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Junctional zone of uterus

The junctional zone (JZ) of the uterus is a specialized inner layer of the myometrium, lying between the endometrium and the outer myometrium. It is histologically distinct, characterized by compact smooth muscle fibers with lower water content compared to the outer myometrium. On imaging, the junctional zone is of great clinical relevance because it is best visualized on MRI and serves as a key marker for uterine pathology such as adenomyosis, leiomyomas, and congenital anomalies.

Synonyms

  • Inner myometrium

  • Endo-myometrial junctional zone

  • Uterine junctional layer

Location and Structure

  • The junctional zone forms the inner third of the myometrium of the uterus.

  • It lies immediately beneath the endometrium and is distinct from the outer myometrium.

  • Composed of tightly packed smooth muscle with less extracellular fluid than the outer layer.

  • Appears as a sharply demarcated low-signal band on MRI, distinguishing it from the endometrium and outer myometrium.

Relations

  • Internal (luminal): Adjacent to the endometrium

  • External: Continuous with the outer myometrium

  • Superiorly/Inferiorly: Extends from uterine fundus to cervix

Function

  • Regulates uterine peristalsis and sperm transport

  • Plays a role in embryo implantation and early pregnancy support

  • Acts as a barrier controlling spread of endometrial processes into the myometrium

  • Provides contractile force during menstruation and labor

Clinical Significance

  • Adenomyosis: Characterized by thickening (>12 mm) or irregularity of the junctional zone

  • Congenital uterine anomalies: Alterations in JZ continuity can be seen in septate or bicornuate uterus

  • Endometriosis and infertility: Dysfunctional JZ peristalsis linked to subfertility

  • Myometrial tumors (leiomyomas): May distort or compress the JZ

  • Serves as an important diagnostic landmark in pelvic MRI interpretation

MRI Appearance

T1-weighted images:

  • Junctional zone appears as a thin, poorly visible band with low signal intensity

  • Lesions such as hemorrhagic adenomyosis may appear hyperintense within JZ

T2-weighted images:

  • Normal JZ appears as a hypointense band (dark) surrounding the hyperintense endometrium and hypointense outer myometrium

  • Thickening (>12 mm) or focal irregularity suggests adenomyosis

STIR (Short Tau Inversion Recovery):

  • JZ itself remains hypointense

  • Pathologic lesions such as adenomyosis appear hyperintense

T1 Fat-Sat Post-Contrast:

  • JZ enhances mildly and homogeneously

  • Pathologic foci (adenomyosis, tumor) show variable heterogeneous enhancement

CT Appearance

Non-Contrast CT:

  • Junctional zone is not well visualized due to limited contrast between endometrium and myometrium

  • May appear as part of overall uterine soft tissue density

Post-Contrast CT:

  • Mild uniform enhancement of uterine wall

  • Junctional zone is indistinct but diffuse or focal uterine wall thickening may suggest pathology

MRI image

Myometrium of uterus MRI sagittal image anatomy image-img-00000-00000

CT image

junctional zone of uterus CT sagittal image anatomy image-img-00000-00000