Hypertrophy and hyperplasia of adult chicken anterior latissimus dorsi muscles following stretch with and without denervation

Abstract

In the anterior latissimus dorsi muscle of the chicken hypertrophy was obtained by stretching, whether the muscle was innervated or not. The teres minor muscle, which was stretched at the same time, also hypertrophied; the posterior latissimus dorsi muscle, consisting of red and white fibers, did not change significantly in weight, but its position in the body is such that it was not stretched adequately. The rate of certain histological changes which accompany hypertrophy keeps pace with the rate of the hypertrophy. These changes include enlargement of nuclei, nuclear proliferation, and invasion with subsequent splitting of the units producing fibers of varying diameters. ATPase differentiation of fibers weakens when hypertrophy exceeds 35–40%, producing muscle fibers exhibiting, in general, a homochromatic, light ATPase response. The amount of hypertrophy and hyperplasia is proportional to the degree of stretch. Excluding an increase caused by fibers splitting, fiber counts of complete cross sections of anterior latissimus dorsi muscles taken at intervals after the onset of stretch showed a steady increase in the total number of fibers. This began promptly under extensive stretch, much later under mild stretch. The presence of new fibers appeared to be a function of both time and the amount of hypertrophy; the degree of hypertrophy had to be at least 70%. The increase in total fibers lagged behind the number of small fibers interpreted as “new.” The fate of many of the new fibers is not known but a sufficient number matured in the 8 wk of the experiment to produce the increase in total number.