Stage dependent effects of vitamin A excess on limbs during ontogenesis and regeneration in tadpoles of the toad Bufo melanostictus Schneider

Abstract

Effects of variable treatments with vitamin A excess on ontogenesis and regeneration of hind limbs in toad tadpoles were investigated. In tadpoles at spatula/paddle stage of limb development right leg was kept intact and regeneration initiated in the left leg by amputation through shank. Following this, group A was reared in water from day 0 to 15 and groups B, C and D were kept in vitamin A solution during days 0-15, 3-15 and 0-3, respectively. In group A development of both intact limbs and regenerates was normal. In group B, foot morphogenesis was suppressed in intact limbs whose entire myogenic mesoderm also became necrotic; regeneration in the operated limbs proceeded only up to blastema stage. In group C, morphogenesis of intact limbs was not affected but histogenesis was relatively retarded and regenerates did not develop beyond blastema stage. In group D, foot morphogenesis was suppressed in the intact limbs but in them cartilage and muscle differentiation was normal. Regenerates however, ' developed in all cases and were well formed. , Vitamin A treatment induced many supernumerary regenerates in groups B and D which, in the former group remained blastema-like outgrowths but in the latter developed into well formed limbs. Differential effects observed on intact and regenerating limbs were related to their respective specific developmental stages at the time and duration of exposure to the vitamin; the stage of limb mesodermal differentiation and morphogenesis during ontogeny and regeneration susceptible to vitamin A could be identified. Developing anuran limb is suggested as a suitable system for investigations on the mechanisms of vitamin A teratogenesis.