Hormonal Control of Female Secondary Sex Characteristics
1. The hypothalamus releases GnRH, which stimulates the anterior pituitary gland.
2. The anterior pituitary gland secretes FSH and LH.
3. FSH stimulates the maturation of a follicle.
4. Granulosa cells of the follicle produce and secrete estrogens; LH stimulates certain cells to secrete estrogen precursor molecules.
5. Estrogens are responsible for the development and maintenance of most of the female secondary sex characteristics.
6. Concentrations of androgen affect other secondary sex characteristics, including skeletal growth and growth of hair.
7. Progesterone, secreted by the ovaries, affects cyclical changes in the uterus and mammary glands.
of GnRH and release stored LH. The resulting surge in LH concentration, which lasts for about thirty-six hours, weakens and ruptures the bulging follicular wall. At the same time, the oocyte and follicular fluid escape from the ovary (ovulation).
Following ovulation, the remnants of the follicle and the theca interna within the ovary rapidly change.
Major Events in a Menstrual Cycle
1. The anterior pituitary gland secretes FSH and LH.
2. FSH stimulates maturation of a follicle.
3. Granulosa cells of the follicle produce and secrete estrogens.
a. Estrogens maintain secondary sex traits.
b. Estrogens cause the uterine lining to thicken.
4. The anterior pituitary gland releases a surge of LH, which stimulates ovulation.
5. Follicular and thecal cells become corpus luteum cells, which secrete estrogens and progesterone.
a. Estrogens continue to stimulate uterine wall development.
b. Progesterone stimulates the uterine lining to become more glandular and vascular.
c. Estrogens and progesterone inhibit secretion of FSH and LH from the anterior pituitary gland.
6. If the egg cell is not fertilized, the corpus luteum degenerates and no longer secretes estrogens and progesterone.
7. As the concentrations of luteal hormones decline, blood vessels in the uterine lining constrict.
8. The uterine lining disintegrates and sloughs off, producing a menstrual flow.
9. The anterior pituitary gland, no longer inhibited, again secretes FSH and LH. 10. The menstrual cycle repeats.
The space containing the follicular fluid fills with blood, which soon clots, and under the influence of LH, the fol-licular and thecal cells expand to form a temporary glandular structure within the ovary, called a corpus luteum (see fig. 22.26).
Follicular cells secrete some progesterone during the first part of the menstrual cycle. However, corpus lu-teum cells secrete abundant progesterone and estrogens during the second half of the cycle. Consequently, as a corpus luteum is established, the blood concentration of progesterone increases sharply.
Progesterone causes the endometrium to become more vascular and glandular. It also stimulates the uterine glands to secrete more glycogen and lipids (secretory phase). As a result, the endometrial tissues fill with fluids containing nutrients and electrolytes, which provide a favorable environment for the development of an embryo.
High levels of estrogens and progesterone inhibit the release of LH and FSH from the anterior pituitary gland. Consequently, no other follicles are stimulated to develop when the corpus luteum is active. However, if the oocyte released at ovulation is not fertilized by a sperm cell, the corpus luteum begins to degenerate (regress) about the twenty-fourth day of the cycle. Eventually, connective tissue replaces it. The remnant of such a corpus luteum is called a corpus albicans (see fig. 22.26).
When the corpus luteum ceases to function, concentrations of estrogens and progesterone decline rapidly, and in response, blood vessels in the endometrium constrict. This reduces the supply of oxygen and nutrients to the thickened endometrium, and these lining tissues (decidua) soon disintegrate and slough off. At the same time, blood escapes from damaged capillaries, creating a flow of blood and cellular debris, which passes through the vagina as the menstrual flow (menses). This flow usually begins about the twenty-eighth day of the cycle and continues for three to five days, while the concentrations of estrogens are relatively low.
The beginning of the menstrual flow marks the end of a menstrual cycle and the beginning of a new cycle. This cycle is diagrammed in figure 22.33 and summarized in table 22.4.
Because the blood concentrations of estrogens and progesterone are low at the beginning of the menstrual cycle, the hypothalamus and anterior pituitary gland are no longer inhibited. Consequently, the concentrations of FSH and LH soon increase, and a new follicle is stimulated to mature. As this follicle secretes estrogens, the uterine lining undergoes repair, and the endometrium begins to thicken again.
After puberty, menstrual cycles continue at regular intervals into the late forties or early fifties, when they usually become increasingly irregular. Then, in a few months or years, the cycles cease altogether. This period in life is called menopause (men'o-pawz) (female climacteric).
The cause of menopause is aging of the ovaries. After about thirty-five years of cycling, few primary follicles remain to respond to pituitary gonadotropins. Consequently, the follicles no longer mature, ovulation does not occur, and the blood concentration of estrogens plummets, although many women continue to synthesize some estrogens from adrenal androgens.
As a result of reduced concentrations of estrogens and lack of progesterone, the female secondary sex
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