The analysis of complex developmental programmes: amphibian metamorphosis.

Metamorphosis in frogs and toads is a set of complex developmental programmes controlled by thyroid hormone (TH). The pervasive and dramatic changes that occur as a tadpole turns into a frog have captured the attention of life scientists from many disciplines. For evolutionary biologists, the extent to which related organisms incorporate metamorphosis in their life cycle is of paramount interest. For specialists who study cell death, a burgeoning field of research interest, apoptosis is a major event in metamorphosis. It can be induced in many tissues, even in whole organs, by TH. For endocrinologists, TH-induced metamorphosis is a model for the general problem of the molecular basis of TH action and the interaction of the hypothalamus, the pituitary and the thyroid glands. However, the emphasis in this essay is the use of amphibian metamorphosis to study complex developmental programmes such as vertebrate organogenesis which can be initiated by the simple addition of TH to the tadpole's rearing water.