ABSTRACT
The endometrium undergoes dynamic reorganization during the menstrual cycle in preparation for implantation. If no implantation occurs the superficial layer is partially or completely shed and remodeled in preparation for the next cycle (Wynn, 1989). Our understanding of the physiology and molecular biology of the endometrium has increased greatly over the past decade (Edwards, 1995). A myriad array of compounds has been identified in the endometrium, some of which exert local effects which may be important in implantation and in maintaining the integrity of the nonpregnant endometrium. These compounds are diverse in nature and functions, and include enzymes, hormones and bioactive peptides (Edwards, 1995; Tabibzadeh, 1996; Hulboy et al., 1997). The net effect of this interplay is to provide a unique environment which allows or prevents implantation and subsequent nidation (Psychoyos, 1995). Successful implantation and subsequent nidation requires synchronization of endometrial maturation and embryonic development (Beier-Hellwig et al., 1995). A concept that has become central to our understanding of endometrial function is that of the implantation window, a period of optimal endometrial receptivity (Psychoyos, 1995). The extent of this window and the physiological, molecular and anatomical events in the endometrium that frame it are still being defined.
