The dermomyotome dorsomedial lip drives growth and morphogenesis of both the primary myotome and dermomyotome epithelium

The cellular and molecular mechanisms that govern early muscle patterning i n vertebrate development are unknown. The earliest skeletal muscle to organ ize, the primary myotome of the epaxial domain, is a thin sheet of muscle t issue that expands in each somite segment in a lateral-to-medial direction in concert with the overlying dermomyotome epithelium. Several mutually con tradictory models have been proposed to explain how myotome precursor cells , which are known to reside within the dermomyotome, translocate to the sub jacent myotome layer to form this first segmented muscle tissue of the body . Using experimental embryology to discriminate among these models, we show here that ablation of the dorsomedial lip (DML) of the dermomyotome epithe lium blocks further primary myotome growth while ablation of other dermomyo tome regions does not, Myotome growth and morphogenesis can be restored in a DML-ablated somite of a host embryo by transplantation of a second DML fr om a donor embryo, Chick-quail marking experiments show that new myotome ce lls in such recombinant somites are derived from the donor DML and that cel ls from other regions of the somite are neither present nor required, In ad dition to the myotome, the transplanted DML also gives rise to the dermomyo tome epithelium overlying the new myotome growth region and from which the mesenchymal dermatome will later emerge, These results demonstrate that the DML is a cellular growth engine that is both necessary and sufficient to d rive the growth and morphogenesis of the primary myotome and simultaneously drive that of the dermomyotome, an epithelium containing muscle, dermis an d possibly other potentialities.