Myometrium

The myometrium is the thick, middle muscular layer of the uterus, lying between the inner endometrium and the outer serosa (perimetrium). It is composed primarily of smooth muscle fibers embedded in connective tissue and a rich vascular network. In many descriptions, the myometrium consists of three muscle layers: an outer longitudinal layer, a middle circular (often thick) layer, and an inner longitudinal layer, with some sources noting variant orientations. The muscle cells are organized into interlacing bundles and are connected by gap junctions, enabling coordinated contractions. Nerves and blood vessels traverse the tissue to regulate its activity.

Functionally, the myometrium generates contractions that propel sperm, shed the endometrium during menstruation, and, most importantly, expel the fetus and placenta during childbirth. Its activity is modulated by hormones and signaling molecules: estrogen increases muscle mass, excitability, and gap junction formation; progesterone promotes quiescence during pregnancy; oxytocin released from the posterior pituitary stimulates labor contractions; prostaglandins also promote contraction and contribute to dysmenorrhea when elevated.

Clinical relevance and developmentarily related changes: during pregnancy the myometrium grows substantially through hypertrophy (and some hyperplasia) to accommodate the fetus, and after delivery it undergoes involution to return toward its nonpregnant state. Pathologies of the myometrium include leiomyomas (fibroids), benign tumors that can cause heavy bleeding and pressure symptoms. Abnormal contractions or uterine atony after birth can lead to postpartum hemorrhage. The myometrium is a key target in obstetric management, with therapies aimed at promoting or inhibiting contraction as needed.