The uterus is a muscular, hollow organ central to female reproductive health, typically about 8 cm long, 5 cm wide, and 4 cm thick in non-pregnant women. This organ is composed of three primary layers: the innermost lining called the endometrium, the thick muscular middle layer known as the myometrium, and the outermost protective covering, the serosa or perimetrium. While the uterus contains distinct regions, a specific area known as the “junctional zone” is important for understanding uterine health and function.
Defining the Junctional Zone
The junctional zone (JZ) is the innermost portion of the myometrium, the muscular wall of the uterus. It lies directly adjacent to the endometrium, forming an interface between the uterine lining and the myometrium.
This zone was identified through magnetic resonance imaging (MRI) in the early 1980s, appearing as a distinct low signal intensity band on T2-weighted sequences. This unique appearance distinguishes it from the brighter signal of the endometrium and the intermediate signal of the outer myometrium. While MRI remains the preferred method for visualizing the JZ, it can sometimes be observed on ultrasound as a slightly hypoechoic layer beneath the endometrium. This visualization has provided new insights into uterine anatomy and physiology.
Structural Makeup of the Junctional Zone
The junctional zone is structurally distinct from the rest of the myometrium. Its smooth muscle cells are primarily arranged in a concentric or circular pattern around the uterine cavity, contrasting with the more longitudinal orientation of muscle fibers in the outer myometrial layers. This inner region also features a higher density of smooth muscle cells and a lower water content. The JZ is thought to have a distinct origin during development, deriving from the Müllerian duct mesenchyme, which differentiates it from the non-Müllerian origin of the outer myometrial layers. This unique structural composition allows the JZ to function differently from the broader myometrium, particularly in its responsiveness to hormonal changes.
Clinical Relevance of the Junctional Zone
The junctional zone plays an important role in various uterine functions, including contractility. In a non-pregnant uterus, specialized contraction waves originate exclusively from the JZ. Their amplitude, frequency, and direction are influenced by the menstrual cycle phases. These contractions are important for processes such as sperm transport and potentially influencing embryo implantation. During menstruation, JZ contractions also contribute to shedding the endometrial lining.
Alterations in the junctional zone are associated with certain uterine conditions, most notably adenomyosis. Adenomyosis is characterized by endometrial tissue growing into the myometrium, often specifically involving the JZ. When affected by adenomyosis, the JZ often appears thickened and irregular on MRI. While a normal JZ typically measures up to 5 mm, a thickness exceeding 12 mm is a strong indicator for diagnosing adenomyosis. Other MRI findings suggesting adenomyosis include small microcysts within the JZ, which represent ectopic endometrial glands.
These changes can impact its function, potentially leading to symptoms like heavy menstrual bleeding and pelvic pain. Disruptions or abnormalities of the JZ, particularly in cases of adenomyosis, have been linked to fertility challenges, including implantation failure and adverse early pregnancy outcomes. Monitoring the junctional zone through imaging is a valuable tool for diagnosing and managing these conditions.