The effects of experimental unilateral anotia on skull development in the chick embryo. III. Chondrocranial development in anotic embryos of 7-20 days of incubation.

The study of the development of of the chondrocranium in chick embryos with unilateral (right-sided) anotia revealed the following main characteristics. 1. The median axes of the chordal and the prechordal part of the cranial base are not in a straight line but show a deviation toward the right side. The angle between the two axes has its vertex in the region of the foramen hypophyseos. 2. The metotic cartilage and the foramina of the IXth and Xth cranial nerves are normal in position. 3. The tectum synoticum develops later and to a lesser extent than normal. 4. Between the basal plate, the metotic cartilage, the occipital arch and the supracapsular cartilage a foramen is formed which, later in development, is closed by outgrowths of the metotic cartilage and the basal plate. 5. The "optic area" shows a practically normal appearance which indicates that the cartilaginous ventral wall of the lagenal capsule is of basal plate origin. 6. The pro-otic process develops practically normal and, hence, is independent of the ear capsule. 7. The quadrate cartilage and the right lower jaw are displaced ventro-posteriorward. The earliest development of the perichondral bones shows some particularities which are closely correlated with the development of the various cartilaginous structures.     

Postparietal and prehatching ontogeny of the supraoccipital in Alligator mississippiensis (Archosauria, Crocodylia)

The first record of the postparietal bone of Alligator mississippiensis, documented by transverse histological sections, is presented. It is the first evidence of the presence of this bone within Recent reptiles. The postparietal is present in a specimen with a head length of 32.3 mm. The bone is a small dermal plate lying ventrally and posteriorly to the posterior margin of the parietal and dorsally to the trabecular bone, forming a dorsal surface of the supraoccipital portion of the neural endocranium. The trabecular bone develops perichondrally from the dorsal surface of the tectal cartilaginous bridge spanning between the dorsal portions of the otic capsules and occipital pilae. The bridge probably represents the fused tectum synoticum posterior plus tectum posterius. Later in ontogeny, the bridge ossifies endochondrally. The endochondrally ossifying bridge together with its perichondrally ossifying trabecular bone form the future supraoccipital. The trabecular bone is the integral part of the cranial endoskeleton and ontogenetically distinct from the dermal postparietal bone.     

Effects of Enucleation on High-Affinity Binding Sites in Chick Optic Tecta

Abstract: The effect of unilateral eye extirpation on the development of the chick optic tectum has been studied in both the embryo and the newly hatched chick. Although the prevention of normal afferentation of the embryonic tectum retarded its growth, there appeared to be a significant increase of muscarinic acetylcholine binding site in the noninnervated tectum. This phenomenon was repeated also in the posthatch denervated system wherein the functioning optic nerve is severed. A significant increase in the number of binding sites as well as reduced dissociation constant of the interactions of this receptor with [³H]quinuclindinyl benzilate was found in the deafferented optic tectum. This may suggest the presence of a denervation-supersensitivity-like modulation. Similar increases were not detected with other binding sites studied in either the noninnervated embryonic or deafferented posthatch optic lobes. The possibility that acetylcholine is a primary neurotransmitter of the optic system is discussed.     

The differing effects of occipital and trunk somites on neural development in the chick embryo

In all higher vertebrate embryos the sensory ganglia of the trunk develop adjacent to the neural tube, in the cranial halves of the somite-derived sclerotomes. It has been known for many years that ganglia do not develop in the most cranial (occipital) sclerotomes, caudal to the first somite. Here we have investigated whether this is due to craniocaudal variation in the neural tube or crest, or to an unusual property of the sclerotomes at occipital levels. Using the monoclonal antibody HNK-1 as a marker for neural crest cells in the chick embryo, we find that the crest does enter the cranial halves of the occipital sclerotomes. Furthermore, staining with zinc iodide/osmium tetroxide shows that some of these crest-derived cells sprout axons within these sclerotomes. By stage 23, however, no dorsal root ganglia are present within the five occipital sclerotomes, as assessed both by haematoxylin/eosin and zinc iodide/osmium tetroxide staining. Moreover, despite this loss of sensory cells, motor axons grow out in these segments, many of them later fasciculating to form the hypoglossal nerve. The sclerotomes remain visible until stages 27/28, when they dissociate to form the base of the skull and the atlas and axis vertebrae. After grafting occipital neural tube from quail donor embryos in place of trunk neural tube in host chick embryos, quail-derived ganglia do develop in the trunk sclerotomes. This shows that the failure of occipital ganglion development is not the result of some fixed local property of the neural crest or neural tube at occipital levels. We therefore suggest that in the chick embryo the cranial halves of the five occipital sclerotomes lack factors essential for normal sensory ganglion development, and that these factors are correspondingly present in all the more caudal sclerotomes.     

