence of ultrastructural changes associated with TE generation by ICMs from 2 h blastocysts. In contrast, the juxtacoelic cells of similarly aged ICMs observed in situ in ultrasections of intact embryos showed little or no evidence of totipotency expression as judged by the absence of TE characteristics. Since TE expression within presumptive ICM cells is thought to be generated by an asymmetry of cell contacts (Johnson & Ziomek, 1983), we propose that the juxtacoelic TE processes, by providing a cellular cover to the ICM, function in suppressing the expression in situ of ICM totipotency.
Translation and stability of ovalbumin messenger RNA injected into growing oocytes and fertilized ova of mice
Authors: Ebert Karl M. , Paynton Barbara V. , Mcknight G. Stanley , Brinster Ralph L.
Published: April 2021

Growing mouse oocytes and fertilized ova were injected with chicken ovalbumin messenger RNA (mRNAov) and chicken conalbumin mRNA (mRNAcon) and cultured in vitro. Estimation of mRNAov and mRNAcon stability by hybridization of cDNAov and cDNAcon to extracted mRNA from injected oocytes and fertilized ova indicated a half-life of 147 and 366 h in the oocyte and 5 and 3 h in the fertilized ovum respectively. Stability of mRNAov was similar in the fertilized and unfertilized ovum. Oocytes injected with chicken ovalbumin mRNA were also labelled with βH]leucine and ovalbumin synthesis was measured by immunoprecipitation. The amount of ovalbumin synthesized during the initial 7 h was less than during the period of 18–25 or 66–73 h postinjection. The greatest percentage of ovalbumin to total protein synthesis occurred between 66–73 h. Oocytes secreted 12% of the synthesized ovalbumin during each of the 7 h periods (0–7, 18–25 and 66-73 h) indicating a stable mechanism for secretion throughout the culture period. These studies demonstrate: (1) a dramatic difference in stability of injected mRNA between the growing oocyte and the unfertilized or fertilized ovum, and (2) a gradual increase in the translation of injected mRNA by the growing oocyte during in vitro culture.
Vascular regression during the formation of the free digits in the avian limb bud: a comparative study in chick and duck embryos
Authors: Hurle J. M. , Colvee E. , Fernandez-Teran M. A.
Published: April 2021

The pattern and structure of the blood vessels of the interdigital spaces of the leg bud have been studied by means of Indian ink injections and transmission electron microscopy in the chick and duck embryos. The results show that in the chick the interdigital necrotic process responsible for the freeing of the digits is followed by regression of the blood vessels. In the webbed foot of the duck, the interdigital necrotic processes are not followed by vascular regression. Transmission electron microscopic studies show that both in the chick and in the duck, interdigital blood vessels are immature structures lacking basal lamina. Dead cells of presumably endothelial origin were detected in the lumen of the regressing blood vessels of the chick but not in the duck. However, the intensity of this cell death process does not appear to be high enough to account by itself for the disappearance of the interdigital blood vessels. The possible relationships between interdigital mesenchymal cell death and vascular regression are discussed.
Paths taken by sensory nerve fibres in aneural chick wing buds
Authors: Swanson Gavin J.
Published: April 2021

What constrains growing nerves to follow the paths they take during the development of peripheral nerve patterns? This paper examines two, related, topics concerning the pathways taken by sensory nerve fibres in the embryo chick wing: the constraints imposed on the nerves by limb tissues; and the timing of axon outgrowth. Sensory ganglia from 7-day-old chick embryos were grafted into younger host embryo wing buds which had been previously denervated. The resultant nerve patterns revealed that, first, nerve fibres could grow almost anywhere within the wing bud, with the exceptions of cartilage and a region just beneath the growing tip. Secondly, the younger the host wing bud at the time of grafting, the more likely the neurites were to form a thick fascicle which followed the limb’s normal nerve pathways. The wing apparently does not impose a rigid restraint on nerves to grow only along certain routes; however, if a nerve fibre reaches a normal nerve pathway, it prefers to follow it.
The distribution of the polarizing zone (ZPA) in the legbud of the chick embryo.
Authors: Hinchliffe J. Richard , Sansom Anna
Published: April 2021

