Uterine junctional zone: function and disease.

The myometrium is usually thought of as a homogeneous mass of smooth muscle fibres. However, magnetic resonance studies of the uterus have revealed two distinct zones--the subendometrial myometrium or junctional zone and the outer myometrium. The junctional zone is not only structurally but also functionally different from the outer myometrium. For instance, myometrial contractions in a non-pregnant woman originate exclusively from the junctional zone, and their amplitude, frequency, and direction depend on the phase of the cycle. Irregular thickening of the junctional zone has been proposed as the magnetic resonance criterion for the diagnosis of diffuse adenomyosis. However, this magnetic resonance appearance relies on the disruption of the inner myometrial architecture secondary to smooth muscle hyperplasia but does not provide proof of mucosal invasion of the myometrium. We postulate that adenomyosis is a dichotomous disease characterised primarily by disruption of the inner myometrial architecture and function, with secondary infiltration of endometrial elements into the myometrium under certain circumstances. This hypothesis focuses on the inner myometrium and may explain the high incidence of superficial adenomyosis in dysfunctional uterine bleeding.