In Vitro Analysis of Epithelial Surface Changes During Implantation

Blastocyst implantation involves interaction between two independently controlled yet highly interdependent systems, the embryo and its maternal environment. In rodents the preimplantation blastocyst becomes closely surrounded by uterine epithelium in an “implantation chamber” (Enders, 1975), which precedes the first irreversible interaction between the embryo and uterus — adhesion between the apical surfaces of the first embryonic epithelium (trophectoderm) and the uterine epithelium (“1” in Figure 1). In mice, adhesion occurs about 100 hours after the morning in which the coital vaginal plug is discovered (Potts, 1968). In rats this is brought about by a nidatory surge of estradiol, accompanying increasing levels of progesterone (Psychoyos, 1973). There is no clear separation in time between the appearance of apical adhesion and the initiation of basolateral changes, which occur between about 3.5 and 4.5 days after mating (“2” in Figure 1). These changes are characterized by a loosening of lateral cell associations, involving ultimately the junctional complexes (“3” in Figure 1) and a separation of the epithelium from its basal lamina (Schlafke et al., 1985), preparatory to its subsequent sloughing and cell death (Parr et al., 1987).