Background: Regeneration of mature skeletal muscle recapitulates closely fetal myogenesis. It is initiated by activation of the reserve myogenic precursor cells, the satellite cells, which proliferate, differentiate into myoblasts expressing muscle-specific proteins, fuse into myotubes, and finally mature into myofibers. The MyoD family of transcription factors participates in the regulation of the complex phenomenon of myogenic differentiation during development and in vitro. The function of these transcription factors in the regeneration of injured mature skeletal muscle in vivo is, however, still unclear.
Experimental design: To clarify the primary events in myogenic precursor cell activation, the expression of myogenin was examined in rats 1 to 48 hours after either a contusion injury to the gastrocnemius or after toxic injury to the soleus muscle. Myogenin mRNA expression was studied by Northern blot hybridizations, and the results were correlated with the onsets of the mitotic activity (i.e., incorporation of bromodeoxyuridine) of the satellite cells and of the production of the myogenin and MyoD1 proteins, as well as muscle-specific intermediate filament protein, desmin.
Results: Both forms of muscle injury produced myofiber necrosis, followed by the activation of the satellite cells. The first sign of myogenic differentiation, an increase in myogenin mRNA expression, occurred between 4 and 8 hours after injury. The first desmin-, MyoD1- and myogenin-positive myoblasts were seen after 12 hours, but satellite cell proliferation was not seen until 24 hours after the injury.
Conclusions: The schedule of the events in our study contradicts the general concept that differentiation should follow proliferation. To explain this discrepancy, we propose that there are two populations of precursor cells: committed satellite cells, which are ready for immediate differentiation without preceding cell division, and stem satellite cells, which undergo mitosis before providing one daughter cell for differentiation and another for future proliferation.