Analysis of muscle regeneration using single myofibers in culture.
Proliferation of satellite cells is responsible for formation of new muscle cells during development and regeneration. Signals governing satellite cell growth were studied using a tissue culture system consisting of single myofibers with attached satellite cells. The cells remain quiescent in basal medium but enter the cell cycle in response to growth factor obtained from injured muscle. The growth factor exhibits both source and target specificity and appears to be a polypeptide greater than 30 Kd. Satellite cells exposed to growth factor enter the S phase of the cell cycle by 18 h and proliferate with a generation time of 12 h. The growth factor is active in vivo and increases the proliferation and fusion of satellite cells to myofibers when injected intramuscularly in rat pups or adults. In addition to positive signals, satellite cell growth is modulated by negative feedback from the mature myofiber, as satellite cells removed from single myofibers by centrifugation show increased sensitivity to growth factor. The mitotic inhibition imposed on satellite cells results from contact with the myofiber plasmalemma and not the basal lamina, as shown by comparison of satellite cell mitogenesis in killed and living myofibers.