Preliminary Immunohistochemical Assessment of Slow-Fiber Distribution and Clustering in Skeletal Muscles of 4-Week- and 6-Month-Old Pigs

Type I (slow-twitch) muscle fibers differ in proportion among skeletal muscles, but their local spatial organization remains poorly characterized. This preliminary cross-sectional study investigated whether muscle-specific characteristics were reflected not only in the proportion of slow myosin-positive fibers but also in their clustering architecture. The longissimus, psoas major, semitendinosus, and triceps brachii muscles were collected from pigs at 4 weeks and 6 months of age. All six pigs were female, and live body-weight records were unavailable. Slow myosin-positive muscle fibers (slow fibers) were identified by immunohistochemistry, and adjacent positive fibers were defined as clusters. In pairwise tests at 6 months, the proportion of slow fibers was significantly higher in the psoas major than in the other three muscles, although the main effect of muscle across both ages was not significant (p = 0.0565). Slow fibers occurred as either isolated fibers or contiguous clusters in all muscles, but their clustering patterns differed markedly. Notably, the psoas major muscle had a significantly higher proportion of slow fibers in large clusters than the longissimus and semitendinosus muscles at 6 months of age. These findings suggest that muscle-specific characteristics are reflected not only in the proportion of slow fibers but also in their local spatial arrangement. Thus, clustering architecture may provide a complementary histological parameter for characterizing muscle-specific tissue organization in porcine skeletal muscle.