SOMITIC TOPOGRAPHY, SCLEROTOMAL DIFFERENTIATION AND VERTEBRAL DEVELOPMENT IN THE EARLY CHICK EMBRYO

SUMMARY Following previous work on the three-dimensional morphology of the somites and developing spinal ganglia, sclerotomal differentiation and the first phase of vertebral development were studied by typing the somitic cellular territories using L.M. and T.E.M. for the analysis of 30 chick embryos (from the 51st hour to the 5th day of incubation). Two distinct territories were identified in the ventro-medial portion of the somite: the undifferentiated somitic mesoderm, loose, with polymorphous cells, located in the cranial half of the somite; the sclerotome, dense, with smaller and ovoidal cells, located in the caudal half of the somite and with a trend to extend in a ventro-medial and caudal direction. Moreover, it was possible to reconstruct the mechanism of the formation of the vertebral body, after the fusion, around the notochord, of homologous sclerotomes, situated from the beginning of their differentiation at a more caudal level with respect to the myotomes and spinal nerves. Rejecting the theo...

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