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.