Autoradiographic and Golgi study on the early development of n. isthmi principalis and adjacent grisea in the chick embryo: a tridimensional viewpoint

Neurogenesis, cell migration and early histogenesis of the isthmic nuclear complex in chick embryos were investigated in autoradiographic and Golgi material. The aim of the experimental observations was to detect whether the apparent origin of different grisea of this complex at separate matrix territories (neuromeres) was accompanied by peculiar generation patterns, consistent with predictions of neuromeric theory. Differential birthday patterns were indeed obtained for a) n. semilunaris--born in the rh1 a rhombomere, b) n. isthmi principalis pars parvocellularis, nn. lemnisci lateralis dorsalis and ventralis, and n. isthmi ventralis--born in the isthmic rhombomere, and c) n. isthmi principalis pars magnocellularis--born at the m1 mesomere. Only the nuclear group at (b) shows a clear-cut gradient of generation. The morphological analysis aimed to describe isthmic neuroblast cell form before, during and immediately after migration into the mesencephalic optic lobe. Golgi data indicate that isthmic neuroblasts emerge as free cells from the matrix and aggregate into a dense superficial mantle layer. Between stages HH26 and 30, the whole mass of cells translocates tangentially in a rostrolateroventral direction, invading the m2 mesomere. The individual migrating neuroblasts have a leading axonal process which rapidly grows into the tectum in advance of the cell body, which follows at a slower pace. As the migration runs to an end the neuroblasts start to differentiate, sprouting dendritic processes. A joint origin in the isthmic mantle primordium is proposed for the nuclear group at (b) (above), whereas n. isthmi principalis pars magnocellularis is formed separatedly from the rest, and shows no tangential migratory behaviour of its neuroblasts. The complex histogenetic and morphogenetic processes at the isthmo-mesencephalic boundary may be explained on the basis of these new data, but this requires a tridimensional viewpoint that is exposed in the Discussion.

Induction of gastrulation in the chick embryo

Interaction between the epiblast and the primary hypoblast in chick blastula results in induction of the primitive streak (PS) in the epiblast. Alpha-amanitin, a specific inhibitor of poly A-containing RNA synthesis, inhibits formation of the definitive PS. This inhibition is associated with qualitative changes in the pattern of protein synthesis in the hypoblast but not in the epiblast. The protein pattern of the component areas of the epiblast shows increase in some polypeptides after treatment with alpha-amanitin. By contrast, alpha-amanitin resulted in a decrease in synthesis of several polypeptides, which are either undetectable or weakly present in the hypoblast. The alpha-amanitin-sensitive translational products of the embryonic genome that are observed in the hypoblast may have specific functions in the control of PS induction and stabilization.

Studies on the role of 1,25-dihydroxyvitamin D in chick embryonic development

Vitamin D-deficient laying hens were repleted with 25-hydroxy[26,27-3H]vitamin D3 or 1,25-dihydroxy[26,27-3H]vitamin D3. Egg production returned to normal for both groups of hens by the third week. Eggs from hens fed either 25-hydroxy[26,27-3H]vitamin D3 or 1,25-dihydroxy[26,27-3H]vitamin D3 contained 1,25-dihydroxy[26,27-3H]vitamin D3. Eggs from hens fed 25-hydroxy[26,27-3H]vitamin D3 contained substantial amounts of 25-hydroxy[26,27-3H]vitamin D3, while those from hens fed 1,25-dihydroxy[26,27-3H]vitamin D3 contained none. Plasma from 18-day embryos from hens fed 1,25-dihydroxy[26,27-3H]vitamin D3 contained little or no 1,25-dihydroxy[26,27-3H]vitamin D3, while that from 18-day embryos from hens given 25-hydroxy[26,27-3H]vitamin D3 had normal levels of 1,25-dihydroxy[26,27-3H]vitamin D3. No eggs from hens fed 1,25-dihydroxy[26,27-3H]vitamin D3 hatched, while eggs from hens fed 25-hydroxy[26,27-3H]vitamin D3 achieved a hatchability of 90%. It appears that embryos from hens maintained on 1,25-dihydroxyvitamin D3 as their sole source of vitamin D are essentially vitamin D deficient.