The effects of experimental unilateral anotia on skull development in the chick embryo. V. The development of the brain and its meningeal envelope in embryos of 9-20 days of incubation.

In a total number of 27 normal and 27 unilaterally anotic chick embryos, varying in age from 9-19 days, the morphogenetic relationships between the neurocranium on the one hand and the brain and its meningeal envelope on the other were studied. The results showed that, in general, unilateral anotia slightly interferes with the development of the bain. Only the homolateral nucleus tangentialis and cerebellar auricle proved to be underdeveloped. However, the brain of the anotic embryos progressively develops asymmetrically: In an antero-posterior and a ventro-dorsal direction abnormal flexures are present, and various components of the brain on the anotic side undergo changes in shape and position. These changes in brain morphology are interpreted as secondary features. Hence, the conclusion is drawn that the neurocranium, through the intermediary of the meningeal envelope, controls--or at least is capable to control--the general morphological development of the brain.     

Vasculogenesis in the chick embryo optic tectum

The initial formation and further development of the intraneural blood vessel network in the tectum opticum of the chick from the 4th to the 14th incubation day have been analyzed and some quantitative data morphometrically recorded. Vessels have been filled by intracardial injection of India ink in vivo. As inferred from our previous investigations on the vasculogenesis of several districts of the central and peripheral nervous system in the chick embryo, also in the developing optic tectum growth and distribution pattern of the vessels seem to unfold step by step under the local influence of earlier occurring morpho-histogenetic processes of the corresponding neural substratum.     

The development of the chondrocranium of the lacertid lizard,Acanthodactylus boskiana

The trabeculae cranii are at first quite separate from each other, after few days their anterior two fifths are connected by a trabecular plate which is obliterated throughout development. The paired origin of the parachordal plate is not observed. The fused posterior orbital cartilages chondrify in the form of a wide short plate, traversed by the oculomotor and trochlear nerves. The basicranial fenestra and fenestra ovalis are formed by the degeneration of pre-existing cartilage. The cochlear portion is completely fused with the parachordal plate from the very beginning. The elements of the pterygoquadrate are fused together. The quadrate and Meckel's cartilage are in close contact from the very beginning. While the lower part of the interorbital septum is derived from the trabecula communis, its upper part is derived from the anterior orbital cartilages. The lateral parts of the fused posterior orbital cartilages give rise to most of the taeniae and pilae of the orbitotemporal region. There is only one commissure between the auditory capsule and parachordal plate. A cartilaginous connection between the distal portion of the columella auris and ceratohyal persists for some time. The parietotectal and paranasal cartilages are fused together from the very beginning. The processus paroticus originates from the columella auris. In the fully formed stage the notochord is completely embedded in the occipital condyle. The union between the condyle and odontoid process persists. The auditory capsules and occipital arches contribute to the formation of the tectum synoticum plus posterius. The prefacial commissure and facial foramen lie in front of the cochlear portion. The columella auris possesses a processus internus (connected with the quadrate), but the processes a dorsalis has completely disappeared. The orbitotemporal region is quite complete. A medial fenestra is formed in the planum supraseptale. A fenestra is observed in each of the interorbital and nasal septa. The lamina transversalis anterior is fused with the parietotectal cartilage. A complete zona annularis is present. The outer wall of the paranasal cartilage is perforated by a large fenestra lateralis. The parietotectal and paranasal cartilages and the posterior process of the lamina transversalis anterior contribute to the formation of the concha nasalis. There is a contact between the planum antorbitale and nasal septum. The pterygoid process has disappeared. The common characters of the lacertid chondrocranuium are deduced.     