The stage-21 to 22 legbud polarizing zone (ZPA) was mapped by transplanting small blocks of posterior marginal mesenchyme preaxially into stage-20 to -22 chick wing buds and assessing the degree of duplication of the wing digital skeleton produced in the host. Blocks taken from the posterior flank, from the angle between posterior flank and the proximal base of the limb bud, and from the most anterior distal position chosen (under the AER), all had very low activity. Blocks taken from the posterior margin of the legbud, plus the next distal block under the posterior part of the AER, all had high activity. We consider that barrier and amputation results on wing and legbud, when interpreted in the light of maps of the ZPA in both limb buds, are consistent with the hypothesis that both leg and wing have their growth and anteroposterior axis of pattern formation controlled by the ZPA.
Genetic identification of tissue of origin of cellular populations within the mouse placenta
Authors: Rossant J. , Croy B. A.
Published: April 2021

The mouse haemochorial placenta is a complex mixture of maternal cells and foetal trophectoderm and inner cell mass (ICM)-derived cells. The majority of the placental tissue is assumed to be trophoblast in origin but the exact extent and localization of the ICM and maternal contribution has not previously been determined. Using embryo transfer and reconstituted blastocyst techniques, combined with isozymal and in situ genetic markers, we have established that about 70% of the 13 to 15-day placenta is trophectoderm-derived, 30% is maternal in origin, and 4% develops from the ICM. Nearly all of the maternal contribution was confined to the spongiotrophoblast region and all of the ICM contribution was confined to the labyrinthine trophoblast region, where it formed the foetal blood capillaries and the endodermal sinuses. Using the same genetic markers, we showed that cell suspension techniques commonly used to produce ‘trophoblast’ cell preparations from placenta do not enrich for trophoblast, and, indeed, that collagenase, the preferred dissociation technique for cell viability, produced cell suspensions in which ICM and maternal cells were preferentially dissociated. No method for producing pure trophoblast populations has yet been found.

Some unusually high ICM contributions to the placenta were found in reconstituted blastocyst experiments using ICMs isolated from early 3·5-day blastocysts, suggesting that these ICMs may have contributed to the trophectoderm layer of the blastocyst. These and other experiments suggest that the inner cell mass lineage may not be closed until some time after formation of the blastocyst.
Stimulation of division in mouse 3T3 cells by coculture with embryonic chick limb tissue
Authors: Bell K. M. , McLachlan J. C.
Published: April 2021

Two regions of the chick limb bud — the apical ectodermal ridge and the zone of polarizing activity — have been shown to influence cell division and pattern formation during normal development and following surgical manipulation. In this study, using a simple coculture system, together with autoradiography, we have shown that these morphogenetically active regions of the limb bud can stimulate quiescent 3T3 cells to initiate DNA synthesis to a significantly greater degree than comparable but morphogenetically inactive regions of the limb bud.
Normal fates and states of specification of different regions in the axolotl gastrula
Authors: Cleine J. Herman , Slack Jonathan M. W.
Published: April 2021

A fate map was constructed for four regions of the early gastrula of Ambystoma mexicanum using orthotopic grafts from donors labelled with FLDx (fluoresceinated-lysinated-dextran). The region around the animal pole gave rise to epidermis only and did not include prospective neural plate. The dorsal marginal zone contributed to cephalic endoderm and to the whole length of the axial mesoderm (notochord and somites), the lateral marginal zone to lateroventral and somitic mesoderm, and the ventral marginal zone to lateroventral mesoderm. It was found that the dorsal marginal zone contributed relatively more to the anterior regions of the mesodermal mantle and the ventral marginal zone more to its posterior parts.

The same regions of the gastrula and also vegetal yolky tissue were cultured as explants and labelled with tritiated mannose. Their glycoprotein synthesis pattern was compared to those of the neurula tissues to which they contribute in vivo. Animal pole explants synthesized large amounts of the epidermis-specific marker epimucin. Dorsal marginal zone explants did not synthesize epimucin but did make amounts of S2 and S6 indicative of mesoderm, as well as the notochord-specific markers S2·2 and S3·2. Lateral marginal zone explants showed the same pattern as the dorsal marginal zone including the two notochord-specific markers, although they do not contribute to notochord in vivo. Ventral marginal zone explants were more variable in their behaviour. Yolky tissue from the vegetal hemisphere of the gastrula or the archenteron floor of the neurula synthesized mainly polydisperse material of high molecular weight rather than discrete glycoproteins.

