Scar formation after skeletal muscle injury. A histological and autoradiographical study in rats

The formation of a connective tissue scar was studied autoradiographically 2-42 days following standard contusion injury in the gastrocnemius muscle of 30 rats. The rats were injected intraperitoneally with 3H-labeled proline 4 h before the muscle samples were taken. On day 2 the majority of cells in the injured area were identified as inflammatory without radioactive labeling. Abundant labeling was observed, however, over the extracellular substance, and moderate labeling with 3H-proline in the cells identified as fibroblasts or their precursors. During the first 2 weeks the number of fibroblasts increased and the majority of them were labeled with silver grains, which were also present over the connective tissue matrix. Six weeks after injury only a few fibroblasts showed labeling, and no labeling was observed extracellularly in the connective tissue. Thus, it seems that the synthesis of extracellular connective tissue components in injured skeletal muscle occurs as early as 2 days after trauma, is intensive between days 5 and 21 after trauma, and decreases markedly during the following 3 weeks.