Embryo-dependent induction of uterine receptivity assessed by an in vitro model of implantation in mice.

To investigate the molecular basis of initial interactions between the trophoblast and the uterine epithelium at implantation, we developed an in vitro model for implantation in mice using primary uterine organ cultures as the substrate. In this model, embryos attached to the endometrial explants within 60 min. We were able to observe the attachment between embryos and endometrial explants in real time. Using this model, we investigated the attachment of embryos to uterine strips cultured from pregnant mice, pseudopregnant mice, pseudopregnant mice that had received hCG injections, and pregnant mice whose utero-tubal junctions had been ligated on Day 2 of pregnancy. The serum levels of progesterone in pregnant and pseudopregnant mice were also compared. The percentages of embryos that attached to uterine strips cultured from pregnant mice on Days 1-4 were low (0.0-15.0%). However, by Days 5-7, percentages of attachment increased markedly (66.8-74.8%). These levels were significantly higher than those observed for pseudopregnant mice (0.0-18.0%) and pseudopregnant mice that had received hCG injections (0.0-15.0%). The percentages of attachment to the ligated horns of pregnant Day 5 uteri (58.3-61.5%) were significantly higher than that of pseudopregnant mice (10.0%). The percentages of attachment to the ligated horns of pregnant Day 6 and 7 uteri decreased significantly (3.2%-19.4%). On the other hand, the percentages of attachment to the unligated horns of pregnant Day 6 and 7 mouse uteri did not decrease significantly (61.1-70.0%). The serum progesterone levels of pseudopregnant mice were not significantly lower than those of pregnant mice on Days 1-5.(ABSTRACT TRUNCATED AT 250 WORDS)