PITUITARY-THYROID AXIS CONTROLS THE FINAL DIFFERENTIATION OF THE DORSAL SKELETAL-MUSCLE IN URODELAN AMPHIBIANS

A histoenzymological study of the ATPase activity of myosin in the dor sal axis muscle (dorsalis trunci) was carried out on two species of ur odelan amphibians: Pleurodeles waltlii, a euthyroid species with spont aneous metamorphosis and Ambystoma mexicanum, a neotenic hypothyroid s pecies. P. waltlii and A. mexicanum underwent an operation after which cytological analysis of the remaining pituitary were carried out in p arallel. The muscle phenotype of urodelan amphibians varies according to the thyroid status of the species. In euthyroid adults, IIA fibers are dominant whereas in hypothyroid adults, IIC fibers are dominant. T he number of type IIB (fast) and type I fibers (slow) are similar in b oth species. Physiological or experimental modulation of the concentra tion of circulating thyroid hormones results in a modification of the muscle fiber type profile pertaining to the considered species. We fou nd that pituitary (TSH) plays a dominant role in the maturation of typ e IIC fibers in both species. Moereover, it seems to modulate the deve lopment of IIA fibers in P. waltlii and that of IIB fibers in A. mexic anum. Its action is thus species specific. Through partial or total hy pophysectomy experiments, we have been able to demonstrate the influen ce of the hypophysothyroidian axis on the appearance of the adult musc le phenotype during metamorphosis.