31.28 : Hormonal Regulation of the Menstrual Cycle

The ovarian cycle regulates endometrial changes throughout a single menstrual cycle via the coordinated action of gonadotrophin-releasing hormone (GnRH) and gonadotrophins.

At puberty, GnRH begins a pulsatile release pattern, which triggers the anterior pituitary gland to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The frequency and amplitude of GnRH pulses vary across the menstrual cycle, with faster pulses favoring LH release and slower pulses favoring FSH release. FSH stimulates the granulosa cells of ovarian follicles to proliferate and produce estrogen through the aromatization of androgens supplied by the theca cells under LH stimulation. Rising estrogen levels induce a positive feedback loop, enhancing GnRH secretion, which causes a surge in LH, leading to ovulation.

Following ovulation, high LH levels promote corpus luteum formation from the ruptured follicle. The corpus luteum predominantly secretes progesterone with moderate estrogen. Progesterone prepares the uterus for potential pregnancy by further enhancing the endometrium.

If fertilization occurs, human chorionic gonadotropin (hCG) secreted by the embryo maintains the corpus luteum and endometrium. If fertilization does not occur, the corpus luteum degenerates, leading to a drop in estrogen and progesterone levels. This hormonal decrease causes the endometrium to shed during menstruation. The decline in progesterone and estrogen levels also removes negative feedback on the hypothalamus, allowing GnRH pulsatility to resume and initiate a new cycle.