The results indicate that at the early gastrula stage states of specification exist which correspond to the three germ layers, ecto-, meso- and endoderm.

The ectodermal specification of animal pole explants is quite robust and cannot easily be changed by variation of the culture conditions. However treatment with a concentrated pellet of vegetalizing factor does induce a change to mesodermal specification, which is clearly detectable in the pattern of glycoprotein synthesis. Similar inductive interactions between different regions of the early embryo are thought to occur during normal development.
Comparative effects of cathepsin inhibitors on rat embryonic development in vitro. Evidence that cathepsin D is unimportant in the proteolytic function of yolk sac.
Authors: Freeman Stuart J. , Brown Nigel A.
Published: April 2021

The effects of two proteinase inhibitors, leupeptin and pepstatin on the development of 9·5-day rat conceptuses in vitro has been studied. All cultures were of 48 h duration and the inhibitors were present throughout the entire period. When pepstatin was added to the culture medium (5–25 μg/ml) conceptuses developed and grew to an extent that did not differ from untreated controls. However, leupeptin (1–4 μg/ml) caused severe growth retardation and abnormal development of conceptuses.

The effects of the two inhibitors on the hydrolysis of 125I-labelled BSA and haemoglobin by homogenates of 10·5-day yolk sac indicated the biochemical basis for the differential toxic effects of the two inhibitors on development. Leupeptin was highly inhibitory of the degration of both substrates whereas pepstatin caused no inhibition of 125I-labelled BSA hydrolysis, and only a slight inhibition of haemoglobin hydrolysis.

These observations demonstrate that cathepsin D, a lysosomal aspartic proteinase that is specifically inhibited by pepstatin is not involved in yolk-sac-mediated protein utilization by early organogenesis-phase conceptuses and that lysosomal cysteine proteinases, specifically inhibited by leupeptin, are of paramount importance in this yolk sac function.
Developmental variability within and between mouse expanding blastocysts and their ICMs
Authors: Chisholm Julia C. , Johnson Martin H. , Warren Paul D. , Fleming Tom P. , Pickering Susan J.
Published: April 2021

We have attempted to reduce the developmental heterogeneity amongst populations of mouse blastocysts by synchronizing embryos to the first visible signs of blastocoel formation. Using embryos timed in this way, we have examined the extent of variation of inside and outside cell number and of inside cell size, nuclear DNA content and developmental potential, between and within embryos of a similar age postcavitation.

The overall impression gained is one of wide heterogeneity in inside: outside cell number ratios and in cell cycling and its relation to cavitation among embryos of similar age postcavitation. However, the simplest explanation of our results suggests that cavitation generally begins at a time when most outside cells are in their sixth developmental cell cycle and that outside cells, as a population, are a little ahead of inside cells in their cell cycling. Additionally we present evidence that, within at least some individual inner cell masses (ICM), there is intraembryo variation in the time at which inside cell developmental potential becomes restricted.
Cell interactions in preimplantation embryos: evidence for involvement of saccharides of the poly-N-acetyllactosamine series
Authors: Rastan S. , Thorpe S.J. , Scudder P. , Brown S. , Gooi H.C. , Feizi T.
Published: April 2021

Roles of cell surface carbohydrates containing the 3-fucosyl-N-acetyllactosamine and poly-N-acetyllactosamine sequences (SSEA-1 and I antigens, respectively) in the compaction of mouse embryos have been investigated using the endo-β-galactosidase of Bacteroides fragilis to modify the surface of cleavage-stage embryos. Treatment with this enzyme abolished SSEA-1 activity and diminished I antigen activity on the embryonic cell surface. Embryos cultured in the presence of endo-β-galactosidase from the 2-to 4-cell stage onwards, or treated with the enzyme at the compacting 8-cell stage, continued to compact and proceeded to form blastocysts at the normal rate. However, when compacted 8-to 16-cell embryos were experimentally decompacted in calcium-free medium, treated for 1 h with endo-β-galactosidase and returned to normal culture medium, the time taken for 50 % of the embryos to recompact was prolonged five-fold. There was an even greater delay if these embryos were maintained in culture medium containing the enzyme. Blastocysts were eventually formed under both conditions. Thus, endo-β-galactosidase did not affect compaction unless the embryos were first decompacted. On the assumption that recompaction and de novo compaction occur by similar mechanisms, we propose that carbohydrate-binding molecules are involved which have high affinities for poly-N-acetyllactosamine structures and protect them from digestion by endo-β-galactosidase.
The origin of pattern duplications in segment polarity mutants of Drosophila melanogaster
Authors: Martinez-Arias Alfonso , Ingham Philip W.
Published: April 2021

