600
Chapter 17
SECTION A
General Terminology and Concepts;
Sex Determination and Differentiation
The primary reproductive organs are known as the
gonads
:
the
testes
(singular,
testis
) in the male and the
ovaries
(singu-
lar,
ovary
) in the female. In both sexes, the gonads serve dual
functions: The fi rst of these is
gametogenesis,
which is the
production of the reproductive cells, or
gametes.
These are
sper-
matozoa
(singular,
spermatozoan,
usually shortened to
sperm
)
in males and
ova
(singular,
ovum
) in females. Secondly, the
gonads secrete steroid hormones, often termed
sex hormones
or
gonadal steroids.
The major sex hormones are
testosterone
in the male and
estradiol
and
progesterone
in the female.
As described in Chapter 11, testosterone belongs to a
group of steroid hormones that have similar masculinizing
actions and are collectively called
androgens.
In the male,
most of the circulating testosterone is synthesized in the tes-
tes. Other circulating androgens are produced by the adrenal
cortex, but they are much less potent than testosterone and are
unable to maintain male reproductive function if testosterone
secretion is inadequate. Some adrenal androgens, like dehy-
droepiandrosterone (DHEA), are sold as dietary supplements
and touted as miracle drugs. Supposedly, DHEA can stop or
reverse the aging process and the diseases associated with it,
cure depression, strengthen the immune system, and improve
athletic performance. Despite the claims, there are few long-
term studies demonstrating the benefi
ts or risks of this hor-
mone. DHEA is itself a weak androgen but can be converted
in the body to testosterone in both men and women.
Estrogens
are a class of steroid hormones secreted in
large amounts by the ovaries and placenta. There are actually
three major estrogens:
Estradiol
is the predominant estrogen
in the plasma, is produced by the ovary and placenta, and is
often used synonymously with the generic term estrogen.
Estrone
is also produced by the ovary and placenta.
Estriol
is usually found only in pregnant women in whom it is pro-
duced by the placenta. In all cases, estrogens are produced
from androgens by the enzyme
aromatase.
Because levels of
the different estrogens vary widely depending on the circum-
stances, and because they have similar actions in the female,
we will refer to them throughout this chapter as
estrogen.
Estrogens are not unique to females, nor are andro-
gens to males. Plasma estrogen in males is derived from the
release of small amounts by the testes and from the conversion
of androgens to estrogen by the aromatase enzyme in some
nongonadal tissues (notably adipose tissue). Conversely, in
females, small amounts of androgens are secreted by the ova-
ries and larger amounts by the adrenal cortex. Some of these
androgens are then converted to estrogen in nongonadal tis-
sues, just as in men, and contribute to the plasma estrogen.
Progesterone in females is a major secretory product of
the ovary at specifi c times of the menstrual cycle as well as
from the placenta during pregnancy. Progesterone is also an
intermediate in the synthetic pathways for adrenal steroids,
estrogen, and androgens.
As described in Chapters 5 and 11, all steroid hormones
act in the same general way. They bind to intracellular recep-
tors, and the hormone-receptor complex then binds to DNA
in the nucleus to alter the rate of formation of particular
mRNAs. The result is a change in the rates of synthesis of the
proteins coded for by the genes being transcribed. The result-
ing change in the concentrations of these proteins in the tar-
get cells accounts for the responses to the hormone.
The duct systems through which the sperm or eggs are
transported and the glands lining or emptying into the ducts
are termed the
accessory reproductive organs.
In the female,
the breasts are also included in this category. The
secondary
sexual characteristics
comprise the many external differences
between males and females. Examples are hair distribution,
body shape, and average adult height. The secondary sexual
characteristics are not directly involved in reproduction.
Reproductive function is largely controlled by a chain of
hormones (
Figure 17–1
). The fi rst hormone in the chain is
gonadotropin-releasing hormone (GnRH).
As described in
Chapter 11, GnRH is one of the hypophysiotropic hormones
involved in the control of anterior pituitary gland function. It
is secreted by neuroendocrine cells in the hypothalamus, and
it reaches the anterior pituitary via the hypothalamo-pituitary
portal blood vessels. The brain is, therefore, the primary regu-
lator of reproduction.
The cell bodies of the GnRH neurons receive input from
throughout the brain as well as from hormones in the blood.
Secretion of GnRH is triggered by action potentials in GnRH-
producing hypothalamic neuroendocrine cells. These action
potentials occur periodically in brief bursts, with virtually no
secretion in between. The pulsatile pattern of GnRH secre-
tion is important because the cells of the anterior pituitary that
secrete the gonadotropins lose sensitivity to GnRH if the con-
centration of this hormone remains constantly elevated.
In the anterior pituitary, GnRH stimulates the release
of the pituitary
gonadotropins—follicle-stimulating hor-
mone (FSH)
and
luteinizing hormone (LH)
(see Figure
17–1). These two glycoproteins were named for their effects
in the female, but their molecular structures are the same
in both sexes. The two hormones act upon the gonads, the
result being (1) the maturation of sperm or ova and (2) stimu-
lation of sex hormone secretion. In turn, the sex hormones
exert many effects on all portions of the reproductive system,
including the gonads from which they come and other parts
of the body as well. In addition, the gonadal steroids exert
feedback effects on the secretion of GnRH, FSH, and LH.
Gonadal protein hormones such as
inhibin
also exert feed-
back effects on the anterior pituitary.
Each link in this hormonal chain is essential. A decrease
in function of the hypothalamus or the anterior pituitary can
result in failure of gonadal steroid secretion and gametogen-
esis just as if the gonads themselves were diseased.
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