C h a p t e r 3 9
Disorders of the Male Genitourinary System
997
The bulbourethral glands are peanut-sized glands
located on either side of the membranous urethra (see
Fig. 39-1). A duct of each gland joins the initial portion
of the penile urethra. The glands, which structurally
resemble mucus-secreting glands, produce a clear mucus-
like secretion.
2
Sexual stimulation causes release of the
secretion, which constitutes the major portion of the pre-
seminal fluid and serves to lubricate the penile urethra.
Hypothalamic/Pituitary Control
of Testicular Function
The hypothalamus and the anterior pituitary gland play
an essential role in promoting spermatogenesis in the
testes and maintaining the endocrine function of the tes-
tes by means of the gonadotropic hormones (i.e., ante-
rior pituitary hormones that promote the function and
growth of the testes in the male).
1,3
The synthesis and
release of the gonadotropic hormones from the pituitary
gland are regulated by gonadotropin-releasing hormone
(GnRH), which is synthesized by the hypothalamus and
secreted into the hypothalamohypophysial portal circu-
lation (Fig. 39-5).
Two gonadotropic hormones are secreted by the pitu-
itary gland: FSH and luteinizing hormone (LH). In the
male, LH also is called
interstitial cell–stimulating hor-
mone.
The production of testosterone by the interstitial
cells of Leydig is regulated by LH. Follicle-stimulating
hormone binds selectively to Sertoli cells surrounding the
seminiferous tubules, where it functions in the initiation
of spermatogenesis. Under the influence of FSH, Sertoli
cells produce androgen-binding protein, plasminogen
activator, and inhibin. Androgen-binding protein binds
testosterone and serves as a carrier and storage site for
testosterone in Sertoli cells. Although FSH is necessary
for the initiation of spermatogenesis, full maturation of
the spermatozoa requires testosterone. Androgen-binding
protein also serves as a carrier of testosterone from the
testes to the epididymis. Plasminogen activator, which
converts plasminogen to plasmin, functions in the final
detachment of mature spermatozoa from Sertoli cells.
Circulating levels of the gonadotropic hormones are
regulated in a negative feedback manner by testoster-
one.
1,3
High levels of testosterone suppress LH secretion
through a direct action on the pituitary and an inhibi-
tory effect on the hypothalamus. Follicle-stimulating
hormone is thought to be inhibited by a substance called
inhibin,
produced by Sertoli cells. Although two forms
of inhibin have been identified, only inhibin B has been
found in males.
3
Inhibin suppresses FSH release from the
pituitary gland without effecting LH release. The pitu-
itary gonadotropic hormones and Sertoli cells in the tes-
tes form a classic negative feedback loop in which FSH
stimulates inhibin secretion and increased inhibin levels
suppress FSH release from the hypothalamus. Unlike the
cyclic hormonal pattern in the woman, in the man, FSH,
LH, and testosterone secretion and spermatogenesis
occur at relatively unchanging rates during adulthood.
Sexual Function
The penis is themale external genital organ throughwhich
the urethra passes. It functions both as a sexual organ
and as an organ of urine elimination. Anatomically, the
external penis consists of a shaft that ends in a tip called
the
glans
(see Fig. 39-1). The loose skin of the shaft folds
to cover the glans, forming what is called the
prepuce
,
or
foreskin
. It is the foreskin that is removed during cir-
cumcision. The glans of the penis contains many sen-
sory nerves, making this the most sensitive portion of the
penile shaft. The shaft of the penis is composed of three
masses of erectile tissue held together by fibrous strands
and covered with a thin layer of skin (Fig. 39-6B). The
two dorsal masses of tissue are called the
corpora cav-
ernosa
and the third ventral mass, in which the spongy
part of the urethra is enclosed, is called the
corpus spon-
giosum.
2
The corpora cavernosa and corpus spongiosum
are cavernous sinuses that normally are relatively empty
but become engorged with blood during penile erection.
The physiology of the male sex act involves a com-
plex interaction between autonomic nervous system–
mediated spinal cord reflexes, higher neural centers,
and the vascular system
1,2
(Fig. 39-6A). It involves erec-
tion, emission, ejaculation, and detumescence.
Erection
involves increased flow of blood into the corpora cav-
ernosa and penile rigidity,
emission
is the contraction
of the vas deferens and ampulla with expulsion of
sperm into the internal urethra, and
ejaculation
is the
Hypothalamus
Anterior
pituitary
GnRH
Feedback
inhibition
Maturation of
spermatozoa
Testosterone
Initiation of
spermatogenesis
Target tissues
Inhibin
Interstitial
cells of Leydig
Sertoli
cells
LH FSH
Testis
FIGURE 39-5.
Hypothalamic-pituitary feedback control
of spermatogenesis and testosterone levels in the male.
The dashed line represents negative feedback. FSH, follicle-
stimulating hormone; GnRH, gonadotropin-releasing hormone;
LH, luteinizing hormone.
