Adult male albino rats were assigned randomly to control (CON) and weight-lifting (WL) groups. The WL rats were subjected to a progressive weight-lifting program against high resistance for 8 weeks. During the last 2 weeks, each WL rat lifted a load equal to 130% of its body weight. The mean weight of the adductor longus muscle was significantly increased in the WL group ( p < 0.05). This increased muscle weight was shown to be due to an increase in the number of fibers per unit cross-sectional area ( p < 0.05), and the mean sizes of both fast-twitch oxidative glycolytic and slow-twitch oxidative fibers were significantly smaller in the WL rats than in the CON rats (p < 0.05). Light and electron microscopic examination showed that five out of eight WL rats exhibited longitudinally split muscle fibers, while only one CON rat had a few centrally placed nuclei. The splitting process appeared as either a "pinching-off" of a small segment from the parent fiber or an invagination of the sarcolemma deep into the muscle fiber in a plane parallel to the sarcomeres. There were preliminary indications that this work-induced fiber-splitting process may be a physiological adaptation of muscle to the stress of exercise.
Histochemical techniques were used to study the postnatal muscle fibre differentiation patterns in the plantaris and soleus muscles of male Sprague-Dawley rats. Nine groups of animals (n = 6/group) were killed at 1, 6, 11, 16, 21, 26, 31, 36, and 140 days of age. Serial transverse sections of the two muscles were stained with H & E, NADH-D, and myofibrillar ATPase with acid (pH 4·35) or alkali (pH 10·4) preincubation. In each of the age groups, all available fibres across the muscle sections were classified. Obtained data show that fibre types are basically undifferentiated at birth in both muscles.
In the plantaris muscle there are about 99% type IIA and less than 1% type I fibres at 6 days of age. Type IIB fibres can be identified at 11 days of age. There are increases in the percentages of type I fibres (from 0·7% to 3·5%) and type IIB fibres (from 1·1% to 6·5%) between 6 and 11 days and between 16 and 21 days respectively. By 36 days of age the relative numbers of type IIA, IIB, and type I fibres in the plantaris are approximately 80%, 14%, and 6%, respectively. A gradual change in fibre-type composition continues until it becomes 47% for type IIA, 43% for type IIB, and 10% for type I at 140 days of age.
In the soleus muscle there are approximately 73% type IIA and 26% type I fibres at both 6 and 11 days of age. However, type IIA fibres decrease to 44% and type I fibres increase to 56% at 16 days of age. This rapid shift in fibre composition continues up to 31 days of age when the distribution becomes 25% for type IIA and 74% for type I fibres. Thereafter, the differentiation rate is much slower. At 140 days of age, there are 17% type IIA and 83% type I fibres in the soleus muscle.
The results of this study show that the fibre populations in the plantaris and soleus muscles of the rat undergo a postnatal differentiation process. In both muscles the adult fibre population is established by 140 days of age. Although relatively rapid increases of type I and type IIB fibres occur in the plantaris during the second and third weeks of life, differentiation in that muscle appears to be an essentially continuous process. There is a notable shift in the fibre composition of the soleus muscle during the second postnatal week. Differences between the patterns of differentiation in the two muscles are apparent.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.