The Ovarian Cycle

The ovarian cycle describes hormonally induced changes in ovarian function that follow the onset of menstrual flow. The first half of the cycle, referred to as the follicular phase, is marked by a rise in FSH and LH concentrations stimulated by the pulsatile release of GnRH. During the follicular phase, a pattern of low

Figure 1 An idealized menstrual cycle of 28 days. Top panel—the pituitary hormone cycle: cycling of gonadotrophins (FSH, LH). Center panel—the ovarian hormone cycle: cycling of ovarian hormones (estradiol, progesterone). Bottom panel—The endometrial cycle: corresponding cyclical development of the endometrium. Abbreviations: FSH, follicle-stimulating hormone; LH, luteinizing hormone.

Figure 1 An idealized menstrual cycle of 28 days. Top panel—the pituitary hormone cycle: cycling of gonadotrophins (FSH, LH). Center panel—the ovarian hormone cycle: cycling of ovarian hormones (estradiol, progesterone). Bottom panel—The endometrial cycle: corresponding cyclical development of the endometrium. Abbreviations: FSH, follicle-stimulating hormone; LH, luteinizing hormone.

amplitude, high-frequency GnRH pulses is thought to preferentially stimulate the secretion of FSH relative to LH. The rise of FSH and LH causes the follicles within the ovaries to grow. After a week or more of follicular growth—but before ovulation occurs—usually, a single follicle outgrows the others, begins to secrete high concentrations of estrogens (notably estradiol), and then matures. Heightened estrogen production by this dominant follicle creates feedback inhibition of the pituitary secretion of FSH and LH, which in turn causes the remaining ovarian follicles to involute (a process known as atresia).

In an idealized 28-day cycle, ovulation occurs at midcycle, 14 days after the onset of menstruation. An elevated concentration of LH is necessary for final follicular growth and ovulation. High amplitude, low-frequency GnRH pulses mediate the preferential stimulation of LH. In response, approximately two days before ovulation, the rate of secretion of LH increases markedly (6 to 10 fold), peaking about 18 hours prior to ovulation (the LH surge) (Fig. 1, top panel). Concurrently, FSH increases about 2-fold. FSH and LH act synergisti-cally to induce ovulation, that is, the rupture of the mature follicle and release of the mature ovum.

The LH surge is a marker of ovulation. Mild unilateral abdominal pain experienced around the time of ovulation by some women, known as Mittle-schmerz (German for "midpain"), may be related to the leakage of blood and fluid from the ruptured follicle.

The second half of the ovarian cycle is known as the luteal phase. Most of the variation in menstrual cycle length (21-35 days) is due to variation in the follicular phase; the luteal phase is relatively constant at 14 days from ovulation to menses (3). Under the influence of LH during the last few days prior to ovulation, and continuing for a day or so after ovulation, the granulosa cells of the follicle undergo a physical and biochemical change called luteinization. The mass of cells remaining at the site of the ruptured follicle becomes the corpus luteum and begins secreting large quantities of the hormones progesterone and estrogen (Fig. 1, center panel). A small increase in body temperature occurs because of heightened progesterone secretion. Feedback inhibition by these hormones, and by other nonsteroidal factors such as inhibin, reduce the secretion of FSH and LH, thereby, preventing the growth of the new ovarian follicles.

At about day 26 of the idealized cycle, the corpus luteum degenerates. The resulting drop in progesterone and estrogen is followed by menstruation. The loss of progesterone acts as a trigger for endometrial desquamation and the onset of menstrual flow. Concurrently, feedback suppression of the gonadotrophic hormones is lost. As a result, the anterior pituitary once again secretes high levels of FSH and moderate levels of LH in response to GnRH stimulus, re-initiating the ovarian cycle.

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