UTERUS: FUNCTION, PARTS, LAYERS AND DISEASES - ANATOMY AND PHYSIOLOGY - 2025

The uterus is a pear-shaped, hollow muscular organ located in the midline of the pelvis, between the urinary bladder and the rectum. It is part of the female reproductive system and the embryo and placenta are implanted in it during pregnancy.

The female reproductive system is made up of internal reproductive organs and external genitalia. The uterus, vagina, two fallopian tubes, and two ovaries are part of what are known as the internal reproductive organs; while the clitoris, the labia majora and the labia minora comprise the external genitalia.

Schematic of the uterus and surrounding organs (Source: NIH Medical Arts via Wikimedia Commons)

The uterus expands considerably during pregnancy, increasing in length from 7 cm to more than 30 cm during the last weeks of gestation.

Outside the pregnancy period, the uterus undergoes periodic hormonal modifications that prepare the endometrium (internal lining of the uterus) for the possible implantation of the embryo. If fertilization does not occur and, therefore, there is no implantation, the endometrium is shed and the cycle repeats (menstrual cycle).

Like any other organ in the body, the uterus is susceptible to different pathologies of infectious, metabolic, traumatic or tumor origin (benign or malignant).

Function

During pregnancy, the uterus receives the morula, which is the product of the first cell divisions of the zygote, the cell that results from the fusion between an egg and a sperm.

In this context, the main function of this organ is to support the growth and development of the embryo and its placenta, which will later give rise to the fetus.

Parts of the uterus

The uterus has an inverted pear-like morphology, that is, the widest part is in the "upper" region and the thinnest part is towards the vagina. It is about 7 cm long, 4 cm wide and 2.5 cm thick.

Uterus wall thickness (Source: Jmarchn via Wikimedia Commons)

This organ is divided into three zones or regions: the body, the fundus and the cervix.

- The body is the widest portion and it is where the oviducts open, that is, it is connected to the ovaries through the fallopian tubes.

- The fundus, on the other hand, is the rounded base of the uterus, located in the upper part of the mouth of the oviducts.

- The cervix, also called the neck, corresponds to the narrowest and most elongated portion, which projects and opens into the upper part of the vagina (it is located at the opposite pole of the body).

Illustration of the uterus (Source: Plim79 via Wikimedia Commons)

Layers

The wall of the body and the fundus of the uterus is made up of three layers, namely:

- The endometrium

- The myometrium

- The perimetrium, an adventitious or serous layer

Endometrium

This layer is the inner layer of the uterus and represents the mucous lining of said cavity. It is composed of two layers, a functional superficial and a basal one, in the deepest zone of the endometrial layer.

In the superficial layer there is a simple columnar epithelium that has secretory columnar cells devoid of cilia and some intercalated ciliated cells. The basal lamina or lamina propria of this layer contains simple or branched tubular glands that extend into the lower layer (the myometrium).

This last layer of the endometrium also contains collagenous dense connective tissue with stellate cells, macrophages, leukocytes, and abundant reticular fibers.

The functional layer of the endometrium is the one that is "shed" during each menstrual cycle, meanwhile the basal layer is responsible for the proliferation and regeneration of the cells of the functional layer in each menstrual cycle.

Myometrium

The myometrium is the uterine muscular layer. It is a set of three layers of smooth muscle: an internal longitudinal one, a middle circular one, and an external longitudinal one. These layers of muscle tissue gradually diminish and are replaced by connective tissue in the portions near the neck or cervix, where only a few scattered smooth muscle fibers remain.

Such a region of the wall of the uterus is highly vascularized and houses the "arched" arteries, which is why it is known as the stratum vascular.

Arterial vasculature of the uterus (Source: Mikael Häggström via Wikimedia Commons)

The size and number of muscle cells in the myometrial layer is related to the concentration of estrogens. During pregnancy, they increase in size and number, but decrease in size when menstruation ends.

The muscular layer of the uterus is the one that contracts during labor to expel the baby that is formed in the endometrium.

Perimetrium, adventitial or serous

The adventitial or serous layer, also known as the perimetrium, is the outermost layer and covers the peritoneal or visceral surface of the uterus. It facilitates the movement of this in the pelvic cavity when necessary.

Some textbooks state that the perimetrium covers the entire posterior surface of the uterus, but only a portion of the anterior surface, which is lined by connective tissue that forms an adventitious layer.

This layer continues with the pelvic and abdominal peritoneum; It is composed of a thin layer of loose connective tissue and a mesothelium, under which there is usually a prominent layer of highly elastic tissue.

The perimetrium contains sympathetic ganglia and nerve fibers from the hypogastric plexus, in addition to parasympathetic fibers from the sacral nerves. Branches of some of these nerves connect to blood vessels, muscles, and endometrial glands.

Diseases of the uterus

There are some pathological clinical manifestations related to the uterus, among which are sarcomas or tumors of different origins, menstrual disorders, fibroids, endometriosis, etc. Furthermore, the uterus can also be affected by microbial infections.

Sarcomas

Uterine sarcomas are “rare” tumors that represent about 7% of cancers of the female genital tract.

Menstrual disturbances

Menstrual alterations, such as primary and secondary amenorrhea, menorrhagia, dysmenorrhea, etc., are those that are related to aberrant patterns both in the duration, periodicity, quantity and volume of menstrual flow.

Fibroids

Fibroids are associated with the formation of benign tumors derived from the smooth muscle of the myometrium.

Endometriosis

Endometriosis consists of the presence of endometrial glands in "abnormal" locations, such as the ovaries, uterine ligaments, etc. These causes infertility, dysmenorrhea (excessive menstrual pain), and general pelvic pain.

References

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