Myometrial zonal differentiation and uterine junctional zone hyperplasia in the non-pregnant uterus

Human non-gravid myometrium differentiates in response to ovarian sex steroids into a subendometrial layer or junctional zone and an outer myometrial layer. Compared to the outer myometrial layer, the junctional zone myocytes are characterized by higher cellular density and lower cytoplasmic-nuclear ratio. These structural differences allow in-vivo visualization of the myometrial zonal anatomy by T2-weighted magnetic resonance (MR) imaging. The human myometrium is also functionally polarized. Video-vaginosonography studies have shown that propagated myometrial contractions in the non-pregnant uterus originate only from the junctional zone and that the frequency and orientation of these contraction waves are dependent on the phase of the menstrual cycle. The mechanisms underlying zonal myometrial differentiation are not known, but growing evidence suggests that ovarian hormone action may be mediated through cytokines and uterotonins locally released by the basal endometrial layer and endometrio-myometrial T-lymphocytes. Irregular thickening of the junctional zone due to inordinate proliferation of the inner myometrium, junctional zone hyperplasia, is a common MR finding in women suffering from menstrual dysfunction. Preliminary data suggest that junctional zone hyperplasia is further characterized by loss of normal inner myometrial function. Although irregular thickening of the junctional zone has been associated with diffuse uterine adenomyosis, the precise relationship between subendometrial smooth muscle proliferation and myometrial invasion by endometrial glands and stroma remains to be established.