Publisher Summary This chapter (1) shows that heterogeneity exists among myogenic cells and (2) suggest that this heterogeneity may play an important role in both muscle histogenesis and pathology. It discusses the evidence for the existence of different classes of myogenic cells, emerging at different periods during development and embryologically related to specific aspects of muscle differentiation and further maturation. Additional reasons to examine myogenic cell heterogeneity lies in the well-known histopathological picture of primary myopathies, where certain muscle fibers appear unaffected by the disease while the majority degenerate. Such heterogeneity reflects the existence of different populations of fibers, which might not express, or express to a different level, the mutated gene. The chapter reviews the literature on early and late myoblasts in somites and limbs and on satellite cells, and discusses possible relationships of these cell types with muscle fiber development and with fiber heterogeneity in primary myopathies. Immunocytochemical evidence of heterogeneity among somitic cells with respect to the expression of myosin heavy-chain (MHC) isoforms is obtained. Proteins which cross-react with antibodies directed against slow MHC are expressed by a subpopulation of differentiated muscle cells. This subpopulation apparently migrates to the limb, since it can be observed in cultures of limbs from 10-day and, to a lesser extent, 12-day-old embryos.