Rôle du mésoderme somitique dans le développement du plumage dorsal chez l’embryon de Poulet:I. Origine, capacités de régulation et détermination du mésoderme plumigène

Abstract

ABSTRACT The role of somitic mesoderm in the development of dorsal plumage in chick embryos. I. Origin, regulation capacity and determination The role of somitic mesoderm in the development of the dorsal plumage has been studied in chick embryos. The operations were performed at 2–2·5 days of incubation. The replacement of a portion of somitic mesoderm by somitic mesoderm labelled with [3H] thymidine or obtained from Japanese quail embryos (whose nuclei bear distinctive specific markers) showed that cells originating from the dermatomes build up the dermis of the dorsal skin only. They do not migrate farther than approximately midway down the flank. Beyond this limit, dermal cells originate from the somatopleural mesoderm. The unsegmented somitic mesoderm is capable of extensive regulation, which leads to the development of a dorsal plumage, normal in the number and arrangement of its feathers according to the characteristic pattern of the spinal pteryla. Uni-or bilateral excision of segmented somitic mesoderm resulted in dorsal plumage deficiencies, the extent and frequency of which was related to the state of differentiation of the excised mesoderm. Thus, the excision of somites generally led to an incomplete spinal pteryla (absence of feather rows, apteria). However, the somitic mesoderm is still capable of regulation even though it has already undergone its differentiation into dermatome, myotome and sclerotome. These results show that somitic mesoderm retains its regulative capacity, even though it has already acquired its feather-forming determination. The replacement of unsegmented somitic mesoderm by various implants (agar, tantalum, gut, neural tube, somatopleural mesoderm), intended to block the regulation processes, abolished the differentiation of the spinal feathers on the operated side. In some cases, the implantation of somatopleural mesoderm resulted in the formation of a supernumerary tract. No tissue other than somitic mesoderm -not even the somatopleural mesoderm, which is normally in part feather-forming -is able to give rise to region-specific spinal pteryla dermis. The excision and replacement of somitic mesoderm prevented the differentiation of dense dermis, whereas these operations had no effect on the early histogenesis of the epidermis, with the formation of arches and anchor filaments.