Repairing process in the transected muscle fibers of the mouse tibialis anterior.

Repairing process at the injury site in the transected muscle fibers of the mouse tibialis anterior was studied by light and electron microscopy. Immediately after the transection, the cut end (approximately 10 µm) was filled with dense network of disorganized myofilaments, in which disrupted membranous structures and degraded mitochondria were scattered. In the portion next to the portion exhibiting sudden necrotic changes, morphological features of the myofilaments, mitochondria and membranous structures appeared to be almost normal. The degradation of disorganized myofilaments at the cut end began within 1 hour after the transection, and at 1hour after the transection, the degenerating areas were noted in most of muscle fibers up to 150-250 µm from the cut end. Following the degradation, accumulation of mitochondria occurred between the necrotic and myofilament-predominant living portions, and several transverse tubules (T-tubules) and sarcoplasmic reticula were found between the mitochondria-accumulated and myofilament-predominant areas. In most cases, demarcation membrane formed between the mitochondria-accumulated and myofilament-predominant areas, and the fusion of T-tubules and sarcoplasmic reticula was encountered in these areas, suggesting that at least some parts of the demarcation membranes formed through fusion of T-tubules and sarcoplasmic reticula. This repairing process was completed in a number of muscle fibers within 6 hours after the transection. Macrophages were first found in the injured portions at 6 hours after the transection, increased in number with time, and several macrophages were distributed at 1 to 3 days after the transection. Some spindle-shaped cells were first found in the degenerating portions of the muscle fibers at 1 day after the transection. Since they were located along the basal lamina of the muscle fiber, and had a long oval pale nucleus and relatively abundant cytoplasm, they can be regarded as activated satellite cells. They gradually increased in number with time, and became larger and longer. On and after 5 days, thin regenerating muscle fibers exhibiting centrally located nuclei were observed, and they became gradually thicker with time. These findings indicate that the muscle regeneration was actively occurring during these periods. The repairing process is followed by the invasion of macrophages, and then the occurrence of muscle regeneration in the sequential order. These findings suggest that there might be close chronological relationship among these events.