[Somite patterning and segregation of different somite lineages].

The somite is a transient embryonic mesodermal structure, found only in vertebrates. In amniotes, somites give rise to the dermis of the back, to the striated skeletal muscles of the trunk and limbs and to the vertebral column and ribs. Segregation of these different lineages is linked to the establishment of two somitic polarity axes, a dorso-ventral one and a medio-lateral one. While the establishment of the former relies essentially on extrinsic cues, that of the latter obeys to both intrinsic and extrinsic mechanisms. Concerning the environmental cues regulating the establishment of both axes, somitic regionalisation results from antagonistic or combinatorial influences mediated by diffusible factors, such as Sonic Hedgehog, Wnt and Bmp-4, that act in gradients.