Regulation Of The Reproductive Cycle

The central event of each ovarian cycle is ovulation, which is triggered by a massive increase in blood LH

Regulation of the Reproductive Cycle concentration. This surge of LH secretion must be timed to occur when the ovum and its follicle are ready. The corpus luteum must secrete its hormones to optimize the opportunity for fertilization and establishment of pregnancy. The period after ovulation during which the ovum can be fertilized is brief and lasts less than 24 hours. If fertilization does not occur, a new follicle must be prepared. Coordination of these events requires two-way communication between the pituitary and the ovaries and between the ovaries and the reproductive tract. Examination of the changing pattern of hormones in blood throughout the ovarian cycle provides some insight into these communications.

Pattern of Hormones in Blood During Ovarian Cycle

Figure 10 illustrates daily changes in the concentrations of major hormones in a typical cycle extending from one menstrual period to the next. The only remarkable feature of the profile of gonadotropin concentrations is the dramatic peak in LH and FSH that precedes ovulation. Except for the 2 to 3 days of the midcycle peak, LH concentrations remain at nearly constant low levels throughout the follicular and luteal phases. The concentration of FSH is also low during both phases of the cycle, but a secondary peak is evident early in the follicular phase and diminishes as ovulation approaches. Blood levels of FSH remain low throughout most of the luteal phase but begin to rise 1 or 2 days before the onset of menstruation.

The ovarian hormones follow a different pattern. Early in the follicular phase, the concentration of estra-diol is low. It then gradually increases at an increasing rate until it reaches its zenith about 12 hours before the peak in LH; thereafter, estradiol levels fall abruptly and reach a nadir just after the LH peak. During the luteal phase, there is a secondary rise in estradiol concentration, which then falls to the early follicular level a few days before the onset of menstruation. Progesterone is barely or not at all detectable throughout most of the follicular phase and then begins to rise along with LH at the onset of the ovulatory peak. Progesterone continues to rise and reaches its maximum concentration several days after the LH peak has ended. Progesterone levels remain high for about 7 days and then gradually fall and reach almost undetectable levels a day or two before the onset of menstruation. Inhibin B concentrations are low early in the follicular phase and then rise and fall in parallel with FSH. The apparent peak that coincides with ovulation is thought to result from the absorption of inhibin B already present in high concentration in the expelled follicular fluid rather than concurrent secretion by

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