1 Gonadotropin - Releasing Hormones : Why Do Fish Need Multiple Forms ?

Vertebrate skeletal muscles contain muscle fibers of several types, which can be broadly classified as slow or fast fibers, on the basis of differences in contraction speeds, metabolic activities, and motoneuron innervation. We have examined the potential roles of growth factors of members of the hedgehog and TGF-ß gene families in the formation of slow muscle fibers including muscle pioneer (MP) slow muscle cells and non-muscle pioneer (NMP) slow muscle cells in zebrafish. Overexpression of hedgehog protein in zebrafish embryos induced formation of both NMP and NMP slow muscle cells. The induction of these extra slow muscle cells was likely a result of the conversion of fast muscle precursors into slow muscle cells. In contrast, ectopic expression of dorsalin-1, a member of the TGF-ß gene family, in zebrafish notochord completely inhibited the development of MP slow muscle cells, but had no effect on the development of NMP slow muscle cells. Moreover, notochord dorsalin-1 expression inhibited the formation of MP cells induced by overexpression of hedgehog, but had no effect on the induction of NMP slow muscle cells. We propose that hedgehog and TGF-ß signals exert competing positive and negative influences on the differentiation of slow muscle cells. (Results of this study have been previously published as Du et al., 1997).