The role of retinoic acid in embryonic and post-embryonic development.

Retinoic acid (RA) is the bioactive metabolite of vitamin A (retinol) which acts on cells to establish or change the pattern of gene activity. Retinol is converted to RA by the action of two types of enzyme, retinol dehydrogenases and retinal dehydrogenases. In the nucleus RA acts as a ligand to activate two families of transcription factors, the RA receptors (RAR) and the retinoid X receptors (RXR) which heterodimerize and bind to the upstream sequences of RA-responsive genes. Thus, in addition to the well-established experimental paradigm of depriving animals of vitamin A to determine the role of RA in embryonic and post-embryonic development, molecular biology has provided us with two additional methodologies: knockout the enzymes or the RAR and RXR in the mouse embryo. The distribution of the enzymes and receptors, and recent experiments to determine the endogenous distribution of RA in the embryo are described here, as well as the effects on the embryo of knocking out the enzymes and receptors. In addition, recent studies using the classical vitamin A-deprivation technique are described, as they have provided novel insights into the regions of the embryo which crucially require RA, and the gene pathways involved in their development. Finally, the post-embryonic or regenerating systems in which RA plays a part are described, i.e. the regenerating limb, lung regeneration, hair cell regeneration in the ear and spinal cord regeneration in the adult.