Matrix metalloproteinases in normal menstruation

Matrix metalloproteinases (MMP) are strongly implicated in menstruation, Messenger RNA for proMMP-1 and -3 was detectable in normal cycle endometrium only peri-menstrually and menstrually, although mRNA for their tissue inhibitors, TIMP-1 and TIMP-2, was present throughout the cycle, MMP-1, -3 and -9 were demonstrated immunohistochemically to be specifically associated with degraded tissue in menstrual endometrium, Activated mast cells and eosinophils, which release regulators of MMP expression and activators of latent enzymes, were also a marked feature of menstrual endometrium. Cultured endometrial stromal cells released MMP-1, -2, -3 and -9 and TIMP-1 and -2, whereas production by epithelial cells was minimal, Progesterone withdrawal from stromal cell cultures (for the final 4 days of a 10 day culture) increased the release of all four enzymes: all but MMP-2 were also stimulated by interleukin-1 or tumour necrosis factor alpha added to short-term stromal cultures, We postulate that an alteration in the balance of MMP and their inhibitors and the activation of MMP are prerequisites for tissue degradation at menstruation, and that this is regulated by a combination of progesterone withdrawal and paracrine factors from epithelial and stromal cells and from mast cells and eosinophils.