Retinoid signalling and retinoid receptors in amphibian limb regeneration

Retinoic acid (RA) is able to respecify the positional identity of a regenerating urodele amphibian limb. The regenerate arises from a mesenchymal growth zone or blastema, composed of mesenchymal progenitor cells. Is RA an endogenous ligand and does it act to respecify axial identity? Two recent lines of evidence in favour of an endogenous role are the activation of an RA reporter gene after transfection into the blastema, and the demonstration that the wound epidermis at the end of the regenerate is a source of RA. To analyse the mechanism of action at the receptor level, we have constructed chimaeric newt RA receptor/Xenopus thyroid hormone receptor molecules that allow a single RA receptor to be activated by thyroid hormone.