Fig. 9. Coherent development of the DM epithelium followed by the myotome and dermis, from the entire mediolateral aspect of the dorsal somite. The entire mediolateral aspect of the dorsal epithelial somite (T0) proportionally grows to give rise to the DM epithelium (T1 and T2). Despite local differences in morphogenetic traits, active growth occurs in at least three consecutive domains (medial, central and lateral as illustrated by the green, blue and red colors, respectively). Further dissociation of the DM into dermis (T3) also comprises a contribution of epithelial precursors from all three domains. This coherent growth pattern also applies for the development of the myotome, which occurs by progressive intercalation of fibers along the mediolateral aspect of the segment, as they originate in all four DM lips and contribute fibers in a regionalized manner (see text for references). An exception to this uniform and regionalized contributions are the pioneer myofibers (yellow), which (despite of originating at the medial epithelial somite) finally span the entire mediolateral extent of the growing myotome (yellow circles). Initial dermis dissociation from the entire mediolateral DM already occurs, while the latter is largely confined to the epaxial domain. During later development, while the expanding myotome and accompanying VLL colonize the somatopleura to give rise to hypaxial muscles (intercostal and abdominal), the somite-derived dorsal dermis remains restricted to the epaxial domain. LPM, lateral plate mesoderm.

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