Differential regulation of epaxial and hypaxial muscle development by paraxis.

In vertebrates, skeletal muscle is derived from progenitor cell populations located in the epithelial dermomyotome compartment of the each somite. These cells become committed to the myogenic lineage upon delamination from the dorsomedial and dorsolateral lips of the dermomyotome and entry into the myotome or dispersal into the periphery. Paraxis is a developmentally regulated transcription factor that is required to direct and maintain the epithelial characteristic of the dermomyotome. Therefore, we hypothesized that Paraxis acts as an important regulator of early events in myogenesis. Expression of the muscle-specific myogenin-lacZ transgene was used to examine the formation of the myotome in the paraxis-/- background. Two distinct types of defects were observed that mirrored the different origins of myoblasts in the myotome. In the medial myotome, where the expression of the myogenic factor Myf5 is required for commitment of myoblasts, the migration pattern of committed myoblasts was altered in the absence of Paraxis. In contrast, in the lateral myotome and migratory somitic cells, which require the expression of MyoD, expression of the myogenin-lacZ transgene was delayed by several days. This delay correlated with an absence of MyoD expression in these regions, indicating that Paraxis is required for commitment of cells from the dorsolateral dermomyotome to the myogenic lineage. In paraxis-/-/myf5-/- neonates, dramatic losses were observed in the epaxial and hypaxial trunk muscles that are proximal to the vertebrae in the compound mutant, but not those at the ventral midline or the non-segmented muscles of the limb and tongue. In this genetic background, myoblasts derived from the medial (epaxial) myotome are not present to compensate for deficiencies of the lateral (hypaxial) myotome. Our data demonstrate that Paraxis is an important regulator of a subset of the myogenic progenitor cells from the dorsolateral dermomyotome that are fated to form the non-migratory hypaxial muscles.