In amniotes, muscle progenitors develop from the dermomyotome, an epithelium that overlays the somite. These myogenic progenitors bring about muscle growth and are well characterised; however, in teleosts these precursors have, until now, not been identified. Now, Devoto and colleagues use lineage-tracing and gene-expression studies to identify a population of cells that resides at the anterior border of zebrafish somites as the elusive myogenic precursors(). Surprisingly, these pax7-expressing precursors remain undifferentiated before they migrate to the outer lateral surface of the somite, where they initiate myotome growth. Unusually, anterioposterior somite patterning can, as shown in this study, generate subsequent myogenic precursor mediolateral patterning. While some progenitor cells remain at the surface of the myotome, others penetrate the slow-twitch muscle layer, moving between cells, to contribute to fast-twitch musculature. Even though the occurrence of external myogenic precursors is now shown to be conserved among vertebrate lineages, the origin and movement of the teleost precursor cells is unexpectedly different.
