Gynecology Sexual anatomy

endometrium Wikipedia

The endometrium is the inner uterine membrane in mammals which is developed in preparation for the implantation of a fertilized egg upon its arrival into the uterus. It grows to be rich in glands and blood vessels, which are eventually to be connected to by the pregnancy, forming the placenta through which the embryo, as it becomes a fetus and eventually gestates fully, receives oxygen and is nourished.

The endometrial lining undergoes cyclic regeneration. Most mammals are subject to an estrous cycle while humans and the great apes display the menstrual cycle. In either situation the endometrium proliferates initially under the influence of estrogen. Once ovulation occurs, in addition to estrogen the ovary will also start to produce progesterone and thereby change the proliferative pattern of the endometrium to a secretory lining. In time the secretory lining provides a hospitable environment to one or more fertilized eggs. If no fertilized egg is detected, the progesterone level drops and the endometrial lining is either reabsorbed (estrous cycle) or shed (menstrual cycle). In the latter, the process of shedding involves the breaking down of the lining, tearing small connective blood vessels, and the loss of the tissue and blood which had constituted it through the vagina, over a series of days. This may be accompanied by some uterine contractions to help expel the menstrual endometrium. In case of implantation, however, the endometrial lining does not get absorbed or shed, but remains as decidua, provides support and protection for the gestation, and becomes part of the placenta.

If there is inadequate stimulation of the lining due to lack of hormones, the endometrium remains thin and inactive. In humans this will result in amenorrhea. The lining after the menopause is often described to be atrophic. In contrast, endometrium that is chronically exposed to estrogens, but not to progesterone, may become hyperplastic.

In humans, the cycle of building and shedding the endometrial lining is 28 days long on average, though it varies among individuals. The endometrium develops at different rates in different mammals. Its formation is sometimes affected by seasons, climate, stress and other factors. The endometrium itself produces certain hormones at different points in the cycle affecting other portions of the reproductive system.

More about the patterns of endometrial growth in different mammals can be found in the article about the estrous cycle.