AGE-DEPENDENT CD45 EXPRESSION IN SKELETAL MUSCLE FOLLOWING INJURY IN WHITE OUTBRED RATS: AN IMMUNOHISTOCHEMICAL STUDY

Skeletal muscle injury triggers complex inflammatory and regenerative processes that vary with age. In this study, we evaluated the expression of CD45, a pan-leukocyte marker, in skeletal muscle of 3-, 6-, 9-, and 12-month-old white outbred rats following mechanical injury. Immunohistochemical staining using DAB chromogen and quantitative analysis with QuPath 0.4.0 revealed age-dependent differences in immune cell infiltration. Early neutrophil recruitment was observed across all ages, followed by macrophage-mediated tissue clearance and myoblast activation. Younger animals exhibited higher initial CD45 expression, whereas older rats showed delayed but sustained immune responses. Our results demonstrate that CD45-positive leukocytes play a pivotal role in muscle regeneration, coordinating inflammation, tissue repair, and remodeling.