[A study of absolute and relative growth on rat skeletal muscle]

Standard development of plantaris muscle in Wistar rats in relation to growth (2-86 weeks) was studied to define more reliable criteria of muscle hypertrophy and hyperplasia. The following variables were measured as indicators of respective factors; body weight and lower hind-limb length as physical factors, whole muscle weight and length as whole-muscle factors, and muscle fiber cross-sectional area, total fiber number and number of proliferating cell as intramuscular factors. When absolute growths were plotted against age, two stages were clearly observed for all factors. The transitional point for the two stage was observed at 10 weeks of age (about 300g in body weight). The first stage was a "growing phase" and the second was a "steady phase". When the relative growth was considered in relation to body weight, most factors increased in logarithmic functions, while muscle weight and muscle fiber cross-sectional area showed a linear relation. It was suggested, therefore, factors concerned with muscle mass and transverse growth developed more with increase in body weight than age. Furthermore, the increase in total fiber number and the decrease in number of proliferating cell in the "growing phase" showed mirror images, suggesting that most proliferating cells detected in the muscles in the "growing phase" may be myoblasts. These results assure the validity of muscle weight/body weight ratio in the evaluation of muscle hypertrophy or atrophy. However, it is remember that animals over 10 weeks of age (300g in body weight) should be used for correct evaluation of muscle hyperplasia for comparative or developmental studies of Wistar rats.