Changes in weight and proportional distribution of fiber types and diameters in the M. supracoracoideus and M. pectoralis of male chickens [Gallus gallus domesticus] following thyroidectomy

The present study was conducted to examine the effects of thyroid hormone deficiency on the weight and proportion and diameter of each fiber type in M. supracoracoideus, in comparison with M. pectoralis. Male White Leghorn chickens were surgically thyroidectomized at 10- to 11 - days old. Experimental birds were divided into intact, autotransplant and thyroidectomy groups. After collecting blood samples, chickens were slaughtered under anesthesia with pentobarbital sodium at 40- to 41-days old. Plasma T3 and T4 levels were immediately determined. The 2 specimens were rapidly dissected. Myofibers were classified according to reactivities of myosin ATPase and NADH-tetrazolium reductase, and percentage compositions and fiber diameters of myofiber types were calculated. Removal of thyroid glands caused marked reductions in plasma T3 and T4 levels. Body weights and weights of M. supracoracoideus and M. pectoralis in the thyroidectomy group were significantly lower than in the other groups. Relative weight of M. supracoracoideus to carcass weight in the thyroidectomy was much less than intact or autotransplant group while the relative weight of M. pectoralis was similar to one another. Muscle fibers of M. supracoracoideus and M. pectoralis in all groups were divided into types IIA and IIB. Percentage distribution of fiber types in M. supracoracoideus changed following thyroidectomy, indicating the transformation of fast-twitch glycolytic (IIB) fibers to fast-twitch oxidative glycolytic (IIA). No such transformation was observed in M. pectoralis. Conversely, diameter of each fiber type in M. supracoracoideus following thyroidectomy was significant smaller compared to other groups. These results suggest that muscle growth in M. supracoracoideus and M. pectoralis is retarded by thyroid hormone deficiency, and thyroid hormones are closely related to the growth of M. supracoracoideus, and particularly to the differentiation of type IIA fibers in this muscle.