[Induction of cell differentiation and programmed cell death in amphibian metamorphosis]

Both the phenomena, the cell growth and differentiation, and the programmed cell death, are involved in the reconstruction of body from larval type to adult type during amphibian metamorphosis. Since all of metamorphic events are triggered by increase of plasma thyroid hormone level, the reconstruction of body during metamorphosis will provide useful biological system for analyzing the mechanism of hormonal induction of cell differentiation and programmed cell death. Induction of death of skein-type epidermal basal cells in tail skin of bullfrog tadpoles was involved in cell cycle arrest caused by triiodothyronine (T3). On the other hand, induction of adult type stratified and cornified epidermis was involved in T3-induction of the growth and differentiation of non-skein type epidermal basal cells in body skin. Developmental origin of these two different basal cells should be clarified. Analysis of adult type epidermal keratin expression during Xenopus metamorphosis revealed that enhanced transcription of adult type gene by synergism between T3 and glucocorticoid. Since receptors of these two hormones are belong to the same members of steroid receptor super family, interaction of hormones and hormone receptors should be studied for clarifying the molecular mechanism of the hormonal synergism at a transcription level. Programmed muscle cell death is important in tail degeneration as well as in the larval-to-adult conversion of the dorsal body muscle cell of Xenopus laevis during metamorphosis. In the tail, only death of larval type muscle cells occur. However, in the body, both the death of larval type muscle cells and new myogenesis (the growth and differentiation of adult type cells) occur to form adult skeletal muscles. The new myogenesis area expanded in a wave-like fashion with an anteroposterior gradient during metamorphic climax stage. The area of muscle cell death expanded like a wave from the base of the tail to the anterior and posterior side. Growth, differentiation and death of muscle cells were induced by T3. The origin of adult satellite cells for new myogenesis should be clarified in order to understand the mechanism of skeletal muscle development during metamorphosis.