The junctional zone endometrium (JZE) is a compacted layer of smooth muscle cells with little extracellular matrix. The innermost myometrium adjacent to the endometrium, JZE is best visualized and evaluated on T2-weighted magnetic resonance imaging (MRI) and two-dimensional/three-dimensional transvaginal ultrasound (TVUS) scanning. Increased thickness of JZE >12 mm on MRI images has been associated with myometrial and subendometrial pathologic conditions, such as, adenomyosis, and is considered a poor prognostic factor for implantation. Gonadotrophin-releasing hormone analogue (GnRHa) has been proposed as a treatment for adenomyosis and fibroids larger than 7 cm, and overall improvement in symptoms and disease progression were attributed to JZE thinning after GnRHa treatment. JZE contractility and frequency of contractions are affected by ovarian hormone cyclic activity and pathologic changes adjacent to JZE, such as fibroids and polyps. However, JZE contractility is not evaluated by TVUS during gynecological examinations because guidelines do not exist and the process is time consuming. The present data indicate that JZE is an important part of the nongravid uterus anatomy, structure, and functionality. When more evidence is available, the morphologic features, thickness, and contractility of JZE may potentially be used as markers for diagnosis and prognosis of normal and abnormal uterine function, for early stages of pregnancy, and possibly for early detection of endometrial cancer. A new tool for JZE measurements should be further investigated to fill this clinical gap. Key Message: JZE is an important component of the nongravid uterus anatomy, structure, and functionality. The thickness and contractility of JZE could potentially be used as markers for diagnosis and prognosis of normal and abnormal uterine function, early stages of pregnancy, and early detection of endometrial cancer. A new tool for JZE measurements should be further investigated.
© 2020 S. Karger AG, Basel.

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