At the same time, secretions
from the female reproductive tract
cause sperm to become activated
or undergo
c a p a c ita tio n
.
Sperm
motility increases further, and the
acrosome—a
vesicle
containing
enzymes that aid penetration—is
stripped of cholesterol and glyco-
proteins as the sperm rubs against
the walls of the uterus and uterine tube; both of these events
make it easier for the sperm to fuse with the secondary
oocyte. The enzymes of the acrosome digest through the
corona radiata cells that surround the egg, allowing physi-
cal contact between the membranes of the egg and sperm.
Despite the fact that as many as 500,000,000 sperm could
be present in a single ejaculate, a mere 100 of these sperm
locate the egg, and only 1
gets to fertilize it.
To prevent fertilization, several contraceptive meth-
ods can be used during or before intercourse. Let’s take a
closer look at some of the options.
Contraceptive Methods
Interrupt Different Stages
of the Fertilization Process
There are several strategies for
c o n t r a c e p t io n
.
These
methods
accomplish
their
common
goal
by interrupting different stages
of the fertilization process. Some
contraceptive methods are more
successful than others, and we’ll
discuss their effectiveness rates based on typical use.
A b stin en ce.
Total
abstinence,
or
the
avoidance
of
sexual intercourse, is the only method of preventing
pregnancy that is 100% reliable.
S te r iliza tio n .
Blocking sperm or eggs from moving
through
their
respective
tubes.
Sterilization
is
a
procedure that renders an individual incapable of
reproduction. Sterilization methods are very effective
at preventing pregnancy, with a failure rate of less
than 1%
. The most common means of sterilization
in males is
vasectom y,
in which a portion of each vas
deferens is removed. Even though sperm production
continues in the testes, sperm can no longer reach the
exterior. Instead, they degenerate and are destroyed
by phagocytosis. Blood testosterone level is normal,
so a vasectomy has no effect on sexual desire or
performance. Sterilization in females most often is
achieved by
tu b a l lig a tio n ,
in which both uterine tubes
are tied closed and then cut. As a result, the secondary
oocyte cannot pass through the uterine tubes, and
sperm cannot reach the oocyte.
H o rm o n a l m ethods.
Preventing ovulation. Aside from
total abstinence or surgical sterilization, hormonal
methods are the most effective means of birth control.
In typical use, they may be
97% effective.
Used by
50 million women worldwide,
oral contraceptives
(“the
pill”) contain various mixtures of synthetic estrogens
and progestins (chemicals with actions similar to those
of progesterone). They prevent pregnancy mainly by
negative
feedback
inhibition
of anterior pituitary
secretios of FSH and LH. The low levels of these
hormones usually prevent development of a dominant
follicle.
B a rrie r
m ethods.
Preventing
sperm
from
accessing
the
uterus
and
uterine
tubes.
Barrier
methods
are
effective 80% to 85% of the
time in typical
use. Barrier methods include a condom, a vaginal
pouch, or a diaphragm. A
condom
is a nonporous,
latex covering placed over the penis that prevents
deposition of sperm in the female reproductive tract.
A
v a g in a l p o u ch
is made of two flexible rings connected
to a polyurethane sheath. The pouch is fitted within
the vagina, from the external genitals to the cervix.
A
d ia p h ra g m
is a rubber, dome-shaped structure that
fits over the cervix and is used in conjunction with a
spermicide.
Sperm icides.
Killing sperm. Spermicides are about 75%
effective in typical use. Spermicides come in various
foams, creams, suppositories, and douches that contain
sperm-killing agents. A spermicide is more effective
when used with a condom or a diaphragm.
In tra u te rin e devices.
Preventing implantation of a fer-
tilized egg into the uterus. An IUD is a small object
made of plastic, copper, or stainless steel that is in-
serted into the cavity of the uterus. IUDs cause changes
in the uterine lining that prevent implantation of a fer-
tilized ovum and are about 99% effective.
Of the various contraceptive methods, sterilizations
(for example, vasectomy, tubal ligation) are generally irre-
versible; however, expensive surgical techniques are avail-
able to attempt to reverse these procedures. Hormonal
methods carry risks of heart attack and stroke, especially
in female smokers and those with histories of heart dis-
ease and hypertension.
While some contraceptive methods (condoms, total
abstinence) help prevent the spread of sexually transmit-
ted diseases (STDs), most methods do not. STDs can be
caused by bacteria, viruses, protozoa, and fungi.
c a p a c ita tio n
(ka-
pas-i-TA-shun) The
functional changes
that sperm undergo in
the female reproduc-
tive tract that allow
them to fertilize a
secondary oocyte.
c o n tr a c e p tio n
(kon'-tra-SEP-shun)
The prevention of
fertilization or impreg-
nation without the
destruction of fertility.
Fertilization Requires the Egg and Sperm to Get Very Close to One Another
485
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