Chapter 5 Summary

  1. Sex hormones may be steroids, proteins and peptides, prostaglandins, or monoamines. The sex steroids are synthesized from cholesterol and fall into three groups: progestins (e.g., progesterone), estrogens (e.g., estradiol), and androgens (e.g., testosterone and 5α-dihydrotestosterone, or DHT). A set of enzymes carries out the synthesis and interconversion of the steroids. The enzyme aromatase converts testosterone to estradiol, and the enzyme 5α-reductase converts testosterone to DHT.
  2. Testosterone is secreted by Leydig cells in the testes and by thecal cells in the ovaries as well as by the adrenal glands. Some tissues convert testosterone to the more potent androgen DHT. Other tissues, including the testes, ovaries, and brain, convert testosterone to estradiol. In women, the main source of estradiol is the ovarian follicles, and the blood levels of estradiol fluctuate with the menstrual cycle. Progesterone is a female hormone secreted by the corpus luteum–the reorganized remnants of the follicle that ruptures at ovulation.
  3. Peptide and protein hormones are gene products. The peptide sex hormones include two hormones synthesized by neuroendocrine cells in the hypothalamus: oxytocin and gonadotropin-releasing hormone (GnRH). Oxytocin is released into the general circulation from the posterior lobe of the pituitary gland during orgasm, breast-feeding, and childbirth. GnRH is secreted into portal vessels that carry it to the anterior pituitary gland, where it stimulates the release of the gonadotropins into the general circulation.
  4. Protein hormones include the two gonad-stimulating hormones (gonadotropins) secreted by the anterior lobe of the pituitary gland: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Inhibins are protein hormones that are secreted by the gonads and influence the pituitary gland.
  5. Prostaglandins are lipid hormones that help prepare the uterus and cervix for childbirth. The monoamine dopamine is a hypothalamic hormone that regulates the release of prolactin from the pituitary gland.
  6. In men, testosterone levels and sperm production are regulated by a negative feedback hormonal loop. This involves GnRH secreted by the hypothalamus, the gonadotropins LH and FSH secreted by the pituitary, and testosterone and inhibin secreted by the testes.
  7. Testosterone supports the normal structure and function of male genital tissues, has a broad anabolic (tissue-building) effect, guides development in a male direction, and stimulates sexual feelings and behavior. This last effect is seen in both sexes.
  8. Menstruation–the sloughing off of the uterine lining (endometrium)–is the outward manifestation of the menstrual cycle; the cycle usually lasts between 24 and 32 days. The menstrual cycle involves hormonal interactions among the ovaries, the hypothalamus, and the pituitary gland. The resulting cyclical changes in hormone levels regulate the development of ovarian follicles and the state of the endometrium.
  9. The menstrual cycle has three phases: the menstrual phase, the follicular phase, and the luteal phase. During the menstrual phase, progesterone and estrogen levels are low, allowing much of the endometrium to be shed as menstrual flow. This change prepares the uterus for its role in sperm transport. LH and FSH levels rise during this phase.
  10. During the follicular phase, LH and FSH promote the development of a set of follicles, of which one becomes dominant. The follicles secrete estrogens. Near the end of the follicular phase, rising estrogen levels trigger a surge of LH and FSH secretion, which drives the dominant follicle toward ovulation. In the late follicular phase the uterus is in optimal condition for sperm transport, and the cervical mucus has a highly elastic quality. Nearly all conceptions result from coitus during the 6 days before and during ovulation.
  11. Ovulation is the release of an ovum from one ovary. The ovum enters the oviduct, where, if sperm are present, it may be fertilized.
  12. During the luteal phase, the remains of the dominant follicle are transformed into a corpus luteum, which secretes progesterone and estrogens. Progesterone causes the endometrium to thicken and prepare itself for implantation of a conceptus. If implantation does not occur by about 14 days after ovulation, the corpus luteum regresses and menstruation begins.
  13. Women experience sexual feelings and engage in sexual behavior at all phases of the menstrual cycle, but there is a tendency for their interest in sex to peak in the late follicular phase, when testosterone levels are high.
  14. Women may experience painful menstrual periods (dysmenorrhea), a variety of physical and psychological symptoms prior to menstruation (premenstrual syndrome), absence of menstrual periods (amenorrhea), or excessively heavy menstrual bleeding (menorrhagia). There are many causes for these conditions, but effective treatments are usually available.
  15. Compounds related to sex hormones are found in some foods and supplements, as well as in the environment. Hormone-related drugs are used as contraceptives, for prevention of menopausal symptoms, for cancer therapy, and for restoration or suppression of the sex drive.