The endometrium has the ability to experience cyclical changes in response to different hormonal stimuli that occur during the menstrual cycle. These changes are known as the endometrial cycle. The endometrial cycle is divided into three phases: the proliferative phase, the secretory phase and the menstrual phase.
1- The proliferative phase:
Starting from the end of the menstrual period to ovulation, ie, between the 4th to 14th day of the cycle. During this phase, egg development occurs and is mainly mediated by estrogen. The endometrium reaches a thickness of 1-3 mm. During this phase the endometrial lining grows due to development of the endometrial glands, the stroma and the vascular component.
2- The secretory phase:
Also known as the luteal phase begins at the time of ovulation up to menstruation, ie between the 14th to 28th days of the cycle. It is this phase the corpus luteum produces high levels of estrogen and progesterone. The endometrium reaches 5-6 mm thickness. The glands have some morphological changes and secretory activity becoming more tortuous and dilated. The endometrial spiral arteries are also developed.
3- The menstrual phase:
In the absence of pregnancy, a sudden decline of estrogen and progesterone production by the corpus luteum produces endometrial ischemia due to vasoconstriction of the spiral arteries between 1 to 24 hours before menstruation. After the period of vasoconstriction there is return of blood flow to the superficial layers of the endometrium, resulting in detachment only of the basal layer. During this phase, uterine contractions occur to facilitate the expulsion of endometrial tissue.
These changes occurring during the endometrial cycle, give the endometrium different hysteroscopic patterns typical of each phase, allowing the assessment of normal “hysteroscopic” endometrial cycle.
1- Proliferative phase:
The endometrium has a light pink color due to the presence of small vessels. The surface is smooth and glands are small and rounded, appearing as small and uniform dots. The endometrial notch is small and generally hemorrhagic.
2- Secretory phase:
The endometrium has a pale pink color. The surface is slightly wavy and irregular. The glands are larger, open and rose at the level of the superficial layer. The vessels disappear from the endometrial surface due to stromal edema. The endometrium reaches its maximum thickness at this stage, showing a deep avascular endometrial notch.
3- Menstrual phase:
The endometrium takes on a reddish color, with indentations and bleeding. In this phase there are alternating areas of detaching and well preserved endometrium.