Abstract: Objective：To analyse the expression of polarity protein Par3, and its possible effects, in the mouse endometrium during embryo implantation. Methods：The mice models of early pregnancy （1-8 days of pregnancy）, pseudo-pregnancy and the steroid treatment （steroid injection in the emasculated mice） were used in this study. The expression of Par3 in uterus were tested by immunohistochemistry and analyzed by Image-Pro Plus system. Results： Expression of Par3 was increased in the uterine luminal epithelium on 1 to 4 days of early pregnancy, and then lowered on the day 5 of pregnancy. Interestingly, the positive expression of Par3 was found in the secondary decidual zone on the day 6 to 8 of pregnancy, while the negative expression in the primary decidual zone. Pseudo-pregnant models have a similar expression pattern as the early pregnant models, with low expression levels. In ovariectomized mice, both E2 and P4 treatment, or the combined treatment, could down-regulate the expression of Par3 in the luminal epithelium. Conclusions： The expression of Par3 in the uterine luminal epithelium of early pregnancy can be briefly down-regulated by E2 and P4, which is benefit for embryo implantation.

Key words: Embryo implantation, Endometrium, Cell polarity, GTP-binding proteins, Mice