IS POLYGONAL PATTERN OF CROSS-SECTIONAL MUSCLE FIBER RELATED TO METABOLIC AND CONTRACTILE PROPERTIES WITH AGING?

Abstract

When a skeletal muscle is viewed in cross-section, various polygonal fibers make a geometrical pattern, called Dirichlet domain. However, little has been done with distribution and changes in polygonal patt erns of cross-sectional muscle fibers with aging. PURPOSE The purpose of this study was to examine polygonal patterns of cross-sectional muscle dominated in soleus muscle with aging. METHODS In six young (6 wk old, Y group) and six young adult □@(10 wk old, YA group) male Wistar Kyoto rats, fiber type was histochemically classified in soleus muscle. The cross-sectional area of muscle fiber (CSA) was morphometrically determined. The polygon of cross-sectional muscle fibers was calculated as a circular degree (C.D.) by using the formula: C.D. = 4fIS/l 2, where 1 is peri meter and S is cross-sectional area of fiber (circle=1.0, hexagon=0.9, square=0.8, triangle=0.6). RESULTS The C.D. for slow twitch fiber (SO) was in greater in Y group than YA group (0.724 ±0.067 vs. 0.680 ±0.072, P □ f > 0.001), whereas CSA was smaller in Y group than YA group (1681 ±379fEm vs. 2321 ±665fEm2, P □f > 0.001). In the FOG (fast twitch fiber, oxidative and glycolytic) fibers, Y group also demonstrated a greater circular degree than YA group (0.717 ±0.067 vs. 0.669 ±0.082, P □f > 0.001), while CSA for FOG fibers was also smaller in Y group than YA group (1350 ±224fEm 2vs. 2126 ±590fEm2, P □ f > 0.001). Growth decreased C.D. in SO and FOG fibers and became more polygonal, whereas both muscle fiber sizes increased with age. CONCLUSION These data suggest that the growth-related shift to more polygonal pattern s increased metabolic surface areas of fibers. For contractile property, more polygonal pattern may decrease a balanced tension in a bundle of microfilaments due to mo re irregular polygons, wh ereas muscle fibers might enhance functional diversity.