Disappearance of anionic sites from the surface of the rat endometrial epithelium at the time of blastocyst implantation

Studies have been carried out to analyze possible changes in the distribution of anionic sites on the endometrial epithelium that might play a role in blastocyst implantation. Anionic sites were stained in nonfixed specimens at physiological pH and ionic strength utilizing polycationic ferritin and were subsequently observed by electron microscopy. Experiments were carried out using both in situ and in vitro labelling conditions, with either pregnant or pseudopregnant rats. There was a general reduction of anionic sites from Days 2-6 of pregnancy or pseudopregnancy until at Day 6 anionic sites were no longer detected. The results were similar on the mesometrial and antimesometrial sides of the uterus and on presumptive implantation sites (visualized by pontamine blue staining) and elsewhere on the endometrial epithelium. These findings suggest that the uterus undergoes a hormonally controlled loss of cell surface anionic sites, which may be related to the ability of the blastocyst to implant about Days 5-6 but not earlier.