A transvaginal ultrasound of a premenstrual woman in the sagittal plane (left) reveals a normal view of the uterus with characteristic premenstrual appearance. Image on the right is an overlay showing the components of the endometrium and subendometrial layers. The stripe is almost homogeneously echogenic and thick but also shows a hypoechoic halo of the junctional zone or inner myometrium. (salmon) The homogeneous stripe is made up from two histological layers (barely distinguished by this ultrasound)– the inner stratum functionalis (deep orange) that will shed once the spiral arteries vasoconstrict, and the outer stratum basalis (deep yellow) that will not shed, and will be the basis for regenerating the endometrium in the next cycle. The next layer as stated above is the compact myometrium – the junctional zone (aka inner myometrium) , and is followed by the thicker outer myometrium (maroon).
The junctional zone is hypoechoic due to decreased water content, and is formed by smooth muscle cells that are tightly packed. The extracellular matrix and water content are sparse. It usually measures less than 8mm.
Adenomyosis is a disease of the myometrium caused by misplaced or ectopic endometrium in the myometrium resulting in thickening of the junctional zone. Leiomyomas can affect the junctional zone or the myometrium beyond the junctional zone.