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Chapter 3 The Menstrual Cycle
Figure 3-1 The three phases of a 28-day menstrual cycle showing ovarian and uterine events. The first two phases of the ovarian cycle (early and late follicular phase) represent growth of selected ovarian follicles. The luteal phase, when the corpus luteum is present, lasts from ovulation to the beginning of the next cycle. The uterine cycle describes important uterine events during the 28-day cycle. The menstrual phase, or period of menstrual flow, is followed by the phase of endometrial proliferation. After ovulation, the uterus enters the secretory phase. Both ovarian and uterine events are depicted in the more commonly used combined menstrual cycle (lower figure). Here, day 1 is the beginning of the menstrual phase, which lasts about 5 days. The menstrual phase is followed by the follicular phase, which ends at ovulation on about day 14. Next is the luteal phase, ending 14 days later with the beginning of menstruation. The combined menstrual cycle will be used in the text
Figure 3-2 Average blood levels of hormones during a 28-day menstrual cycle, along with changes in the ovaries and uterine endometrium. Note that an estradiol peak precedes the LH surge. The ratio of estradiol to progesterone is high during the follicular phase and lower during the luteal phase. The ovaries also secrete small amounts of another estrogen (estrone), androgens, and 17-hydroxyprogesterone (not shown). Levels of progesterone are in nanograms (one thousandth of a milligram) and of estradiol in picograms (one thousandth of a nanogram). Levels of FSH and LH are in milli-international units (one thousandth of an international unit). An international unit is an amount of a hormone that produces a given biological response in a target tissue. Although average FSH and LH blood amounts are shown, the levels of these hormones and GnRH exhibit hourly pulses (Fig. 3-4).
Figure 3-3 The effect of increasing blood levels of estradiol during the follicular phase of the menstrual cycle on secretion of gonadotropin (FSH and LH) from the pituitary. Note that in the early follicular phase (EFP), estradiol levels are low and gonadotropin output is high due to the absence of negative feedback of estradiol on gonadotropin secretion. In the midfollicular phase (MFP), estradiol levels are moderately high and exert a negative feedback on gonadotropin secretion. In the late follicular phase (LFP), the very high estradiol levels now exert a positive feedback on gonadotropin secretion, resulting in the LH surge
Figure 3-4 The pattern of hourly pulses of LH and FSH in the blood during the menstrual cycle
Figure 3-5 Variations in length of menstrual cycle phases. Note that the length oftheluteal phase is usually 14 days. Lengths of the follicular phases of these cycles are 6, 9, and 16 days. The length of the menstrual phase, although shown as constant here, can vary.