Analysis of a repulsive axon guidance molecule, draxin, on ventrally directed axon projection in chick early embryonic midbrain

The mesencephalic V neurons and tectobulbar axons in chick embryo project over long distances that appear during the early development of the chick optic tectum. The mesencephalic V neuron and tectobulbar axonal growth begin at Hamburger and Hamilton stage 14 and stage 18, respectively. Both fibers proceed downward from the dorsal to the ventral side of the lateral wall of the optic tectum and then turn caudally and join the medial longitudinal fasciculus. Their axons appear in the most superficial layer of the tectum at early stages and do not cross the dorsal midline of the tectum. Here, we report the role of draxin, a recently identified axon guidance protein, in the formation of the ventrally directed tectum axonal tracts in chicken embryo. draxin is expressed in a high dorsal to low ventral gradient in chick optic tectum. In vitro experiments show that draxin repels neurite outgrowth from dorsal tectum explants. In vivo overexpression resulted in inhibition or misrouting of axon growth in the tectum. Therefore, draxin may be an important member of the collection of repulsive guidance molecules that regulate the formation of the ventrally directed tectum axon tracts.     

In vitro development of the hamster and chick secondary palate

A series of experiments were undertaken to compare the in vitro behaviour of the medial edge epithelium (MEE) of hamster, in which palatal shelves normally fuse, and chick, in which they do not fuse. Homotypic pairs of hamster and chick embryo palatal processes, single palatal processes, and heterotypic palatal shelves of both animals were grown in vitro. The results indicated that contact between palatal shelves may not be crucial for MEE differentiation in mammals. The ability to acquire pre-fusion characteristics may be present in mammalian palatal tissue from their early development and may be expressed by cessation of DNA synthesis in the MEE, elevation of cAMP, and MEE cell death. Isolated chick palatal shelf cultured under identical conditions did not express these mammalian pre-fusion characteristics. When MEE of hamster and chick palatal shelves were placed in contact with one another, the intervening epithelia underwent cytolysis. This could be due to either the destruction of chick MEE by lysosomal enzymes liberated from adjacent degenerating hamster MEE cells, or by induction of cell death in chick MEE by hamster mesenchyme. Heterotypic palatal tissue combinations also suggest that release of lysosomal enzymes in the hamster MEE, which leads to its dissolution, may be the terminal event in epithelial differentiation prior to the establishment of mesenchymal continuity. It is suggested that an inverse relationship exists between DNA synthesis and cAMP levels during palatogenesis: when palate closes (as in mammals) the MEE is eliminated by increasing cAMP levels, whereas when palate remains open (as in birds) low level of cAMP preserve the integrity of MEE by supporting DNA synthesis.     

Anatomy of the fully formed chondrocranium of Podocnemis unifilis (Pleurodira: Podocnemididae)

This study describes the anatomy of the chondrocranium of Podocnemis unifilis (Pleurodira, Podocnemididae), based on recently hatched specimens, and cleared and double‐stained specimens. The orbitotemporal region is dramatically different from those observed for other species of turtles in that the: (1) planum supraseptale is greatly reduced and present only as tiny projections on the posterodorsal margin of the interorbital septum, (2) pila metoptica is free from all neighbouring structures and bifurcates distally, (3) pila antotica is greatly reduced, (4) foramina for optic nerve, ophthalmic artery and oculomotor nerves are open dorsally by virtue of this species lacking the taenia marginalis and taenia medialis, and (5) tectum synoticum is present and invested dorsally by the supraoccipital, despite the fact that this bone forms by replacement of the supraoccipital. The unique morphology of the pila metoptica is explained either as de novo formation of processes on the terminus of this cartilage or by retention of portions of the taenia medialis (anteriorly) and pila antotica or pila accessoria (posteriorly). Variation in the orbitotemporal region presented here is discussed for two other pleurodiran turtles (Phrynops hilarii and Emydura subglobosa) and briefly compared with the anatomy observed in Cryptodira.     

The ontogeny of the chondrocranium in Clarias gariepinus: trends in siluroids

The ontogeny of the chondrocranium of 31 different stages of the African catfish Clarias gariepinus (Siluroidei: Clariidae) was studied, both from cleared and stained, and sectioned material. The fish ranged from 4.1 (1 day post-hatching) to 127.0 mm SL (100 days post-hatching). The chondrocranium of C. gariepinus seemed to correspond to the general adaptive trends in siluroids, especially in relation to the reduction of eye size and the dorso-ventral flattening of the skull. The platybasic neurocranium involved several modifications related to the trabecular bars, the hypophyseal fenestra, the ethmoid region and even the olfactory nerves. Certain reductions were present, which have been observed in all siluroids (e.g. absence of the pila lateralis, the commissura lateralis, the myodomes) or are part of a variable trend within siluroids (e.g. reduction of the taenia marginalis anterior and the tectum synoticum). Compared with some other siluroid species, the neurocranium of C. gariepinus is well developed, for example in the otic region. The same was observed in the splanchnocranium where some general siluroid trends persist (e.g. isolation of palatine from pterygoquadrate, presence of 'hyo-symplectic-pterygoquadrate' plate). Some trends, as observed in other siluroids, were present also (e.g. interhyal continuous with suspensorium and ceratohyal, Meckel's cartilage initially continuous with the suspensorium). The branchial basket is well developed as all expected elements are present (basibranchials I-IV, hypobranchials I-IV, ceratobranchials I-V, epibranchials I-IV). Based on the observed ontogeny of C. gariepinus and data from the literature, a hypothesis was formulated which indicated the presence of a general reductional trend within siluroids. In C. gariepinus , all four (I-IV) infrapharyngobranchials develop, although the anterior two are much reduced and fused with each other.     

