[Effect of temperature, ageing and hormonal conditions (thyroxine) on cell proliferation, in the young tadpole and during metamorphosis of the toadBufo bufo L. (Amphibia, Anura)].

Generation time, duration of cell cycle phases and growth fraction were measured for three tissues: telencephalon, anterior limb-bud mesenchyme and intestinal epithelium.By rearing young larvae at different temperatures (12, 16 and 24° C), it was demonstrated that: cell cycle duration (T) and the duration of S, G and M phases are shorter at higher temperature; the G phase is characteristically lengthened or shortened, depending on the tissue concerned at a given stage, the growth fraction (CP) is characteristic for each tissue, and does not vary with temperature the sensitivity to temperature conditions is also characteristic for each tissue; anterior limb bud mesenchyme is the least sensitive. Ageing and differentiation of cell populations during larval life and natural metamorphosis (at the constant temperature of 16° C) lengthen the generation time (T) and the duration of S, M and G phases; simultaneously the growth fraction decreases. But at the metamorphic climax, the growth rate of some tissues (telencephalon, secondary intestinal epithelium) is suddenly and temporarily increased, as a result of both a shortening of T and an increase of CP. On the contrary, other tissues (primary intestinal epithelium) no longer proliferate, and collapse. These different modifications to the different organs seem to be related to variations in the amount of thyroxine in the circulating blood. They can be considered as one of the aspects of the differential tissue sensitivity to thyroxine.