The Menstrual Cycle

The menstrual cycle consists of three phases: the menstrual phase (days 1-5), when the endometrium sheds its functional layer; the proliferative phase (days 6-14), when estrogen stimulates endometrial rebuilding and ovulation occurs; and the secretory phase (days 15-28), when progesterone prepares the endometrium for potential embryo implantation.

If fertilization does not occur, the corpus luteum degenerates, causing progesterone and estrogen levels to drop sharply. This hormonal decline triggers constriction of uterine spiral arterioles, leading to oxygen deprivation, cell death, and detachment of the functional endometrial layer, resulting in menstruation.

During the proliferative phase, estrogen released by growing ovarian follicles stimulates endometrial cells in the stratum basalis to proliferate. The endometrium thickens as a new stratum functionalis forms, doubling in thickness by the end of this phase, which concludes with ovulation.

Menstrual discharge is a mixture of blood, tissue fluid, mucus, and epithelial cells shed from the stratum functionalis layer of the endometrium. This bloody discharge results from the shedding process triggered by declining estrogen and progesterone levels during the menstrual phase.

During the secretory phase, progesterone and estrogen from the corpus luteum promote growth and coiling of endometrial glands, increase vascularization of the superficial endometrium, and thicken the endometrial layer. These changes create an optimal environment for potential embryo implantation and support early pregnancy.

The menstrual cycle typically lasts between 21 and 38 days, with an average of 28 days. This cycle is regulated by fluctuating levels of ovarian hormones, primarily estrogen and progesterone, which control the sequential changes in the uterine endometrium throughout each phase.

The stratum basalis is the deeper, permanent layer of the endometrium that remains intact during menstruation. After the stratum functionalis sheds, cells from the stratum basalis proliferate during the proliferative phase to regenerate a new functional layer for the next cycle.