Morphogenetic effects of 9-cis-retinoic acid on the regenerating limbs of the axolotl.

9-cis-retinoic acid has recently been found to be a high affinity ligand for the retinoic X receptor (RXR). RXRs are believed to be involved in metabolic activities rather than in morphogenetic ones. Interestingly, RXR has been found to form heterodimers involving other receptors from the steroid family, such as the thyroid hormone receptor, vitamin D receptor or retinoic acid receptors (RARs). In this paper we examined whether or not 9-cis-retinoic acid had any morphogenetic properties on the regenerating axolotl limb. It is shown that 9-cis-retinoic acid proximalized regenerating limbs and was somewhat more potent in this action than all-trans-retinoic acid. Based on these observations, the possible roles of other receptors during pattern formation is discussed.