Endometrium

The wall of the uterus is made up of three layers of tissue, which from the inside out are called:

• endometrium: mucous membrane that lines the uterine cavity internally
• myometrium: muscular tunic, consisting of a layer of smooth muscle that contributes approximately 90% to the overall thickness of the organ
• perimetry: serous tunic, peritoneal sheet that covers it externally only in the body and bottom (missing in the sides and supravaginal portion of the cervix)

Before going into the description of the endometrium, let us briefly recall how in the uterus, from a macroscopic point of view, four different regions are identified:

• body of the uterus: upper portion, more expanded and voluminous
• neck of the uterus or cervix: lower portion, smaller and narrower, about 3-4 cm long. Facing down, it looks towards the vagina into which it protrudes through the so-called “tench snout”.
• isthmus of the uterus: narrowing that divides the body and neck of the uterus
• fundus or base of the uterus: portion of the uterine cavity located above the imaginary line that joins the two fallopian tubes , facing forward

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 consists of two layers that differ in structure and function: a deeper basal layer and a superficial layer represented by the superficial epithelium, endometrial glands and stroma.

• Coating epitheliumcylindrical (syn. Batiprismatic or columnar) simple (syn. Monostratified) ciliated ; it is composed of a single layer of hair cells, interspersed with mucus- secreting goblet cells. The hair cells cause a direct flow to the cervix
• Proper tunic (lamina)or stroma, formed by loose connective tissue poor in fiber and rich in vessels and glands, for the most part 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 constitutes the so-called basal layer, very thin and insensitive to hormones, which adheres to the underlying myometrium.

The cervical canal has a mucosa that is different from the endometrium in function and structure. In the cervix, the endometrium is not as smooth as the areas above it, but rises in folds that make up the arbor vitae. In the lower part of the cervix (ectocervix) of the lining epithelium – passing through a so-called zone of transformation – from simple cylindrical ciliated becomes squamous multilayered (squamous) devoid of glands, similar to the vaginal

The arterial vessels that supply the uterine wall are of two types:

• Straight arteries that cross the myometrium, reach the proper tunic and capillarize in the depths of the endometrium.
• Spiral-like arteries that once crossed the myometrium enter the endometrium taking a tortuous course until they reach under the lining epithelium where they give rise to capillarization.

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

• Basal area: very thin, close to the myometrium, contains the bottom of the uterine glands, the capillarization of the straight arteries and the trunk of the spiral-shaped arteries
• Functional zone: overlying the basal, it contains the body of the uterine glands, the body of the spiral-shaped arteries with their most tortuous portion, and the lining epithelium on the surface. The functional zone therefore contains the ciliated lining epithelium and the more superficial layers of the lamina propria.

These two areas respond differently to female sex hormones; in fact, the basal area does not undergo cyclical changes, while the functional area is subject to periodic changes 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 fundus.

While the fundus and body endometrium undergoes continuous functional changes between the beginning of one menstruation and the end of the next, the mucous membrane of the neck does not undergo the same cyclical changes.

Endometrium: what are its functions?

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

From puberty (11-13 years) to menopause (45-50 years), the endometrium of the body and fundus undergoes cyclical and massive changes, 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 secretory phase (days 14-28): in this phase the endometrium reaches its maximum thickness, the cells enlarge and fill with fatand glycogen , the tissue becomes edematous → the uterus is functionally and structurally ready to accommodate 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 favorable state for implantation would be, for the organism, too expensive from an energy point of view . For this reason, in the event that the egg cell is not fertilized, the most superficial layer of the endometrium undergoes apoptosis, flaking off; the leakage of small amounts of blood and dead tissue residues gives rise to menstrual flow.

PLEASE NOTE: at the level of the cervix, the mucosa does not undergo cyclical 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 plug that hinders the ascent of spermatozoain the cervix, in the days between ovulation , it undergoes a fluidification, becomes stringy, watery, acellular, alkaline and easily penetrable by the spermatozoa

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