Menstrual Cycle
Overview
The menstrual cycle consists of many changes in the uterus, ovaries, and breasts and in the hypothalamus and anterior pituitary gland’s secretion of hormones (Figure 23-15). In the majority of women, these changes occur with almost precise regularity throughout their reproductive years. The first indication of changes comes with the first menstrual period. The first menses or menstrual flow is referred to as the menarche.
A typical menstrual cycle covers a period of about 28 days. However, the length of the cycle varies among women. Some women, for example, may have a regular cycle that covers about 24 days. The length of the cycle also varies within one woman. Some women, for example, may have irregular cycles that range from 21 to 28 days, whereas others may be 2 to 3 months long.
Phases
Each cycle consists of three phases. The three periods of time in each cycle are called the menses, the proliferative phase, and the secretory phase. Refer often to Figure 23-16 as you read about the events occurring during each phase of the cycle in the hypothalamus and pituitary gland, the ovary, and in the uterus. Be sure that you do not overlook the event that occurs around day 14 of a 28-day cycle.
The menses is a period of 4 or 5 days characterized by menstrual bleeding. The first day of menstrual flow is considered day 1 of the menstrual cycle.
The proliferative phase begins after the menstrual flow ends and lasts until ovulation. During this period the follicles mature, the uterine lining thickens (proliferates), and estrogen secretion increases to its highest level.
The secretory phase of the menstrual cycle begins at ovulation and lasts until the next menses begins. It is during this phase of the menstrual cycle that the uterine lining reaches its greatest thickness and the ovary secretes its highest levels of progesterone.
Ovulation
As a general rule, during the 30 or 40 years that a woman has periods, only one ovum matures each cycle. However, there are exceptions to this rule. Some cycles, more than one matures, and some cycles no ovum matures.
Ovulation occurs 14 days before the next menses begins. In a 28-day cycle, this means that ovulation occurs around day 14 of the cycle, as shown in Figure 23-16. (Recall that the first day of the menses is considered the first day of the cycle.) In a 63530-day cycle, however, ovulation would not occur on the 14th cycle day, but instead on the 16th. And in a 25-day cycle, ovulation would occur on the 11th cycle day.
The time of ovulation has great practical importance because the possibility of fertilization—the fusion of a sperm and egg—can occur only during a short period of time during each menstrual cycle. Although a few “super” sperm may remain viable for up to 5 days, most sperm retain their fertilizing power for only 24 to 72 hours after being deposited in the female reproductive tract following ejaculation. And, the oocyte remains viable and capable of being fertilized for only about 12 to 24 hours after ovulation. A woman’s fertile period therefore lasts only a few days each cycle—from between 3 to 5 days before, and no later than 24 hours after, ovulation.
Control of the menstrual cycle
The anterior pituitary gland plays a critical role in regulating the cyclic changes that characterize the functions of the female reproductive system. As noted earlier, secretion of GnRH from the hypothalamus stimulates the anterior pituitary gland to secrete the gonadotropins FSH and LH. From day 1 to about day 7 of the menstrual cycle, GnRH selectively stimulates the anterior pituitary gland to secrete increasing amounts of FSH. A high blood concentration of FSH stimulates several immature ovarian follicles to start growing and secreting estrogen (see Figure 23-16).
Working together, increasing levels of estrogen and GnRH in blood stimulate the anterior pituitary gland to release increasing amounts of LH. LH causes maturing of a follicle and its ovum, ovulation (rupturing of mature follicle with ejection of ovum), and luteinization (formation of a yellow body, the corpus luteum, from the ruptured follicle).
Which hormone—FSH or LH—would you call the “ovulating hormone”? Do you think ovulation could occur if the blood concentration of FSH remained low throughout the menstrual cycle? If you answered LH to the first question and no to the second, you answered both questions correctly. Ovulation cannot occur if the blood level of FSH stays low because a high concentration of this hormone is essential to stimulation of ovarian follicle growth and maturation. With a low level of FSH, no follicles start to grow, and therefore none become ripe enough to ovulate. Ovulation is caused by the combined actions of FSH and LH. Birth control pills that contain estrogen substances suppress FSH secretion. This indirectly prevents ovulation.
Ovulation occurs, as we have said, because of the combined actions of the two anterior pituitary hormones, FSH and LH. The next question is: what causes menstruation? A brief answer is this: a sudden, sharp decrease in estrogen and progesterone secretion toward the end of the secretory phase causes the uterine lining to break down and another menstrual period to begin.