To evaluate the intrauterine environment on embryo development, we examined the effects of co-culture with uterine epithelial cells and of uterine fluids on mouse embryo development in vitro. In the co-cultures of mouse uterine epithelial cells and 8-cell embryos, the rate of expanded blastocyst after 24 h of co-culture and the rates of hatching blastocysts 48 h and 72 h after the initiation of co-culture (29.4%, 56.7% and 78.9%, respectively) were significantly higher than those in the control

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... (11.3%, 26.5% and 49.7%, respectively). The incorporation of L-[4,5-3H] leucine by the morulae after 24 h of co-culture was significantly higher than that of the control (an increase of 50.1%). The similar results were obtained in the cultures of 8-cell embryos in the medium added with uterine fluids. Moreover, the stimulating effect was exerted not only by uterine fluids from random stages of the estrous cycle but also by the fluids collected in three different stages, suggesting that the effect observed in this study might be independent of the estrous cycle. These results suggest that the intrauterine environment promotes the embryo development throughout the estrous cycle.