Retinoids in embryonal development

The key role of vitamin A in embryonal development is reviewed. Special emp hasis is given to the physiological action of retinoids, as evident from th e retinoid ligand knockout models. Retinoid metabolism in embryonic tissues and teratogenic consequences of retinoid administration at high doses are presented. Physiological and pharmacological actions of retinoids are outli ned and explained on the basis of their interactions as ligands of the nucl ear retinoid receptors. Immediate target genes and the retinoid response el ements of their promoters are summarized. The fundamental role of homeobox genes in embryonal development and the actions of retinoids on their expres sion are discussed. The similarity of the effects of retinoid ligand knocko uts to effects of compound retinoid receptor knockouts on embryogenesis is presented. Although much remains to be clarified, the emerging landscape of fers exciting views for future research.