Endometrial leukocytes and menstruation

This review examines evidence supporting the concept that menstruation occu rs as a result of an inflammatory process, In the endometrium, leukocyte nu mbers rise in the late secretory phase following the fall in serum progeste rone concentrations. It is postulated that products released following acti vation of these leukocytes are critically important for menstruation. Mast cells, eosinophils, neutrophils and macrophages in particular are involved. Endometrial granular lymphocytes may also play a role, although their incr ease in numbers is somewhat earlier during the menstrual cycle than that of the others, suggesting perhaps a primary role in embryo implantation. Leuk ocyte products include a range of proteases, chemokines and cytokines which in concert result in focal production and activation of matrix metalloprot einases by endometrial cells and the subsequent breakdown of tissue that ch aracterizes menstruation. Regulation of leukocyte entry, proliferation, dif ferentiation and activation within the endometrium is not Set well understo od, although both chemokines and cytokines produced locally by endometrial cells are clearly implicated. The role of progesterone in regulating these events is still not understood although the lack of progesterone receptors on endometrial leukocytes suggests indirect actions.