Chapter
28: The Reproductive System
The reproductive system is the only system that is not essential to the
life of the individual. Many primitive societies failed to discover the
basic link between sexual activity and childbirth, and assumed that cosmic
forces were responsible for producing new individuals.
Male and female reproductive cells
are known as gametes.
All the cells in a human body are the mitotic descendants of a zygote, the single cell created by
the fusion of a gamete from the father and a gamete from the mother.
During this period, hormones produced by the
reproductive system direct gender–specific patterns of development.
The reproductive system includes the following components:
- Gonads, or reproductive organs that produce gametes and hormones.
- Ducts that receive and transport the gametes.
- Accessory glands and organs that secrete fluids into
the ducts of the reproductive system or into other excretory ducts.
- Perineal structures that are collectively known as the external genitalia
In adult males, the testes, or male gonads, secrete sex hormones called androgens
, principally testosterone. The
testes also produce the male gametes, called spermatozoa, or sperm –one–half billion each day. During emission
, mature spermatozoa
travel along a lengthy duct system, where they are mixed with the secretions of
accessory glands. The mixture created is known as semen. During ejaculation , semen is expelled from the body.
In adult females, the ovaries , or female gonads, typically release only one
immature gamete, an oocyte, per month. This immature gamete travels along
short uterine tubes ,
which end in the muscular organ called the uterus. If a sperm reaches the oocyte and initiates
the process of fertilization, the oocyte matures into an ovum.
Proceeding from a
testis, the spermatozoa travel within the epididymis; the ductus deferens , or vas deferens ; the ejaculatory duct ; and the urethra before leaving the body. Accessory organs–the seminal
vesicles , the prostate gland , and the bulbourethral glands –secrete various fluids into the ejaculatory
ducts and urethra.
In cryptorchidism,
one or both of the testes have not descended into the scrotum by the time of
birth. Typically, the cryptorchid testes are lodged in the abdominal cavity or
within the inguinal canal. Cryptorchidism occurs in about 3 percent of
full–term deliveries and in roughly 30 percent of premature births. In most
instances, normal descent occurs a few weeks later, but the condition can be
surgically corrected if it persists. Corrective measures should be taken before
puberty (sexual
maturation), because cryptorchid (abdominal) testes will not produce
spermatozoa. Thus, the individual will be sterile ( infertile ),
or unable to bear children. If the testes cannot be moved into the scrotum, in
most cases they will be removed, because about 10 percent of males with
uncorrected cryptorchid testes eventually develop testicular cancer. This surgical procedure is called an orchiectomy.
.
The inguinal canals form during development
as the testes descend into the scrotum; at that time, these canals link the
scrotal cavities with the peritoneal cavity. In normal adult males, the
inguinal canals are closed, but the presence of the spermatic cords creates
weak points in the abdominal wall that remain throughout life. As a result, inguinal
hernias –protrusions of a
portion of the intestine into the inguinal canal–are relatively common in
males. The inguinal canals in females are very small, containing only the ilioinguinal
nerves and the round
ligaments of the uterus. The
abdominal wall is nearly intact, so inguinal hernias in women are rare.
Contraction of the cremaster muscle during sexual arousal
or in response to cold temperature tenses the scrotum and pulls the testes
closer to the body. Normal development of spermatozoa in the testes requires
temperatures about 2 deg F lower
than those elsewhere in the body. The cremaster muscle relaxes or contracts to
move the testes away from or toward the body as needed to maintain acceptable
testicular temperatures.
Structure of the
Testes
Below the tunica vaginalis covering the
testis is the tunica
albuginea, a dense layer of
connective tissue rich in collagen fibers. These fibers are continuous with
those surrounding the adjacent epididymis and extend into the substance of the
testis. There they form fibrous partitions, or septa.
that converge toward
the region nearest the entrance to the epididymis. The connective tissues in
this region support the blood vessels and lymphatic vessels that supply the
testis and the efferent ductules , which transport spermatozoa to the epididymis.
Histology of the
Testes
The septa subdivide the testis into a
series of lobules
. Roughly 800 slender,
tightly coiled seminiferous
tubules are distributed among
the lobules. Each tubule averages about 80 cm (31 in.) in length, and a typical
testis contains nearly one–half mile of seminiferous tubules. Sperm production
occurs within these tubules.
Interstitial
cells are responsible for
the production of androgens ,
the dominant sex hormones in males. Testosterone is the most important androgen.
Spermatozoa are
produced by the process of spermatogenesis.
