Current concepts of the mechanisms of menstruation.

Menstruation is the process whereby the superficial or functionalis layer of the endometrium lining the uterine cavity disintegrates and is removed from the uterine lumen towards the end of the luteal phase of a non-pregnant cycle. Within a period of 5-6 days the old lining is removed and a new lining regenerated without scarring occurring, a remarkable example of controlled tissue remodelling unparalleled in other organs. This chapter describes the phenomenon of menstruation and tissue repair and reviews the vasoconstrictor and inflammatory hypotheses for the initiation of menstruation. Particular emphasis is placed on the roles of steroids, prostaglandins, endothelin, lysosomal enzymes and matrix metalloproteinases, and the involvement of the fibrinolytic system and apoptosis. Repair of the endometrium is compared to the processes and mechanisms of wound healing.