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Reproduction is essential for any species to sustain its population. In
the simplest sense, the most important function of every living organism
is reproduction. Organs of the male and female reproductive systems play a
central role in sexual reproduction by creating, nourishing, and housing
sex cells called
The human male reproductive system consists of gonads called testes, a
series of ducts (epididymis,
, ejaculatory duct, urethra) that serve to transport spermatozoa to the
female reproductive tract, and accessory sex glands (seminal vesicles,
prostate, and bulbourethral glands).
The testes (singular, testis) are paired structures that originally
develop in the abdomen and descend into the scrotum, a sac of skin and
positioned outside the pelvic cavity. This scrotal location is important
for maintaining a testicular temperature, approximately 1.5 to 2.5 degrees
Celsius (34.7 to 36.5 degrees Fahrenheit) below body temperature, required
for spermatogenesis (sperm production). Testes also serve important
functions as the source of male sex
called androgens. The most abundant androgen is testosterone.
Inside each testis is a network of fine-diameter tubes called seminiferous
tubules. Sertoli cells form the walls of a seminiferous tubule. Sertoli
nourish, support, and protect developing germ cells, which undergo cell
to form spermatozoa (immature sperm). During spermatogenesis, germ cells
begin near the wall of a seminiferous tubule, and after division they are
shed into the tubule.
produced by Sertoli cells are required for spermatogenesis, as is
A photomicrograph of human testis showing spermatogenesis.
Surrounding the tubules are clusters of interstitial cells, which
synthesize testosterone and secrete it into the bloodstream. Testosterone
is present in infant boys, although synthesis increases dramatically at
puberty around age thirteen. This increase stimulates the onset of
spermatogenesis and development of accessory sex glands. All male
reproductive organs require testosterone for functions such as protein
, cell growth, and cell division. Androgens also play important roles in
the male sexual response and stimulate secondary sex characteristics such
as skeletal development, facial hair growth, deepening of the voice,
, and enlargement of the testes, scrotum, and penis.
Sperm production and androgen synthesis are controlled by a complex
loop involving the testes, hypothalamus, and pituitary gland. The
pituitary controls testis function by producing follicle-stimulating
(FSH) and luteinizing hormone (LH). FSH stimulates spermatogenesis, in
part by affecting Sertoli cells, while LH stimulates androgen production
by interstitial cells. Pituitary production of these hormones depends on
secretion of gonadotropin-releasing hormone (GnRH) by the hypothalamus.
Elevated levels of GnRH initiate puberty.
How does this feedback loop prevent testosterone levels from getting too
high or too low? While some may joke that the testis controls the brain in
boys, there is some truth to this statement: the testis can control brain
function. The production of LH is controlled by the actions of
testosterone on the hypothalamus and pituitary. If testosterone
concentration is elevated, testosterone inhibits production of GnRH by the
hypothalamus; subsequently, LH and FSH production decreases.
Spermatozoa leave each testis through small tubes called efferent
ductules. Fluid pressure from secretions in the testis and
cells in the efferent ductules help move spermatozoa into the epididymis.
Testicular spermatozoa are immature because they cannot swim and lack the
ability to penetrate an egg.
Sperm maturation occurs in the epididymis. Located adjacent to the testis,
the epididymis contains a single, highly coiled tubule nearly 6 meters
(19.6 feet) long. Sperm transport through the epididymis takes
approximately twenty days. As sperm transit the epididymis, they are
bathed in a specialized fluid rich in proteins,
, and a number of other molecules. Complex interactions between
spermatozoa and epididymal fluid contribute to sperm maturation. The
epididymis is also a site for sperm storage and for the protection of
sperm against chemical injury.
The male reproductive system provides for the formation, maturation,
storage, and ejaculation of sperm. Both sperm and urine exit through
Sperm Formation and Ejaculation
From the epididymis, spermatozoa enter a muscular tube called the vas
deferens (approximately 45 centimeters [17.7 inches] long). The vas
deferens contracts during the release of sperm—a process called
ejaculation—to move spermatozoa out of the epididymis and into the
ejaculatory duct, where sperm are mixed with secretions from the seminal
vesicles. The ejaculatory duct enters the urethra as it passes through the
prostate gland. In males, the urethra serves a dual purpose transporting
sperm to the penis and urine from the urinary bladder.
The accessory sex glands consist of a single prostate gland and paired
seminal vesicles and bulbourethral glands. The prostate and seminal
vesicles secrete seminal fluid, or semen, a
mixture of spermatozoa and fluid from accessory sex glands. Spermatozoa
constitute 10 percent of semen volume and number approximately 50 to 150
million sperm per milliliter. Combined secretions from the seminal
vesicles and prostate account for roughly 90 percent of semen volume. The
seminal vesicle secretions are rich in fructose (which serves as an energy
source for spermatozoa),
, and proteins that facilitate clotting of ejaculated semen in the female.
Prostate secretions are rich in zinc, citric acid, antibioticlike
important for sperm function. A protein called prostate-specific
can show elevated levels in the blood under conditions such as prostate
growth. During sexual excitation, the bulbourethral glands produce a
fluid that neutralizes residual urine in the urethra, protecting the
sperm from its acidity.
Anatomy of the human sperm.
The penis contains two bodies of tissue (corpora cavernosa) above the
urethra and a lower cylinder of tissue (corpus spongiosum) surrounding the
urethra. The enlarged tip of the penis is called the glans penis.
Mechanisms responsible for penile erection are complex. During sexual
arousal, penile arteries dilate and a large volume of blood fills the
penis, resulting in erection. The nervous system plays an important role
in controlling erection and ejaculation.
The parasympathetic division of the
nervous system regulates erection, whereas ejaculation is triggered by
sympathetic impulses. Medical and emotional conditions can cause clinical
disorders of erectile dysfunction. Drugs such as Viagra increase erectile
function by improving blood flow into penile tissue. Many factors result
in poor fertility or infertility in males including hormone imbalances,
reproductive tract blockages, decreased sperm concentration, and abnormal
The journey of spermatozoa from formation to release is complicated.
Reproduction of any individual is never guaranteed, yet through combined
functions of male and female reproductive organs, nature has provided a
sophisticated sequence of biological events designed to maximize the
likelihood that we pass our
to future generations.
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