Ultrastructural events following acute muscle trauma.

The purpose of this study was to determine the ultrastructural events occurring in skeletal muscle following acute blunt trauma. Male rats weighing 250 g were subjected to a single impact trauma to the medial gastrocnemius muscle while under general anesthesia. Hemorrhage, inflammation, non-necrotic degeneration, and later regeneration were observed. In the short term following impact (6-24 h), the damaged segments showed gross tearing and degeneration. A large number of mononuclear cells were seen in the intercellular connective tissue and within the damaged muscle cells. By 24-48 h, there was an increase in the number of sarcolemmal nuclei, some of which were likely of satellite cell origin. By day 3, regenerating muscle cells displayed central nuclei and reorganizing sarcomeres. By day 6, further progression of regeneration was seen. Moreover, focal interstitial collagen formation suggested minimal to mild scar formation. On days 14, 21, and 30 after trauma, the muscle appeared to have healed and no abnormalities could be found at the site of injury. In parallel with the ultrastructural events noted, the injured muscles underwent a marked catabolic response and showed a reproducible fall (-27%, P less than 0.001) in total protein content within 48 h. Muscle protein accumulation commenced after day 3; however, complete repletion of the loss did not occur until day 21 post-injury.