RAT YOLK-SAC EXPLANTS AS A SYSTEM FOR STUDYING THE REGULATION OF ENDODERMAL GENES - DOWN-REGULATION OF THE ALPHA-FETOPROTEIN GENE BY DEXAMETHASONE AND PHORBOL ESTER

The visceral yolk sac is a fetal membrane with essential placental fun ctions. It is the major site of synthesis of alpha-fetoprotein (AFP), the most abundant plasma protein in the fetus. We developed a system o f rat yolk sec explants in serum-free culture medium to study the regu lation of endodermal gene expression in yolk sac, The explanted yolk s ac tissues retained their double-sided morphology for up to 48 hours. The epithelial cells of both layers remained tightly joined on a basem ent membrane as seen by light and electron microscopy. This probably a ccounts for the continued expression of several endodermal cell-specif ic markers, The levels of mRNA encoding AFP, vitamin D-binding protein (DBP), hepatocyte nuclear factor 1 alpha and beta transcription facto rs did not change during the 48-hour culture period. This reflects the stability of the differentiation state of the yolk sac endodermal cel ls. Dexamethasone and phorbol ester (TPA) specifically reduced the AFP mRNA level without affecting that of DBP, This suggests that these tr ansduction pathways are functional in the yolk sac during this period of gestation and could be involved in the physiological down-regulatio n of AFP gene expression before birth, AII these results show that thi s serum-free culture of rat yolk sac explants is a valuable system for further investigating the action of natural compounds and pharmacolog ical drugs on endodermal gene expression during the embryonic and feta l periods.