Spermatogenesis begins at the outermost layer of cells in the seminiferous
tubules and proceeds toward the lumen. At each step in this process, the daughter
cells move closer to the lumen. First, stem cells called spermatogonia divide by mitosis to produce daughter cells,
some of which differentiate into primary spermatocytes. Primary spermatocytes are the cells that begin meiosis
, a specialized form
of cell division involved only in the production of gametes (spermatozoa in
males, ova in females). Spermatocytes give rise to spermatids–immature gametes that subsequently
differentiate into spermatozoa. The spermatozoa lose contact with the wall of
the seminiferous tubule and enter the fluid in the lumen.
Mitosis and Meiosis
In both males and females, mitosis and
meiosis differ significantly in terms of the events that take place in the
nucleus. Mitosis is part of the process of somatic
(body) cell division, which produces two daughter cells, each containing 23 pairs of chromosomes.
Each pair consists of
one chromosome provided by the father and another by the mother at the time of
fertilization. Because the daughter cells contain both members of each
chromosome pair (for a total of 46 chromosomes), they are called diploid cells.
Meiosis involves two cycles of cell division ( meiosis
I and meiosis
II ) and produces four
cells, each of which contains 23 individual chromosomes. Because
these cells contain only one member of each pair of chromosomes, they are
called haploid cells.
The events in the nucleus shown in Figure 28–6b are the same whether you
consider the formation of spermatozoa or ova.
As a cell prepares to
begin meiosis,
At this point, the close similarities
between meiosis and mitosis end. In meiosis, the corresponding maternal and
paternal chromosomes now come together, an event known as synapsis. Synapsis involves 23 pairs of chromosomes; each member of each pair consists of two
chromatids (one from Mom, one from Dad). A matched set of four chromatids is
called a tetrad. Much exchange of genetic material can occur
between the chromatids of a chromosome pair at this stage of meiosis. Such an
exchange, called crossing–over ,
increases genetic variation among offspring.
Meiosis includes two division cycles,
referred to as meiosis I and meiosis II. The stages within each phase are
identified similarly (as prophase I, metaphase II, etc.). The nuclear envelope
disappears at the end of prophase I. As metaphase I begins, the tetrads line up
along the metaphase plate. As anaphase I begins, the tetrads break up–the
maternal and paternal chromosomes separate. This is a major difference between
mitosis and meiosis: In mitosis, each daughter cell receives one of the two
copies of every chromosome, maternal and paternal; in meiosis I, each daughter
cell receives both copies of either the maternal chromosome or the paternal chromosome from each tetrad.
As anaphase proceeds,
the maternal and paternal components are randomly and independently
distributed. That is, as each tetrad splits, you cannot predict which daughter
cell will receive copies of the maternal chromosome rather than copies of the
paternal chromosome. As a result, telophase I ends with the formation of two
daughter cells containing unique combinations of maternal and paternal
chromosomes. Both cells contain 23 chromosomes. Because the first meiotic
division reduces the number of chromosomes from 46 to 23, it is called a reductional division . Each of these chromosomes still consists of
two duplicate chromatids. The duplicates will separate during meiosis II .
The interphase
separating meiosis I and meiosis II is very brief, and no
In males, the mitotic division of a
spermatogonium produces two daughter cells. One is a spermatogonium that
remains in contact with the basement membrane, and the other is a primary
spermatocyte that is displaced toward the lumen.
Spermiogenesis
In spermiogenesis ,
the last step of spermatogenesis, each spermatid matures into a single
spermatozoon, or sperm
Oogenesis.
In oogenesis, a single primary oocyte produces an ovum and two or three
nonfunctional polar bodies.
Sustentacular Cells play
a key role in spermatogenesis. These cells have six important functions that
directly or indirectly affect mitosis, meiosis, and spermiogenesis within the
seminiferous tubules:
The Maintenance of the
Blood–Testis Barrier.
The Support of Mitosis and
Meiosis.
The Support of Spermiogenesis.
The Secretion of Inhibin.
The Secretion of Androgen–Binding
Protein.
The Secretion of
Müllerian–Inhibiting Factor. Müllerian– inhibiting factor (MIF) is secreted by sustentacular cells in the
developing testes. This hormone causes regression of the fetal Müllerian ducts , passageways that participate in the formation
of the uterine tubes and the uterus in females. In males, inadequate MIF production during fetal development leads to
the retention of these ducts and the failure of the testes to descend into the
scrotum.
Testicular
cancer occurs at a relatively low rate
of about 3 cases per 100,000 males per year. Although only about 7200 new cases
occur each year in the
More than 95 percent of testicular
cancers are the result of abnormal spermatogonia or spermatocytes, rather than
abnormal sustentacular cells, interstitial cells, or other cells of the testes.
