Fig. 3

pax3 and fast MyHC gene expression show reciprocal regulation. Wild-type (A, D, G, J), smu(smo)^-/- (B, E, H, K), or yot(gli2)^-/- (C, F, I, L) embryos were analyzed for pax3 (A, B, C, G, H, I) and fast Myosin Heavy Chain (MyHC_f: D, E, F, J, K, L), expression. By the 19-somite stage (A–F), when pax3 expression in somite anterior cells is down-regulated in wild-type embryos (A), expression of MyHC_f begins to span the full anterior–posterior extent of the somite (D). In contrast, segmented expression of pax3 is maintained in anterior cells within somites of smu(smo) mutants (B), while expression of MyHC_f is only found in the posterior cells of each somite (E). Arrowheads in panels B and E indicate the anterior cells of the somite, which in smu(smo) mutants express pax3 but not MyHC_f. In yot(gli2) embryos (C, F), the segmented expression of pax3 is also down-regulated (C), and MyHC_f is expressed in the posterior part of the somite. By the 24-somite stage (G–L), expression of MyHC_f has increased in wild-type, smu(smo), and yot(gli2) mutants (J–L). However, in smu(smo) mutant embryos cells in the anterior of each somite still do not express MyHC_f(K), and the segmental pax3 expression is still maintained (H). In yot(gli2) mutant embryos, anterior cells of each somite down-regulate pax3 and up-regulate MyHC_f similar to wild type (I). The arrow indicates the horizontal myoseptum where muscle pioneers, not expressingMyHC_f, are present in wild type. All panels are anterior to the left, dorsal up; all images are of somites 7–11. Scale bar = 100 μm.