Three-dimensional magnetic resonance microscopy of the developing chick embryo

Magnetic resonance imaging microscopy was performed on live chick embryos. A combination of high gradient strength (0.47 mT/cm), special purpose radiofrequency coils and 3-dimensional Fourier imaging was used to obtain images with effective thickness of 1.25 mm and pixel dimensions as small as 200 mu in the live chick embryo. The signal-to-noise ratio was sufficient to allow unequivocal identification of the individual chambers of the heart, spinal cord, ventricles in the brain, and vascular structures in the liver of a live 11-day embryo. Anatomical assignment was accomplished with the aid of correlated histologic sections. Because there are no external landmarks, the plane of imaging is frequently oblique, making the 3-dimensional acquisition particularly useful.

A description of caudal migration during growth leading to the formation of the pericardial and pleural coeloms, to caudal movement of the aortic arches, and to development of the shoulder

Formation of the shoulder in the chick embryo results from movement of cells into the base of the wing from more cranial regions. Between stage 23 and stage 27, the lateral body wall overlying the pleural coelom is caused to move into the base of the wing because the cranial margin of the pleural coelom moves from the level of the tenth somite to the level of the 16th somite. Thus, formation of the shoulder is related to the formation of the pleural coelom. Textbook descriptions of the formation of the pleural coelom did not permit this relationship to be delineated. Therefore, investigation of the relationship between the formation of the shoulder and the formation of the pleural coelom seemed warranted. At stage 12 the coelom is lateral to the gut from the level of the second somite to the level of the 16th somite and is continuous with the extraembryonic coelom. During stage 13 the coelom obtains a ventral margin at the level of the third and fourth somites as the heart fuses with the lateral body wall to form the lateral mesocardium. Between stage 16 and stage 23, the second (cervical) flexure develops, causing the lateral mesocardium to form a ventral margin of the coelom from the level of the first to the 17th somites. Between stage 16 and stage 23, the coelom becomes ventral to the pharynx and lung to the level of the ninth somite. From stage 23 to stage 27, the lateral body wall from the cranial margin of the lung to the caudal margin of the hyoid arch moves in a caudal direction. During stage 27, the lateral body wall fuses with the lateral mesocardium to form the pleuro-pericardial membrane. The wing remains ventral to the 16th to 19th somites, causing a pileup of cells at the cranial base of the wing, the shoulder.

Regulation of extraembryonic calcium mobilization by the developing chick embryo

During development, the chick embryo mobilizes the calcium it needs from two extraembryonic sources, first the yolk and then the eggshell. Since previous studies have strongly suggested that vitamins D and K may regulate chick embryonic calcium metabolism, we have examined here how these vitamins might be involved in regulating the calcium mobilization processes. We used as our experimental system chick embryos which were maintained in long-term in vitro culture in the absence of the eggshell. Our results showed that exogenous vitamin D3, in the form of the active 1,25-dihydroxylated metabolite, was hypercalcaemic in both control embryos and the calcium-deficient, shell-less embryos. Since the eggshell was absent in the latter, the vitamin D-induced hypercalcaemia must involve mobilization of calcium from the yolk and, or, the embryonic skeleton. The latter was unlikely since concomitant hyperphosphataemia was not observed. By radiolabelling the yolk with 45Ca2+ and subsequently monitoring its distribution, we showed that vitamin D3 stimulated yolk calcium mobilization. However, exogenous vitamin D3 did not appear to influence the calcium uptake activity of the chorioallantoic membrane (CAM), the tissue responsible for translocating eggshell calcium. On the other hand, when embryos were rendered vitamin K deficient by the administration of its antagonist, Warfarin, CAM calcium activity was significantly depressed, an effect which was remedied by vitamin K supplementation. We conclude that, during normal chick embryonic development, vitamin D is primarily involved in regulating yolk calcium mobilization whereas vitamin K is required for eggshell calcium translocation by the CAM.

