Influence of experimental hypothyroidism on chick myogenesis

The influence on chick myogenesis of hypothyroid status induced in ovo by methimazole was histologically (number and types of muscle fibers) and biochemically studied. A single injection of methimazole induced an hypothyroid status during embryogenesis, as shown by the plasma T4 levels which were separately assayed in male and female controls and treated embryos from day 12 of incubation to day 4 after hatching. In both sexes, control plasma T4 reached a peak on day 20 of incubation, but the female values were significantly higher; plasma T4 in the treated embryos remained at a low level independently of sex. Only methimazole-treated males showed a significant increase (P less than 0.01) in the total number of fibers of the two muscles (tibialis posterior and flexor digitorum) studied. Moreover, the histochemical results on succinate dehydrogenase and myofibrillar ATPase evidenced that, of the three fiber types (alpha R, alpha W and beta R) constituting the skeletal muscles, only the alpha fibers increased significantly. Aldolase (glycolytic) and NADP isocitrate dehydrogenase (tricarboxylic acid cycle) activities, measured on the adductor and pectoralis muscles, showed a similar developmental pattern in control and treated animals, but was retarded in the latter due to a 5-day delay in hatching. It is not known whether the tissular differences were related directly to the hypothyroid status, to alterations in nervous system differentiation or, as suggested by intersexual differences, to modifications in hormonal balance.