Human Physiology, 14th edition (2016)

Reproduction 711

than estrogen does; this accounts for the larger, thicker bones in

males compared to females.

Interestingly, the growth of hair in the pubic and axillary

(underarm) regions in boys and girls is primarily stimulated by

an increased secretion of androgens from the adrenal cortex at

the time of puberty. The maturation of the pituitary-adrenal axis

is separate from the pubertal changes in the pituitary-gonad axis.

is inhibited by neurons that release the neurotransmitter GABA,
whereas this inhibition is reduced at puberty. Studies also sug-
gest that stimulation of GnRH neurons by kisspeptins is required
for puberty. Through these and other changes in the hypothal-
amus, increased secretion of LH (and FSH) from the anterior
pituitary drive the other endocrine changes that cause puberty.
Because of the increased pulses of LH at puberty, the gonads
secrete increased amounts of sex steroid hormones. Increased
secretion of testosterone from the testes and of estradiol – 17 b
(estradiol is the major estrogen, or female sex hormone) from
the ovaries during puberty, in turn, produce body changes char-
acteristic of the two sexes. Such secondary sex characteristics
( tables 20.2 and 20.3 ) are the physical manifestation of the hor-
monal changes that occur during puberty.
For example, the rising secretion of estradiol in girls stimu-
lates the epiphyseal growth plates of bones, causing a growth
spurt; this is the first sign of puberty. Estradiol also stimulates
breast development, and, a little later (at an average age of 11
to 13 years), menarche —the first menstrual flow. In boys, the
rising secretion of testosterone causes growth of the penis and
testes, as well as most of the other male secondary sex char-
acteristics. Testosterone also stimulates growth in boys (via its
conversion to estrogen within the epiphyseal growth plates),
but the growth spurt in boys occurs at a later age than in girls
( fig. 20.10 ). Boys gain about 35% more muscle mass at puberty
as a result of testosterone-stimulated anabolism.
Estrogen produced from testosterone in the epiphyseal
growth plates of males stimulates cell division of chondrocytes,
synthesis of new cartilage matrix, and calcification of the bone
matrix. Testosterone directly stimulates the addition of new bone
under the periosteum (the membrane surrounding bones) more

Table 20.2 | Development of Secondary
Sex Characteristics and Other Changes
That Occur During Puberty in Girls

Characteristic

Age of First

Appearance

Hormonal

Stimulation

Appearance of breast

buds

8–13 Estrogen, progester-

one, growth hor-

mone, thyroxine,

insulin, cortisol

Pubic hair 8–14 Adrenal androgens

Menarche (first

menstrual flow)

10–16 Estrogen and

progesterone

Axillary (underarm)

hair

About 2 years

after the

appearance

of pubic hair

Adrenal androgens

Eccrine sweat glands

and sebaceous

glands; acne (from

blocked sebaceous

glands)

About the

same time as

axillary hair

growth

Adrenal androgens

Table 20.3 | Development of Secondary

Sex Characteristics and Other Changes

That Occur During Puberty in Boys

Characteristic

Age of First

Appearance

Hormonal

Stimulation

Growth of testes 10–14 Testosterone, FSH,

growth hormone

Pubic and axillary

hair

10–15 Adrenal androgens

Body growth 11–16 Testosterone,

growth hormone

Growth of penis 11–15 Testosterone

Growth of larynx

(voice lowers)

Same time as

growth of penis

Testosterone

Facial hair About 2 years

after the

appearance of

pubic hair

Testosterone

Eccrine sweat glands

and sebaceous

glands; acne

(from blocked

sebaceous glands)

About the same

time as facial

and axillary

hair growth

Testosterone

Figure 20.10 Growth as a function of sex and

age. Notice that the growth spurt during puberty occurs at an

earlier age in females than in males.

2

4

6

8

10

Height gain (cm per year)

618110 12 14 68

Age (years)

Females

Males