Vascular system in the developing wing bud of normal and talpid mutant chick embryos

The vascular limb mesoderm probably plays a prominent role in limb pattern formation in both normal and talpid chick embryos. The differential vascularization, or its consequences, is a factor controlling the initial stages of differentiation in the developing bud. The axial artery runs from the subclavian artery to the distal region of the normal limb bud, whereas in the talpid3 only secondary blood vessels develop. In the talpid, the gene permits the chondrogenic regions to grow and at the same time keeps the peripheral regions to a certain size. The mesenchyme tissue lies within the effective range of a metabolic gradient extending from either the ectodermal surface or the peripheral vessels to the limb axis.

The patterns on chondrogenesis of cells from facial primordia of chick embryos in micromass culture

Chondrogenesis of mesenchymal cells from the frontonasal mass, mandibles and maxillae of stage-24 chick embryos has been investigated in micromass (high-density) cultures. Distinct differences in the amount and pattern of cartilage differentiation are found. In cultures of frontonasal mass cells, a central sheet of cartilage develops; in cultures of mandible cells, less cartilage differentiates and nodules form; while in cultures of maxillae cells, virtually no chondrogenesis takes place. The same patterns of cartilage are found in cultures established from stage-20 embryos. At stage 28, frontonasal mass cultures form cartilage nodules and the number of nodules in mandible cultures is markedly decreased. There are striking parallels between the chondrogenic patterns of cells from the face and limb buds in micromass culture. The frontonasal mass cell cultures of stage-20 and -24 chick embryos resemble those established from the progress zone of limb buds. The progress zone is an undifferentiated region of the limb in which positional cues operate. Cultures established from the frontonasal mass of stage-28 chick embryos and from the mandibles of all stages resemble cultures of whole limb buds. These contain a mixture of committed and uncommitted cells. Ectoderm from facial primordia locally inhibits chondrogenesis in micromass cultures and this could provide a positional cue. The differences in chondrogenic potential of cells from facial primordia may underlie the specific retinoid effects on the frontonasal mass.

Evaluation of the roles of intrinsic and extrinsic factors in occlusion of the spinal neurocoel during rapid brain enlargement in the chick embryo

The spinal neurocoel normally occludes during the second day of chick embryogenesis as the lateral walls of the spinal cord become apposed closely in the midline. Concomitantly, the brain initiates its rapid and substantial enlargement. Occlusion, although short-lived, might play a major role in brain enlargement. As a result of occlusion, the brain ventricles are sealed off from the external milieu prior to closure of the posterior neuropore, establishing a closed fluid-filled system. The present study focuses on the mechanisms of occlusion of the spinal neurocoel. We tested two postulated intrinsic factors (microtubule-mediated neuroepithelial cell elongation and microfilament-mediated apical neuroepithelial cell constriction) and five extrinsic factors (three mediad pushing forces generated by the somites, perineural extracellular matrix and expanding surface ectoderm; and two stretching forces generated either vertically by pulling of the elongated notochord or longitudinally by elongation of the embryo) in maintaining occlusion. Our results suggest that occlusion is maintained by other, untested intrinsic factors and/or by forces generated within a perineural collar, composed of cellular and extracellular materials, intimately associated with the basal aspects of the spinal cord. Cytoskeletal-mediated changes in cell shapes, pushing forces and vertical and longitudinal tensions are not involved. Further studies are needed to examine the intrinsic properties of the neuroepithelium and the factors initiating occlusion and reopening.

Transcriptional inhibition in early chick embryos as a result of polyamine depletion

In the early chick embryo, inhibition of polyamine synthesis by alpha-difluoromethylornithine (DFMO), an enzyme-activated irreversible inhibitor of ornithine decarboxylase, blocks development at gastrulation. This effect was paralleled by a marked suppression of RNA and protein synthesis. There was no major change in cell cycle distribution in DFMO-treated embryos. Nevertheless, analysis of DNA synthesis and mitotic index indicated a prolongation of the cell cycle, possibly affecting all the phases. The inhibition of RNA synthesis in polyamine-depleted embryos, as evaluated by [3H]uridine incorporation, was not a result of reduced uptake or expansion of the UTP pool, and there was no deficiency or major imbalance among the ATP, GTP, and CTP pools. On the basis of agarose gel electrophoretic analyses of the various RNA species, and experiments using RNA synthesis inhibitors with different modes of action (actinomycin D, alpha-amanitin, and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole), it was concluded that the DFMO-induced gastrular arrest was due to general inhibition of transcription.

