Activation of myogenic precursor cells after muscle injury.

The time and site of satellite cell activation in contusive myofiber rupture with segmental necrosis were studied in rat by labeling the cells in S phase of mitotic cycle with bromodeoxyuridine. The labeled satellite cell nuclei did not appear before phagocytosis of the necrotized debris had started, usually 1 d after the trauma. Most labeled satellite cell nuclei were found in the necrotized part of the ruptured myofibers, whereas very few of them were detected in the surviving part. The proliferation of satellite cells was extensive during the first 4 d in the regeneration zone; thereafter, it declined and only few labeled nuclei were observed on day 7. These results suggest that necrosis alone is not sufficient as a signal for the satellite cell activation, but it is speculated that (growth?) factors released by the macrophages are needed. A great majority of myoblasts derive from local precursor cells, and satellite cells recruited from the surviving part do not significantly contribute to the regeneration. Most myonuclei needed in regenerating myofibers are produced early within the preserved basal lamina cylinders to be later distributed into those parts of myofibers, which extend into connective tissue between the stumps.