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Cryptic emergence of craniofacial secondary myoblasts

Most maps of myogenesis in amniote embryos focus on the emergence and movements of primary myoblasts. The assumption that progenitors of secondary myoblasts develop congruently has been shown inaccurate in the trunk, where these cells emerge spatially and temporally apart from their predecessors. Two approaches were used to examine this possible dichotomy in the head of chick embryos. First, patterns of gene expression both within and surrounding precursors of extra‐ocular muscles (EOMs) were probed in situ. Next, the effects on myogenesis of elevated periocular fgf8 levels were assessed. Expressions of pitx2, hgf, sema3a, and myoR revealed a population of myf5/myoD‐negative muscle lineage‐committed cells located within paraxial mesoderm spatially and temporally behind aggregated myf5/myoD‐positive primary myocytes en route to their terminal locations. Treatment with fgf8, applied by implanting protein‐releasing beads, retarded primary myogenesis and resulted in the ectopic differentiation of the myoR‐positive muscle progenitors described above. These results suggest the presence of secondary EOM myoblasts that become committed early to the muscle lineage but remain spatially separate and molecularly cryptic during the period of initial EOM differentiation and morphogenesis. Supported by NIH grant EY01597.