Retinoids as endogenous components of the regenerating limb and tail.

The role of retinoids (vitamin A derivatives) in limb and tail regeneration is the focus of this review. The dramatic effects of excess retinoids on regeneration are considered, which consist of the respecification of each of the cardinal axes of the limb and the transformation of tail regenerates into limbs. The evidence that retinoids are endogenous components of the regeneration blastema and are required for normal regeneration is then addressed. This evidence consists of the fact that various techniques can detect endogenous retinoic acid in the blastema and that retinoic acid receptors are present, and an individual function in regeneration has been established for each of three receptors. Finally, experiments are described in which retinoic synthesis has been inhibited, and this results in the inhibition of both limb development and limb and tail regeneration.