Temporal expression of regulatory and structural muscle proteins during myogenesis of satellite cells on isolated adult rat fibers.

Myogenic precursors in adult skeletal muscle (satellite cells) are mitotically quiescent but can proliferate in response to a variety of stresses including muscle injury. To gain further understanding of adult myoblasts, we analyzed myogenesis of satellite cells on intact fibers isolated from adult rat muscle. In this culture model, satellite cells are maintained in their in situ position underneath the fiber basement membrane. In the present study patterns of satellite cell proliferation, expression of myogenic regulatory factor proteins, and expression of differentiation-specific, cytoskeletal proteins were determined, via immunohistochemistry of cultured fibers. The temporal appearance and the numbers of cells positive for proliferating cell nuclear antigen (PCNA) or for MyoD were similar, suggesting that MyoD is present in detectable amounts in proliferating but not quiescent satellite cells. Satellite cells positive for myogenin, alpha-smooth muscle actin (alpha SMactin), or developmental sarcomeric myosin (DEVmyosin) appeared following the decline in PCNA and MyoD expression. However, expression of myogenin and alpha SMactin was transient, while DEV-myosin expression was continuously maintained. Moreover, the number of DEVmyosin + cells was only half of the number of myogenin + or alpha SMactin + cells--indicating, perhaps, that only 50% of the satellite cell descendants entered the phase of terminal differentiation. We further determined that the number of proliferating satellite cells can be modulated by basic FGF but the overall schedule of cell cycle entry, proliferation, differentiation, and temporal expression of regulatory and structural proteins was unaffected. We thus conclude that satellite cells conform to a highly coordinated program when undergoing myogenesis at their native position along the muscle fiber.