In vivo regulation of somite differentiation and proliferation by Sonic Hedgehog

In vertebrates, somite differentiation is mediated in part by Sonic Hedgeho g (Shh), secreted by the notochord and the floor plate. However, Shh-null m ice display close to normal expression of molecular markers for dermomytome , myotome, and sclerotome, indicating that Shh might not be required for th eir initial induction. In this paper, we have addressed the capacity of Shh to regulate in vivo the expression of the somite differentiation markers P ax-1, MyoD, and Pax-3 after separation of paraxial mesoderm from axial stru ctures. We show that Pax-1, which is lost under these experimental conditio ns, is rescued by Shh. In contrast, Shh maintains, but cannot induce MyoD e xpression, while Pax-3 expression is independent of the presence of axial s tructures or Shh. Finally, we demonstrate that Shh is a potent mitogen for semitic cells, supporting the idea that it may serve to expand subpopulatio ns of cells within the somite. (C) 1999 Academic Press.