Myotome formation: A multistage process

The epaxial muscles of the body are localized in a dorsomedial position wit h respect to the axial structures, attach to the vertebral column and are c oncerned with maintenance of posture and movements of the vertebral column. The epaxial musculature derives from the myotome, a transient embryonic st ructure whose formation is initiated at the epithelial somite stage and is accomplished following complete dissociation of the epithelial dermomyotome . Recent results suggest that myotome development is a multistage process, characterized by addition of sequential waves of muscle progenitors. A firs t wave originates along the medial part of the epithelial somite and gives rise to a primary myotomal structure, a second wave arises from the rostral and caudal lips of the epithelial dermomyotome and from the dorsomedial li p, which contributes indirectly through the rostral and caudal edges, and a third wave which is composed of mitotically active resident progenitors ac counts for significant growth of the myotomal mass and for its transition i nto epaxial muscle. In this review we discuss the origin, migration and kno wn cellular and molecular features that characterize each wave of progenito rs that colonize the myotome.