Ganglioside, protein, hexose, and sialic acid changes in the trisected optic tectum of the chick embryo.

Spatiotemporal changes in membrane constituents of cells from the optic tectum of the chick embryo were analyzed during the period of maximum differentiation and synaptogenesis. Each tectum from 6-, 8-, 10-, and 12-day embryos was cut into three subregions along the topological gradient of differentiation. Electrophoretic analysis of proteins revealed an already complex population by Day 6 which remained relatively unchanged through later stages, with little if any topological variations. In contrast, chromatographic analysis of gangliosides showed an increasingly complex pattern as differentiation proceeded, with a growing preponderance of multisialogangliosides. Total membrane protein increased symmetrically with tissue mass in each subregion. However, hexose concentration and sialic acid/hexose ratios showed strikingly asymmetrical topological distributions as early as Day 8, and tended to fluctuate reversibly within brief (1 day or less) time periods. These results suggest that during the period of maximal differentiation and retino-tectal synaptogenesis in the optic tectum of the chick, the membrane protein population remains relatively stable and topologically invariant, whereas the polysaccharide chains of membrane macromolecules fluctuate according to topological position and developmental state in a complex, relatively rapid, and apparently oscillatory fashion.     

The Prosencephalon Has the Capacity to Differentiate into the Optic Tectum: Analysis by Chick-Specific Monoclonal Antibodies in Quail-Chick-Chimeric Brains

The alar plate of the prosencephalon of the quail embryo was heterotopically transplanted into the alar plate of the mesencephalon of the chick embryo at the 7–10 somite stage. Chick and quail cells in chimeric brains were distinguished after Feulgen-Rossenbeck staining and/or immunohistochemical staining with a species specific monoclonal antibody MAb-37F5 which recognized cytoplasmic components of chick brain cells. Neural connections between the transplant and the host were studied by monoclonal antibodies, MAb39-B11, which recognizes a species specific antigen on chick nerve fibers, and MAb-29B8, which reacts to 160 kD neurofilaments of both chick and quail.
When the transplant was completely integrated into the host mesencephalon, the transplant developed a laminar morphology closely resembling that of the optic tectum. Immunohistochemical staining with MAb-39B11 showed that the host optic nerve fibers innervated both the host tectum and the tectum-like transplant. However, optic nerve fibers did not invade transplants that failed to develope a laminar structure characteristic of the tectum. These findings suggest that the prosencephalon has a capacity to differentiate into the optic tectum at the 7–10 somite stage.     

Ipsilateral retinal projections into the tectum during regeneration of the optic nerve in the cichlid fish Haplochromis burtoni: a Dil study in fixed tissue.

Retinal projections were experimentally manipulated in a bony fish to reveal conditions under which considerably enlarged ipsilateral projections developed and persisted. Three experimental groups were studied: animals after unilateral enucleation, after unilateral nerve crush, and after enucleation and crush of the remaining optic nerve. At 29 days after unilateral enucleation alone, no enhanced ipsilateral projection had developed. After nerve crush, however, large numbers of retinal fibers regenerated into the ipsilateral tectum. Retrogradely filled, ipsilaterally projecting ganglion cells were distributed throughout the entire retina. After 15 days regenerating retinal fibers covered the entire ipsilateral tectum. At later stages the ipsilateral projection showed progressive reduction in coverage of the tectum. Combining enucleation with nerve crush led to an ipsilateral projection that covered the tectum at 28 days and later. In this experimental situation the development of an ipsilateral projection appears to be a two-step process: (1) Fibers are rerouted to the ipsilateral side at the diencephalon, and (2) ipsilateral fibers persist in the tectum only in the absence of a contralateral projection while they appear to be eliminated in the other cases.     

