The enigmatic uterine junctional zone: the missing link between reproductive disorders and major obstetrical disorders?

While there is a growing realization that the origins of major obstetrical complications associated with defective deep placentation, such as pre-term labour, fetal growth restriction and pre-eclampsia, may lie in the very early pregnancy events, the underlying mechanisms are not understood. Impaired deep placentation is foremost a vascular pathology, characterized by a lack of endovascular trophoblast invasion and remodelling of a segment of the spiral arteries embedded within the inner myometrium of the uterus. Outside pregnancy, the inner myometrium represents a highly specialized, hormone-dependent structure, termed the junctional zone (JZ), which plays an integral part in the implantation process. The JZ changes with age and is disrupted in several reproductive disorders, such as endometriosis and adenomyosis, which in turn may account for the increased risk of adverse pregnancy outcome. Unlike the endometrium, the myometrial JZ is not readily accessible to biochemical or molecular studies, yet its structure and function can be assessed using imaging techniques, such as high-resolution ultrasound and magnetic resonance imaging. Thus, non-invasive assessment of the JZ prior to conception may turn out to be useful in identifying those women at risk of major obstetrical complications.