Localized vascular regression during limb morphogenesis in the chicken embryo: II. Morphological changes in the vasculature

The regression of blood vessels in the distal wing bud of chicken embryos from stages 19 to 31 was examined by light and electron microscopy. The vessels were double-labelled by an injection of Monastral blue B (MB) to label the regressing endothelial cells, followed 6-48 hours later with an injection of India ink which marked the lumens of patent vessels. Prior to stage 26 the vessels contained only India ink since the endothelial cells were not phagocytic at this stage. Vessels at stage 26 or later were often double-labelled, with MB sequestered in the endothelial cell cytoplasm and India ink in the vessel lumens. After stage 27 cells not associated with lumens, but labelled with MB, were observed in areas undergoing vascular regression. Ultrastructural changes in the endothelial cells as the vessels regressed included formation of luminal and abluminal processes, long complex junctions, and vacuoles containing MB. In many involuting vessels the endothelial cells appeared normal even though the lumens were collapsed. Occasionally, isolated pyknotic cells were observed in regions that had been previously vascularized. At stage 31 cells in the developing cartilage had vacuoles containing MB. Our study suggests that blood vessels may disappear from the prechondrogenic zone of the distal wing bud by several mechanisms. These could include a type of cell death that does not elicit a cellular infiltrate, migration of the endothelial cells away from vascularized regions, and/or transdifferentiation into cells that resembled chondrocytes.

Localized vascular regression during limb morphogenesis in the chicken embryo. I. Spatial and temporal changes in the vascular pattern

Morphogenesis of the vertebrate limb bud depends upon reciprocal interactions between epithelial and mesenchymal tissues. A characteristic limb vascular pattern is essential for normal limb outgrowth. The vascular pattern in the distal portion of the wing bud was examined by ink injection and compared to the sites of cartilage differentiation, as indicated by [35S]-incorporation. During development, avascular areas arose in three distinct locations within the vascularized mesoderm. These areas corresponded to the distal skeletal elements, referred to as digits 2, 3, and 4. Incorporation of radioactive sulfate was high in the avascular areas and low in the adjacent vascular tissue. Examination of autoradiographs of ink-injected limbs suggested that the appearance of avascular regions preceded the accumulation of sulfated cartilage matrix. These results indicate that remodeling of the limb vasculature is related to the formation of the skeletal pattern.

Autoshaped discrimination learning in chicks incubated under normobaric hyperoxia

White Leghorn chicken eggs were exposed to either 60% O2 or room air (21% O2) for the first 19 days of incubation. Chicks that hatched from these eggs were then tested in discrimination learning tasks in which keypecking was autoshaped to colored lights that were paired with either access to food (Experiment 1) or heat (Experiment 2). Chronic prenatal exposure to 60% O2 reduced hatchability but did not affect mean hatching time. Although previous research has shown that hyperoxic treatment accelerates growth in chick embryos until the 18th day of incubation, experimental chicks weighed either the same (Experiment 1) or less (Experiment 2) than controls at hatching. Prenatal exposure to hyperoxia depressed rate of acquisition, but not final performance level in both discrimination tasks. The initial performance deficit appeared to reflect a temporary depression of activity or arousal, possibly due to a relatively greater hypothermia in experimental chicks. This general pattern of results was attributed to premature depletion of essential nutrients within the egg as a result of oxygen-induced growth acceleration.

Changes in plasma amino acids in the developing chick

We have sampled arterial blood from chicken embryos during development and measured the changes in plasma amino acids from mid-gestation to hatching. During gestation, several amino acids rise to a peak concentration at 16 days and fall prior to hatching. After hatching, most amino acids fall, although the plasma concentrations of aspartate, glutamate and taurine rise significantly.

