Fig. 6. Time course of muscle regeneration induced by focal freeze injury. Transverse cryosections from TA muscles of 3-month-old C57 mice subjected to focal damage in vivo were analyzed by HE staining. Uninjured muscle (U) shows a typical pattern of evenly sized myofibers with peripheral nuclei. 30 minutes after injury no histological changes are evident, whereas hyper-contracted fibers (arrowhead) and infiltrating mononuclear cells (arrow) appear by 2 and 6 hours, respectively. Mononucleated cells (inflammatory cells and myoblasts) increase in number in the lesion over the next 3 days. By 7 days post injury, myoblast fusion results in centrally nucleated regenerating fibers (arrowhead) of a smaller caliber than the adjacent uninjured fibers. The cross-sectional area of regenerating fibers matures to that of undamaged fibers over the next week (14d).