Ganong's Review of Medical Physiology, 23rd Edition

398
SECTION IV
Endocrine & Reproductive Physiology

infertility with or without gynecomastia. When the loss of
receptor function is complete, the
testicular feminizing
syndrome,
now known as
complete androgen resistance
syndrome,
results. In this condition, MIS is present and testos-
terone is secreted at normal or even elevated rates. The exter-
nal genitalia are female, but the vagina ends blindly because
there are no female internal genitalia. Individuals with this
syndrome develop enlarged breasts at puberty and usually are
considered to be normal women until they are diagnosed when
they seek medical advice because of lack of menstruation.
It is worth noting that genetic males with congenital block-
age of the formation of pregnenolone are pseudohermaphro-
dites because testicular as well as adrenal androgens are
normally formed from pregnenolone. Male pseudohermaph-
roditism also occurs when there is a congenital deficiency of
17
α
-hydroxylase (see Chapter 22).

PUBERTY

As noted above, a burst of testosterone secretion occurs in
male fetuses before birth (Figure 25–8). In the neonatal period
there is another burst, with unknown function, but thereafter
the Leydig cells become quiescent. There follows in all mam-
mals a period in which the gonads of both sexes are quiescent
until they are activated by gonadotropins from the pituitary to
bring about the final maturation of the reproductive system.
This period of final maturation is known as
adolescence.
It is
often also called
puberty,
although puberty, strictly defined, is

the period when the endocrine and gametogenic functions of

the gonads have first developed to the point where reproduc-

tion is possible. In girls, the first event is

thelarche,

the devel-

opment of breasts, followed by

pubarche,

the development of

axillary and pubic hair, and then by

menarche,

the first men-

strual period. Initial menstrual periods are generally anovula-

tory, and regular ovulation appears about a year later. In

contrast to the situation in adulthood, removal of the gonads

during the period from soon after birth to puberty causes only

a small increase in gonadotropin secretion, so gonadotropin

secretion is not being held in check by the gonadal hormones.

In children between the ages of 7 and 10, a slow increase in es-

trogen and androgen secretion precedes the more rapid rise in

the early teens (Figure 25–9).

The age at the time of puberty is variable. In Europe and the

United States, it has been declining at the rate of 1 to 3 mo per

decade for more than 175 y. In the United States in recent

years, puberty generally occurs between the ages of 8 and 13

in girls and 9 and 14 in boys.

Another event that occurs in humans at the time of puberty

is an increase in the secretion of adrenal androgens (see

Figure 22–12). The onset of this increase is called

adrenarche.

It occurs at age 8 to 10 y in girls and age 10 to 12 y in boys.

Dehydroepiandrosterone (DHEA) values peak at about age 25

in females and slightly later than that in males. They then

decline slowly to low values in old age. The rise appears to be

due to an increase in the activity of 17

α

-hydroxylase.

Control of the Onset of Puberty

The gonads of children can be stimulated by gonadotropins;

their pituitaries contain gonadotropins and their hypothalami

contain gonadotropin-releasing hormone (GnRH)

(see Chap-

ter 18). However, their gonadotropins are not secreted. In im-

mature monkeys, normal menstrual cycles can be brought on

by pulsatile injection of GnRH, and they persist as long as the

pulsatile injection is continued. Thus, it seems clear that pulsa-

tile secretion of GnRH brings on puberty. During the period

FIGURE 25–7
Summary of four possible defects produced
by maternal nondisjunction of the sex chromosomes at the time
of meiosis.
The YO combination is believed to be lethal, and the fetus
dies in utero.

44

XO

44

XX

22

O

Gonadal dysgenesis

22X

44

XXX

44

XX

22

XX

Superfemale

Ovum Zygote Sperm

22X

44

YO

44

XX

22

O

Lethal

22Y

44

XXY

44

XX

22

XX

Seminiferous

tubule dysgenesis

22Y

Abnormal

meiosis

FIGURE 25–8

Plasma testosterone levels at various ages in

human males.

Fetal Neo-

natal

Pre-

pubertal

Pubertal Adult Senes-

cence

600

500

400

300

200

100

0

Plasma testosterone (ng/dL)