Mutations of the segment polarity group in Drosophila melanogaster produce additional denticles with reversed polarity in every segment of the larval cuticle. We have investigated the effect of mutations in different elements of the bithorax complex on the segmental identity of these additional pattern elements. Our results suggest that they are derived, primarily, from the anterior compartment of each segment.
Modulation of epidermal cell shaping and extracellular matrix during caudal fin morphogenesis in the zebra fish Brachydanio rerio
Authors: Dane P. J. , Tucker J. B.
Published: April 2021

Distinct changes in epidermal cell shaping largely define the overall pattern of growth and form during generation of the ectodermal ridge and early stages of fin fold morphogenesis. The epidermal portion of the ridge and early fin fold are formed from a strip of epidermal cells that is only six to nine cells wide. There is apparently no increase in the number of these cells during initial formation of the ridge and its subsequent conversion into a fin fold which contains extracellular matrix fibres.

Epidermal cells adopt a wedge-shaped morphology during ridge production. Distinct changes in the shaping and contact relationships between basal portions of these cells generate intercellular spaces at several discrete loci within the ridge. These spaces become continuous with each other to form a subepidermal space. Hence, the subepidermal space is not produced by straightforward folding of an epidermal sheet.

Cells flanking the sides of the ridge start to flatten as it is converted into a fin fold. A continuous row of distinctive cells is positioned along the apex of the developing fold. The term ‘cleft cells’ is suggested for these apical cells. Each cleft cell retains a wedge-shaped form during fold formation and develops a basal cleft-shaped invagination. Invaginations are aligned in neighbouring cleft cells so that these cells cap the distal boundary of the subepidermal space where collagenous extracellular fibres called actinotrichia run anteroposteriorly along the length of the fin fold. This orientation is in direct contrast to the proximodistal orientation of actinotrichia within the remainder of the subepidermal space. During early stages of fold production a temporary set of previously unreported extracellular cross fibres spans the subepidermal space at right angles to actinotrichia. These configurations of extracellular fibres could be advantageous for maintaining the structural integrity of the early fin fold.
pH affects fruiting and slug orientation in Dictyostelium discoideum
Authors: Bonner J.T. , Hay A. , John D.G. , Suthers H.B.
Published: April 2021

We have demonstrated two interesting facts about the transition from the migration stage to the final fruiting stage of Dictyostelium discoideum. One is that fruiting is favoured on acid substrata, and secondly, migrating slugs tend to migrate towards the acid side of a pH gradient. The suggestion is offered that these results can be interpreted in terms of the effects of NH3. It appears to be an additional mechanism (besides phototaxis and thermotaxis) to assure that the final fruiting takes place in a favourable environment.
The development of fused− embryos of Drosophila melanogaster
Authors: Martinez-Arias Alfonso
Published: April 2021

The mutant fused (1−59.5) belongs to a class of lethal mutations in Drosophila melanogaster that produce pattern duplications in every segment of the mature embryo. A study of the embryonic development of fused− embryos derived from fused− mothers shows that extensive cell death occurs early in development. This cell death accounts for the smaller size of the segments in fused− embryos. The pattern duplication observed is, probably, a secondary consequence of the pattern deletion.
The role of microfilaments in cranial neurulation in rat embryos: effects of short-term exposure to cytochalasin D
Authors: Morriss-Kay Gillian , Tuckett Fiona
Published: April 2021

During the late stages of cranial neurulation in mammalian embryos, the neural epithelium becomes concave. A thick subapical band of microfilament bundles, attached to junctions which are both vertical and horizontal in orientation, can be seen by TEM. Prior to this the neural epithelium is first biconvex and then V-shaped in transverse section, microfilament bundles are absent, and the subapical junctions are only vertical in orientation.

In order to determine the role of microfilaments in cranial neurulation, rat embryos were exposed to cytochalasin D (0 ·15 μgml-1) for Ih at three stages of development: convex neural fold stage, early concave (prior to midline apposition at the forebrain/midbrain junction: ‘preapposition’) and later concave (‘postapposition’). They were subsequently washed and cultured in addition-free medium for 5,12, 24 or 36h, then examined alive and by LM, TEM, or SEM.

