Molecular biology of amphibian metamorphosis A new approach to an old problem

Amphibian metamorphosis has attracted the interest of developmental biologists for decades. Its dependency on thyroid hormone (TH) makes it a unique system to study postembryonic organ development as well as hormone action. Recent cloning o f the TH receptors and the demonstration that these receptors are transcription factors suggest that TH controls metamorphosis by regulating a cascade of gene expression. Systematic analyses of TH early-response genes in three very different organs (the tail, limb, and intestine) have been conducted. The developmental profiles of their expression and their identities suggest that these genes play important roles during metamorphosis, for example, controlling the expression of intermediate and/or TH lateresponse genes. In addition, the gene-regulation program that governs amphibian metamorphosis as revealed by these and related studies appears to resemble that induced by the hormone ecdysone during insect metamorphosis. Further investigation on how the hormone regulates early-response genes and how the products of these genes, in turn, participate in tissue remodeling will provide a better understanding of the molecular basis of the hormonal control of metamorphosis.