Uptake and fate of ferritin in the uterine epithelium of the rat during early pregnancy.

The ingestion of ferritin injected into the uterine lumen for 20 min was followed by light and electron microscopy. Uptake occurred on Days 4 to 6, with a distinct peak of activity on Day 5 when blastocysts are present in the uterine lumen. At this time, the antimesometrial cells took up more tracer than those on the mesometrial side. The ferritin was located in various types of organelles which contained acid phosphatase, indicating that the tracer entered the lysosomal system. Ferritin-containing lysosomes were essentially all located in the apical half of the cells at 1 h after the injection, but thereafter were distributed throughout the cell. There appeared to be some digestion of ferritin by 48 h, but no evidence for exocytosis of the tracer at the base of the cells was found with light or electron microscopy.