Progesterone-mediated endometrial maturation limits matrix metalloproteinase (MMP) expression in an inflammatory-like environment: a regulatory system altered in endometriosis.

The human endometrium exhibits regular cycles of growth, differentiation, and breakdown in response to changing levels of ovarian steroids. Following the tissue loss and repair processes of menstruation, rising levels of estradiol initiate a development-like process leading to a complete restructuring of the endometrial surface. In contrast, while under the predominate influence of progesterone, proliferation declines as cell-specific differentiation prepares the endometrium for pregnancy over a 5- to 6-day period. In the absence of nidation, steroid support is lost; the endometrial surface begins a complex process of tissue breakdown and bleeding, producing a viscous mixture of cellular debris within a bloody menstrual effluent. Although most of the menstrual fluid exits the body, reflux of some material occurs in most women, providing a poorly understood opportunity for ectopic endometrial growth and establishment of the disease endometriosis. The cyclic restructuring of the endometrium requires numerous matrix metalloproteinases (MMPs) that mediate normal and pathological tissue turnover throughout the reproductive tract. The expression of multiple MMPs facilitates degradation of extracellular matrix during growth-related remodeling as well as tissue breakdown at the time of menstruation. However, these enzymes are absent during the early and mid-secretory phase and the suppression of endometrial MMPs remains important to maintaining the integrity of the endometrium during the highly invasive events required to establish a normal hemochorial placenta. Several research groups have suggested that steroid-mediated expression and action of MMPs during the menstrual cycle may provide a key mechanistic link between endometrial turnover and the invasive processes necessary for establishment of endometriosis.