The Endometrium

The endometrium is the mucosal lining of the uterine cavity. It has two functionally different layers: - The functionalis is the hormonally responsive surface layer. It thickens, becomes secretory, supports implantation, decidualises in pregnancy and is shed during menstruation. - The basalis lies next to the myometrium. It is retained during menstruation and provides the tissue from which the next functionalis regenerates. This arrangement explains both normal cycling and some pathology. Superficial shedding is expected; destruction or scarring that reaches the basalis can impair future regeneration and fertility. Menstruation is an active inflammatory process, not simply the passive wearing away of tissue. When the corpus luteum regresses in the absence of pregnancy, progesterone levels fall. Progesterone withdrawal promotes inflammatory-cell influx, endothelin-1 and prostaglandin F2-alpha activity, and intense spiral-arteriole vasoconstriction. Reduced perfusion produces hypoxia and reactive oxygen species; matrix metalloproteinases then break down the extracellular matrix. The damaged functionalis separates and is expelled, while the basalis remains available for repair. The clinical consequence is a useful one: bleeding caused by normal cyclical shedding is not the same pattern as bleeding from...