The degree of neural fold collapse varied with the stage of development: at the convex stage there was only slight opening out of the neural groove; early concave (preapposition) neural folds collapsed laterally to a horizontal position; later concave (postapposition) neural folds showed widening of the midbrain/hindbrain neuropore and slight neuroepithelial eversion at the anterior neuropore. Neural epithelium which had been concave prior to cytochalasin D treatment changed in structure so that the cells were broader and shorter; most of the subapical junctions were vertical in orientation, and microfilament bundles were represented either as a mass of amorphous material adjacent to the junctions, or as separated and broken filaments.

Re-elevation of neural folds in ‘recovery’ cultures was accompanied by regeneration of apical microfilament bundles and horizontal junctions. Embryos which had been exposed to cytochalasin D at the convex or later concave stage of cranial neural fold development were able to complete cranial neural tube closure; none of the early-concave-stage embryos achieved apposition at the forebrain/midbrain junction, and all had major cranial neural tube defects.

The results suggest that contraction .of apical microfilament bundles plays an essential role in elevation of the neural folds and in the generation of concave curvature during the later stages of cranial neurulation. During the convex neural fold stage, microfilaments are important in maintaining neuroepithelial apposition in the neural groove, but are not crucial to maintenance of the convex shape.

Successful formation and maintenance of the forebrain/midbrain apposition point at the appropriate time is considered to be essential for subsequent brain tube closure.
Gene expression in Xenopus embryogenesis
Authors: Dawid Igor B. , Haynes Susan R. , Jamrich Milan , Jonas Erzsebet , MiyatañI Seiji , Sargent Thomas D. , Winkles Jeffrey A.
Published: April 2021

This article considers some aspects of the storage of macromolecules in the oocyte of Xenopus laevis and the activation of previously unexpressed genes during early embryogenesis. The large quantity and complex nature of poly(A)+ RNA accumulated in the egg provides the cleavage embryo with a supply of mRNA sufficient to sustain protein synthesis for several hours of development. Onset of gene activity at the midblastula transition (MBT) leads to the synthesis and accumulation of molecules of various RNA classes, including tRNAs, rRNAs, mRNAs and mitochondrial RNAs. At gastrulation the poly(A)+ RNA population is still qualitatively similar to that of the egg but some sequences not present in egg RNA have accumulated by this time. Through the use of a subtractive cDNA cloning procedure we have prepared a library of sequences that represent genes activated for the first time between MBT and gastrula. A study of several of these cDNA clones suggests that genes in this class are restricted in their activity to embryonic and tadpole stages.
Actin genes in Xenopus and their developmental control
Authors: Gurdon J. B. , Mohun T. J. , Brennan S. , Cascio S.
Published: April 2021

The results summarized here have established the temporal and regional activation of three kinds of Xenopus actin genes. The cardiac and skeletal muscle actin genes are among the first cell-type-specific genes to be expressed in early development. The first transcripts to be synthesized by these genes appear to be correctly initiated, spliced, and at once translated into proteins. Both cardiac and skeletal actin genes are strongly transcribed in the axial skeletal muscle of embryos.

The mechanism by which the cardiac actin gene is first transcribed in only the somite region of an embryo depends, at least in part, on materials already localized in the subequatorial region of a fertilized but uncleaved egg. Cells which acquire this material seem able to activate their cardiac actin genes without requiring normal contact with other cells.
Regional specificity of glycoconjugates in Xenopus and axolotl embryos
Authors: Slack J. M. W. , Cleine J. H. , Smith J. C.
Published: April 2021

This paper reviews work on the presence, synthesis and developmental regulation of glycoconjugates (proteoglycans, glycoproteins and glycolipids) in the early amphibian embryo.

In the axolotl there is little regional specificity of protein synthesis until the tailbud stage, but substantial regional specificity of glycoprotein synthesis from the end of gastrulation. Glycolipid synthesis is more uniform although a number of unique species are made in the epidermis.

Isolated explants from axolotl early gastrulae show three types of behaviour in terms of glycoprotein synthesis, corresponding to the classical germ layers. Xenopus embryos at this stage show a higher degree of mosaicism.

Changes of glycoprotein synthesis in response to mesodermal or neural induction follow the predicted course depending on the regional character of the induced tissue.

