UTERINE JUNCTIONAL ZONE - FUNCTION AND DISEASE

The myometrium is usually thought of as a homogeneous mass of smooth m uscle fibres. However, magnetic resonance studies of the uterus have r evealed two distinct zones-the subendometrial myometrium or junctional zone and the outer myometrium. The junctional zone is not only struct urally but also functionally different from the outer myometrium. For instance, myometrial contractions in a non-pregnant woman originate ex clusively from the junctional zone, and their amplitude, frequency, an d direction depend on the phase of the cycle. Irregular thickening of the junctional zone has been proposed as the magnetic resonance criter ion for the diagnosis of diffuse adenomyosis. However, this magnetic r esonance appearance relies on the disruption of the inner myometrial a rchitecture secondary to smooth muscle hyperplasia but does not provid e proof of mucosal invasion of the myometrium. We postulate that adeno myosis is a dichotomous disease characterised primarily by disruption of the inner myometrial architecture and function, with secondary infi ltration of endometrial elements into the myometrium under certain cir cumstances. This hypothesis focuses on the inner myometrium and may ex plain the high incidence of superficial adenomyosis in dysfunctional u terine bleeding.