Endocytosis at the basal and lateral membranes of rat uterine epithelial cells during early pregnancy.

By 20 min after intravenous injection of horseradish peroxidase on Day 5 of pregnancy the tracer was present between the lateral epithelial cell membranes, in coated and non-coated invaginations in the lateral and basal membranes of epithelial cells, and in pinocytotic vesicles at the periphery of epithelial cells. By 60 min after injection the tracer was also distributed in vesicles throughout the epithelial cell cytoplasm. By 2 h the tracer was present only in a few vesicles in the apical region of epithelial cells, and some of these vesicles appeared to be fusing with the apical cell membrane. Pinocytotic invaginations in the basal membrane of uterine epithelial cells were 4--5-fold more numerous on Day 5 of pregnancy than on Days 1--4 and 7, suggesting a specific role for this activity during early pregnancy. The observations show that uterine epithelial cells can pinocytose substances derived from blood, transport them to the apical cytoplasm, and release them into the uterine lumen by exocytosis.