Blastocyst surface charge and implantation in the mouse

During delay of implantation, the mouse blastocyst possesses several negatively charged sites on its surface and it is surrounded closely by the uterine epithelium. After activation for implantation by an injection of estrogen, the surface charge of the blastocyst remains unchanged for 8 hrs after the injection, although the blastocyst now lies free in the uterine secretion. Fourteen hrs after the injection, the surface charge is slightly less negative,but at 20 hrs the number of negative sites is markedly reduced. At this stage the blastocyst has resumed close contact with the uterine surface and has begun to invade the endometrium.It is concluded that the blastocyst can form close contact with the uterine surface both when inactive and possessing many negative surface sites and also when invasive and possessing few negative sites. The change in surface charge, which precedes blastocyst attachment, seems to occur simultaneously with a decrease in trophoblast antigenicity. Attention is drawn to the similarity in changes for the trophoblast at implantation to those of cultured cells at transformation.