Human Physiology, 14th edition (2016)

Reproduction 709

much higher levels than in the intact animal. This demonstrates

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 progester-

one 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 secre-

tion 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 simi-

lar in males and females, there are important differences. Secre-

tion of gonadotropins 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 hypo-

thalamus is pulsatile rather than continuous, stimulating a simi-

lar pulsatile secretion of FSH and LH from the anterior pituitary.

This pulsatile secretion is needed to prevent desensitization and

downregulation of the target glands (chapter 11, section 11.1).

The embryonic testes during the first trimester of pregnancy
are active endocrine glands, secreting the high amounts of tes-
tosterone needed to masculinize the male embryo’s external
genitalia and accessory sex organs. Ovaries, by contrast, do
not mature until the third trimester of pregnancy. Testosterone
secretion in the male fetus declines during the second trimester
of pregnancy, however, so that the gonads of both sexes are
relatively inactive at the time of birth.
Before puberty, there are equally low blood concentrations
of sex steroids —androgens and estrogens—in both males and
females. Apparently, this is not due to deficiencies in the ability
of the gonads to produce these hormones, but rather to lack of suf-
ficient stimulation. During puberty, the gonads secrete increased
amounts of sex steroid hormones as a result of increased stimula-
tion by gonadotropic hormones from the anterior pituitary.

Interactions Between the

Hypothalamus, Pituitary

Gland, and Gonads

The anterior pituitary produces and secretes two gonadotropic
hormones— FSH (follicle-stimulating hormone) and LH
(luteinizing hormone). Although these two hormones are named
according to their actions in the female, the same hormones are
secreted by the male’s pituitary gland. (In the male, LH is some-
times called interstitial cell stimulating hormone and abbrevi-
ated ICSH; because this is identical to the LH in females, it is
simpler and more common to refer to it as LH in both females and
males.) The gonadotropic hormones of both sexes have three pri-
mary effects on the gonads: (1) stimulation of spermatogenesis or
oogenesis (formation of sperm or ova); (2) stimulation of gonadal
hormone secretion; and (3) maintenance of the structure of the
gonads (the gonads atrophy if the pituitary gland is removed).
The secretion of both LH and FSH from the anterior pitu-
itary is stimulated by a hormone produced by the hypothala-
mus and secreted into the hypothalamo-hypophyseal portal
vessels (chapter 11, section 11.3). This releasing hormone is
sometimes called LHRH (luteinizing hormone-releasing hor-
mone). Because attempts to find a separate FSH-releasing hor-
mone have thus far failed, and LHRH stimulates FSH as well
as LH secretion, LHRH is often referred to as gonadotropin-
releasing hormone (GnRH).
If a male or female animal is castrated (has its gonads sur-
gically removed), the secretion of FSH and LH increases to

LEARNING OUTCOMES

After studying this section, you should be able to:

3. Describe the interactions among the hypothalamus,
   anterior pituitary, and gonads.


4. Explain mechanisms that may be responsible for
   puberty, and describe the male and female sexual
   response.

Figure 20.8 A simplified biosynthetic pathway for

the steroid hormones. The sources of the sex hormones

secreted in the blood are also indicated.

Cholesterol

Progesterone (secreted by ovaries and placenta)

Androstenedione (secreted by adrenal cortex)

Testosterone (secreted by testes)

Estradiol-17b (secreted by ovaries and placenta)