Vitamin A and embryonic development: an overview.

Vitamin A is an essential micronutrient throughout the life cycle. Its active form, retinoic acid via retinoid receptors, is involved in signal transduction pathways regulating development. Both the lack and excess of vitamin A during embryonic development result in congenital malformations. Approaches to examine the function of vitamin A in embryonic development have included treatment with excess retinoids and the use of retinoid receptor knock-out mice, which have provided important insights into the complexity of the retinoid signaling system. A recently explored model is the retinoid ligand knock-out, i.e., the vitamin A-deficient embryo. Early development can be successfully examined in the vitamin A-deficient avian embryo, in which bioactive retinoids can rescue the deficient genotype as well as phenotype. In this model it has been possible to unequivocally link the physiological function of vitamin A to development of heart, embryonal circulatory and central nervous systems and the regulation of heart asymmetry. Several developmental genes regulated by endogenous vitamin A during early embryogenesis have been identified. Retinoid receptors and their endogenous ligands, the vitamin A-active forms, are present in the early embryo. It is the developmentally regulated biogeneration of the vitamin A-active forms via distinct spatio-temporal metabolic pathways that is critically linked to the initiation of retinoid signal transduction during embryonic development.