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A histoenzymological study of the ATPase activity of myosin in the dorsal axis muscle (dorsalis trunci) was carried out on two species of urodelan amphibians: Pleurodeles waltlii, a euthyroid species with spontaneous metamorphosis and Ambystoma mexicanum, a neotenic hypothyroid species. P. waltlii and A. mexicanum underwent an operation after which cytological analysis of the remaining pituitary were carried out in parallel. 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. The number of type IIB (fast) and type I fibers (slow) are similar in both species. Physiological or experimental modulation of the concentration of circulating thyroid hormones results in a modification of the muscle fiber type profile pertaining to the considered species. We found that pituitary (TSH) plays a dominant role in the maturation of type IIC fibers in both species. Moreover, it seems to modulate the development of IIA fibers in P. waltlii and that of IIB fibers in A. mexicanum. Its action is thus species specific. Through partial or total hypophysectomy experiments, we have been able to demonstrate the influence of the hypophysothyroidian axis on the appearance of the adult muscle phenotype during metamorphosis.

Pituitary-thyroid axis controls the final differentiation of the dorsal skeletal muscle in urodelan amphibians.