Intramuscular collagen accumulation in different types of skeletal muscle fibers in middle-aged male rats.

This study focused on the histological characterization of age-related intramuscular collagen accumulation in different skeletal muscle fiber types, specifically fast- and slow-twitch fibers, in young and middle-aged male rats, in relation to the number of nuclei between muscle fibers. The extensor digitorum longus (EDL) and soleus (SOL) muscles from male Sprague-Dawley (SD) rats were collected and sectioned. Hematoxylin and eosin staining was performed for histological examination, while Picrosirius Red and hematoxylin staining were used for morphometric analyses. The SOL, a slow-twitch dominant muscle, tended to have a more distinct and thicker interstitium, as well as more collagen fibers and nuclei between muscle fibers, than the EDL, a fast-twitch dominant muscle. The degree of collagen accumulation between muscle fibers was positively correlated with the number of nuclei. Intramuscular collagen fibers increased with age in both the EDL and SOL, particularly in the latter. The number of nuclei remained unchanged with age. These results suggest that the increase in intramuscular collagen fibers with age is due to increased collagen production by existing fibroblasts rather than fibroblast proliferation. Given that middle-aged male SD rats fed