Anatomical morphology
On a frontal section, the organ has a triangular shape. The wide base (fundus of the uterus) faces upward, and the uterine tubes open into its corners. The apex points downward and transitions into the cervix, which protrudes into the vagina. The lumen of the organ is narrow and has a stellate shape on cross-section.
Development of the fetus takes place in the uterine cavity, starting from the 5th day after fertilization until birth. For the first two days, the embryo remains free in the cavity, after which implantation occurs — penetration into the mucous membrane.
Histological structure of the wall
The uterine wall is significantly thicker than that of the uterine tubes and consists of three layers:
- Endometrium (mucous membrane). Covered by a simple columnar epithelium dominated by mucus-producing cells. Near the mouths of the glands by the end of the menstrual cycle, ciliated cells appear. The lamina propria is formed by loose fibrous connective tissue and is penetrated by simple tubular uterine glands running perpendicular to the surface.
- Myometrium (muscular coat). The thickest part of the wall, which performs the function of mechanical protection of the fetus and its expulsion during childbirth. It consists of three layers of smooth myocytes: submucosal (inner), the most powerful vascular (middle), and supravascular (outer). Bundles of myocytes run obliquely and longitudinally and intersect, with layers of connective tissue containing elastic fibers lying between them.
- Perimetrium (serous coat). Consists of loose fibrous connective tissue with vessels and fat cells, covered externally by mesothelium. The accumulation of adipose tissue around the cervix is distinguished separately as the parametrium.
Endometrial layers and menstrual cycle
Functionally and morphologically, the endometrium is divided into two layers with different types of vascularization:
- Functional layer (superficial). Supplied with blood by transient spiral arteries. It is this layer that is shed (desquamated) during menstruation.
- Basal layer (deep and narrow). Supplied by straight arteries that do not extend beyond its limits. This layer is preserved during menstruation and serves as the source of regeneration (restoration proceeds through the epithelium of the fundi of the glands).
The mechanism of menstruation is triggered by spasm of the spiral arteries. Ischemia and necrosis of the functional layer tissues occur. Subsequent vasodilation leads to the shedding of the dead tissue under the pressure of blood. In the postmenstrual period, under the action of estrogens, the layer regenerates, while in the premenstrual period, under the influence of progesterone, it swells, accumulates fluid, and the glands begin to actively produce mucus.
Features during pregnancy and postpartum
During pregnancy, cells of the endometrial lamina propria transform into large decidual cells rich in glycogen and lipoproteins. They become a huge component of the placenta (which provides respiration, fetal nutrition, and hormone synthesis).
The muscular coat adapts to fetal growth through marked myocyte hypertrophy. Progesterone reduces the sensitivity of the myometrium to oxytocin, ensuring uterine "quiescence".
The organ's vessels always have a tortuous, spiral course, allowing them to straighten without rupture during massive wall stretching. After childbirth, the vessels (including the main uterine artery) become tortuous again: this adapts them to the reduction in uterine size (involution) and facilitates lumen closure to stop bleeding (hemostasis). Decidual cells exhibit lytic activity after childbirth, aiding in placental detachment.
Briefly: answers to topical questions
What histological phases are distinguished in the endometrial (menstrual) cycle?
The endometrial cycle includes three consecutive histological phases: menstrual, proliferative, and secretory.
- Menstrual phase (desquamation) — shedding of the necrotic functional layer of the endometrium occurs, accompanied by bleeding from open vessels.
- Proliferative phase — under the influence of estrogens, proliferation of the epithelium, endometrial glands, and basal layer cells takes place, leading to the restoration of the functional layer.
- Secretory phase — under the action of progesterone, secretory vacuoles appear in the epithelium, and secretion is released into the lumen of the glands. Growth of spiral arteries and development of decidua-like changes occur in the stroma.
What are the histological features of the structure of the cervix and its canal?
The cervix has mucous and muscular coats; the structure of the mucous membrane varies depending on the zone.
- Mucous membrane (Tunica mucosa) of the cervical canal forms longitudinal branching folds. It is lined with simple columnar glandular epithelium, and its lamina propria contains large branched cervical glands that produce mucus.
- Outer surface (vaginal part) is covered with stratified squamous non-keratinizing epithelium continuous with the vaginal walls.
- Muscular coat (Tunica muscularis) is formed by a powerful circular layer of smooth myocytes that forms the sphincter of the uterus.
How does the shape of the uterine glands change during various phases of the menstrual cycle?
The shape of simple tubular uterine glands changes throughout the menstrual cycle.
Most of the time, the glands appear as straight, unbranched tubules. In the premenstrual period, i.e., the secretory phase, the uterine glands become tortuous; sources also note that branching of the glands is possible in the premenstrual period.
The names of the myometrial layers are easy to remember from inside out: Submucosal, Vascular, Supravascular. The main thing is not to confuse the first layer with the submucosa (tela submucosa), which physically does not exist in the uterus.
Class and exam questions
Why is the endometrium not shed to its full thickness during menstruation?
Due to the peculiarities of blood supply. Only the spiral arteries supplying the superficial functional layer undergo spasm. The basal layer, which receives blood from straight arteries, does not experience critical ischemia and survives.
How are hormones and uterine contractility related?
Estrogens increase the sensitivity of smooth myocytes to oxytocin, promoting contractions (important during childbirth). Progesterone, conversely, decreases this sensitivity, relaxing the organ for successful pregnancy maintenance.
Where does mucosal restoration after menstruation originate from?
The source of regeneration of the functional layer of the endometrium is the epithelium of the fundi of the uterine glands located in the undamaged basal layer.
What are decidual cells and what is their purpose?
These are large rounded cells of the endometrial lamina propria that accumulate glycogen. During pregnancy, they actively form the maternal part of the placenta, and after childbirth, they perform a macrophage function, facilitating the expulsion of the afterbirth.
What else to study on this topic
- Cytological differences between mucus-producing and ciliated cells
- Synthesis of relaxin by decidual cells: modern hypotheses
- Placental structure and interaction between maternal and embryonic tissues
- Histoarchitectonics of elastic fibers in the myometrium
- Dynamics of uterine gland branching in different phases of the cycle
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