Molecular Lesions in Infertility

Abstract

Exquisite mechanisms control the processes that drive endometrium through the characteristic phases of the menstrual cycle and prepare this tissue for the implantation (1). Implantation is a complex process that initially requires the interaction of the blastocyst and subsequently, the developing embryo and placenta, with the endometrium. In implantation, both endometrial and embryonic factors are involved. During the menstrual cycle, endometrium becomes receptive to implantation during a defined period called the “receptivity period” or “implantation window.” Outside this window, endometrium remains either neutral or refractory to implantation (2–3). Therefore, the acquisition of endometrial receptivity is a regulated process. A receptive endometrium is likely to establish a dialogue with the implanting blastocyst and to actively participate in the implantation process. For example, the trophoblasts of the implantation state blastocysts can invade, quite deeply, tissues other than endometrium (4). Despite the invasive behavior in vitro, in some animals, such as the pig, the trophoblasts attach to—but do not invade—the uterine epithelium (5). When transplanted ectopically in the uterine wall, however, the same cells can invade tissues. Therefore, it is likely that endometrium is actively involved in the regulation of the blastocyst implantation.