|Uterus and uterine tubes. (Endometrium labeled at center right.)|
|Latin||tunica mucosa uteri|
|Gray's||subject #268 1262|
The endometrium is the innermost glandular layer and 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 or estrous 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. The endometrium is central, echogenic (detectable using ultrasound scanners), and has an average thickness of 6.7 mm.
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 foetus.
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 a blastocyst implants, then the lining 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, 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 progestins can also induce endometrial atrophy.
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 the stroma, a layer of connective tissue that varies in thickness according to hormonal influences. 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:
- The functional layer is adjacent to the uterine cavity. This layer is built up after the end of menstruation during the first part of the previous menstrual cycle. Proliferation is induced by estrogen (follicular phase of menstrual cycle), and later changes in this layer are engendered by progestrone from the corpus luteum (luteal phase). It is adapted to provide an optimum environment for the implantation and growth of the embryo. This layer is completely shed during menstruation.
- The basal layer, adjacent to the myometrium and below the functional layer, is not shed at any time during the menstrual cycle, and from it the functional layer develops.
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|
Chorionic tissue can result in marked endometrial changes, known as an Arias-Stella reaction, that have an appearance similar to cancer. Historically, this change was diagnosed as endometrial cancer and it is important only in so far as it should not be misdiagnosed as cancer.
Pathological conditions 
Endometriosis is the growth of endometrial tissue outside the uterus.
Asherman's syndrome, also known as intrauterine adhesions occurs when the basal layer of the endometrium is damaged by instrumentation (e.g., D&C) or infection (e.g., endometrial tuberculosis) resulting in endometrial sclerosis and adhesion formation partially or completely obliterating the uterine cavity.
Thin endometrium may be defined as an endometrial thickness of less than 8 mm. It usually occurs after menopause. Treatments that can improve endometrial thickness include Vitamin E, L-arginine and sildenafil citrate.
Gene expression profiling using cDNA microarray can be used for the diagnosis of endometrial disorders. The European Menopause and Andropause Society (EMAS) released Guidelines with detailed information to assess the endometrium. 
See also 
Additional images 
The initial stages of human embryogenesis
Micrograph showing endometrial stromal condensation, a finding seen in menses.
- Effects of hormone therapy on the endometrium, http://www.ncbi.nlm.nih.gov/pubmed/8426860
- William's Gynecology, McGraw 2008, Chapter 8, Abnormal Uterine Bleeding
- Blue Histology - Female Reproductive System. School of Anatomy and Human Biology - The University of Western Australia http://www.lab.anhb.uwa.edu.au/mb140/CorePages/FemaleRepro/FemaleRepro.htm Accessed 20061228 20:35
- Arias-Stella, J. (Jan 2002). "The Arias-Stella reaction: facts and fancies four decades after.". Adv Anat Pathol 9 (1): 12–23. PMID 11756756.
- Takasaki A, Tamura H, Miwa I, Taketani T, Shimamura K, Sugino N (April 2010). "Endometrial growth and uterine blood flow: a pilot study for improving endometrial thickness in the patients with a thin endometrium". Fertil. Steril. 93 (6): 1851–8. doi:10.1016/j.fertnstert.2008.12.062. PMID 19200982.
- Tseng, L.; Chen, I.; Chen, M.; Yan, H.; Wang, C.; Lee, C. (2010). "Genome-based expression profiling as a single standardized microarray platform for the diagnosis of endometrial disorder: an array of 126-gene model". Fertility and Sterility 94 (1): 114–119. doi:10.1016/j.fertnstert.2009.01.130. PMID 19328470.
- Dreisler E, Poulsen LG, Antonsen SL, Ceausu I, Depypere H, Erel CT, Lambrinoudaki I, Pérez-López FR, Simoncini T, Tremollieres F, Rees M, Ulrich LG. (2013). "EMAS clinical guide: Assessment of the endometrium in peri and postmenopausal women.". Maturita 75. PMID 23619009.
- SUNY Figs 43:05-15 - "The uterus, uterine tubes and ovary with associated structures."
- BU Histology Learning System: 18902loa - "Female Reproductive System uterus, endometrium"
- Swiss embryology (from UL, UB, and UF) gnidation/role02
- Histology at OU 20_01
- Histology at utah.edu. Slide is proliferative phase - click forward to see secretory phase