The regional binding patterns of a number of lectins and monoclonal antibodies specific for particular carbohydrate determinants are presented and their significance discussed.
Information transfer during embryonic induction in amphibians
Authors: Grunz Horst
Published: April 2021

Neural induction and differentiation has been studied using Concanavalin A, cyclic AMP, tunicamycin and calcium ionophore A 23187. Competent ectoderm of Xenopus laevis treated with Concanavalin A differentiates into neural (archencephalic) structures. Binding studies with gold-labelled ConA indicate that the superficial ectodermal layer contains fewer ConA-sensitive sites (α-D-mannoside and α-D-glucoside residues of glycoproteins) than the inner ectodermal layer. The small number of ConA-sensitive sites can be correlated with the fact that the isolated superficial ectoderm layer, in contrast to the inner layer, does not differentiate into neural structures. The gold-ConA particles bound to inner ectoderm are quickly (within 30 minutes) internalized, presumably by receptor-mediated endocytosis. However, endocytosis is not a prerequisite for neural induction. On the contrary ConA apparently must be bound to the plasma membrane for a certain period to initiate neural induction. The rapid internalization of ConA could explain why neural inductions are evoked only if ectoderm is incubated in ConA-containing medium for longer than 30 minutes.

On the other hand cyclic AMP or calcium ionophore A 23187 does not elicit neural inductions. On the contrary calcium ionophore A 23187 apparently, inhibits neural and mesodermal differentiation. This effect could be correlated with an increase of intracellular calcium level of the ectodermal target cells, which could influence the permeability of gap junctions resulting in a loss of cell communication, followed by a change of differentiation and pattern formation.
Intracellular pH and ribosomal protein S6 phosphorylation: role in translational control in Xenopus oocytes
Authors: Taylor Mark A. , Robinson Kenneth R. , Smith L. Dennis
Published: April 2021

The induction of amphibian oocyte maturation with progesterone as well as the activation of sea urchin eggs at the time of fertilization result in increased protein synthesis. The increase in both cases involves the recruitment of maternal mRNA onto polysomes. Further, it has been reported that sea urchin eggs, like full-grown Xenopus oocytes, contain no spare translational capacity based on the observation that injected heterologous mRNA is translated only at the expense of endogenous messages. The nature of the limiting component defined by such experiments is not known, but two factors which have been proposed to play a role in regulating protein synthesis are ribosomal protein S6 phosphorylation and intracellular pH. In the current paper, we review the literature and present new evidence on the roles intracellular pH and S6 phosphorylation have in regulating protein synthesis in Xenopus oocytes. We report that pHi does not increase between stage 3 and stage 6, yet the protein synthetic rate increases at least eight fold during the same period. Hence, we conclude that increasing pHi is not a prerequisite for increasing protein synthesis. Moreover, we present three arguments against increased ribosomal protein S6 phosphorylation being sufficient or necessary for increased protein synthesis in Xenopus oocytes. First, the level of S6 phosphorylation does not increase between stages 4 and 6, a period exhibiting a two to three fold increase in protein synthesis. Second, the injection of globin mRNA into stage-4 oocytes increases total protein synthesis two to three fold, but has no effect on S6 phosphorylation. Third, when the injection of globin mRNA into stage-4 oocytes is followed by an injection of MPF, a dramatic increase in S6 phosphorylation is seen, but total protein synthesis is not further stimulated.
The Transplantation of Individual Rat and Guinea-pig Whisker Papillae
Authors: Cohen J.
Published: April 2021

A method is described for the removal and transplantation of vibrissa follicle bases (end bulbs). The whole papilla may be dissected from the capsule and follicle wall, and the dermal papillae may also be dissected out. Transplants have been made to various sites. No whisker was produced from end bulbs implanted into body skin, but whiskers were produced from end bulbs in the spleen, and from both end bulbs and whole papillae under ear skin. Several generations of whisker have been produced from some of the implants, but retention of the club whisker has not been observed. It is concluded that the whisker papilla is a permanent entity in the same sense as is the feather papilla, despite follicular changes during the growth-cycle. Dermal papillae may cause local ear epidermis to invest them, and pre-sumably this produces hair of local kind, as no whisker or whisker follicle was discovered. It is suggested that the whisker papilla is a non-specific organizer, speci-ficity residing in the ectoderm concerned.