Patterning of neural crest derivatives in the avian embryo: in vivo and in vitro studies.

In this article we review recent findings from our laboratory on the developmental fate of the neural crest as studied in the avian embryo using the quail-chick marker system in vivo. Quail-chick chimeric experiments carried out at the late neurula stage have revealed the contribution of the neural crest to the skull vault, i.e., the frontal and parietal bones, previously thought to be mesodermally derived. At early developmental stages, the fate of the cephalic mesoderm (free of neural crest cells) could be analyzed. The striking result was that the mesoderm does not contribute to the head and face dermis except in the occipital and otic areas where the skull is derived from the cephalic and somitic mesoderm. Thus, the neural crest forms the dermis, the membrane and cartilage bones of the skull vault, the skull basis, and the face. The limit of the mesoderm-derived skeleton in the skull basis is located in the sella turcica between the basipre- and the basipostsphenoid and coincides with the tip of the notochord. We thus define a "chordal" and an "achordal" skull, the latter being all derived from the neural crest. These results are discussed in the framework of the "New Head" concept of Gans and Northcutt (1983). The second part of this review deals with the role of the environment in the morphogenesis and diversification of neural crest derivatives. The role of the rostrocaudal heterogeneity of the somites in establishing the metameric pattern of the truncal neural crest derivatives is analyzed. The respective contributions of the "in embryo" and in vitro approaches to our understanding of the neural crest cell differentiating potentialities are reviewed. It is pointed out that the search for survival and proliferation factors acting locally on neural crest derivatives when they are wandering and/or settling in various embryonic locations constitutes the new challenge for further understanding their complex patterning and the highly diversified variety of their phenotypes.

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.

Mapping of the early neural primordium in quail-chick chimeras. II. The prosencephalic neural plate and neural folds: implications for the genesis of cephalic human congenital abnormalities.

Mapping of the avian neural primordium was carried out at the early somitic stages by substituting definite regions of the chick embryo by their quail counterpart. The quail nuclear marker made it possible to identify precisely the derivatives of the grafted areas within the chimeric cephalic structures. A fate map of the prosencephalic neural plate and neural folds is presented. Moreover the origin of the forebrain meninges from the pro- and mesencephalic neural crest is demonstrated. In the light of the data resulting from these experiments, we present a rationale for the genesis of malformations of the face and brain and of congenital endocrine abnormalities occurring in man.

The triple origin of skull in higher vertebrates: a study in quail-chick chimeras.

We have used the quail-chick chimera technique to study the origin of the bones of the skull in the avian embryo. Although the contribution of the neural crest to the facial and visceral skeleton had been established previously, the origin of the vault of the skull (i.e. frontal and parietal bones) remained uncertain. Moreover formation of the occipito-otic region from either the somitic or the cephalic paraxial mesoderm had not been experimentally investigated. The data obtained in the present and previous works now allow us to assign a precise embryonic origin from either the mesectoderm, the paraxial cephalic mesoderm or the five first somites, to all the bones forming the avian skull. We distinguish a skull located in front of the extreme tip of the notochord which reaches the sella turcica and a skull located caudally to this boundary. The former ('prechordal skull') is derived entirely from the neural crest, the latter from the mesoderm (cephalic or somitic) in its ventromedial part ('chordal skull') and from the crest for the parietal bone and for part of the otic region. An important point enlighten in this work concerns the double origin of the corpus of the sphenoid in which basipresphenoid is of neural crest origin and the basipostsphenoid is formed by the cephalic mesoderm. Formation of the occipito-otic region of the skeleton is particularly complex and involves the cooperation of the five first somites and the paraxial mesoderm at the hind-brain level. The morphogenetic movements leading to the initial puzzle assembly could be visualized in a reproducible way by means of small grafts of quail mesodermal areas into chick embryos. The data reported here are discussed in the evolutionary context of the 'New Head' hypothesis of Gans and Northcutt (1983, Science, 220, 268-274).

The cephalic neural crest provides pericytes and smooth muscle cells to all blood vessels of the face and forebrain.

Most connective tissues in the head develop from neural crest cells (NCCs), an embryonic cell population present only in vertebrates. We show that NCC-derived pericytes and smooth muscle cells are distributed in a sharply circumscribed sector of the vasculature of the avian embryo. As NCCs detach from the neural folds that correspond to the future posterior diencephalon, mesencephalon and rhombencephalon, they migrate between the ectoderm and the neuroepithelium into the anterior/ventral head, encountering mesoderm-derived endothelial precursors. Together, these two cell populations build a vascular tree rooted at the departure of the aorta from the heart and ramified into the capillary plexi that irrigate the forebrain meninges, retinal choroids and all facial structures, before returning to the heart. NCCs ensheath each aortic arch-derived vessel, providing every component except the endothelial cells. Within the meninges, capillaries with pericytes of diencephalic and mesencephalic neural fold origin supply the forebrain, while capillaries with pericytes of mesodermal origin supply the rest of the central nervous system, in a mutually exclusive manner. The two types of head vasculature contact at a few defined points, including the anastomotic vessels of the circle of Willis, immediately ventral to the forebrain/midbrain boundary. Over the course of evolution, the vertebrate subphylum may have exploited the exceptionally broad range of developmental potentialities and the plasticity of NCCs in head remodelling that resulted in the growth of the forebrain.

The developmental fate of the cephalic mesoderm in quail-chick chimeras.

The developmental fate of the cephalic paraxial and prechordal mesoderm at the late neurula stage (3-somite) in the avian embryo has been investigated by using the isotopic, isochronic substitution technique between quail and chick embryos. The territories involved in the operation were especially tiny and the size of the transplants was of about 150 by 50 to 60 microns. At that stage, the neural crest cells have not yet started migrating and the fate of mesodermal cells exclusively was under scrutiny. The prechordal mesoderm was found to give rise to the following ocular muscles: musculus rectus ventralis and medialis and musculus oblicus ventralis. The paraxial mesoderm was separated in two longitudinal bands: one median, lying upon the cephalic vesicles (median paraxial mesoderm--MPM); one lateral, lying upon the foregut (lateral paraxial mesoderm--LPM). The former yields the three other ocular muscles, contributes to mesencephalic meninges and has essentially skeletogenic potencies. It contributes to the corpus sphenoid bone, the orbitosphenoid bone and the otic capsules; the rest of the facial skeleton is of neural crest origin. At 3-somite stage, MPM is represented by a few cells only. The LPM is more abundant at that stage and has essentially myogenic potencies with also some contribution to connective tissue. However, most of the connective cells associated with the facial and hypobranchial muscles are of neural crest origin. The more important result of this work was to show that the cephalic mesoderm does not form dermis. This function is taken over by neural crest cells, which form both the skeleton and dermis of the face. If one draws a parallel between the so-called "somitomeres" of the head and the trunk somites, it appears that skeletogenic potencies are reduced in the former, which in contrast have kept their myogenic capacities, whilst the formation of skeleton and dermis has been essentially taken over by the neural crest in the course of evolution of the vertebrate head.