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The male reproductive system, like that of the female, consists of
those organs whose function is to produce a new individual, i.e., to
accomplish reproduction. This system consists of a pair of testes, a network of excretory ducts
(epididymis, ductus deferens, and ejaculatory ducts), seminal vesicles, the
prostate, the bulbourethral glands, and the penis.
Testes
The male gonads, testes, or testicles, begin their development high in the
abdominal cavity, near the kidneys. During the last two months before birth,
or shortly after birth, they descend through the inguinal canal into the
scrotum, a pouch that extends below the abdomen, posterior to the penis.
Although this location of the testes, outside the abdominal cavity, may seem
to make them vulnerable to injury, it provides a temperature about 3°C below
normal body temperature. This lower temperature is necessary for the
production of viable sperm.The scrotum consists of skin and subcutaneous
tissue. A vertical septum, or partition, of subcutaneous tissue in the center
divides it into two parts, each containing one testis. Smooth muscle fibers,
called the dartos muscle, in the subcutaneous tissue contract to give the
scrotum its wrinkled appearance. When these fibers are relaxed, the scrotum is
smooth. Another muscle, the cremaster muscle, consists of skeletal muscle
fibers and controls the position of the scrotum and testes. When it is cold or
a man is sexually aroused, this muscle contracts to pull the testes closer to
the body for warmth.
Structure
Each testis is an oval structure about 5 cm long and 3 cm in diameter. A
tough, white fibrous connective tissue capsule, the tunica albuginea,
surrounds each testis and extends inward to form septa that partition the
organ into lobules. There are about 250 lobules in each testis. Each lobule
contains 1 to 4 highly coiled seminiferous tubules that converge to form a
single straight tubule, which leads into the rete testis. Short efferent ducts
exit the testes. Interstitial cells (cells of Leydig), which produce male sex
hormones, are located between the seminiferous tubules within a lobule.
Spermatogenesis
Sperm are produced by spermatogenesis within the seminiferous tubules. A
transverse section of a seminiferous tubule shows that it is packed with cells
in various stages of development. Interspersed with these cells, there are
large cells that extend from the periphery of the tubule to the lumen. These
large cells are the supporting, or sustentacular cells (Sertoli's cells),
which support and nourish the other cells.
Early in embryonic development, primordial germ cells enter the testes and
differentiate into spermatogonia, immature cells that remain dormant until
puberty. Spermatogonia are diploid cells, each with 46 chromosomes (23 pairs)
located around the periphery of the seminiferous tubules. At puberty, hormones
stimulate these cells to begin dividing by mitosis. Some of the daughter cells
produced by mitosis remain at the periphery as spermatogonia. Others are
pushed toward the lumen, undergo some changes, and become primary
spermatocytes. Because they are produced by mitosis, primary spermatocytes,
like spermatogonia, are diploid and have 46 chromosomes.
Each primary spermatocytes goes through the first meiotic division, meiosis
I, to produce two secondary spermatocytes, each with 23 chromosomes (haploid).
Just prior to this division, the genetic material is replicated so that each
chromosome consists of two strands, called chromatids, that are joined by a
centromere. During meiosis I, one chromosome, consisting of two chromatids,
goes to each secondary spermatocyte. In the second meiotic division, meiosis
II, each secondary spermatocyte divides to produce two spermatids. There is no
replication of genetic material in this division, but the centromere divides
so that a single-stranded chromatid goes to each cell. As a result of the two
meiotic divisions, each primary spermatocyte produces four spermatids. During
spermatogenesis there are two cellular divisions, but only one replication of
DNA so that each spermatid has 23 chromosomes (haploid), one from each pair in
the original primary spermatocyte. Each successive stage in spermatogenesis is
pushed toward the center of the tubule so that the more immature cells are at
the periphery and the more differentiated cells are nearer the center.
Spermatogenesis (and oogenesis in the female) differs from mitosis because
the resulting cells have only half the number of chromosomes as the original
cell. When the sperm cell nucleus unites with an egg cell nucleus, the full
number of chromosomes is restored. If sperm and egg cells were produced by
mitosis, then each successive generation would have twice the number of
chromosomes as the preceding one.
The final step in the development of sperm is called spermiogenesis. In
this process, the spermatids formed from spermatogenesis become mature
spermatozoa, or sperm. The mature sperm cell has a head, midpiece, and tail.
The head, also called the nuclear region, contains the 23 chromosomes
surrounded by a nuclear membrane. The tip of the head is covered by an
acrosome, which contains enzymes that help the sperm penetrate the female
gamete. The midpiece, metabolic region, contains mitochondria that provide
adenosine triphosphate (ATP). The tail, locomotor region, uses a typical
flagellum for locomotion. The sperm are released into the lumen of the
seminiferous tubule and leave the testes. They then enter the epididymis where
they undergo their final maturation and become capable of fertilizing a female
gamete.
Sperm production begins at puberty and continues throughout the life of a
male. The entire process, beginning with a primary spermatocyte, takes about
74 days. After ejaculation, the sperm can live for about 48 hours in the
female reproductive tract.
Duct System
Sperm cells pass through a series of ducts to reach the outside
of the body. After they leave the testes, the sperm passes through
the epididymis, ductus deferens, ejaculatory duct, and urethra.
Epididymis
Sperm leave the testes through a series of efferent ducts that
enter the epididymis. Each
epididymis is a long (about 6 meters) tube that is tightly coiled to form a
comma-shaped organ located along the superior and posterior margins of the
testes. When the sperm leave the testes, they are immature and incapable of
fertilizing ova. They complete their maturation process and become fertile as
they move through the epididymis. Mature sperm are stored in the lower
portion, or tail, of the epididymis.
