Myometrium

Myometrium (from Greek mys – muscle, metra – uterus) is the middle, most massive layer of the uterine wall of mesenchymal origin, formed by smooth muscle cells (leiomyocytes).

These cells are bundled together by connective tissue and form a complex three-dimensional functional syncytium that provides the contractile activity of the organ.

Etiology and pathophysiology

The structural organization of myometrium includes three indistinctly differentiated layers: submucous (subendometrial), vascular (middle, the most powerful, with oblique and circular course of fibers) and subserosal (external, with longitudinal course of fibers).

Uterine leiomyocytes have a unique phenotypic plasticity: during pregnancy, under the influence of estrogens and mechanical stretching, they undergo tremendous hypertrophy (cell volume increases 50-fold) and hyperplasia.

The contractile activity is regulated by the concentration of intracellular calcium and expression of receptors for oxytocin, prostaglandins and alpha-adrenomimetics, the density of which increases dramatically by the time of labor. A special role is played by the subendometrial layer, which provides uterine peristalsis outside of pregnancy.

Clinical significance

Myometrium is the source of leiomyoma development – the most frequent benign monoclonal neoplasm in women of reproductive age. The architectonics of the middle layer of myometrium provides mechanical hemostasis in the third period of labor: crossed muscle bundles contract and constrict the lumen of the gaping spiral arteries of the placental site (the mechanism of “living ligatures”). Violation of contractility (atonia or hypotonia) leads to massive, life-threatening obstetric bleeding. Also myometrial pathology includes adenomyosis, in which endometrium-like tissue is defined in the muscle layer, causing reactive myometrial hyperplasia and globular transformation of the uterus.