In situ hybridization analysis for expression of myogenic regulatory factors in regenerating muscle of mdx mouse.

Precise temporal and spatial coordination of expression of the myogenic regulatory factors (MRF) plays a critical role in the development of skeletal muscle. Whether this pattern is recapitulated postnatally during regeneration of mature muscle after injury is not known. The aim of this study was to determine the cellular distribution of mRNA expression of the various myogenic regulatory factors (MyoD, Myogenin, Myf-5 and Myf-6) during regeneration in mature skeletal muscle. We used the mdx mouse, an animal model for Duchenne muscular dystrophy, which undergoes necrosis and vigorous regeneration of muscle fibers, as a natural model to study muscle fibers at various stages of maturity ranging from satellite cells to mature muscle fibers. In situ hybridization analysis revealed that MyoD expression was the most widespread and was expressed in satellite cells and small regenerating muscle fibers surrounding necrotic fibers. Myogenin, Myf5, and Myf6 were expressed weakly in some immature fibers and none of the MRF's could be detected in mature muscle. These findings, taken together with previous studies, suggests that the pattern of expression of the various MRF's in regenerating skeletal muscle differs from that in developing muscle in embryos and that MyoD may play a critical role in the initiation and progression of events which lead to regeneration in mature skeletal muscle.