ABSTRACT
The basic biology of the human endometrium is intimately linked to cyclic ovarian steroid production, creating a unique endometrial architecture which supports and controls the highly invasive events of implantation and placentation. During a woman’s reproductive life the endometrium consists of morphologically and functionally distinct layers: the stratum basalis, which lies adjacent to the myometrium, the stratum spongiosum or intermediate layer, and the stratum compactum. It is from the stratum basalis that the surface of the endometrium regenerates after each episode of cyclic tissue shedding, whereas the two uppermost layers, referred to jointly as the stratum functionalis, undergo coordinated histologic and cytologic changes in preparation for pregnancy.1 In the absence of nidation, steroids drive the reassembly of the endometrial stratum functionalis by coordinating distinct patterns of cell-cell communication across each phase of the menstrual cycle. Although rare among adult tissues, extensive tissue restructuring is a hallmark of the human reproductive cycle and numerous matrix metalloproteinases (MMPs), as well as their natural inhibitors (TIMPs), exhibit cellspecific and cycle-dependent expression in both the ovary and the endometrium.2
