CONTROL OF SOMITE PATTERNING BY SIGNALS FROM THE LATERAL PLATE

The body musculature of higher vertebrates is composed of the epaxial muscles, associated with the vertebral column, and of the hypaxial mus cles of the limbs and ventro-lateral body wall. Both sets of muscles a rise from different cell populations within the dermomyotomal componen t of the somite. Myogenesis first occurs in the medial semitic cells t hat will form the epaxial muscles and starts with a significant delay in cells derived from the lateral semitic moiety that migrate to yield the hypaxial muscles. The newly formed somite is mostly composed of u nspecified cells, and the determination of semitic compartments toward specific lineages is controlled by environmental cues. In this report , we show that determinant signals for lateral somite specification ar e provided by the lateral plate. They result in a blockade of the myog enic program, which maintains the lateral semitic cells as undifferent iated muscle progenitors expressing the Pax-3 gene, and represses the activation of the MyoD family genes. In vivo, this mechanism could acc ount for the delay observed in the onset of myogenesis between muscles of the epaxial and hypaxial domains.