ABSTRACT
There is increasing interest in the role of the endometrium in trophoblast attachment and invasion during implantation.1,2 The endometrium represents a barrier to implantation except under appropriate and defined hormonal conditions. To understand the regulatory biology that governs uterine receptivity requires an appreciation not only of the many endometrial peptides, cytokines, and signaling molecules that are expressed during the reproductive cycle, but also of the eloquent interplay between the embryo and the endometrium. Synchrony between the corpus luteum, endometrium, and embryo appears to be a critical element of normal implantation. The developmental strategy of the blastocyst requires cooperation with the maternal tissues that is highly regulated and temporally coordinated. Because of this complexity, it is not surprising that these mechanisms sometimes fail, resulting in pregnancy loss or infertility. By understanding the role of the endometrium in defining receptivity for the embryo, we begin to understand how the process of implantation itself works.
