Final answer:
The endometrium is the inner lining of the uterus that thickens during the menstrual cycle to prepare for potential pregnancy and sheds if fertilization does not occur, known as menstruation. It is regulated by hormones and is part of a process that supports embryo nourishment in the event of pregnancy.
Step-by-step explanation:
The endometrium is the inner lining of the uterus, which plays a vital role in the menstrual cycle and pregnancy. During the menstrual cycle, the endometrium thickens and develops a rich blood supply in preparation for a potential pregnancy. If fertilization occurs, the endometrium supports the embryo by providing nourishment. However, if the egg is not fertilized, the built-up endometrial lining is shed during menstruation. The uterus itself is divided into three regions—the fundus, body, and cervix—and has three layers: the external perimetrium, the middle muscular myometrium, and the inner endometrium. During labor, the smooth muscle in the uterus's myometrium layer helps in delivering the baby through contractions.
The process of endometrium thickening and shedding is regulated by hormones such as estrogen and progesterone. Particularly, the endometrium responds to estrogen released by follicles and grows thicker with an increased number of blood vessels. The menstrual cycle approximately spans a 28-day period and includes phases such as menses, proliferative, and secretory. When an egg is not fertilized, the corpus luteum ceases to produce progesterone, leading to the sloughing off of the inner endometrium, known as menses or menstruation.