Postnatal growth of skeletal muscle fibres of the rat.

Cross sectional area [micron] of Red, Intermediate, and White fibres of M. extensor carpi radialis longus of Wistar rats at frequent age intervals were measured and then calculated the growth equations, the growth rate equations, the growth rates, and the asymptotes of each kind of fibre. Muscled fibres of 12 d old rats were round in shape, presenting a somewhat uniform SDH activity. By 23 d and thereafter, muscle fibres were polygonal in shape, with different degree of enzyme activity and cross sectional areas. There was a close relationship between cross sectional area and in age. Individual fibres revealed distinct growth curves. The higher growth rates were observed at 23 and 45 d. After this ages, a markedly fall in growth rates for the 3 kind of fibres were observed. Growth rates of White fibres were higher than that of Red and Intermediate fibres.