Development of uterine chick embryos after storage at 5 degrees C for 15 hours

Eggs obtained from the uterus of hens by hormonal administration 14 to 10 h before expected oviposition were kept at 5 degrees C for 15 h. Forty-two of these uterine eggs were incubated as whole eggs; in the case of a further 70 eggs the blastoderms were isolated and incubated whilst immersed in liquid egg albumen. Thirty-two of the eggs incubated whole (76%) developed to the primitive streak stage, but only 6 of the incubated blastoderms (9%) reached this stage of development.

Mapping of the early neural primordium in quail-chick chimeras. I. Developmental relationships between placodes, facial ectoderm, and prosencephalon

Defined fragments of the anterolateral neural ridge and of the associated region of the neural plate of presomitic to three-somite stage quail embryos were grafted isotopically and isochronically into chick hosts. This resulted in the development of apparently normal brain and facial structures to which the contribution of the grafted tissue could be observed by means of the quail nuclear marker. It was shown that the anterolateral neural ridge contains the progenitor cells of the adenohypophyseal and olfactory placodes and also of the superficial ectoderm lining the nasal cavity and conchae and the superficial ectoderm of the beak. When the appropriate region of the neural ridge was involved in the quail-chick substitution, the egg tooth was made up of graft-derived cells. Grafting of the neural plate area adjacent to the "ridge" territory containing the placodal ectoderm revealed that the presumptive region of the hypothalamus is in contiguity with that of the adenohypophyseal placode. The same observation was made for the olfactory placode and the floor of the telencephalon from which the olfactive bulb later develops.

Induction of chick embryo feather malformations by an influenza C virus

The effect of influenza C virus, strain JJ/50, on the development of chicken embryos infected at 10 or 12 days was documented by microscopic techniques, as well as by gross observations of embryos or chicks at hatching. The infected, newly hatched chicks displayed marked abnormalities in their feathering. Such abnormalities were observed neither in mock-infected embryos nor in embryos injected with virus which had been previously treated with specific influenza C virus antibody. At a microscopic level, the abnormalities apparently are a result of hypertrophy and/or hyperplasia of the developing barb and barbule cells. Further, the additional development of integumental necrotic foci was correlated with the development of relatively high viral titers (greater than 256) as measured by hemagglutination (HA). Embryos infected after 12 instead of 10 days incubation showed normal feathering at hatching. Infection at 12 days, however, was correlated with the development of relatively low viral titers (HA = 4) and limited degeneration of the respiratory epithelium. The relationship of teratogenic effects to the site of viral replication in rapidly differentiating tissue is discussed.

Modification of the N-linked oligosaccharides in cell surface glycoproteins during chick embryo development. A using lectin affinity and a high resolution chromatography study

Important differences in asparagine-linked glycopeptides were observed in vitro cultured fibroblasts derived from chick embryo at different stages of development. Cells from 8-day and 16-day embryos were labeled metabolically with [3H]mannose. Cell surface glycopeptides obtained after mild trypsin treatment were extensively digested with pronase and then chromatographed on concanavalin-A-Sepharose and other immobilized lectins. The most important changes concerned the complex type chains. The ratio between triantennary plus tetraantennary and biantennary chains increased about 2.5-fold from the 8th to the 16th day of development. In the same way, complex chains with bisecting N-acetylglucosamine increased from 8-day to 16-day cells as shown by Phaseolus-vulgaris-erythroagglutinin--agarose chromatography. In 16-day cells, the majority of triantennary chains (60%) with alpha-linked mannose substituted at C2 and C6 positions and biantennary chains (50%) were shown to contain fucosyl (alpha 1----6)N-acetylglucosaminyl structure in the core region by their ability to bind to a lentil lectin affinity column. Similarly, in 8-day cells, triantennary chains (50%) were more fucosylated than biantennary chains (35%). Thus, complex structures exhibited an increased fucosylation of their invariable core from the 8th to the 16th day of development, except for fucosylated triantennary chains which were retained on Phaseolus vulgaris Leucoagglutin and on lentil lectin. These latter structures were present at the surface of 8-day cells and absent at the surface of 16-day cells. After chromatography on Bio-Gel P6 and treatment with endo-beta-N-acetylglucosaminidase H, the [3H]-mannose-labeled glycopeptides were separated by high resolution chromatography into glycopeptides with complex chains and glycopeptides with high-mannose chains. Analysis of the high-mannose oligosaccharides released after endo-beta-N-acetylglucosaminidase H treatment by chromatography on Bio-Gel P4 indicated that the same type of high-mannose chains were present at the surface of 8-day and 16-day cells. Quantification of mannose, galactose and sialic acid residues using gas liquid chromatography was consistent with a decrease of the relative amount of oligomannose chains and an increase of the relative amount of complex type chains in 16-day cells compared to 8-day cells. Thus N-linked oligosaccharides derived from cell surface glycoproteins undergo changes during embryo development resulting in greater complexity of carbohydrate chains.

