ABSTRACT
Human endometrium undergoes cyclic, hormonally driven changes in tissue morphology and functional differentiation.1,2 Following menstruation, estrogen drives proliferation of the remaining epithelium and underlying stroma to regenerate the full thickness of the mature endometrium. Predominance of progesterone during the ovarian luteal phase pushes the epithelial cells towards a secretory phenotype and increases the complexity of the glandular structures within the endometrium. During the mid-to lateluteal phase, the subepithelial stromal cells undergo a round of proliferation and begin differentiation towards a decidualized phenotype. In addition, while under the influence of progesterone and estrogen, uterine spiral arteries grow, increasing in both length and tortuosity. Ovarian hormones also regulate infiltration of lymphoid cells into the endometrial stroma, particularly in the subepithelial, decidualizing region. All of this growth, differentiation, and tissue patterning takes place in preparation for the arrival and eventual implantation of the developing embryo. The secretory epithelial cells provide nutrients and stimuli that are required for maturation of the embryo. Proteins elaborated by the uterine epithelium activate the embryo to initiate attachment and implantation. The decidualized stromal cells participate in the formation of the maternal placenta and they secrete proteins that regulate the implantation reaction of the invading trophoblasts.
