Transfer of uterine implantation blastocysts to the oviduct in mice.

Although ectopic pregnancy is a common occurrence in humans, it occurs only rarely in other species. In the mouse, confinement of the blastocyst to the oviduct fails to result in implantation at that level. In prior studies, these confined blastocysts had not been exposed to the uterine environment. We transferred blastocysts from the uterus, at a time when implantation would normally occur, to ligated oviducts of pseudopregnant mice. Although enlargement of the blastocysts, loss of the zona pellucida, and adherence to the tubal epithelium were noted, implantation did not occur. The contributions to the embryo by the uterine environment were not sufficient to allow implantation within the oviduct.