PATTERNING OF THE CRANIAL NERVES IN THE CHICK-EMBRYO IS DEPENDENT ON CRANIAL MESODERM AND RHOMBOMERIC METAMERISM

The vertebrate peripheral nervous system (PNS) consists of two groups of nerves that have a metamerical series of proximal roots along the b ody axis: the branchial and spinal nerves. Spinal nerve metamerism is brought about by the presence of somites, while that of the branchial nerves is, in part, intrinsic to rhombomeres, the segmental compartmen ts of the hind-brain. As the distribution pattern of neural crest cell s prefigures the morphology of the PNS, we constructed tissue-recombin ant chick embryos in order to determine factors that might regulate th e crest cell distribution pattern. When the segmental plate was transp lanted between the hind-blain and the head mesoderm before crest cell emigration, it developed into ectopic somites that inhibited the dorso lateral migration of crest cells such that formation of the cranial ne rve trunks was disturbed. Even so, proximal portions of the nerve root s were intact. An ectopic graft of lateral mesoderm did not inhibit th e directional migration of the crest cells, but allowed their ectopic distribution, resulting in the fusion of cranial nerve trunks. When sp inal neurectoderm was transplanted into the hind-brain, the graft beha ved like an even-numbered rhombomere and caused the fusion of cranial nerve roots. The identity of the spinal neurectoderm was preserved in the ectopic. site analyzed by the immunolocalization of Hoxb-5 protein , a spinal cord marker. We conclude that the spatial distribution of c ephalic crest cells is regulated by successive processes that act on t heir proximal and distal distribution. The migratory behavior of crest cells is achieved partly by an embryonic environment that is dependen t upon the presence of somitomeres, which do not epithelialize as somi tes, in the trunk.