The Mechanism of Menstruation

Abstract

Menstruation is a result of the profound tissue remodeling that occurs each month in reproductive-aged women. After withdrawal of steroid hormone support, the functionalis layer of the endometrium undergoes extensive changes, resulting in complete tissue breakdown. With each menstrual cycle, most of the endometrium is completely shed at menses and subsequently regenerated. Menstruation is seen in only a few animals that have hemochorial placentation. In hemochorial placenta-tion, trophoblasts invade the maternal blood vessels and chorionic villi are in direct contact with maternal blood. Thus the invasive nature of hemochorial placenta requires a correspondingly defensive uterus. In pregnancy, under the influence of progesterone, the endometrial stroma undergoes extensive decidualization. Decidualization limits trophoblastic invasion; however, protection from invasive trophoblasts requires the development of a barrier, a process that results in terminal differentiation. This differentiated state is renewable only by regeneration from progenitor cells located in the basalis layer of the endometrium; a strategy that requires monthly bleeding events and introduces multiple potential opportunities for mechanistic failure and the emergence of abnormal uterine bleeding. An appre-ciation of normal endometrial physiology as it pertains to the regulation of menstruation is essential to understand disorders of menstruation. The Endometrium During the Menstrual Cycle The endometrium is composed of the basalis and functionalis layers. The basalis layer is deep and adjacent to the myometrium, while the functionalis layer comprises the superficial two-thirds of the endometrium. The functionalis is divided into stratum compactum and stratum spongiosum. The stratum compactum is a superficial thin layer with gland necks and dense stroma, while the stratum spongiosum is the deeper part of functionalis composed of glands and loosely arranged stroma. Only the functionalis layer of endometrium is shed with each cycle. The basalis layer contains the progenitor cells that regenerate the functionalis layer in each cycle. endometrial tissue responds to sex steroid hormones produced in the follicular and luteal phases of the ovarian cycle. The menstrual cycle is divided into proliferative and secretory phases, and cytoarchitectural and molecular differences between the phases reflect endometrial responses to cyclic changes in ovarian hormone exposure.