Comparison of morphological and biochemical parameters of growth in rat skeletal muscles

Morphological and biochemical parameters of cellular growth of four skeletal muscles were examined in male Sprague-Dawley rats from 25 to 165 days of age. The number of muscle fibres decreased in the soleus (41%), plantaris (20%), extensor digitorum longus (25%), and biceps brachii (19%) from 25 to 165 days of age, while nuclear number increased in each muscle until approximately 81 days, with no additional change up to 165 days of age. Fusion of muscle fibres may possibly explain the apparent loss of fibres. During this period of growth, nuclear number correlated equally well with both fibre length and cross sectional area. However, transverse growth accounts for approximately 60% to the post-weanling increase in nuclear number.