Age Related Differences in Muscle Fiber Composition and Capillary Supply of the Human Masseter Muscle

Abstract: The aim of the study was to test the hypothesis that aging causes changes in fiber composition and vascular supply in the human masseter muscle that contribute to impaired jaw function in elderly. The myosin heavy chain (MyHC) composition and capillary supply of muscle fibers in functionally different parts of the masseter muscle of six elderly and six young subjects (mean age 74 and 22 years) were analyzed with immunohistochemical and morphological methods. The mean muscle fiber area in the old masseter was decreased by 27% compared to the young subjects (1100 vs. 1507 m2, p=0.038). Smaller mean fiber area was observed for all fibers containing only slow MyHCI or fast MyHCII isoforms, but not for fibers co-expressing slow and fast MyHCs. There were no significant differences in the numbers of capillaries around fiber (CAF 1.85 vs. 1.92). When CAF was related to individual fiber area, capillaries around fiber area (CAFA), the capillary supply was significantly higher in elderly (CAFA 1.10 vs. 1.65, p=0.004). This was reflected by a higher capillary density in the old masseter (CD 574 vs. 794, cap/mm2, p=0.002). The loss of muscle mass without any reduction in capillary supply, suggests that the capillary network in the human masseter muscle is rather stable against degradation during aging. This finding is in contrast to previous findings in human limbs, where aging has been reported to decrease both fiber size and capillary network in muscles. We conclude that the ageing process might have different impact on jaw and limb muscles.