The endometrium is the inner membrane of the mammalian uterus. It develops in preparation for the implantation of a blastocyst upon its arrival into the uterus. During pregnancy, the endrometrium becomes rich in glands and blood vessels. These all become interconnected, forming the placenta. The placenta supplies oxygen and nourishment to the embryo as it becomes a foetus and eventually, fully gestates.
The endometrial lining undergoes cyclic regeneration. Humans and the great apes display the menstrual cycle. Whereas, most other mammals are subject to an estrous cycle cycle. In both cases, the endometrium initially proliferates under the influence of estrogen. However, once ovulation occurs, in addition to estrogen, the ovary will also start to produce progesterone. This changes the proliferative pattern of the endometrium to a secretory lining. Eventually, the secretory lining provides a hospitable environment for one or more blastocysts. If no blastocyst is detected, the progesterone level drops and the endometrial lining is either reabsorbed (estrous cycle) or shed (menstrual cycle). In the latter case, the process of shedding involves the breaking down of the lining, the tearing of small connective blood vessels, and the loss of the tissue and blood that had constituted it through the vagina. The entire process occurs over a period of several days. Menstruation may be accompanied by a series of uterine contractions; these help expel the menstrual endometrium. In case of implantation, however, the endometrial lining is neither absorbed nor shed. Instead, it remains as decidua. The decidua becomes part of the placenta; it provides support and protection for the gestation.
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. After menopause, the lining is often described as being 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 lasts an average of 28 days. 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 along the cycle. This affects other portions of the reproductive system.
The growth of the endometrium into the muscle layer of the uterus, myometrium, is called adenomyosis. The growth of endometrial tissue outside the uterus is a pathological condition known as endometriosis. Endometrial cancer is the most common cancer of the human female genital tract.
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