MENON J.*; FRESSE, D.: Effects of hormones/morphogens in anuran metamorphosis

Retinoids, a large family of natural and synthetic compounds related to vitamin A, have pleiotropic effects on embryonic development. The role of retinoids in processes such as cell proliferation, differentiation and apoptosis is well documented. Heterodimerization of retinoid X receptors (RXRs) with receptors of thyroid hormone (TRs) is crucial for action of the latter, responsible for tissue remodeling during metamorphosis. Presently we found that retinol palmitate (10 ^-6 M) treatment during prometamorphic stage (stage 55/56 Nieuwkoop and Faber, 1967) accelerates metamorphic events in tadpoles of Xenopus laevis as confirmed by morphological, histological and ultrastructural observations in various organs. A further acceleration was seen when treated with retinol (10^-6 M) plus thyroxine (10^-6 M) indicating their synergistic mode of action. In contrast, no such positive effects were seen in Rana catesbeiana tadpoles when treated during mid prometamorphic stage. However, retinol treatment during premetamorphic stage, inhibited metamorphosis in both the species of tadpoles. We used high performance liquid chromatography (HPLC) to identify, and quantify concentrations of retinoid metabolites present in tissues such as tail, body skin, liver and limbs of both the species under different experimental conditions. Our results indicate significant differences in retinol metabolism between the two species, which are correlated with the species differences in response to retinol treatment.