Two-dimensional changes of muscle fiber types in growing rat hind limb

In the present study, we examined the changes in two-dimensional distribution of fiber types in the whole area of the rat skeletal muscle and the effect of growth on this distribution. Muscles of rats aged 3 (body weight 58 g), 4 (89 g), 8 (276 g), 12 (312 g), 18 weeks (368 g), and 6 months (450 g) were stained for myofibrillar adenosine triphosphatase (mATPase) with preincubation at pH 4.35. Muscle fibers were classified into type I (slow oxidative), IIA (fast oxidative), IIB (fast glycolytic), and IIX (fast oxidative glycolytic). The x-y coordinates of each fiber were used to analyze the growth-related changes using an image analyzing system. In the tibialis anterior (TA) muscle, type I fibers were predominant in the deep and middle regions at 3 to 4 weeks of age, but became restricted to the deeper region with growth. In the extensor digitorum longus (EDL) muscle, type I fibers were predominant in the deep region at 3 to 8 weeks of age, but decreased gradually with growth and completely disappeared at 6 months of age. Compared with the TA and EDL, type I fibers of the soleus (SOL) muscle were spread throughout the muscle and the number of these fibers tended to increase with growth. Type IIA and IIX fibers of the SOL decreased in number and became restricted to the superficial region with growth. No type IIB fibers were detected in the SOL throughout life. Our results indicated that the growing process influences the distribution, proportion and characteristics of individual muscle fiber types in the rat hind limb muscles.