During every menstrual cycle the extracellular matrix in the uterus undergoes dramatic changes associated with release and shedding of the endometrium. Classical investigations made in monkeys assumed that menstruation is caused by vasoconstriction, hypoxia and subsequent necrosis. More recent work indicates the importance of the system of matrix metalloproteinases (MMP) for the extracellular architecture of the endometrium. The submitted paper presents a review of crucial characteristics of the MMP system and its importance for cyclic transformation of the endometrium during the menstrual cycle. From the review it is apparent that impairment of the MMP equilibrium and its tissue inhibitors are an essential prerequisite of tissue degradation and menstruation. MMP activation is requlated by hypoxia, a drop progesterone and paracrine factors released by epithelial, stromal and migratory cells.

        Key words: metalloproteinase, endometrium, menstrual cycle