Endometrium

The wall of the uterus is made up of three layers of fabric, which from the inside towards the outside take the name of:

• endometrium : mucous membrane that covers the uterine cavity internally
• myometrium : muscular tunic, consisting of a layer of smooth muscle that contributes about 90% to the overall thickness of the organ
• perimetric : serous tunic, peritoneal leaflet which covers it externally only in the body and in the fundus (missing in the sides and in the supraginal portion of the cervix)

Before going into the description of the endometrium, we briefly recall how in the womb, from the macroscopic point of view, four different regions are identified:

• body of the uterus : upper portion, more expanded and voluminous
• cervix or uterine cervix : lower portion, smaller and narrower, about 3-4 cm long. Facing down, look towards the vagina in which it protrudes through the so-called "tench snout".
• isthmus of uterus : narrowing that divides body and cervix
• bottom or base of the uterus : portion of the uterine cavity located above the imaginary line that connects the two fallopian tubes, facing towards the front

Endometrium: histology

The endometrium internally covers the walls of the uterus. It therefore represents the innermost layer of the organ, facing the lumen or uterine cavity, which rests directly on the underlying musculature (myometrium)

As we will see better in the next chapter, in women of childbearing age the endometrium has a thickness of 1-7 mm depending on the phase of the menstrual cycle. The endometrium is formed by two layers that differ in structure and function: a deeper basal layer and a surface layer represented by the superficial epithelium, the endometrial glands and the stroma.

• Cylindrical covering epithelium (syn. Batiprismatic or columnar) simple (left monostratified) ciliated; it is composed of a single layer of ciliated cells, interspersed with mucus-secreting goblet cells. Hair cells cause a direct flow to the cervix
• Own tunic (lamina) or stroma, formed by loose connective tissue poor in fibers and rich in vessels and glands, mostly tubular and simple, which produce a secretion rich in glycoproteins and glycogen. It is a highly vascularized functional layer, which varies in thickness during the various phases of the cycle and can provide the implant bed for the blastocyst (fertilized egg cell). The deepest part is the so-called basal layer, very thin and insensitive to hormones, which adheres to the underlying myometrium.

The cervical canal has a different mucosa from the endometrium in function and structure. In the cervix the endometrium is not as smooth as the overlying areas, but rises in folds that make up the arbor vitae. In the lower part of the cervix (exocervice) the covering epithelium - passing through a so-called transformation zone - from a cylindrical simple ciliate becomes paved pluristratificato (scaly) devoid of glands, similar to the vaginal

• Straight arteries that cross the myometrium, reach their own frock and become capillary in the depths of the endometrium.
• Spiral arteries that once crossed the myometrium enter the endometrium taking a tortuous course until they reach the epithelium where they give rise to capillarization.

Based on the distribution of these vessels the endometrium of a woman of childbearing age is distinguished into:

• Basal zone : very thin, close to the myometrium, it contains the fundus of the uterine glands, the capillarisation of the straight arteries and the trunk of the spiraling arteries
• Functional area : above at basal level, it contains the body of the uterine glands, the body of the spiraling arteries with their most tortuous portion, and the surface epithelium also on the surface. The functional zone therefore contains the ciliated covering epithelium and the more superficial layers of the lamina propria.

These two areas respond differently to female sex hormones; in fact the basal zone does not undergo cyclical modifications, while the functional zone is subject to periodic modifications based on the levels of estrogen and progesterone. The basal zone therefore represents the layer from which, after menstruation, the previously lost functional zone is restored.

Before menstruation, during the so-called secretory phase, three layers are recognized in the endometrium of the body and the fundus.

While the endometrium of the fund and the body undergoes continuous functional modifications between the beginning of a menstruation and the end of the next, the mucosa of the neck does not undergo the same cyclic changes.

Endometrium: what are its functions?

In addition to representing the innermost layer of the organ, the endometrium is also the most dynamic tissue of the uterus.

From puberty (11-13 years) up to menopause (45-50 years), the endometrium of the body and of the fund undergoes cyclic and imposing modifications, which occur every 28 days (approximately) under the influence of ovarian hormones :

• regenerative and proliferative phase (days 5-14): the uterine endometrium is gradually enriched with new cells and blood vessels, the tubular glands lengthen and overall the endometrium increases its thickness
• glandular or secretive phase (days 14-28): in this phase the endometrium reaches its maximum thickness, the cells become larger filling up with fat and glycogen, the tissue becomes oedematous → the uterus is functionally and structurally ready to receive the cell fertilized egg and to support it in its development.
• desquamative or menstrual phase (days 1-4): the constant maintenance of the endometrium in a state favorable to the implant would, for the organism, be too wasteful from the energy point of view. For this reason, if the egg cell is not fertilized, the most superficial layer of the endometrium undergoes necrosis, crumbling; the leakage of small quantities of blood and dead tissue residues gives rise to menstrual flow.

NOTE: at the level of the cervix, the mucosa does not undergo cyclic changes as striking as those described above. What varies is above all the mucous secretion of the cervical glands:

• generally very dense, to the point of forming a real cork that hinders the rise of the spermatozoa in the cervix, in the days straddling ovulation, it undergoes a fluidization, becomes stringy, watery, acellular, alkaline and easily penetrated by spermatozoa

The mucous secretion of the uterine cervix also protects the innermost genital organs from ascending infections.

Endometrial disorders

• Endometriosis
• Endometrial tumor