NEURAL-TUBE CAN INDUCE FAST MYOSIN HEAVY-CHAIN ISOFORM EXPRESSION DURING EMBRYONIC-DEVELOPMENT

We investigated the role of the neural tube in muscle cell differentia tion in developing semitic myotome of chick embryo, particularly throu gh fast myosin heavy chain (MHC) isoform expression. An embryonic fast MHC labeled with EB165 mAb was expressed in semitic cells from stage 15 of Hamburger and Hamilton (H.H.) (24 somites). Moreover, a distinct early embryonic fast MHC was expressed only from stage 15 of H.H. to stage 36 (E10). Like neonatal MHC, this isoform was labeled with 2E9 m Ab but differed in its immunopeptide mapping. Expression of EB165-labe led embryonic fast MHC occurred in semitic myotomes deprived of neural tube influence by in ovo ablation as well as in somite explants cultu red alone in vitro. Conversely, ablation of the neural tube prevented semitic expression of MHC labeled with 2E9 mAb. The neural tube induce d in vitro expression of this MHC in explants of somites which failed to express it when cultured alone. These results indicate that signals emanating from the neural tube are required for the expression of ear ly embryonic fast MHC isoform in developing semitic myotome.