C h a p t e r 3 9
Disorders of the Male Genitourinary System
995
Testosterone exerts a number of biologic effects in the
male (Chart 39-1). In the male embryo, it is essential for
appropriate differentiation of the internal and external
genitalia, and in the fetus it is necessary for descent of
the testes. Testosterone is essential to the development
of primary and secondary male sex characteristics dur-
ing puberty and for the maintenance of these character-
istics during adult life. It causes growth of pubic, chest,
and facial hair; it produces changes in the larynx that
result in the male bass voice; and it increases the thick-
ness of the skin and the activity of the sebaceous glands,
predisposing to acne.
Most of the actions of testosterone and other andro-
gens result from increased protein synthesis in target
tissues. Androgens function as anabolic agents in males
and females to promote metabolism and musculoskeletal
growth. They greatly affect the development of muscle
mass during puberty, with boys averaging approximately
50% more of an increase in muscle mass than girls.
1
Spermatogenesis
Spermatogenesis, which occurs in the seminiferous
tubules of the testes, is the sequence of events that leads
to the formation of male germ cells or spermatozoa.
The process begins shortly before puberty and continues
throughout a man’s life.
The seminiferous tubules are composed of two types of
cells: Sertoli cells and spermatogenic cells
1–3
(Fig. 39-3A).
Sertoli cells are tall columnar cells with extensive cyto-
plasmic processes that surround the spermatogenic cells
and form tight junctions with other Sertoli cells. Also
known as
supporting
or
sustentacular cells
, Sertoli cells
provide structural organization for the tubules; they sup-
ply both physical and nutritional support for the devel-
oping germ cells; and their tight junctions prevent the
passage of proteins from the interstitial space into the
lumen of the seminiferous tubules, thus forming a blood-
testis barrier.
2,3
In addition, these cells have been shown
to be responsible for the movement of spermatozoa
from the base of the tubule toward the lumen and for
the release of mature sperm into the lumen. They also
phagocytose damaged germ cells and residual bodies
formed during the last stage of spermatogenesis. Finally,
in response to follicle-stimulating hormone (FSH) or
testosterone, the Sertoli cells secrete androgen-binding
protein. This substance, which has a high affinity for
androgens, provides high concentrations of testosterone
to the developing germ cells during spermatogenesis.
Spermatogenic cells, the source of spermatozoa, are
organized in poorly defined layers of progressive develop-
ment between adjacent Sertoli cells. Both spermatogen-
esis and oogenesis, its counterpart in the female, involve
a unique kind of nuclear division in which the chromo-
some number is reduced from 46 to 23 (see Chapter 5).
Spermatogonium
46
Primary
spermatocyte
23
23
Secondary
spermatocyte
Spermatids
Spermatozoa
23 23
23 23
Head
Middle
piece
Tail
Sertoli cells
Interstitial cells
of Leydig
Lumen of
seminiferous tubule
Spermatogenic
cells
A
B
FIGURE 39-3.
(A)
Cross-section of seminiferous tubule and
(B)
stages of development of spermatozoa. (46, 23 = number of
chromosomes.)
CHART 39-1
Main Actions of Testosterone
Induces differentiation of the male genital tract during
fetal development
Induces development of primary and secondary sex
characteristics
Gonadal function
External genitalia and accessory organs
Male voice timbre
Male skin characteristics
Male hair distribution
Anabolic effects
Promotes protein metabolism
Promotes musculoskeletal growth
Influences subcutaneous fat distribution
Promotes spermatogenesis (in follicle-stimulating
hormone [FSH]-primed tubules) and maturation of
sperm
Stimulates erythropoiesis
