Myometriums

The myometrium is the thick, muscular middle layer of the uterus, lying between the inner endometrium and the outer serosa. The plural form myometriums is occasionally used to refer to this tissue in multiple specimens or individuals, though the singular term is more common in medical literature.

It is composed predominantly of smooth muscle fibers arranged in interlacing bundles and layered in three orientations: an outer longitudinal layer, a middle circular layer, and an inner longitudinal (or oblique) layer. The muscle is supported by connective tissue, blood vessels, and nerves, and its thickness is greatest in the fundus of the uterus.

Functionally, the myometrium contracts rhythmically during the menstrual cycle and especially during labor to expel the fetus and placenta. Its contractions also help constrict uterine blood vessels after delivery to limit bleeding. Hormones such as estrogen and progesterone, as well as oxytocin, regulate the strength and pattern of contractions, with receptor density increasing near term.

Clinical relevance includes conditions arising from myometrial tissue. Leiomyomas, or fibroids, are benign tumors of the myometrium that can cause abnormal bleeding and fertility issues. Adenomyosis occurs when endometrial tissue invades the myometrium, producing pain and heavy menses. Myometrial changes also accompany pregnancy, postpartum involution, and infections such as myometritis.

In summary, the myometrium is essential to uterine function, providing the force for delivery and contributing to postpartum hemostasis, while its structure and hormonal responsiveness underlie both normal physiology and related pathologies.