Distinct spatiotemporal roles of hedgehog signalling during chick and mouse cranial base and axial skeleton development

The cranial base exerts a supportive role for the brain and includes the occipital, sphenoid and ethmoid bones that arise from cartilaginous precursors in the early embryo. As the occipital bone and the posterior part of the sphenoid are mesoderm derivatives that arise in close proximity to the notochord and floor plate, it has been assumed that their development, like the axial skeleton, is dependent on Sonic hedgehog (Shh) and modulation of bone morphogenetic protein (Bmp) signalling. Here we examined the development of the cranial base in chick and mouse embryos to compare the molecular signals that are required for chondrogenic induction in the trunk and head. We found that Shh signalling is required but the molecular network controlling cranial base development is distinct from that in the trunk. In the absence of Shh, the presumptive cranial base did not undergo chondrogenic commitment as determined by the loss of Sox9 expression and there was a decrease in cell survival. In contrast, induction of the otic capsule occurred normally demonstrating that induction of the cranial base is uncoupled from formation of the sensory capsules. Lastly, we found that the early cranial mesoderm is refractory to Shh signalling, likely accounting for why development of the cranial base occurs after the axial skeleton. Our data reveal that cranial and axial skeletal induction is controlled by conserved, yet spatiotemporally distinct mechanisms that co-ordinate development of the cranial base with that of the cranial musculature and the pharyngeal arches.     

Apoptosis during chick inner ear development: some observations by TEM and TUNEL techniques

In order to clarify the occurrence, distribution and possible role of apoptosis during inner ear development, the ultrastructural aspects (by TEM) (at 9-19 incubation day and 1 day after hatching) and the distribution of the apoptotic phenomenon (by the TdT-mediated dUTP nick end-labeling technique), were studied in the crista ampullaris of chick embryo at 5-19 days of incubation to hatching and of postnatal 1-day old chick. We found, in the sensorial epithelium, dark supporting cells in chick embryos and mainly dark hair cells in postnatal chicks, both with ultrastructural features consistent with those of apoptosis. The presence of apoptotic phenomena was confirmed by the TUNEL technique. According to our findings, it is hypothesized that apoptosis in the inner ear may be involved: 1) at first, in macroscopic remodelling of the membranous labyrinth in early developmental stages, 2) later, in the correct differentiation of the hair and of the supporting cells, leading to characteristic cellular pattern formation and 3) finally, in physiological cell turnover of the postnatal chicken sensorial epithelium of the crista.     

gamma-Butyrobetaine as a specific antagonist for carnitine in the development of the early chick embryo

The effect of γ-butyrobetaine alone and with the addition of carnitine on the development of the early excised chick embryo has been studied. γ-Butyrobetaine in appropriate amounts exerts an inhibitory effect which can be relieved or annulled by the inclusion of appropriate amounts of carnitine. This has been interpreted as a metabolite-antimetabolite relationship, in which the normal metabolite, carnitine, is antagonized by the structurally closely related γ-butyrobetaine, and is regarded as evidence of an important role of carnitine in the metabolism of the developing chick embryo.     

Ipsilateral retinal projections into the tectum during regeneration of the optic nerve in the cichlid fish Haplochromis burtoni: A dil study in fixed tissue

Retinal projections were experimentally manipulated in a bony fish to reveal conditions under which considerably enlarged ipsilateral projections developed and persisted. Three experimental groups were studied: animals after unilateral enucleation, after unilateral nerve crush, and after enucleation and crush of the remaining optic nerve. At 29 days after unilateral enucleation alone, no enhanced ipsilateral projection had developed. After nerve crush, however, large numbers of retinal fibers regenerated into the ipsilateral tectum. Retrogradely filled, ipsilaterally projecting ganglion cells were distributed throughout the entire retina. After 15 days regenerating retinal fibers covered the entire ipsilateral tectum. At later stages the ipsilateral projection showed progressive reduction in coverage of the tectum. Combining enucleation with nerve crush led to an ipsilateral projection that covered the tectum at 28 days and later. In this experimental situation the development of an ipsilateral projection appears to be a two-step process: (1) Fibers are rerouted to the ipsilateral side at the diencephalon, and (2) ipsilateral fibers persist in the tectum only in the absence of a contralateral projection while they appear to be eliminated in the other cases. © 1992 John Wiley & Sons, Inc.