The Menstrual Cycle
In the female, as in the male, reproductive function is controlled by pituitary hormones that are regulated by the hypothalamus. Female activity differs, however, in that it is cyclic; it shows regular patterns of increases and decreases in hormone levels. These changes are regulated by hormonal feedback. The typical length of the menstrual cycle varies between 22 and 45 days, but 28 days is taken as an average, with the first day of menstrual flow being considered the first day of the cycle (Fig. 19-13).
Beginning of the Cycle
At the start of each cycle, under the influence of pituitary FSH, several follicles, each containing an ovum, begin to develop in the ovary. Usually, only one of these follicles will ultimately release an ovum from the ovary in a single month. The follicle produces increasing amounts of estrogen as it matures (see Fig. 19-13). (Estrogen is the term used for a group of related hormones, the most active of which is estradiol.) The estrogen is carried in the bloodstream to the uterus, where it starts preparing the endometrium for a possible pregnancy. This preparation includes thickening of the endometrium and elongation of the glands that produce the uterine secretion. Estrogen in the blood also acts as a feedback messenger to inhibit the release of FSH and stimulate the release of LH from the pituitary. (Note that there is an unexplained rise in FSH at the time of ovulation, as shown in Fig. 19-13.)
In an average 28-day cycle, ovulation occurs on day 14 and is followed two weeks later by the start of the menstrual flow. However, an ovum can be released any time from day 7 to 21, thus accounting for the variation in the length of normal cycles. About 1 day before ovulation, there is an LH surge, a sharp rise of LH in the blood. This hormone causes ovulation and transforms the ruptured follicle into the corpus luteum, which produces some estrogen and large amounts of progesterone. Under the influence of these hormones, the endometrium continues to thicken, and the glands and blood vessels increase in size. The rising levels of estrogen and progesterone feed back to inhibit the release of FSH and LH from the pituitary. During this time, the ovum makes its journey to the uterus by way of the oviduct. If the ovum is not fertilized while passing through the uterine tube, it dies within 2 to 3 days and then disintegrates. During each menstrual cycle, changes occur in both the ovary and the uterus (see Fig. 19-13).
Figure 19-13 The menstrual cycle. Changes in hormones, the ovary, and the uterus are shown during a typical 28-day menstrual cycle with ovulation on day 14. (Pituitary hormones are shown with dashed lines, ovarian hormones with solid lines.)
The time before ovulation is described as the follicular phase in the ovary, because it encompasses development of the ovarian follicle. The uterus during this same time is in a proliferative phase, marked by growth of the endometrium. After ovulation, the ovary is in a luteal phase, with conversion of the follicle to the corpus luteum, and the uterus is described as being in a secretory phase, based on activity of the endometrial glands.
The Menstrual Phase
If fertilization does not occur, the corpus luteum degenerates, and the levels of estrogen and progesterone decrease. Without the hormones to support growth, the endometrium degenerates. Small hemorrhages appear in this tissue, producing the bloody discharge known as menstrual flow, or menses. Bits of endometrium break away and accompany the blood flow during this period of menstruation. The average duration of menstruation is 2 to 6 days. Even before the menstrual flow ceases, the endometrium begins to repair itself through the growth of new cells. The low levels of estrogen and progesterone allow the release of FSH from the anterior pituitary.
FSH causes new follicles to begin to ripen within the ovaries, and the cycle begins anew. The activity of ovarian hormones as negative feedback messengers is the basis of hormonal methods of contraception (birth control). Estrogen and progesterone inhibit the release of FSH and LH from the pituitary, resulting in a menstrual period but no ovulation.