G. Couly

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 angiogenic potentials of the cephalic mesoderm and the origin of brain and head blood vessels

We have used two molecular markers to label blood vessel endothelial cells and their precursors in the early avian embryo. One marker, called Quek1, is the avian homologue of the mammalian VEGF receptor flk-1 and the other is the MB1/QH1 monoclonal antibody. Quek1 is expressed in a subset of mesodermal cells from the gastrulation stage. Quek1 positive cells later form blood vessel endothelial cells and express the MB1/QH1 antigen which is specific for endothelial and hemopoietic cells of the quail species. These two markers allowed us first to show that the cephalic paraxial mesoderm has angiogenic potentials which are much more extended than its trunk counterpart (the somites). Secondly, the origin of the endothelial cells lining the craniofacial and head blood vessels was mapped on the 3-somite stage cephalic mesoderm via the quail-chick chimera technique, in which well defined mesodermal territories are exchanged between stage-matched embryos of both species in a strictly isotopic manner. We found that the anterior region of the cephalic paraxial mesoderm is largely recruited to provide the forebrain and the upper face with their vasculature. This means that large volumes of tissues are vascularized by a discrete region of the cephalic mesoderm, the fate of which is otherwise to give rise to muscles. The widespread expansion of the angiogenic cells arising from the anterior paraxial mesoderm must be related to the high growth rate of the anterior region of the neural primordium, yielding the telencephalon and of the neural crest-derived facial structures which are themselves devoid of angiogenic potencies.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of the SOX10 gene during human development

SOX10, a new member of the SOX gene family, is a transcription factor defective in the Dom (Dominant megacolon) mouse and in the human Shah‐Waardenburg syndrome. To help unravel its physiological role during human development, we studied SOX10 gene expression in embryonic, fetal, and adult human tissues by Northern blot and in situ hybridization. As in mice, the human SOX10 gene was essentially expressed in the neural crest derivatives that contribute to the formation of the peripheral nervous system, and in the adult central nervous system. Nevertheless, it was more widely expressed in humans than in rodents. The spatial and temporal pattern of SOX10 expression supports an important function in neural crest development.

Patterning of the hyoid cartilage depends upon signals arising from the ventral foregut endoderm

Hyoid bone is a part of the visceral skeleton which arises from both Hox‐expressing (Hox+) and Hox‐nonexpressing (Hox‐) cephalic neural crest cells. In a previous work, we have demonstrated that the Hox‐ neural crest domain behaves as a naïve entity to which the ventral foregut endoderm confers patterning cues to specify the shape and orientation of the nasal and mandibular skeleton. By using ablation and grafting approaches, we have extended our study to the formation of the hyoid bone and tested the patterning ability of more caudal levels of the lateroventral foregut endoderm in the chick embryo at the early neurula stage. In this study, endodermal stripes have first been delineated according to the projection of mid‐ and posterior rhombencephalic structures. The extirpation of endodermal transverse stripes along the anteroposterior axis selectively hampers the formation of the ceratobranchials and epibranchials. Thus defined, the patterning ability of the endodermal stripes was further explored in their medial and lateral parts. When homotopically engrafted on the migration pathway of cephalic neural crest cells, ventromedial zones of endoderm lead to the formation of supernumerary basihyal and basibranchial, while lateral zones generate additional cartilaginous pieces recognizable as ceratobranchial and epibranchial. Taken together, our data demonstrate that, early in development, the ventral foregut endoderm exerts a regionalized patterning activity on the cephalic neural crest to build up the primary facial and visceral skeleton in jaws and neck and enable a map of the endodermal skeletogenic areas to be drawn. This map reveals that a cryptic metamerization of the anterior foregut endoderm precedes the formation of the branchial arches. Developmental Dynamics 228:239–246, 2003.

Cephalic ectodermal placodes and neurogenesis

Abstract In the vertebrate embryo, the primary neural anlage, or neural plate, develops from the superficial ectoderm as a result of an inductive stimulus arising from the chordomesoderm. Although it is well established that the neural plate itself gives rise to CNS, the fate of those cells located at the junction of the neural and superficial ectoderm (also called neural ridges or neural folds), from which the neural crest and placodes are derived, has been the subject of controversy. Tracing these cells during ontogeny has been made possible by using the quail/chick chimaera system. Such studies have revealed the contribution of the neural ridge and the neurogenic placodes to various cephalic structures, and have allowed their relationships with the CNS to be followed from the early stages of neurulation throughout the whole period of morphogenesis.

Rhombomères, code Hox, crête neurale et malformations de la face

Les recherches realisees sur lembryon doiseau au moyen des chimeres caille-poulet ont montre que les structures faciales et hypobranchiales sont derivees de la crete neurale. Les cellules issues de cette structure transitoire expriment le meme code Hox que celles du rhombomere dont elles sont issues. Ce code est defini par lexpression de genes a homeobox selon une combinatoire determinee, caracteristique de chaque rhombomere. Lalteration experimentale du code Hox a certains stades critiques du developpement provoque des malformations de la region branchiorhombomerique correspondante