Successful embryo implantation requires functional luminal epithelia to establish uterine receptivity and blastocyst-uterine adhesion. During the configuration of uterine receptivity from prereceptive phase, the luminal epithelium undergoes dynamic membrane reorganization and depolarization. This timely regulated epithelial membrane maturation and precisely maintained epithelial integrity are critical for embryo implantation in both humans and mice. However, it remained largely unexplored with respect to potential signaling cascades governing this functional epithelial transformation prior to implantation. Using multiple genetic and cellular approaches combined with uterine conditional Rac1 deletion mouse model, we demonstrated herein that Rac1, a small GTPase, is spatiotemporally expressed in the periimplantation uterus, and uterine depletion of Rac1 induces premature decrease of epithelial apical-basal polarity and defective junction remodeling, leading to disrupted uterine receptivity and implantation failure. Further investigations identified Pak1-ERM as a downstream signaling cascade upon Rac1 activation in the luminal epithelium necessary for uterine receptivity. In addition, we also demonstrated that Rac1 via P38 MAPK signaling ensures timely epithelial apoptotic death at postimplantation. Besides uncovering a potentially important molecule machinery governing uterine luminal integrity for embryo implantation, our finding has high clinical relevance, because Rac1 is essential for normal endometrial functions in women. The implantation of the blastocyst into the maternal uterus is a crucial step in establishing pregnancy and thus ensuring further embryonic development. 1-3 Similar to many developmental processes, implantation involves an intricate succession of molecular and cellular interactions which must be executed within an optimal time frame. During this period, the acquisition of blastocyst implantation competency is synchronized with the establishment of uterine receptivity. [4][5][6] On the basis of previous findings, uterine sensitivity to implantationcompetent blastocysts is classically divided into three stages: pre-receptive, receptive and refractory phases. 7 In mice, prior to day 4 of pregnancy, the uterus is conventionally considered as the pre-receptive phase. On day 4 and beyond, the uterus becomes fully receptive following the priming actions of ovarian progesterone and preimplantation estrogen; whereas by late day 5, the uterus becomes refractory to initiate the implantation. 2,4,8 Upon entering the receptive phase, uterine luminal epithelium undergoes dynamic transformation. For example, on day 4 morning in mice, luminal epithelial cells cease proliferation under the dominance of increasing levels of ovarian progesterone and preimplantation estrogen. Meanwhile, the epithelial cells undergo differentiation, accompanied with a dynamic junction complex remodeling and membrane maturation, leading to a decrease of epithelial polarity approaching implantation and a cell-shape transition from...