Model Systems for the Study of Vitamin A Function in Embryonic Development.
Vitamin A deficiency and excess have profound effects on the development of the vertebrate embryo (3
4 5 6 7 8) . Almost every organ or tissue can be affected by RA if the embryo is treated with it at a critical time in development (16) . A vast amount of information has been gathered on vitamin A function at the molecular level from cell culture (4 ,10 ,17) and from in vivo studies by perturbing normal embryonic development with exogenous retinoids (4 5 6 7 8 ,10) . The abnormalities observed in these studies, when viewed together with similar abnormalities reported for the vitamin A–deficient (VAD) embryo and fetus, identify common development pathways that are severely disrupted by an imbalance in vitamin A status. The function of vitamin A in development has been addressed recently by the use of transgenic mice with changes in retinoid receptor gene structure (4 ,9 ,18 19 20) . Many of the abnormalities in these mutant mice resemble those observed in fetuses from the VAD animals reported earlier. These studies have provided valuable insights into the pleiotropic effects of vitamin A and have demonstrated the critical role of retinoid receptors in vertebrate ontogenesis. The interpretation of the data has been complicated by retinoid receptor redundancy and by the broad role of the RXR as coreceptors for other nuclear receptors (4 ,9) .
An important approach to the examination of molecular mechanisms of retinoid action in developmental regulation is the use of in vivo embryo model systems in which the function of vitamin A has been diminished or completely eliminated by removing the vitamin. The absolute essentiality of vitamin A for embryogenesis is most clearly demonstrated in the VAD avian embryo, i.e., the quail embryo retinoid ligand knockouts. The completely VAD embryos develop gross abnormalities in the cardiovascular and central nervous systems and trunk and die by d 4 of embryonic life (3 ,4 ,6 ,7 ,21 22 23) . Importantly, the VAD embryo can be "rescued" and normal development restored by administration of the physiologic ligand for RAR, all-trans-retinoic acid, or its precursor, retinol. Bioactive retinoids must be administered to these embryos during early development so as to be present during the critical window of time in which important developmental events are specified, i.e., the formation of heart, cardiovascular system, hindbrain, foregut and probably other events (21 ,23) . With this model, it is possible to examine morphological, anatomical and molecular biology aspects during development that are solely attributable to vitamin A. The ability to rescue the VAD embryo at a precise time during development makes the avian retinoid ligand knockout model a powerful tool for the elucidation of vitamin A function during early development.
Using rats as a mammalian model, it is possible to obtain near vitamin A deficiency in the dams and to target embryonal vitamin A insufficiency to distinct gestational windows (3 ,4 ,6 ,7 ,24 25 26) . These rat embryos exhibit specific cardiac, limb, ocular and central nervous system (CNS) abnormalities, some of which have certain features similar to those reported in retinoid receptor knockout mice (25) . Abnormalities in hindbrain development of the rat embryos (26) are similar to those reported in VAD quail embryos (7 ,27) and suggest that even partial vitamin A deficiency affects the sensitive developing CNS. These studies have also revealed the importance of vitamin A in fetal lung and kidney development (24 ,28 ,29) .
Another in vivo approach to eliminate vitamin A–active forms is to block RA with an anti-RA antibody (30) or to inactivate the RA generation pathways (4 ,7) . Knockout mice embryos of the RA synthesizing enzyme RALDH2 (11) have many abnormalities similar to those of the VAD quail embryo, but complete vitamin A deficiency was not obtained, probably due to the presence of other RA-generating systems. Partial depletion of RA may also be achieved by an overexpression of CYP26, an enzyme that degrades RA (4 ,13) .
Answers to all your Biology Questions
