Blastocyst's state of activity determines the "window" of implantation in the receptive mouse uterus.

Abstract

The present investigation studied the influ-

Uterine RAC1 via Pak1-ERM signaling directs normal luminal epithelial integrity conducive to on-time embryo implantation in mice

“…Implantation sites were visualized and recorded by routine blue dye method. 4 Mouse blood samples were collected on day 4 morning and serum progesterone and 17β-estradiol levels were measured by radioimmunoassay.…”

“…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.…”

“…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.…”

Changes in anandamide levels in mouse uterus are associated with uterine receptivity for embryo implantation

“…The fundamental feature of this process is the synchronized development of the embryo to the activated state of the blastocyst, its escape from the zona pellucida, and differentiation of the uterus to the receptive state (15,16). This results in ''two-way'' interactions between the blastocyst and the uterus to initiate the attachment reaction that occurs at 2200-2300 h on day 4 of pregnancy in the mouse (17).…”

“…This is followed by stromal cell decidualization at the sites of blastocysts (18). In this species, uterine environment with respect to implantation is divided into prereceptive, receptive, and nonreceptive (refractory) phases (15,16,19). The ''window'' of uterine receptivity for implantation occurs only for a limited period during pregnancy or pseudopregnancy (16).…”