Distribution of endogenous retinoids, retinoid binding proteins (RBP, CRABPI) and nuclear retinoid X receptor beta (RXRbeta) in the porcine embryo.

Retinoids are important signalling molecules in the development of limbs and in the determination of the anterior-posterior orientation of the embryo. The present study examined the content and distribution of retinoic acid, retinol and retinyl esters in porcine embryos during early gestation (gestation days 22-30) macroscopically and microscopically by its autofluorescence and by HPLC. Macroscopically, the yellowish-greenish autofluorescence characteristic of vitamin A was observed in tissues affected by morphogenesis, such as the limbs, in a spatial and temporal manner. Changes in the intensity of autofluorescence in the limbs paralleled changes in the concentration of retinoids in these structures. In the limbs and the body, retinol, retinyl palmitate, and all-trans-retinoic acid but neither the isomers of all-trans retinoic acid nor other retinoid metabolites were detected. In addition, the distribution of specific retinoid-binding proteins was investigated; these are involved in vitamin A transport, metabolism and signal transduction. Immunoreactive retinol-binding protein as well as cellular retinoic acid binding protein type I were only localised in the mesonephros, while the retinoid X receptor beta was widely distributed in most of the tissues and organs of the embryo throughout the time period investigated. The combination of autofluorescence and HPLC analysis allowed for the first time to attribute the yellowish-greenish autofluorescence in specific regions of the embryo to vitamin A, and offers a method to study the local cellular distribution of retinol and/or retinyl esters as well as their concentrations in embryonic tissues.