The endometrial-myometrial junction EMJ is the transitional zone between the mucous membrane that is the endometrium and the outer smooth- muscle layer of the myometrium. The EMJ is structurally distinct from other mucosal – muscle interfaces within the human body in that it lacks a submucosal layer. The EMJ is seen as consisting of two distinctive structures: basal endometrium and inner myometrium. In normal uteri, the basal endometrium is seen as a continuous uninterrupted hyperechoic line and in practical terms it represents the endometrial – myometrial interface.
While the superficial and basal layers of the endometrium are clearly distinguishable on light microscopy, there is no histological distinction between the inner myometrium and the outer myometrium on light microscopy. Investigators have demonstrated increased vascularity of the inner myometrium compared with the rest of the myometrium. It was also found that the muscle fibers of this zone were more densely packed compared with other zones of the myometrium. They concluded that these architectural findings would account for the hypoechoic appearance of the inner myometrium on both transvaginal ultrasound and MRI.
It is not just an embryological origin that the endometrium and inner myometrium share. While physiologically distinct, they are both under the cyclical influence of ovarian sex steroids. The main function of the inner myometrium appears to be modulation of uterine peristalsis, an area that increasingly is believed to play a significant role in fertility. Inner myometrial contractions vary in orientation, amplitude and frequency throughout the menstrual cycle, under the influence of estradiol and progesterone. Studies have shown that, in the follicular phase of the cycle, these contractions are predominantly cervico-fundal in orientation and their amplitude and frequency increase significantly as ovulation approaches. There is evidence that this facilitates sperm ascension towards the distal end of the fallopian tubes, where fertilization takes place. After ovulation, the uterus undergoes, under the influence of progesterone, progressive relaxation that culminates during the mid-luteal phase and which may assist proper embryo positioning in the midsection of the uterine cavity and, thus, facilitate blastocyst implantation while also improving the supply of oxygen and nutrients.
Ultrasound assessment
The uterus has been described as comprising three distinct zones on both MRI and transvaginal ultrasound. On MRI, the uterine zonal anatomy is best demonstrated on T2-weighted images. Generally, the endometrium is of high signal intensity and is therefore visualized as a thin white stripe. The inner myometrium is of uniformly low signal intensity, while the outer myometrium is of intermediate signal intensity. Because of this observed contrast of the three uterine zones, MRI is very good at delineating and measuring the size of these zones. This is crucial in the MRI diagnosis of adenomyosis and the early diagnosis of endometriosis and uterine cancer, which is dependent on the increased thickness of the inner myometrium.
The endometrial-myometrial junction (EMJ), is related to peristaltic-like movements in the non-pregnant uterus. Hyperperistalsis and dysperistalsis of uterus underlie many important disorders such as dysmenorrhea, infertility, endometriosis, implantation failure. The maximum thickness of EMJ in patients with endometriosis is significantly greater than in patients without endometriosis (6.5 ± 1.9 mmvs 4.8 ± 1.0 mm.
The major proteins for uterine contraction of the non-pregnant uterus are Oxytocin (OT) and the oxytocin receptor (OTR). The expression level of OTR in the fundus region of the EMJ is positively associated with the severity of dysmenorrhea in the endometriosis (r = 0.870, p < 0.05). Compared to normal uteri, the expression of endometrial-myometrial junction OTR in the secretory phase was significantly higher in the endometriosis uteri (p < 0.05). In the fundus of endometriosis uteri, OTR expression was significantly higher in both the proliferative and secretory phases (p = 0.045 and 0.028, respectively)
On ultrasound, the inner myometrium has been described as a regular hypoechoic band or ‘halo’ that encircles the endometrium. While this zone is distinct from both the endometrium and the outer myometrium, the delineation of uterine zonal anatomy is less clear on sonography than it is on MRI. Ultrasound, however, has the ability to visualize very clearly the basal endometrial layer, which forms the actual interface between the inner myometrium and the endometrium. On two-dimensional (2D) ultrasound, the EMJ is best seen in the longitudinal section, which enables the examination of its anterior and posterior aspects. In this view, the EMJ is seen as consisting of two distinctive structures: basal endometrium and inner myometrium. In normal uteri, the basal endometrium is seen as a continuous uninterrupted hyperechoic line and in practical terms it represents the endometrial – myometrial interface.
The differences in the sonographic appearance of the inner and outer myometrium depend partially on the ovarian sex steroid hormone levels present at the time. Prepubertally, when the levels of the ovarian sex steroids are low, the inner and outer myometrium are indistinct on both ultrasound and MRI.
A similar pattern is also seen on MRI images obtained from healthy postmenopausal women. Others have noted an increase during pregnancy in the signal intensity of the inner myometrium on MRI, which gradually reverted to normal postpartum. Furthermore, serial MRI scans throughout menstrual cycles have shown an increase in EMJ size from the early proliferative phase through to the late luteal phase.
The ability to analyze the entire contour of the uterine cavity facilitates detection of minor irregularities of the EMJ. This has the potential to improve significantly the diagnosis of early adenomyosis and endometriosis and may help to introduce therapeutic measures to arrest the progress of these common and often distressing conditions. A detailed assessment of the EMJ may also help to assess the origin and potential clinical significance of submucosal fibroids. Another area in which 3D imaging might be of use is the diagnosis of early invasive endometrial cancer.
https://obgyn.onlinelibrary.wiley.com/doi/full/10.1002/uog.6432
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5209923/