The Anatomy of a
Spermatozoon
Each spermatozoon has three distinct
regions: (1) the head, (2) the middle piece, and (3) the tail.
Mitochondrial activity
provides the ATP that is needed to move the tail.
The tail is the only
flagellum in the human body. A flagellum , a whiplike organelle, moves a cell from one
place to another. Whereas cilia beat in a predictable, waving fashion, the
flagellum of a spermatozoon has a complex, corkscrew motion.
Unlike other, less
specialized cells, a mature spermatozoon lacks an endoplasmic reticulum, Golgi
apparatus, lysosomes, peroxisomes, inclusions, and many other intracellular
structures. The loss of these organelles reduces the cell's size and mass; it
is essentially a mobile carrier for the enclosed chromosomes, and extra weight
would slow it down. Because the cell does not contain glycogen or other energy
reserves, however, it must absorb nutrients (primarily fructose) from the
surrounding fluid.
The Male Reproductive
Tract
The Epididymis, the start of the male reproductive tract, is a
coiled tube bound to the posterior border of the testis.
A tubule almost 7
meters (23 ft) long, the epididymis is coiled and twisted so as to take up very
little space.
It Monitors and Adjusts the
Composition of the Fluid Produced By the Seminiferous Tubules.
It Acts as a Recycling Center for
Damaged Spermatozoa.
It Stores and Protects Spermatozoa
and Facilitates Their Functional Maturation. A
spermatozoon passes through the epididymis in about two weeks and completes its
functional maturation at that time.
The Ductus Deferens or vas deferens , is 40–45 cm (16–18 in.) long. It begins at the
tail of the epididymis ( Figures 28–1 , p. 1048, and 28–9a ) and, as part of
the spermatic cord, ascends through the inguinal canal ( Figure 28–3 , p.
1050). Inside the abdominal cavity, the ductus deferens passes posteriorly,
curving inferiorly along the lateral surface of the urinary bladder toward the
superior and posterior margin of the prostate gland. Just before the ductus
deferens reaches the prostate gland and seminal vesicles, its lumen enlarges.
This expanded portion is known as the ampulla
The wall of the ductus
deferens contains a thick layer of smooth muscle ( Figure 28–10b ). Peristaltic
contractions in this layer propel spermatozoa and fluid along the duct, which
is lined by a pseudostratified ciliated columnar epithelium. In addition to
transporting spermatozoa, the ductus deferens can store spermatozoa for several
months.
During this time, the
spermatozoa remain in a state of suspended animation and have low metabolic
rates.
The junction of the ampulla with the duct
of the seminal vesicle marks the start of the ejaculatory duct . This short passageway (2 cm, or less than 1
in.) penetrates the muscular wall of the prostate gland and empties into the
urethra near the opening of the ejaculatory duct from the opposite side.
The Accessory Glands
The fluids contributed by the
seminiferous tubules and the epididymis account for only about 5 percent of the
volume of semen. The fluid component of semen is a mixture of the secretions of
many glands, each with distinctive biochemical characteristics. Important
glands include the seminal vesicles , the prostate gland , and the bulbourethral glands , all of which occur only in males. Among the
major functions of these glands are (1) activating the spermatozoa; (2)
providing the nutrients spermatozoa need for motility; (3) propelling
spermatozoa and fluids along the reproductive tract, mainly by peristaltic
contractions; and (4) producing buffers that counteract the acidity of the
urethral and vaginal environments.
The Seminal Vesicles are glands embedded in connective tissue on
either side of the midline, sandwiched between the posterior wall of the
urinary bladder and the rectum.
The seminal vesicles
contribute about 60 percent of the volume of semen. Although the vesicular
fluid generally has the same osmotic concentration as that of blood plasma, the
composition of the two fluids is quite different. In particular, the secretion
of the seminal vesicles contains (1) higher concentrations of fructose, which
is easily metabolized by spermatozoa; (2) prostaglandins, which can stimulate
smooth muscle contractions along the male and female reproductive tracts; and
(3) fibrinogen, which after ejaculation forms a temporary clot within the
vagina.
The secretions of the
seminal vesicles are slightly alkaline, helping to neutralize acids in the secretions
of the prostate gland and within the vagina. When mixed with the secretions of
the seminal vesicles, previously inactive but functional spermatozoa undergo
the first step in capacitation and begin beating their flagella, becoming
highly motile.
