Hormonal Control of the Ovarian Cycle

The hypothalamic-pituitary-gonadal axis orchestrates the ovarian cycle by coordinating hormone release at three levels. The hypothalamus secretes GnRH, which stimulates the pituitary gland to release FSH and LH. These hormones regulate follicle development, estrogen production, and ovulation, ensuring a mature oocyte is released monthly in reproductive-age women.

During the early follicular phase, high FSH levels stimulate granulosa cells within developing ovarian follicles to proliferate and secrete estrogen. This FSH-driven growth promotes multiple follicles to develop, increasing overall estrogen production and thickening the uterine lining in preparation for potential implantation.

As follicles mature, one becomes dominant through enhanced LH receptor expression, superior vascular supply, and increased inhibin secretion. The dominant follicle releases inhibin, which suppresses FSH levels and prevents maturation of additional follicles, ensuring only one follicle reaches full maturity each cycle.

Sustained high estrogen levels from the dominant follicle exert positive feedback on the hypothalamus and anterior pituitary, triggering a surge in GnRH and subsequently LH. This LH surge drives ovulation, releasing the secondary oocyte from the dominant follicle at mid-cycle.

Following ovulation, the ruptured follicle transforms into the corpus luteum during the luteal phase. This structure secretes progesterone and moderate estrogen to prepare the uterus for potential zygote implantation and support early pregnancy if fertilization occurs. The corpus luteum maintains these hormone levels throughout the luteal phase.

If fertilization does not occur, the corpus luteum degenerates into the corpus albicans, causing estrogen and progesterone levels to decline. This hormonal withdrawal triggers menstruation and initiates a new ovarian cycle, restarting the monthly process of follicle development and oocyte maturation.

Before puberty, the hypothalamus releases GnRH in low frequency and low amplitude, resulting in low estrogen levels and an immature hypothalamic-pituitary-gonadal axis. At puberty, GnRH secretion increases in both frequency and amplitude, establishing an adult cyclic pattern that stimulates FSH and LH release to initiate ovarian cycling.