Rôle du mésoderme somitique dans le développement du plumage dorsal chez l'embryon de Poulet

The role of somitic mesoderm in the development of dorsal plumage in chick embryos. II. Regionalisation. Transplantation and inversion experiments were performed on the somitic mesoderm of 2- to 2·5-day chick embryos in order to study the role of regional and axial determinations in the development of the dorsal plumage. The transposition of a piece of somitic mesoderm from the posterior cervical region (where the spinal pteryla is narrow) to the thoraco-lumbar region (where it is wide) leads to a local and unilateral narrowing of the spinal pteryla at the operation site. Conversely, the transposition of somitic mesoderm from the thoraco-lumbar region to the posterior cervical region results in a local and unilateral widening of the spinal pteryla. Consequently at the time of operation the segmented or not yet segmented somitic mesoderm is already determined to give rise to a definite transverse level of the spinal pteryla. The inversion of the cephalo-caudal polarity of a piece of somitic mesoderm without the ectodermal covering, or of a portion of the axial organs deprived of the overlying ectoderm has no effect on the orientation of feather filaments and feather rows. In contrast, the inversion of the cephalo-caudal polarity of a portion of the axial organs together with the overlying ectoderm results in the development of feathers growing in a cephalad direction and feather chevrons opening towards the head of the embryo. The inversion of the dorso-ventral polarity of a piece of somitic mesoderm does not prevent the normal differentiation of feathers in the operated region. The inversion of the medio-lateral polarity of a piece of unsegmented somitic mesoderm has little effect on the development of the spinal pteryla. On the contrary, the medio-lateral inversion of a chain of somites precludes the formation of the feathers at the level of operation. The somitic mesoderm, even when segmented, is endowed with extensive regulative capacity of its axes, except for the medio-lateral polarity, which is fixed irreversibly at the time of segmentation. The regional determination of the feather-forming somitic mesoderm is acquired at an early stage, at any rate before segmentation. However, at a given transverse level of the cephalo-caudal axis, the somitic cells remain totipotent as concerns their histo-genetic destiny (dermatome, myotome, or sclerotome) until after the onset of segmentation.