MUSCLE FIBER NUMBER: RESPONSE TO CHANGES IN MUSCLE GROWTH

Three groups of animals were used to examine the effects of chronic stretch and altered caloric intake on muscle cell proliferation. Muscle enlargement was induced in the anterior latissimus dorsi (ALD) muscle of White Leghorn chickens by hanging weights on the distal portion of one wing. Caloric intake was regulated by fasting weanling rats for 3 days followed by a return to a regular feeding schedule. Adjusting the litter size to small (AG), normal (NorG), and large (RG), numbers of pups per mother varied the caloric intake for newborn rats. The average increase in weight for the ALD muscle was 150%. The soleus muscle weight from the fasted rats was not different from the soleus muscle weights of the control animals. The growth rate of the RG animals was slower than the animals in the AG groups. The fiber number for the ALD and the rat soleus muscles was determined by a digestion-dissection technique where each muscle fiber was counted individually. There was no difference in the fiber number between the control and hypertrophied ALD muscles. There was no difference in the fiber number for the soleus muscles between the fasted, fasted-fed, or control weanling rats or the mature rats from the three growth-rate litters. Branched muscle fibers were found in most ALD and soleus muscles from young and mature animals. The frequency of bifurcated fibers did not change with any experimental perturbation. Growth in these muscles was attributed to hypertrophy of the fiber volume components and not hyperplasia.