The secretions of the
seminal vesicles are discharged into the ejaculatory duct at emission
, when peristaltic
contractions are under way in the ductus deferens, seminal vesicles, and
prostate gland. These contractions are under the control of the sympathetic
nervous system.
The Prostate Gland is a small, muscular, rounded organ about 4 cm
(1.6 in.) in diameter. The prostate gland encircles the proximal portion of the
urethra as it leaves the urinary bladder.
The prostate gland
produces prostatic
fluid , a slightly acidic
solution that contributes 20–30 percent of the volume of semen. In addition to
several other compounds of uncertain significance, prostatic secretions contain
seminalplasmin
(sem–i–nal–PLAZ–min),
an antibiotic that may help prevent urinary tract infections in males. These
secretions are ejected into the prostatic urethra by peristaltic contractions
of the muscular wall.
The Bulbourethral
Glands , or Cowper's
glands , are situated at the
base of the penis, covered by the fascia of the urogenital diaphragm. The
bulbourethral glands are compound, tubuloalveolar mucous glands that secrete a
thick, alkaline mucus. The secretion helps neutralize any urinary acids that
may remain in the urethra and lubricates the glans , or tip of the penis.
Semen
A typical ejaculation releases 2–5 ml of
semen. This volume of fluid, called ejaculate , contains:
Spermatozoa. The normal sperm count ranges
from 20 million to 100 million spermatozoa per milliliter of sperm.
Seminal Fluid. Seminal fluid , the fluid component of semen, is a mixture of
glandular secretions
Enzymes. Several
important enzymes are present in seminal fluid, including (1) a protease that
may help dissolve mucous secretions in the vagina; (2) seminalplasmin, an
antibiotic enzyme from the prostate gland that kills a variety of bacteria,
including Escherichia
coli ; (3) a prostatic
enzyme that converts fibrinogen to fibrin after ejaculation; and (4) fibrinolysin , which subsequently liquefies the clotted
semen.
The determination of male fertility
problems in the absence of abnormal semen analysis results may require
additional tests. In what is often called the "hamster test," a
sample of semen is placed on a slide with the oocyte of a hamster. Normal human
spermatozoa will fertilize the oocyte, although further development is
impossible. If fertilization does not occur, there may be problems with the
enzymes in the acrosomal cap.
Most of the body of
the penis consists of three cylindrical columns of erectile tissue . The parasympathetic innervation of the penile
arteries involves neurons that release nitric oxide (NO) at their synaptic
knobs. The smooth muscles in the arterial walls relax when NO is released, at
which time the vessels dilate, blood flow increases, the vascular channels become
engorged with blood, and erection of the penis
occurs. The prescription drug Viagra
which enhances and prolongs the effects of nitric oxide on the erectile tissue
of the penis, has proven useful in treating many cases of impotence.
Prostatic Hypertrophy and
Prostate Cancer
In most cases, enlargement of the
prostate gland, or benign prostatic
hypertrophy , occurs spontaneously in men over age 50. The
increase in size occurs while testosterone production by the interstitial cells
decreases. For unknown reasons, small masses, called prostatic concretions , may
form within the glands.
Partial surgical removal is the
most effective treatment. In the procedure known as a TURP
( transurethral
prostatectomy ), an instrument pushed along the urethra restores
normal function by cutting away the swollen prostatic tissue. Most of the
prostate gland remains in place, and no external scars result.
Prostate
cancer , a malignant, metastasizing
cancer of the prostate gland, is the second most common cancer and the second
most common cause of cancer deaths in males. In 2002, approximately 189,000 new
cases of prostate cancer were diagnosed in the
Hormones and Male
Reproductive Function
The anterior lobe of
the pituitary gland releases follicle–stimulating hormone (
The hormone GnRH is secreted in pulses
rather than continuously. In adult males, small pulses occur at 60–90–minute
intervals. As levels of GnRH change, so do the rates of secretion of
In males,
LH and Androgen
Production
In males, LH causes the secretion of
testosterone and other androgens by the interstitial cells of the testes.
Testosterone, the most important androgen, has numerous functions, such as (1)
stimulating spermatogenesis and promoting the functional maturation of
spermatozoa, through its effects on sustentacular cells; (2) affecting central
nervous system (CNS) function, including the libido (sexual drive) and related
behaviors; (3) stimulating metabolism throughout the body, especially pathways
concerned with protein synthesis and muscle growth; (4) establishing and
maintaining the secondary sex characteristics, such as the distribution of
facial hair, increased muscle mass and body size, and the quantity and location
of characteristic adipose tissue deposits; and (5) maintaining the accessory
glands and organs of the male reproductive tract.