Ductus Deferens
The ductus deferens, also called vas deferens, is a fibromuscular tube that
is continuous ( or contiguous) with the epididymis. It begins at the bottom
(tail) of the epididymis then turns sharply upward along the posterior margin
of the testes. The ductus deferens enters the abdominopelvic cavity through
the inguinal canal and passes along the lateral pelvic wall. It crosses over
the ureter and posterior portion of the urinary bladder, and then descends
along the posterior wall of the bladder toward the prostate gland. Just before
it reaches the prostate gland, each ductus deferens enlarges to form an
ampulla. Sperm are stored in the proximal portion of the ductus deferens, near
the epididymis, and peristaltic movements propel the sperm through the tube.
The proximal portion of the ductus deferens is a component of the spermatic
cord, which contains vascular and neural structures that supply the testes.
The spermatic cord contains the ductus deferens, testicular artery and veins,
lymph vessels, testicular nerve, cremaster muscle that elevates the testes for
warmth and at times of sexual stimulation, and a connective tissue covering.
Ejaculatory Duct
Each ductus deferens, at the ampulla,
joins the duct from the adjacent seminal vesicle (one of the accessory glands)
to form a short ejaculatory duct. Each ejaculatory duct passes through the
prostate gland and empties into the urethra.
Urethra
The urethra extends from the urinary bladder to the external urethral
orifice at the tip of the penis. It is a passageway for sperm and fluids from
the reproductive system and urine from the urinary system. While reproductive
fluids are passing through the urethra, sphincters contract tightly to keep
urine from entering the urethra.
The male urethra is divided into three regions. The prostatic urethra is
the proximal portion that passes through the prostate gland. It receives the
ejaculatory duct, which contains sperm and secretions from the seminal
vesicles, and numerous ducts from the prostate glands. The next portion, the
membranous urethra, is a short region that passes through the pelvic floor.
The longest portion is the penile urethra (also called spongy urethra or
cavernous urethra), which extends the length of the penis and opens to the
outside at the external urethral orifice. The ducts from the bulbourethral
glands open into the penile urethra.
Accessory Glands
The accessory glands of the male reproductive system are the
seminal vesicles, prostate gland, and the bulbourethral glands.
These glands secrete fluids that enter the urethra.
Seminal Vesicles
The paired seminal vesicles are saccular glands posterior to the
urinary bladder. Each gland has a short duct that joins with the
ductus deferens at the ampulla to form an ejaculatory duct, which
then empties into the urethra. The fluid from the seminal vesicles
is viscous and contains fructose, which provides an energy source
for the sperm; prostaglandins, which contribute to the mobility and viability of
the sperm; and proteins that cause slight coagulation reactions in the semen
after ejaculation.
Prostate
The prostate gland is a firm, dense structure that is located just inferior
to the urinary bladder. It is about the size of a walnut and encircles the
urethra as it leaves the urinary bladder. Numerous short ducts from the
substance of the prostate gland empty into the prostatic urethra. The
secretions of the prostate are thin, milky colored, and alkaline. They
function to enhance the motility of the sperm.
Bulbourethral Glands
The paired bulbourethral (Cowper's) glands are small, about the size of a
pea, and located near the base of the penis. A short duct from each gland
enters the proximal end of the penile urethra. In response to sexual
stimulation, the bulbourethral glands secrete an alkaline mucus-like fluid.
This fluid neutralizes the acidity of the urine residue in the urethra, helps
to neutralize the acidity of the vagina, and provides some lubrication for the
tip of the penis during intercourse.
Seminal Fluid
Seminal fluid, or semen, is a slightly alkaline mixture of sperm cells and
secretions from the accessory glands. Secretions from the seminal vesicles
make up about 60 percent of the volume of the semen, with most of the
remainder coming from the prostate gland. The sperm and secretions from the
bulbourethral gland contribute only a small volume.
The volume of semen in a single ejaculation may vary from 1.5 to 6.0 ml.
There are usually between 50 to 150 million sperm per milliliter of semen.
Sperm counts below 10 to 20 million per milliliter usually present fertility
problems. Although only one sperm actually penetrates and fertilizes the ovum,
it takes several million sperm in an ejaculation to ensure that fertilization
will take place.
Penis
The penis, the male copulatory organ, is a cylindrical pendant organ
located anterior to the scrotum and functions to transfer sperm to the vagina.
The penis consists of three columns of erectile tissue that are wrapped in
connective tissue and covered with skin. The two dorsal columns are the
corpora cavernosa. The single, midline ventral column surrounds the urethra
and is called the corpus spongiosum.
The penis has a root, body (shaft), and glans penis. The root of the penis
attaches it to the pubic arch and the body is the visible, pendant portion.
The corpus spongiosum expands at the distal end to form the glans penis. The
urethra, which extends throughout the length of the corpus spongiosum, opens
through the external urethral orifice at the tip of the glans penis. A loose
fold of skin, called the prepuce, or foreskin, covers the glans penis.
Sexual Response
The male sexual response includes erection and orgasm accompanied by
ejaculation of semen. Orgasm is followed by a variable time period during
which it is not possible to achieve another erection.
Three hormones are the principal regulators of the male reproductive
system. Follicle-stimulating hormone (FSH) stimulates spermatogenesis;
luteinizing hormone (LH) stimulates the production of testosterone;
and testosterone stimulates the development of male secondary sex
characteristics and spermatogenesis.
Source:
SEER's Training Website.
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