THE ROLE OF THYROID-HORMONE IN AMPHIBIAN METAMORPHOSIS

Recent advances in our understanding of thyroid hormone (TH) metabolism, and the nature and function of TH receptors, have shed some light on the mechanisms by which TH initiates the numerous morphologic and biochemical components of amphibian metamorphosis. The data support the view that the successful accomplishment of metamorphosis depends primarily on a series of programmed increases in the rate of secretion of TH. In addition, as climax approaches and the requirement for TH increases, the physiologic effectiveness of the circulating hormone is enhanced as a result of changes in peripheral TH economy and tissue-specific increases in TH receptor concentration. The receptor bears a striking resemblance to its mammalian and avian counterparts, and it is postulated that TH-induced metamorphic events result, at least in part, from changes in transcriptional activity induced by the effect of triiodothyronine-receptor complexes on the response element of the relevant genes.