Myometrium

Myometrium (tunica muscularis uteri)

The thick smooth-muscle middle layer of the uterine wall, which hypertrophies enormously in pregnancy, remains quiet under progesterone, contracts powerfully in labour and clamps its own blood vessels after delivery.

Overview

The myometrium (≈1.5 cm thick in the non-pregnant uterus) is composed of interlacing bundles of smooth muscle with connective tissue, arranged in three ill-defined layers: an inner layer of mainly longitudinal fibres, a thick middle vascular layer of interlacing, circular and oblique fibres containing the arcuate vessels, and an outer layer of longitudinal fibres. The figure-of-eight arrangement of the middle-layer fibres around the vessels means that contraction after delivery compresses them — the "living ligatures" that control postpartum bleeding.

In pregnancy, under estrogen and stretch, smooth muscle cells enlarge up to ≈10-fold in length (hypertrophy) with some hyperplasia, and the uterus grows to ≈1 kg. Progesterone, relaxin, nitric oxide and prostacyclin keep it quiescent (Braxton Hicks contractions are irregular and painless). Towards term, estrogen induces contraction-associated proteins — oxytocin and prostaglandin receptors and connexin-43 gap junctions — so that the muscle becomes an electrically coupled syncytium capable of coordinated, fundally dominant contractions. Contraction depends on Ca²⁺–calmodulin activation of myosin light-chain kinase.

Location

Middle layer of the uterine wall, between endometrium and perimetrium; it is much thinner in the cervix, which is mainly fibrous.

Anatomy

Innervation

Uterovaginal plexus; contractions are largely myogenic and hormonally controlled.

Arterial supply

Arcuate arteries in the middle layer (from the uterine and ovarian arteries) → radial arteries.

Histology

Bundles of smooth muscle cells in three ill-defined layers with collagen, elastic fibres and large vessels.

Function

Expels the fetus and placenta during labour.

Compresses uterine vessels after delivery to prevent haemorrhage.

Contracts mildly during menstruation and orgasm.

Regulation

Quiescence: progesterone, relaxin, NO, prostacyclin, β2-adrenergic stimulation. Contraction: oxytocin, prostaglandins (PGF2α, PGE2), estrogen-induced receptors and gap junctions, stretch, α-adrenergic stimulation.

Clinical significance

Uterine fibroids (leiomyomas)

Benign monoclonal tumours of myometrial smooth muscle; submucosal, intramural or subserosal; hormone-dependent.

Postpartum haemorrhage from atony

Failure of the myometrium to contract after delivery (prolonged labour, overdistension, infection, retained tissue) is the commonest cause of PPH; treated with uterine massage and uterotonics (oxytocin, ergometrine, carboprost, misoprostol), then mechanical or surgical measures.

Tocolysis and uterine rupture

Preterm contractions can be suppressed temporarily (nifedipine, atosiban) to allow corticosteroids to act. A previous caesarean scar can rupture in labour, especially with induction.