BIOLOGY Volume 3 - A Guide to General Biology - 2004

21. REPRODUCTION

21.7. Human Reproductive Systems

21.7.6. Hormonal Regulation of Oogenesis and the Menstrual Cycle

In women, as in men, the Hypothalamus and Pituitary gland play a central role in regulating gamete production. The hypothalamus secretes GnRH, which stimulates the Pituitary Gland to release FSH and LH. As discussed above, FSH and LH are referred to as gonadotropic Hormones because they stimulate the Gonads—in this case, the female gonads, or Ovaries. However, in women, hormone secretion is a cyclical process, repeating approximately every 28 days and referred to as the Menstrual cycle. Typically, only a single egg is produced during each cycle. The first day of menstruation is considered the beginning of a new cycle. The Sequential Stages of the menstrual cycle are briefly described below, while Fig. 21.44 illustrates the hormonal changes associated with the cycle, along with the corresponding events in the Uterus and ovaries.

1. GnRH stimulates the anterior pituitary gland to secrete FSH (follicle-stimulating hormone). FSH is transported via the bloodstream to its target: the ovaries.

2. FSH molecules bind to receptor sites on primordial follicles. They stimulate The Development of several follicles, but only one of these follicles reaches maturity.

3. The granulosa Cells of the developing follicle begin to produce the female sex hormone estrogen. Estrogen is a steroid hormone whose production steadily increases During the first half of the cycle in accordance with follicular growth (Fig. 21.44). Estrogen acts on two target Organs: the uterus and the anterior pituitary. In the uterus, it stimulates the restoration and Development of the lining, the endometrium, which is necessary to prepare the uterus for a potential Pregnancy, as the embryo must implant into the endometrium. In the anterior pituitary, estrogen suppresses FSH secretion via negative feedback, thereby preventing the development of other follicles; thus, only a single egg ovulates during each cycle. By the middle of the cycle, Blood estrogen levels reach a high concentration, which triggers the secretion of LH (luteinizing hormone).

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Fig. 21.44. Changes occurring throughout the menstrual cycle. The levels of FSH, LH, estrogen, and progesterone are shown, along with the hormone-induced Changes in the ovaries and uterus.

4. The release of LH and estrogen peaks (Fig. 21.44). As a result of The Effect of LH on the Ovary, ovulation occurs—that is, the release of the secondary oocyte from the Graafian follicle. The peak of LH release precisely indicates the timing of ovulation. During ovulation, the secondary oocyte separates from the follicular wall and is expelled into the Abdominal cavity, from which it enters the fallopian tube. Typically, only a single oocyte is released from one ovary each month, meaning ovulation alternates between the left and right ovaries. The ovulated oocyte is a Cell whose Nucleus is in metaphase I of Meiosis; it is surrounded by a layer of cells forming the so-called zona pellucida, and a layer of granulosa cells known as the corona radiata, which protects the oocyte until Fertilization takes place.

The remaining portion of the Graafian follicle transforms into the corpus luteum under METABOLISM/18.html">The Influence of LH.

5. The corpus luteum continues to secrete estrogen as well as another hormone, progesterone. Like estrogen, progesterone has two targets: the uterus and the anterior pituitary gland. Under the influence of progesterone, the uterine walls continue to thicken, and it also stimulates The activity of the uterine glands. In the anterior pituitary, progesterone suppresses LH secretion—another example of negative feedback. Similar to estrogen, it also inhibits FSH secretion. The release of progesterone is accompanied by an increase in a woman's body Temperature immediately following ovulation.

6. If fertilization does not occur, the corpus luteum begins to degenerate, typically around the 28th day of the cycle. The exact causes of this are unknown, although it is believed that degeneration may be triggered by chemical substances secreted by the corpus luteum itself. As it begins to degenerate, estrogen and progesterone levels drop, lifting the suppression of FSH secretion. The endometrium breaks down, triggering menstruation, which lasts for approximately the first 5 days of the next cycle.

A diagram illustrating the hormonal regulation of oogenesis is presented in Fig. 21.45.

Fig. 21.45. Hormonal regulation of oogenesis.

The Role of Hormones in Premenstrual Syndrome

Premenstrual syndrome, or premenstrual tension, refers to the "psychological and physical discomfort" regularly experienced by some women toward the end of the menstrual cycle and which "significantly subsides during the rest of the cycle." Tension is not the only symptom of this syndrome; in fact, over 150 symptoms have been attributed to it at various times, and in 1953 the term "premenstrual syndrome" (PMS) was coined to encompass all these manifestations. It is generally estimated that 75% of women experience some form of discomfort during this period. Most commonly, these include depression, mood swings, Water retention, and sharp or dull aches. Little is known about the causes of PMS; it is generally believed to be hormonal in origin. Because the syndrome occurs a few days before menstruation begins, it may be linked to an imbalance between progesterone and estrogen, whose levels drop at different rates during this time. Alternatively, the cause may lie in progesterone deficiency.

Meanwhile, debate continues as to whether the syndrome actually exists. In 1993, some psychologists (and not only male ones) even began to question its existence, viewing it rather as a social construct; they argued that women simply wish to "legitimize" the syndrome and elicit sympathy.

Menopause and Hormone Replacement Therapy

Menopause refers to the cessation of menstruation, marking the end of a woman's reproductive years. In the United Kingdom, menopause occurs at an average age of 51. Initially, periods become irregular and eventually cease altogether. This is caused by the gradual decline in ovarian function. The number of follicles decreases, and they become less sensitive to FSH stimulation, making the monthly production of eggs increasingly unlikely. Estrogen secretion declines, and since estrogen normally suppresses FSH secretion via negative feedback, menopause is typically characterized by elevated FSH levels (and later, LH levels). Many of the symptoms associated with menopause are largely driven by declining estrogen levels (though progesterone levels also drop). The most common symptoms include night sweats, daytime hot flashes, and vaginal dryness. Other frequent symptoms include depression, irritability, fatigue, and Osteomalacia caused by the leaching of minerals, particularly calcium salts. The depletion of calcium from the bones leads to a condition known as Osteoporosis (Section 15.7.2). It is characterized by a decrease in bone mass and an increase in bone fragility, resulting in a higher frequency of fractures. This is because parathyroid hormone—an antagonist of estrogen—stimulates an increase in blood calcium levels. The symptoms listed above can be managed relatively easily using hormone replacement therapy (HRT), which involves taking estrogen in tablet form or via subcutaneous implants. HRT dramatically slows down The rate of calcium loss from bones, bringing it close to that of men. In the UK, 80% of female physicians of the relevant age use this method, whereas among the general female population of the same age, only 15% resort to it. Treatment may be short-term or last for years, though long-term use carries risks of blood clots and other undesirable side effects. Some of these can be avoided by adding progesterone to the estrogen regimen.

It is important to remember that HRT replenishes the deficiency of natural hormones, whereas birth control pills introduce exogenous hormones into the body (see Section 21.9.1). Therefore, the risk of side effects such as thrombosis associated with oral contraceptives is considered higher than the risk associated with HRT.