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

Human non-gravid myometrium differentiates in response to ovarian sex stero ids 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 lon er cytoplasmic-nuclea r ratio. These structural differences allow in-vivo visualization of the my ometrial zonal anatomy by T2-weighted magnetic resonance (MR) imaging. The human myometrium is also functionally polarized. Video-vaginosonography stu dies 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 me nstrual cycle. The mechanisms underlying zonal myometrial differentiation a re not known, but growing evidence suggests that ovarian hormone action mag be mediated through cytokines and uterotonins locally released by the basa l endometrial layer and endometrio-myometrial T-lymphocytes. Irregular thic kening of the junctional zone due to inordinate proliferation of the inner myometrium, junctional zone hyperplasia, is a common MR finding in rr omen suffering from menstrual dysfunction, Preliminary data suggest that junctio nal zone hyperplasia is fut ther characterized by loss of normal inner myom etrial function. Although irregular thickening of the junctional zone has b een associated with diffuse uterine adenomyosis, the precise relationship b etween subendometrial smooth muscle proliferation and myometrial invasion b y endometrial glands and stroma remains to be established.