Test Yourself Before You Continue

1. Define the terms diploid and haploid, and explain how the chromosomal sex of an individual is determined.

2. Explain how the chromosomal sex determines whether testes or ovaries will be formed.

3. List the male and female accessory sex organs and explain how the development of one or the other set of organs is determined.

4. Describe the abnormalities characteristic of testicular feminization syndrome and of 5a-reductase deficiency and explain how these abnormalities are produced.

Clinical Investigation Clue

Remember that Gloria has normal secondary sexual characteristics and used to have regular periods. ■ Do you think it likely that Gloria may suffer from any of the problems in sexual development discussed in this section?

Feminisation Effects Oestrogen

■ Figure 20.8 A simplified biosynthetic pathway for the steroid hormones. The sources of the sex hormones secreted in the blood are also indicated.

that the gonads secrete products that exert a negative feedback effect on gonadotropin secretion. This negative feedback is exerted in large part by sex steroids: estrogen and progesterone in the female, and testosterone in the male. A biosynthetic pathway for these steroids is shown in figure 20.8.

The negative feedback effects of steroid hormones occurs by means of two mechanisms: (1) inhibition of GnRH secretion from the hypothalamus and (2) inhibition of the pituitary's response to a given amount of GnRH. In addition to steroid hormones, the testes and ovaries secrete a polypeptide hormone called inhibin. Inhibin is secreted by the Sertoli cells of the seminiferous tubules in males and by the granulosa cells of the ovarian follicles in females. This hormone specifically inhibits the anterior pituitary's secretion of FSH without affecting the secretion of LH.

Figure 20.9 illustrates the process of gonadal regulation. Although hypothalamus-pituitary-gonad interactions are similar in males and females, there are important differences. Secretion of go-nadotropins and sex steroids is more or less constant in adult males. Secretion of gonadotropins and sex steroids in adult females, by contrast, shows cyclic variations (during the menstrual cycle). Also, during one phase of the female cycle—shortly before ovulation— estrogen exerts a positive feedback effect on LH secretion.

Studies have shown that secretion of GnRH from the hypothalamus is pulsatile rather than continuous, and thus the secretion of FSH and LH follows this pulsatile pattern. This pulsatile secretion is needed to prevent desensitization and downregulation of the target glands (discussed in chapter 11). It appears that the frequency of the pulses of secretion, as well as their amplitude (how much hormone is secreted per pulse), affects the target gland's response to the hormone. For example, it has been proposed that a slow frequency of GnRH pulses in women preferentially stimulates FSH secretion, while faster pulses of GnRH favor LH secretion.

Hypothalamus

GnRH

Anterior pituitary

Gonadotropins

Inhibin

Sex steroids

Gonads

Gametes (sperm or ova)

■ Figure 20.9 Interactions between the hypothalamus, anterior pituitary, and gonads. Sex steroids secreted by the gonads have a negative feedback effect on the secretion of GnRH (gonadotropin-releasing hormone) and on the secretion of gonadotropins. The gonads may also secrete a polypeptide hormone called inhibin that functions in the negative feedback control of FSH secretion.

If a powerful synthetic analogue of GnRH (such as na-farelin) is administered, the anterior pituitary first increases and then decreases its secretion of FSH and LH. This decrease, which is contrary to the normal stimulatory action of GnRH, is due to a desensitization of the anterior pituitary evoked by continuous exposure to GnRH. The decrease in LH causes a fall in testosterone secretion from the testes, or of estradiol secretion from the ovaries. The decreased testosterone secretion is useful in the treatment of men with benign prostatic hyperplasia. In this condition, common in men over the age of 60, testosterone supports abnormal growth of the prostate. The fall in estradiol secretion in women given synthetic GnRH analogues can be useful in the treatment of endometrio-sis. In this condition, ectopic endometrial tissue from the uterus (dependent on estradiol for growth) is found growing outside the uterus—for example, on the ovaries or on the peritoneum. These treatments illustrate the reasons why GnRH and the go-nadotropins are normally secreted in a pulsatile fashion, and are particularly beneficial clinically because they are reversible.

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Essentials of Human Physiology

Essentials of Human Physiology

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