Morphometric and autoradiographic analysis of frontonasal development in the chick embryo

Dimensional changes in the nasal processes were measured in chick embryos from Hamburger and Hamilton (1951) stages 20 through 27.5. Transverse measurements in the frontonasal region of freshly fixed embryos were compared to frontal sections of the nasal region of comparably staged embryos. These observations were correlated with autoradiographic studies of cell movement employing an implant labeling technique. Morphometric analysis indicated that between stages 20 and 25 the separation of the nasal pit orifices increased coincidentally with rapid forebrain enlargement. Since the separation of the nasal pit fundi increased more rapidly, the orientation of the nasal pits changed. Autoradiographic studies indicated that lateral movement of medial nasal process mesenchyme into the base of the nasal groove and medial area at the base of the lateral nasal process had occurred. After stage 25, the separation of the nasal orifices declined dramatically, coincidental with rapid orbital enlargement. In contrast, the separation of the nasal pit fundi was maintained. It is proposed that nasal development of the chick embryo may be governed initially by forebrain enlargement and associated lateral movements of mesenchyme in the medial nasal processes, resulting in reorientation of the invaginating nasal placodes; subsequently, orbital enlargement and an associated medial redirection of growth of the lateral nasal processes assumes greater significance to the continued development of the frontonasal region.

A quantitative analysis of the effect of all-trans-retinoic acid on the pattern of chick wing development

Small, positively charged beads that slowly release known amounts of all-trans-retinoic acid have been implanted below the apical ectodermal ridge at the anterior margin (opposite somite 16) of wing buds of 3 1/2 day-old chick embryos. The continuous release of retinoic acid is shown to create an anteroposterior concentration gradient of retinoic acid in the limb field that is stable with time, despite the fact that this compound is metabolized by the limb tissue. With beads that release increasing amounts of retinoic acid, the normal 234 digit pattern is progressively altered to a 2234, to a 32234, and then to a 432234 pattern. The tissue concentrations of all-trans-retinoic acid required to change the digit pattern in this way range between 1 and 25 nM. When the same amounts of retinoic acid are released from posteriorly implanted beads (placed below the apical ectodermal ridge opposite somite border 19/20 or somite 20), the normal digit pattern is unaffected. Implantations of beads that release all-trans-retinoic acid are thus identical in their effect to grafts of cells from the limb polarizing region, which cause similar dose-dependent changes in the digit pattern when grafted to the anterior margin of the bud (but not when grafted opposite somites 19 or 20). Because of the low concentrations of retinoic acid required for its biological effect, the graded response observed, and the fact that a concentration gradient is established across the limb field, all-trans-retinoic acid closely mimics the putative morphogen that has been postulated to be emitted by polarizing region cells during normal development.

