Correlation of the proximalizing effect of retinoid treatment on limb regeneration in anuran tadpoles with the normal capacity to regenerate

Abstract

The unique effect of retinoids on limb regeneration in amphibians is expressed in the apperance of some or all stump structures in the regenerate; the phenomenon is described as proximalization of blastema.In the anuran larvae and urodeles the proximalized blastema forms the regenerate which may even be a complete limb and not just the part removed distal to the amputation level.Urodeles retain the normal capaciy to regenerate amputated limbs throughout life but in the developing hind limbs of anuran larvae this capacity is progressively reduced along its P-D axis and lost completely with the approach of metamorphosis. The question was whether the proximalization of limb regeneration by retinoids in anuran tadpoles is related with the presence of normal capacity to regenerate. Therefore, the frequency of the type of regenerates produced in controls (untreated) was compared with that in vitamin A treated groups after amputation across thigh, shank and ankle in young and older tadpoles of Bufo melanostictus. In young control groups, good regeneration occurred in nearly all cases at the three levels; and all regenerates were the usual type consisting of the part only distal to amputation level. In older tadpoles this capacity was reduced with regenerates developing in fewer cases, many of them being hypomorphic, and in the oldest group small spikes developed in a few cases only at the ankle level. In young vitamin A treated groups, regeneration at the three levels occurred also in nearly all cases; but all regenerates were the proximalized type and structurally perfect. In the tadpoles of a higher devlopmental stage having reduced regenerative capaity, vitamin A treatment produced both the proximalized and the usual type regenerates, the former type being fewer at thigh but much more than the latter type at shank and ankle levels. Both types were structurally well formed. In the oldest group with almost nil regenerative capacity, vitamin A treatment did promote some regeneration in many cases at all levels but only of small spikes. Results indicated that proximalization of blastema by retinoid in anuran tadpoles is positively related with the extent of the ability to regenerate present in the limbs at any stage.