HUMAN BIOLOGY

reprodUCtIVe systeMs 313

hoW do sperm arise?

• Germ cells in seminiferous tubules of the testes give rise to
  sperm.


• Testosterone, LH, and FSH guide the steps by which sperm
  form. Testosterone also governs the development of male
  secondary sexual traits.


• Feedback loops from the testes to the hypothalamus and
  pituitary gland control the secretion of these hormones.

taKe-hoMe Message

nourishment and chemical signals

from Sertoli cells that line the semi-

niferous tubule.

The testes produce sperm from

puberty onward. Millions are in dif-

ferent stages of development on any

given day. A mature sperm has a tail,

a midpiece, and a head (Figure 16.7).

Inside the head, a nucleus contains DNA

organized into chromosomes. A cap,

the acrosome, covers most of the head.

Enzymes it releases help the sperm pen-

etrate protective material around an egg at

fertilization. In the midpiece, mitochondria

supply energy for the tail’s movements.

Hormones control the formation of sperm

Male reproductive function depends on
several hormones. Leydig cells (also
called interstitial cells) in tissue between
the seminiferous tubules in testes (Fig-
ure 16.6B) release testosterone. This is
the hormone that governs the growth,
form, and functions of the male reproduc-
tive tract. Testosterone stimulates sexual
behavior, and at puberty it promotes the
development of male secondary sexual
traits, including facial hair and deepening of an adolescent
male’s voice.
When the testosterone level in a male’s blood falls below
a set point, the hypothalamus secretes GnRH (Figure
16.8). This releasing hormone
prompts the pituitary’s anterior
lobe to release LH (luteinizing
hormone) and FSH (follicle-
stimulating hormone). These
hormones are named for their
functions in females but are
chemically the same in males.
Both have targets in the tes-
tes. LH stimulates Leydig cells
to release tes tosterone, which
in turn stimulates the sperm-
forming steps shown in Figure
16.6C. FSH acts on Sertoli cells.
It is crucial to launching sperm
formation at puberty.

ALevel of testosterone in blood decreases; the

hypothalamus secretes GnRH, a releasing hormone.

(+)

(+)

(–)

(–) (–)

C LH prompts Leydig cells

in testes to produce and

release testosterone.

E Testosterone is used to stimulate the

formation and development of sperm.

D Sertoli cells bind FSH and

testosterone, and function in

spermatogenesis at puberty.

B GnRH stimulates secretion of LH,

FSH from anterior lobe of pituitary.

Testes

G High sperm

count causes

Sertoli cells to

secrete inhibin,

which inhibits

secretion of

GnRH and LH.

F Elevated

level of

testosterone

in blood

inhibits

secretion

of GnRH.

Hypothalamus

Anterior Pituitary

Figure 16.8 Animated! Negative feedback loops regulate the

release of male reproductive hormones. (© Cengage Learning)

acrosome The cap of the
head of a sperm cell; it con-
tains enzymes.

Leydig cells Cells associ-
ated with seminiferous
tubules and that secrete
testosterone; also called
interstitial cells.

Sertoli cells Cells in semi-
niferous tubules that produce
substances that nourish
developing sperm.

testosterone Sex hormone
that governs the growth,
form, and functions of the
male reproductive tract.

tail, with

its core of

microtubules

head,

with DNA

and a cap

of enzymes

midpiece

with

mitochondria

A high level of testosterone in a male’s blood inhibits

the release of GnRH. Also, when a male’s sperm count is

high, Sertoli cells release inhibin, a hormone that acts on

the hypothalamus and pituitary to inhibit the release of

GnRH and FSH. Now feedback loops to the hypothalamus

begin to operate, so the secretion of testosterone and the

formation of sperm decline.

Figure 16.7 A
mature sperm
has a head, a
midpiece, and
a tail. (© Cengage
Learning)

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