Amphibian metamorphosis as a model for studying the developmental actions of thyroid hormone.

A salient feature of the thyroid hormones. L-thyroxine (T4) and triiodo-L-thyronine (T3), is the multiplicity of their physiological and biochemical actions in a wide variety of vertebrates. Important among these is their profound effects on postembryonic development. Analysis of their developmental action in mammals is vitiated by the exposure of the developing foetus to a number of maternal factors which do not allow one to specifically define the role of thyroid hormone (TH) or that of other hormones and factors that modulate its action. Amphibian metamorphosis is obligatorily dependent on TH which can initiate all the diverse physiological manifestations of this postembryonic developmental process (morphogenesis, cell death, re-structuring, etc.) in free-living embryos and larvae of most anurans. It is therefore an ideal model for studying the mechanisms underlying the hormonal regulation of postembryonic development. This article will first summarize the key features of metamorphosis and its control by TH and other hormones. Emphasis will be laid on the important role played by TH receptor (TR), in particular the phenomenon of TR gene autoinduction, in initiating the developmental action of TH. Finally, it will be argued that the findings on the control of amphibian metamorphosis enhance our understanding of the regulation of postembryonic development by TH in mammals and other vertebrate species.