The Mechanism of Menstruation

N.F. Santoro and G. Neal-Perry (eds.), Amenorrhea: A case-Based, clinical Guide, Contemporary endocrinology, DOI 10.1007/978-1-60327-864-5_2, © Springer Science+Business Media, LLC 2010 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 placentation, 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 appreciation of normal endometrial physiology as it pertains to the regulation of menstruation is essential to understand disorders of menstruation.