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 physiologicalpH and ionicstrengthutilizingpolycationicferritin and were subsequently observed by electronmicroscopy. Experiments were carriedout usingboth in situ and in vitrolabelingconditions, with either pregnant or pseudopregnant rats. There was a general reduction of anionic sitesfrom Days 2-6 of pregnancy or pseudopregnancy untilat 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(visualizedby pontamine blue staining)and elsewhere on the endometrial epithelium. These findingssuggest that the uterus undergoes a hormonally controlledlossof cellsurface anionic sites,which may be relatedto the abilityof the blastocystto implant about Days 5-6 but not earlier.