Editing Endometrium (section)

==Function==
The endometrium is the innermost lining layer of the [[uterus]], and functions to prevent adhesions between the opposed walls of the [[myometrium]], thereby maintaining the patency of the uterine cavity.<ref name="proteinatlas2">{{cite web |title=Dictionary - Normal: Endometrium - The Human Protein Atlas |url=https://www.proteinatlas.org/learn/dictionary/normal/endometrium |website=www.proteinatlas.org |access-date=28 December 2022}}</ref> During the [[menstrual cycle]] or [[estrous cycle]], the endometrium grows to a thick, blood vessel-rich, glandular tissue layer. This represents an optimal environment for the [[implantation (human embryo)|implantation]] of a [[blastocyst]] upon its arrival in the uterus. The endometrium is central, echogenic (detectable using ultrasound scanners), and has an average thickness of 6.7&nbsp;mm.

During [[pregnancy]], the glands and [[blood vessel]]s in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the [[placenta]], which supplies [[oxygen]] and nutrition to the [[embryo]] and [[fetus]].

===Cycle===
The functional layer of the endometrial lining undergoes cyclic regeneration from stem cells in the basal layer.<ref name="Gargett"/> Humans, apes, and some other species display the [[menstrual cycle]], whereas most other mammals are subject to an [[estrous cycle]].<ref name="Emera"/> In both cases, the endometrium initially proliferates under the influence of [[estrogen]]. However, once [[ovulation]] occurs, the ovary (specifically the corpus luteum) will produce much larger amounts of [[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.

Upon fertilization, the egg may implant into the uterine wall and provide feedback to the body with [[human chorionic gonadotropin]] (hCG). hCG provides continued feedback throughout pregnancy by maintaining the corpus luteum, which will continue its role of releasing progesterone and estrogen. In case of implantation, the endometrial lining remains as ''[[decidua]]''. The decidua becomes part of the placenta; it provides support and protection for the gestation.

Without implantation of a fertilized egg, 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.

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]], or the absence of a menstrual period. 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]]. Long-term use of [[oral contraceptives]] with highly potent [[progestin]]s can also induce endometrial [[atrophy]].<ref>{{Cite journal | last1 = Deligdisch | first1 = L. | title = Effects of hormone therapy on the endometrium | journal = Modern Pathology | volume = 6 | issue = 1 | pages = 94–106 | year = 1993 | pmid = 8426860 }}</ref><ref>William's Gynecology, McGraw 2008, Chapter 8, Abnormal Uterine Bleeding</ref>

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. Various factors including the seasons, climate, and stress can affect its development. The endometrium itself produces certain [[hormone]]s at different stages of the cycle and this affects other parts of the [[reproductive system]].