Hormonal Control of the Ovarian Cycle

The ovarian cycle is meticulously regulated by the hypothalamic-pituitary-gonadal axis. This cycle orchestrates the release of a mature oocyte, essential for reproduction.

Before puberty, the hypothalamus releases GnRH in a low frequency, low amplitude pulsatile manner. This along with the immature hypothalamic-pituitary-gonadal axis activity, results in low estrogen levels and the absence of a fully functional ovarian cycle. At puberty, GnRH secretion increases in both frequency and amplitude, establishing an adult cyclic pattern that stimulates the pituitary gland to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones orchestrate the ovarian cycle.

During the early follicular phase, FSH is a vital hormone that stimulates granulosa cells within developing ovarian follicles to proliferate and secrete estrogen. This promotes the thickening of the uterine lining in the proliferative phase.

As follicles mature, typically, one becomes dominant, characterized by enhanced LH receptor expression, a superior vascular supply, and increased inhibin secretion. Inhibin reduces FSH levels, preventing the maturation of additional follicles.As a result, only one follicle continues to full maturity in each cycle. Mid-cycle is characterized by sustained high estrogen levels from the dominant follicle which exert positive feedback on the hypothalamus and anterior pituitary. This results in a surge in GnRH, which leads to a surge in LH, triggering the ovulation of the secondary oocyte from the dominant follicle.

Following ovulation, the ruptured follicle transitions into the corpus luteum during the luteal phase. This structure secretes progesterone and, to a lesser extent, estrogen, preparing the uterus for potential zygote implantation. If fertilization does not occur, the corpus luteum degenerates into the corpus albicans, leading to a decrease in estrogen and progesterone levels. This hormonal withdrawal, in turn, initiates menstruation and the start of a new ovarian cycle. This cyclic activity is fundamental to female fertility and reproductive health.