Endometrial Receptivity to Embryo Implantation: Molecular Cues from Functional Genomics

The endometrium is the mucous lining the uterine cavity comprised of a basal and a functional layer being the latter the one that sheds during menses and regenerates from the basal portion. The main cell populations within the functional stratus are epithelial and stromal cells accompanied by a variable number of leukocytes. Epithelial cells are found covering the luminal surface and tubular glands in basal and functional layers. Endometrial stroma contains reticular connective tissue comprised mainly by uterine fibroblasts that rapidly differentiate into decidualized cells when stimulated by an implanting blastocyst. The stromal compartment contains also abundant lymphocytes, granulocytes and macrophages during luteal phase of the menstrual cycle. These cells along with epithelial and stromal fibroblasts are source and target of paracrine signals of proliferation and differentiation. Both components respond to ovarian steroid hormones and depend on each other for their structure, function and responsiveness to estrogen (E2) and progesterone (P4) (Tabibzadeh, 1998, review). During a normal menstrual cycles, human endometrium display unique features for an adult tissue: undergoes cyclic construction and sloughing. The outer layer of the endometrium is loss while the basal layer containing the deep glandular epithelium gets preserved. Later on, stem cells located in this layer will originate the various endometrial cell types in response to the appropriate hormonal stimulus, regenerating the whole endometrium (Padykula, 1991).