RETINOID-BINDING PROTEINS IN MOUSE YOLK-SAC AND CHORIOALLANTOIC PLACENTAS

In the adult, as well as in the embryo, a number of specific extra- an d intracellular binding proteins such as the plasma retinol binding pr otein (REP), the cellular retinol binding protein type I (CRBP I), and also the cellular receptors for REP are thought to regulate transport and metabolism of retinol (vitamin A). Since the regulation of matern o-fetal transport of vitamin A is not well understood, we examined the localization of these proteins during the development of the mouse ch orio-allantoic and yolk sac placentas. The labyrinthine region of the chorio-allantoic placenta, where exchange of substances can occur betw een the maternal and fetal circulations, did not contain REP (mRNA or protein) or antigen(s) similar to the bovine REP-receptor p63, whereas the visceral endoderm of the yolk sac placenta, the second site for m aterno-fetal transport, did. Furthermore, only the endodermal cells of the visceral yolk sac appeared to strongly accumulate radiolabelled r etinoids. The cellular retinol binding protein (CRBP I) was detected b oth in the trophoblast layer of the placental labyrinth closest to the fetal endothelium (layer III), and in the visceral endoderm of the ye ll; sac. Together, these findings suggest that the yolk sac placenta m ediates retinol transfer to the embryo/fetus throughout the entire ges tation. The chorio-allantoic placenta, on the other hand, does not app ear to have this capacity, while the presence of CRBP I does suggest a retinol-metabolizing capability.