PASSAGE TO THE TERRESTRIAL LIFE IN AMPHIBIANS .2. ENDOCRINE DETERMINISM

In many amphibians, the departure from water associated with the metam orphic events is controlled by a number of hormones. The most importan t are the thyroid hormones. After they bind to the nuclear receptors, the thyroid hormones trigger a number of mechanisms by acting at the g enome level. These hormones control the synthesis of many specialized proteins during cell differentiation and degeneration. Thyroid hormone activity is a necessary condition for metamorphosis and departure fro m water in many amphibians. However, it is not in itself sufficient an d other hormones intervene also. Cortical steroids such as corticoster one or aldosterone play a part in up-regulating metamorphosis. On the contrary, prolactin and growth hormone can inhibit certain metamorphic events in larvae, thus maintaining them in an aquatic environment. Mo reover, neurohypophyseal hormones, aldosterone or prolactin regulate t he hydromineral equilibrium which optimizes water conservation in the metamorphosed animals, while they migrate towards the terrestrial habi tat. Finally, a complex endocrine determinism presides over the enviro nmental transitions observed in modern amphibian development.