The endometrium is the mucosal layer lining the cavity of the uterus.
|Uterus and uterine tubes. (Endometrium labeled at center right.)|
|Opened uterus with cat fetus in midgestation: 1 umbilicus, 2 amnion, 3 allantois, 4 Yolk sac, 5 developing marginal hematoma, 6 maternal part of placenta (endometrium)|
|Latin||tunica mucosa uteri|
|Gray's||subject #268 1262|
The endometrium functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During the menstrual cycle, the endometrium grows to a thick, blood vessel rich, glandular tissue layer. This represents an optimal environment for the implantation of a blastocyst upon its arrival in the uterus.
During pregnancy, the glands and blood vessels 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.
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. 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 endometrium consists of a single layer of columnar epithelium, resting on a layer of connective tissue which varies in thickness according to hormonal influences - the stroma. Simple tubular uterine glands reach from the endometrial surface through to the base of the stroma, which also carries a rich blood supply of spiral arteries. In a woman of reproductive age, two layers of endometrium can be distinguished. These two layers occur only in endometrium lining the cavity of the uterus, not in the lining of the Fallopian tubes:
It is possible to identify the phase of the menstrual cycle by observing histological differences at each phase:
|secretory phase||15-28||thick||columnar. Also visible are helicine branches of uterine artery|
Endometriosis is the growth of endometrial tissue outside the uterus.
The content of this section is licensed under the GNU Free Documentation License (local copy). It uses material from the Wikipedia article "Endometrium" modified November 19, 2007 with previous authors listed in its history.