Identification of cells that give rise to slow and fast muscle during postembryonic growth in zebrafish

Vertebrate muscle development begins when signals from adjacent tissues pattern the paraxial mesoderm. In zebrafish, Hedgehog signaling from the notochord induces the most medial cells in the paraxial mesoderm to become adaxial cells, which subsequently differentiate into embryonic slow muscle fibers. We propose that lateral presomitic cells give rise to embryonic fast muscle (1) and a population of germinal cells which will generate new slow and fast muscle fibers during postembryonic growth. The dorsal and ventral regions of the myotome express muscle regulatory factors and have a higher percentage of proliferating cells, suggesting that the dorsal and ventral extremes are the site of germinal cells (2). These germinal zones can be identified in living embryos by position and morphology. Small-diameter slow and fast fibers are found adjacent to the germinal zones suggesting that germinal zone cells give rise to both cell types. To test this hypothesis we are ablating and fate mapping these cells. Hedgehog induces the differentiation of paraxial mesoderm into embryonic slow muscle fibers. Overexpression of Hh induces all of the paraxial mesoderm to differentiate into adaxial-derived slow muscle. We have shown that the overexpression of Hh leads to the loss of germinal zone cells. 1. S. H. Devoto et al., 1996, Development 122, 3371–3380; 2. M. J. F. Barresi et al., 2001, Curr. Biol. 11, 1432–1438.