Сүтқоректілер жатырының ішкі қабығы (Endometrium)

Inner mucous membrane of the mammalian uterus

The endometrium is the inner epithelial layer, along with its mucous membrane, of the mammalian uterus. It has a basal layer and a functional layer: the basal layer contains stem cells which regenerate the functional layer. The functional layer thickens and then is shed during menstruation in humans and some other mammals, including other apes, Old World monkeys, some species of bat, the elephant shrew and the Cairo spiny mouse. In most other mammals, the endometrium is reabsorbed in the estrous cycle. 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 speculated presence of an endometrial microbiota
has been argued against.

The endometrium consists of a single layer of columnar epithelium plus the stroma on which it rests. The stroma is a layer of connective tissue that varies in thickness according to hormonal influences. In the uterus, simple tubular glands reach from the endometrial surface through to the base of the stroma, which also carries a rich blood supply provided by the spiral arteries. In women of reproductive age, two layers of endometrium can be distinguished. These two layers occur only in the endometrium lining the cavity of the uterus, and not in the lining of the fallopian tubes where a potentially life threatening ectopic pregnancy may occur nearby. Just over 100 of these genes are more specifically expressed in the endometrium with only a handful genes being highly endometrium specific. The corresponding specific proteins are expressed in the glandular and stromal cells of the endometrial mucosa. The expression of many of these proteins vary depending on the menstrual cycle, for example the progesterone receptor and thyrotropin releasing hormone both expressed in the proliferative phase, and PAEP expressed in the secretory phase. Other proteins such as the HOX11 protein that is required for female fertility, is expressed in endometrial stroma cells throughout the menstrual cycle. Certain specific proteins such as the estrogen receptor are also expressed in other types of female tissue types, such as the cervix, fallopian tubes, ovaries and breast.

The uterus and endometrium was for a long time thought to be sterile. The cervical plug of mucosa was seen to prevent the entry of any microorganisms ascending from the vagina. In the 1980s this view was challenged when it was shown that uterine infections could arise from weaknesses in the barrier of the cervical plug. Organisms from the vaginal microbiota could enter the uterus during uterine contractions in the menstrual cycle. Further studies sought to identify microbiota specific to the uterus which would be of help in identifying cases of unsuccessful IVF and miscarriages. Their findings were seen to be unreliable due to the possibility of cross contamination in the sampling procedures used. The well documented presence of Lactobacillus species, for example, was easily explained by an increase in the vaginal population being able to seep into the cervical mucous. 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 fetus.

The functional layer of the endometrial lining undergoes cyclic regeneration from stem cells in the basal layer. 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 hormones at different stages of the cycle and this affects other parts of the reproductive system.

An endometrial thickness (EMT) of less than 7 mm decreases the pregnancy rate in in vitro fertilization by an odds ratio of approximately 0.4 compared to an EMT of over 7 mm. However, such low thickness rarely occurs, and any routine use of this parameter is regarded as not justified. The optimal endometrial thickness is 10mm. Nevertheless, in human a perfect synchrony it is not necessary, if the emdometrium is not ready to receive the embryo a ectopic pregnancy may occur. This consist of the implantation of the blast outside the uterus, which can be extremely dangerous. Observation of the endometrium by transvaginal ultrasonography is used when administering fertility medication, such as in in vitro fertilization. At the time of embryo transfer, it is favorable to have an endometrium of a thickness of between 7 and 14 mm with a triple line configuration, which means that the endometrium contains a hyperechoic (usually displayed as light) line in the middle surrounded by two more hypoechoic (darker) lines. A triple line endometrium reflects the separation of the basal layer and the functional layer, and is also observed in the periovulatory period secondary to rising estradiol levels, and disappears after ovulation. Endometrial thickness is also associated with live births in IVF. The live birth rate in a normal endometrium is halved when the thickness is <5mm.

Estrogens stimulate endometrial proliferation and carcinogenesis. Conversely, progestogens inhibit endometrial proliferation and carcinogenesis caused by estrogens and stimulate differentiation of the endometrium into decidua, which is termed endometrial transformation or decidualization. This is mediated by the progestogenic and functional antiestrogenic effects of progestogens in this tissue. These effects of progestogens and their protection against endometrial hyperplasia and endometrial cancer caused by estrogens is referred to as endometrial protection.