Effects of ovarian hormones on endocytosis at the basal membranes of rat uterine epithelial cells.

Ovariectomized rats treated with progesterone and estradiol-17 beta in sequences that prepared the uterus for implantation showed marked increases in the number of endocytotic invaginations in the basal membranes of luminal epithelial cells and in the amount of intravenously administered horseradish peroxidase taken into the cells. Tight junctions between the epithelial cells blocked the direct passage of horseradish peroxidase into the uterine lumen. The number of basal pinocytotic invaginations in glandular epithelial cells was relatively small and did not increase following hormonal stimulation. The probable occurrence of a pathway for the transepithelial transport of macromolecules from the blood or stroma into the uterine lumen, and its sensitivity to stimulation by ovarian hormones, suggests a likely mechanism for altering the molecular environment of the embryo during the implantation period.