Early ultrastructural events of skeletal muscle damage following cardiotoxin-induced injury and glycerol-induced injury

In this study, we investigated the early changes of skeletal muscle damage in response to injuries induced by cardiotoxin (CTX) and glycerol by using both light microscopy and transmission electron microscopy. Normal, non-dystrophic, adult male mice were used in this study. Tibialis anterior (TA) muscles were injected either with CTX or glycerol. Samples were collected at intervals starting from 1 h up to 4 days after injury. Injured muscles were subjected to both histological and ultrastructural analyses. CTX-induced injury caused mitochondrial accumulation and swelling followed by lysis, while glycerol-induced injury caused accumulation of vesicles with focal disruption of the basal lamina, indicating that the injuries have different mechanisms of damage to myofibers. Moreover, inflammatory cells, including neutrophils and macrophages, were recruited earlier and in larger numbers after CTX-induced injury than after glycerol-induced injury. On the other hand, satellite cells (SCs) activation started at 6 h after both injuries, as indicated by an increase in both the length and cytoplasmic-to-nuclear ratio. However, there were significantly longer SCs with a higher cytoplasmic-to-nuclear ratio in the CTX-injured muscles than in the glycerol-injured muscles at day 4. In conclusion, our results demonstrated a difference between CTX and glycerol in their damage to myofibers; CTX damages myofiber mitochondria, while glycerol damages the myofiber cell membrane and alters osmosis. In addition, CTX-induced injury caused earlier and more extensive inflammatory infiltration than did glycerol-induced injury. This study is the first study to shed light on the early events following skeletal muscle injury induced by CTX and glycerol.