Endometrium

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Endometrium

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
MeSH	Endometrium
Dorlands/Elsevier	t_22/12832078

The endometrium is the inner membrane of the mammalian uterus.

1 Function
2 Cycle
3 Histology
4 Pathological conditions
5 See also
6 Additional images
7 External links

[edit] Function

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 fetus and eventually, fully gestates.

[edit] Cycle

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 oestrogen. However, once ovulation occurs, in addition to oestrogen, 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.

[edit] Without implantation

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.

[edit] With implantation

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.

[edit] Hormonal understimulation and overstimulation

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.

[edit] Factors affecting periodicity

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.

[edit] Histology

It is possible to identify the phase of the menstrual cycle by observing histological differences at each phase:

Phase	Days	Thickness	Epithelium
menstrual phase	1-4	thin	absent
proliferative phase	4-14	intermediate	columnar
secretory phase	15-28	thick	columnar. Also visible are helicine branches of uterine artery