The Thyroid Gland and Its Functions in Cold-Blooded Vertebrates

Abstract

Some of the topics covered in this review are, morphology, embryonic origin, histology-cytology, role of thyroid in metamorphosis, in the sequence of metamorphic events, in neoteny, and in regeneration. In brief summary, the thyroid gland originates from certain cells of the larval endostyle in Cyclostomes and from the embryonic pharyngeal floor in all other vertebrates. The best known function of the thyroid hormone is its control of amphibian metamorphosis. The secretory activity of the thyroid is controlled by the hypo-physeal thyrotropin.Neotenic amphibians pose special problems. Their failure to metamorphose has been variously ascribed to deficiency of the animal s thyroid secretion and to exceptional tissue insensitivity to the influence of the tropic function of the pituitary. In regeneration the present indication is that the loss of regenerative capacity which occurs at metamorphosis in amphibians is to be regarded as an accompaniment of the morphogenetic changes produced in the tissue under the influence of the thyroid hormone.