Anatomic Morphology
In coronal section, the organ has a triangular shape. The wide base (fundus of the uterus) is directed superiorly, and the uterine tubes open into its lateral corners. The apex points inferiorly and transitions into the cervix, which protrudes into the vagina. The uterine lumen is narrow and has a stellate shape on transverse section.
The uterine cavity supports fetal development from the 5th day after fertilization until birth. For the first two days, the embryo remains free in the cavity before undergoing implantation 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). Lined by a simple columnar epithelium dominated by mucus-secreting cells. Near the gland openings, ciliated cells appear toward the end of the menstrual cycle. 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, responsible for mechanical protection of the fetus and its expulsion during childbirth. It consists of three layers of smooth myocytes: an inner subvascular layer, a very thick middle vascular layer, and an outer supravascular layer. Bundles of myocytes run in a spiral-longitudinal direction and intersect, interspersed with connective tissue layers containing elastic fibers.
- Perimetrium (serous coat). Composed of loose fibrous connective tissue containing blood vessels and adipose cells, covered externally by mesothelium. An accumulation of adipose tissue around the cervix is separately designated as the parametrium.
Layers of the Endometrium and the Menstrual Cycle
Functionally and morphologically, the endometrium is divided into two layers with distinct vascularization patterns:
- Functional layer (superficial). Supplied by transient spiral arteries. This is the layer that is shed (desquamated) during menstruation.
- Basal layer (deep and narrow). Supplied by straight arteries that do not extend beyond it. This layer is preserved during menstruation and serves as the source of regeneration (restoration occurs from the epithelium at the base of the glands).
The mechanism of menstruation is triggered by vasospasm of the spiral arteries, leading to ischemia and necrosis of the functional layer. Subsequent vasodilation causes the dead tissue to detach under arterial pressure. In the postmenstrual period, estrogens drive regeneration of the layer, while in the premenstrual period, the influence of progesterone causes it to swell, accumulate fluid, and trigger active mucus production by the glands.
Changes During Pregnancy and Postpartum
During pregnancy, the cells of the endometrial lamina propria transform into large decidual cells rich in glycogen and lipoproteins. They constitute a massive component of the placenta, which provides fetal respiration, nutrition, and hormone synthesis.
The muscular coat adapts to fetal growth through marked myocyte hypertrophy. Progesterone decreases myometrial sensitivity to oxytocin, ensuring uterine quiescence.
The organ's blood vessels maintain a tortuous, spiral course, allowing them to straighten without rupturing during massive wall distension. Postpartum, the vessels (including the main uterine artery) become tortuous again, adapting to uterine involution and facilitating lumen closure for hemostasis. Decidual cells exhibit lytic activity postpartum, aiding in placental detachment.