The endometrium refers to the inner lining of the uterine lumen, composed of endometrial glands surrounded by loose highly cellular connective tissue.
In women of reproductive age, the endometrium is composed of two layers:
- stratum basale (basal layer):
- describes the deeper one-third supporting layer of the endometrium that adheres to the myometrium
- after the superficial two-thirds of the endometrium (stratum functionale) are shed during menstruation, the stratum basale remains to regenerate the endometrium ready for the next cycle
- stratum functionale (functional layer):
- describes the superficial two-thirds that proliferates, secretes and then sheds during the menstrual cycle (in the absence of pregnancy) in response to hormonal factors
- proliferation of the functional layer of the endometrium is predominantly stimulated by estrogen
There is no discrete border between the two layers, however, the layers are distinguishable because stromal tissue is more cellular in the basal layer.
If pregnancy occurs, the endometrium is not shed but remains as the decidua.
- branches of the uterine and ovarian arteries that perforate through the myometrium
- upon the decrease in progesterone before menstruation, the arteries supplying the functional layer of the endometrium constrict, leading to ischemia and shedding
Para-aortic, internal iliac and external iliac nodes via lymphatic vessels through the myometrium.
The endometrium is generally assessed by ultrasound or MRI examination.
Women of reproductive age:
- day 1 to 4 of the menstrual cycle: hyperechoic line measuring 1 to 4 mm
- early proliferative phase (day 5 to 13): hyperechoic line measuring 5 to 7 mm
- late proliferative phase (day 14 to 16): multilayered appearance with hypoechoic functional layer and hyperechoic basal layer, as well as a thin central hyperechoic layer representing overlapped or interfacing opposed layers; typically less than 11 mm
- secretory phase (day 16 to 28): thickened hyperechoic endometrium measuring up to 16 mm
- regular, thin hyperechoic line measuring up to 5 mm, representing the remaining basal layer of endometrium
Please see the separate article on endometrial thickness for a detailed discussion of measurements and pathological correlation.
T2 hyperintense layer with a normal thickness corresponding to the present phase of the menstrual cycle, as with ultrasound.