Gross Anatomy Of The Male Reproductive Tract
The testes are a pair of
oval, slightly flattened bodies measuring about 4 cm in length and 2.5 cm in
diameter. Together with the epididymides, they lie in the scrotum, an
extra-abdominal sac just below the penis. The walls of the cavity in which the
testes and epididymis reside are known as the tunica vaginalis. The
tunica vaginalis forms from intra- abdominal peritoneum that migrates into the
primitive scrotum during development of the male internal genitalia. After
migration of the testis into the scrotum, the channel down which the testis has
moved (processus vaginalis) is obliterated.
The epididymis is a
comma-shaped structure that clasps the posterolateral margin of the testis. It
is formed from the duct of the epididymis, an irregularly twisted tube. The
epididymal duct is about 600 cm long. It begins at the top of the testis as the
head of the epididymis. After an extraordinarily tortuous course it ends as the
tail of the epididymis, then becomes the vas deferens (Fig. 7.1).
The testicular arteries supply
blood to the testes and epididymides. These arteries arise from the aorta just
below the renal arteries. The testicular arteries end in a dense vascular
plexus, the pampiniform plexus, which courses just under the tunica
vaginalis surrounding the testes. The plexus drains into the testicular veins.
The pampiniform plexus dissipates heat out of the scrotum by vasodilatation and
thereby has an important role in temperature regulation of the testes. Like
the ovarian veins, the right testicular vein empties into the inferior vena
cava, and the left testicular vein into the left renal vein. Lymphatic
drainage of the testes is to the para-aortic nodes.
All the blood and lymph vessels
to the testis and epididymis are bundled in a structure known as the spermatic
cord. This structure also contains the vas deferens and any remnants
of the processus vaginalis. The spermatic cord enters the scrotum from the
abdomen through the inguinal canal.
The testis is the site of
spermatogenesis and sex steroid production in the male. The epididymis is the
site of final sperm maturation. The scrotum
is basically a specialized dermal pouch that protects the testis and epididymis
from physical injury and aids in heat regulation of the testes. Spermatozoa are
very heat sensitive. Because the testes and epididymides are outside the body
cavity, intratesticular temperature is typically lower than in the abdomen.
Disorders of urogenital development that result in testicular retention within
the abdominal cavity or inguinal canal can have dramatic effects on future
fertility and increase the risk for testicular tumors (relative risk 3–8). Most
of these are seminomas (Chapter 40).
Vas (ductus) deferens and seminal vesicles
The vas deferens is a direct
continuation of the epididymis. It is a 45-cm-long structure that begins at the
lower end of the epididymis and ascends along the posterior aspect of the
testis in loose coils. After leaving the back of the testis, the vas deferens
traverses the spermatic cord into the abdomen. The vas deferens may be felt as
a firm hard cord on the posterior aspect of the spermatic cord as it traverses
the scrotum toward the superficial inguinal ring. After crossing into the
abdomen, the vas deferens curves medially across the external iliac artery
toward the pelvis. From there, it crosses the obturator nerve and vessels and
the vesicular vessels. The vas deferens then crosses over the ureter to meet the
duct of the seminal vesicle. Together, the vas deferens and the duct of the
seminal vesicle form the ejaculatory duct that opens into the prostatic
portion of the urethra. The ejaculatory duct is short (2.5 cm) and lies very
close to its companion contralateral duct as they pass forward through the
prostate.
The seminal vesicles are a
pair of hollow, sacculated structures located at the base of the bladder in
front of the rectum. Each vesicle is about 5 cm long and more intimately
connected to the bladder than to the rectum. During embryonic development, the
seminal vesicles form as diverticula of the vas deferens. The structures share
common blood and lymphatic supplies.
Blood supply to the vas deferens and seminal vesicles is mainly from the
inferior vesicular artery. The artery accompanies the vas deferens into the
scrotum where it anastomoses with the testicular artery. Lymphatic drainage is
to the internal and external iliac nodes. The vas deferens functions in sperm
transport. The seminal vesicles produce approximately 50–60% of the volume of
the seminal fluid.
Important seminal vesicle-derived
semen components include fructose and prostaglandins.
Prostate gland
The prostate is a partly
glandular, partly muscular organ that surrounds the beginning of the male
urethra, firmly affixed by a connective tissue sheath just behind the symphysis
pubis. The organ is about 2.5 × 3.5 × 4.5 cm. The
median lobe of the prostate, histologically referred
to as the transition zone, is wedge-shaped, directly surrounds the urethra and
separates it from the ejaculatory ducts. When hypertrophied, the median lobe
may obstruct the flow of urine. Median lobe hypertrophy occurs commonly in elderly
men.
The anterior prostate is composed
mostly of fibromuscular tissue. The glandular tissue of the prostate is
situated at the sides of the urethra and immediately posterior to it. This
glandular tissue is sub- divided into a central and peripheral zone based on
embryology (Chapter 6) and histology (Chapter 8). The peripheral zone is much
larger than the central zone and composed of about 50 incompletely defined
lobules. Each lobule contains minute ducts that empty directly into the urethra
just above the ejaculatory ducts.
The blood supply to the prostate
gland is variable, but most commonly arises from the common origin of the
internal pudendal and inferior gluteal arteries off the internal iliac
(hypogastric) arteries. The veins draining the prostate are wide and
thin-walled, forming a plexus that communicates with the plexus draining the
bladder. Both drain into the internal iliac veins. The prostatic plexus also
communicates with the vertebral venous plexuses; therefore, a tumor in the
prostate may give rise to secondary growth in the vertebral column. Lymphatic
drainage of the prostate follows that of the seminal vesicles and bladder neck
into the iliac chain of nodes.
All the muscular tissues in the
vas deferens, prostate, prostatic urethra and seminal vesicles are involved in
ejaculation. Prostate secretions contribute ∼15% of the volume of the
seminal fluid. Important prostate-derived components include acid phosphatases,
zinc, citrate and proteases that aid in semen
liquefaction. Liquefaction enables sperm to escape the very viscous initial
ejaculate.
Penis
The penis is composed chiefly of
cavernous (erectile) tissue and is traversed by the urethra. The posterior
surface of the flaccid penis is nearest the urethra and the opposite, more
extensive surface is dorsal (Fig. 7.2). Most of the
erectile tissue of the penis is arranged in three longitudinal
columns: the paired corpora cavernosa and the single median corpus
spongiosum. The tip of the penis is called the glans. The glans of
the penis also contains erectile tissue and is continuous with the corpus
spongiosum. The glans is covered with a retractable folded layer of thin skin,
called the prepuce or foreskin. Although it is not typically indicated
medically, the operation of circumcision removes the foreskin and is still
widely practiced in some societies.
The internal pudendal arteries
supply blood to the penis, entering the organ on its dorsal surface and
penetrating deeply into the erectile tissue of the corpora cavernosa. Veins
draining the penis enter the prostatic plexus either directly or through the
dorsal vein of the penis. Erection of the penis occurs when the extensive
cavernous spaces of the corpora cavernosa and corpus spongiosum fill with
blood. Engorgement of the penis inhibits venous return and allows maintenance
of erection.
Innervation of the penis is
critical for its erection. Penile nerve supply is derived from the pudendal
nerve (2nd, 3rd, 4th sacral nerves) and from the pelvic autonomic plexuses. The
lymphatic drainage of the penis is into the medial group of superficial
inguinal lymph nodes. The function of the penis is penetration. Penetration of
the vagina of the female allows deposition of semen near
the uterine cervix.
