The third wave of myotome colonization by mitotically competent progenitors: regulating the balance between differentiation and proliferation during muscle development

The myotome is formed by a first wave of pioneer cells originating from the entire dorsomedial region of epithelial somites and a second wave that der ives from all four lips of the dermomyotome but generates myofibers from on ly the rostral and caudal edges. Because the precedent progenitors exit the cell cycle upon myotome colonization, subsequent waves must account for co nsecutive growth. In this study, double labeling with CM-DiI and BrdU revea led the appearance of a third wave of progenitors that enter the myotome as mitotically active cells from both rostral and caudal dermomyotome edges. These cells express the fibroblast growth factor (FGF) receptor FREK and tr eatment with FGF4 promotes their proliferation and redistribution towards t he center of the myotome, Yet, they are negative for MyoD, Myf5 and FGF4, w hich are, however, expressed in myofibers. The proliferating progenitors first appear around the 30-somite stage in ce rvical-level myotomes and their number continuously increases, making up 85 % of total muscle nuclei by embryonic day (E)4, By this stage, generation o f second-wave myofibers, which also enter from the extreme lips is still un der way. Formation of the latter fibers peaks at 30 somites and progressive ly decreases with age until E4, Thus, cells in these dermomyotome lips gene rate simultaneously distinct types of muscle progenitors in changing propor tions as a function of age. Consistent with a heterogeneity in the cellular composition of the extreme lips, MyoD is normally expressed in only a subs et of these epithelial cells. Treatment with Sonic hedgehog drives most of them to become MyoD positive and then to become myofibers, with a concurren t reduction in the proportion of proliferating muscle precursors.