Cell surface glycosaminoglycans (GAGs) of cultured chick fibroblasts. Modifications in relation to the stage of embryo development

We have investigated the changes in glycosaminoglycan (GAG) composition between cultured fibroblasts derived from 8- and 16-day chick embryos. GAG composition has been studied after [3H]glucosamine and [35S]sulfate labeling. Both the 8- and 16-day embryo fibroblasts were found to contain hyaluronic acid (HA), dermatan sulfate (DS), heparan sulfate (HS) and chondroitin sulfates (CS), the latter being the major component in 8- and 16-day cells. These four GAGs were quantified after their separation using cellulose acetate electrophoresis. The amounts of HA and CS were respectively shown to increase 2-fold and 4-fold between the 8th and 16th day of development, whereas the amounts of HS and DS resp. diminished 2.5-fold and 1.2-fold. These results show that the relative proportions of the different GAGs alter during embryo development. The fibroblasts from 8-day-old embryos detached more rapidly from the culture dishes than the cells from 16-day-old embryos when treated with trypsin. However, this difference was not directly related to the different GAG content.

Formation and perforation of closing plates in the chick embryo

The morphology of the closing plates between adjacent pharyngeal arches was examined in chick embryos between stages 11 and 21 (Hamburger-Hamilton). Each closing plate is formed by apposition between the basal surfaces of portions of the pharyngeal pouch endoderm and the ectoderm of the overlying pharyngeal cleft. Initial contact between ectoderm and endoderm occurs at several small points which are separated by regions containing mesenchymal cells and extracellular material. Contact between the opposed epithelia is made by extension of cellular processes through the intervening basal laminae and extracellular space. Endodermal and ectodermal cells then interdigitate to create a cellular layer which rapidly thins. The interposed extracellular material is sequestered into small pools as cellular interdigitation proceeds. Perforations form through certain regions of closing plates 1-3 and persist during the stages studied. Small slit-like depressions appear between cells of the closing plate just prior to perforation. The initial perforations enlarge until they are separated only by thin cellular strands. These strands presumably rupture, leaving small cellular accumulations which persist for a short time marking the junction between ectoderm and endoderm along the walls of adjacent pharyngeal arches. Clear evidence of cell degeneration is rare. These results suggest that cellular reorganization, rather than cell death, is a major mechanism of initial perforation.

Preventive effect of ascorbic acid against glucocorticoid-induced cataract formation of developing chick embryos

Glucocorticoid administration to developing chick embryos is known to promote cataract formation with a decreasing level of glutathione in the lens. To gain further understanding of this process, the level of ascorbic acid, a biological antioxidant, in the lenses was measured during the course of glucocorticoid treatment. When 0.25 mumol of hydrocortisone hemisuccinate sodium (HC) were administered to 15-day-old chick embryos, the level of ascorbic acid in the lens began to decline after 30 hr and became around 40% of the control value at 48 hr after HC treatment. At this time about 90% of the lenses showed opacity in the nuclear region. However, the level of ascorbic acid in the cataractous lens recovered to the control level at 96 hr, a time when the lens has recovered from cataract formation. A triple application of ascorbic acid (20 mumol/egg) at 3, 10 and 20 hr after HC treatment significantly prevented lens opacification. The administration of ascorbic acid prevented the decline of ascorbic acid content and partially that of glutathione content in the lens caused by HC.

The vasculature and limb development

The developing vascular pattern of the embryonic chick limb results from a combination of two properties: the intrinsic self-assembly and branching properties of the vascular cells and the extrinsic information associated with the expanding mitotic population of mesenchymal cells; and the inhibitory factors which restrict the entrance of vessels into particular domains and/or decrease the branching frequency of such vessels. It is hypothesized that an important component of limb pattern formation is the interplay between the dividing population of mesenchymal cells and the intrinsic properties of the vascular cells. It is further asserted that the presence of particular vascular elements may, indeed, be 'positional information'. Two examples are cited involving aspects of limb duplication to support this possibility; it is suggested that vascular vessel size of a host limb may dictate the polarity of duplication events. The presented hypothesis emphasizes that the interplay between the intrinsic properties of self-assembly into tissues and extrinsic factors which establish boundaries and morphologies is involved in both vascular and limb pattern formation.