ASPECTS OF AMPHIBIAN METAMORPHOSIS - HORMONAL-CONTROL

This chapter reviews recent studies on metamorphosis from the perspective of hormonal control. It also focuses on biochemical and cell-biological aspects of amphibian metamorphosis. The developmental stages of larvae of representative species are presented. Amphibian metamorphosis appears to be under multihormonal control and the secretion and function of these hormones are regulated in various ways. Thyroid hormone levels need to be low for premetamorphic tadpoles to grow without showing metamorphic changes. For the final phase of metamorphosis, known as climax, higher levels of thyroid hormones are required because no tail shortening occurs in thyroidectomized tadpoles with relatively low concentrations of thyroid hormone. Thyroid hormone mRNA concentrations decrease in most of the tissues that undergo substantial remodeling. However, in the developing ovary of the juvenile toad, high levels of thyroid hormone α-receptor mRNA are detected. The accumulation of thyroid hormone receptor transcripts in oocytes is sustained and continues to increase throughout further development. The presence of thyroid-stimulating hormone in the amphibian pituitary has been confirmed indirectly by evidence of a difference in thyroid function between normal and hypophysectomized tadpoles at TK stage III and by the stimulating effect of crude extracts prepared from the pituitaries of prometamorphic and climax Xenopus larvae on uptake of 131I by the thyroid. © 1993 Academic Press, Inc.