Ultrastructural studies on maternal-embryonic cell interaction during experimentally induced implantation of rat blastocysts to the endometrium of the mouse.

Rat blastocysts collected around noon on Day 5 of pregnancy were transferred to the uterus of the mouse on Day 3 of pseudopregnancy. Out of 430 rat blastocysts transferred, a total of 37 were recovered as xenogeneic implants from the recipient mice killed 36, 48, 52, 58, 72, 96, and 120 hr after transfer. None of the transferred blastocysts was found surviving in the host uterus beyond 96 hr after transfer. Electron microscopic examination of the recovered implants revealed that rat blastocysts can successfully undergo the stages of ovum implantation in the mouse uterus from the early attachment to the initial phase of the trophoblastic invasion of the endometrium. During the late attachment stage, desmosomes (maculae adhaerentes), intermediate junctions (zonulae adhaerentes), and gap junctions (nexuses) were formed xenogeneically between the foreign trophoblast and the uterine epithelial cells of the host. Trophoblast cells of xenogeneic implants were destroyed shortly after the penetration of the basement membrane of the luminal epithelium of the host endometrium, leading to the degeneration and sloughing off of the transferred embryos.