[Contribution of somite cells to the development of posterior limb buds in mice].

1. The structural changes accompanying the early genesis of the mouse hindlimb bud have been studied in serial Epon thick sections at five successive somitic stages collected during the 10th developmental day. 2. As soon as the 14-somite stage, increased proliferative activities occurring in a limited area of the embryonic somatopleure close to the caudal end of the coelomic cavity point out the onset of limb morphogenesis. From its very beginning on, up to its ceasing at the 27-somite stage, the somatopleural proliferation is more active in the caudal part of the prospective limb territory and in the whole area, it appears stronger ventrally than dorsally. 3. Discrete structural changes occur during this period in the mesoderm interposed between the hindlimb bud territory and the overlying unsegmented sheet of somitic mesoderm, in relation with the genesis of the mesonephritic constituents and of the posterior cardinal vein. A detailed analysis of this area in serial sections reveals that the unsegmented somitic mesoderm does not provide any cell to the limb bud mesoderm. 4. As soon as the somitic mesoderm becomes organized into segmented somites, obvious histological changes located on the lateral aspect of the ventro-lateral somitic edge reveal that numerous undifferentiated somitic cells invade the proximal area of the limb bud mesoderm. Starting at the 27-somite stage in the first two metameres adjacent to the precordonal portion of the limb bud, this processus extends to the five somites of the limb territory at the 33-somite stage. In each metamere, the area of cell migration takes place near the caudal border of the somite and, from one somite to the other, the number of migrating cells increases in a cephalo-caudal direction. No sign of a similar cell migration was observed in the somites located cranially or caudally to the hindlimb bud. 5. The results are discussed in relation with the various experimental and descriptive evidences obtained in non-mammalian Vertebrates about the contribution of the somites to limb morphogenesis and about the factors involved in the onset of cell proliferation in the prospective limb mesoderm.