The Relation between the Inner Myometrium and the Junctional Zone

BACKGROUND: The nature and functions of the innermost layer of the myometrium, which is located immediately below the endometrium, coined the "junctional zone" (JZ), continue to be the subject of debate. The role and significance of the JZ have attracted little attention beyond its relation to the diagnosis of adenomyosis. OBJECTIVES: This review was conducted to update our current understanding of the role of the JZ as a specific uterine region. METHODS: This is a comprehensive review of literature that was published in PubMed and MEDLINE platforms till April 2025 and that addresses the uterine JZ, excluding articles concerned with uterine adenomyosis. OUTCOME: It is not possible to reconcile JZ appearance on imaging with embryological or functional correlates. There are clear histological and immunohistological differences between the inner and outer myometrium, but the change is gradual with no demarcation of the transition. Whether the JZ has a different origin remains controversial because of the lack of supportive embryological evidence. There is evidence that JZ appearance on MRI is hormonally dependent, but it is not always recognizable and is often indistinct before puberty and after menopause. JZ seems to increase in thickness in the secretory and menstrual phases. CONCLUSION: While increased thickness is often considered a sign of adenomyosis, considerable uncertainty remains. We have not been able to identify studies that related features of the JZ per se to clinical outcomes. This supports the need for caution when interpreting the relevance of the JZ.