Animal Reproduction
Chapter 41
How Do Animals Reproduce?

Sexually or asexually

Asexual reproduction -a single animal produces offspring,
usually through repeated mitotic cell division

The offspring are genetically identical to the parent
 Sexual
reproduction - gonads produce haploid
sperm or eggs through meiotic cell division

A sperm and fuse to produce a diploid fertilized egg, undergoes
repeated mitotic cell divisions to produce an offspring
 Not
genetically identical to either parent
Asexual Reproduction

Advantages


Efficient in effort (no need to search for mates, courting, or battle
rivals), materials (no wasted gametes), and genes (the offspring
have all the genes of their parent)
Disadvantages

Genetically identical
Types of Asexual Reproduction

Budding produces a
miniature version of the
adult
 The
bud grows directly
on the body of the adult
 When large enough, it
breaks off

Sponges and cnidarians
Types of Asexual Reproduction

Fission followed by
regeneration can produce a
new individual


Regeneration - the ability to regrow lost body parts
Regeneration is part of
reproduction in species that
reproduce by fission

Annelids, flatworms, corals, sea
jellies, brittle stars divide into
pieces, each regenerates the
missing parts of a complete body
Types of Asexual Reproduction

Parthenogenesis - eggs develop without fertilization

Some parthenogenetically produced offspring are haploid

Male honeybees develop from unfertilized haploid eggs and
remain haploid; their diploid sisters develop from fertilized eggs

Some fish, amphibians, reptiles produce diploid offspring by
doubling the number of chromosomes in the eggs, either before
or after meiosis


The whiptail of the Southwestern US and Mexico, have populations
consisting entirely of parthenogenetically reproducing females
Aphids, can reproduce either sexually or parthenogenetically,
depending on environmental factors such as the season and
availability of food
The Whiptail Lizard
A Female Aphid Gives Birth
offspring being born
adult female aphid
Sexual Reproduction

Requires the union of sperm and egg

No one is certain why sexual reproduction evolved or why
natural selection has made it the most common form of
reproduction in animals

Advantage - The genetic recombination creates new genotypes
and phenotypes that are an important source of variation upon
which natural selection may act
Disadvantage - not efficient in effort (need to search for mates,
courting,& battle rivals), materials (wasted gametes)

Defining Male and Female

In most animals species, an individual is either male or
female, defined by the type of gamete produced

Female gonad - ovary, produces eggs
 Large, nonmotile haploid cells containing food reserves to
provide nourishment for the embryo – an offspring in its early
stages of development before birth or hatching

Male gonad - testis (plural, testes), produces small, motile
haploid sperm, which have little cytoplasm and no food reserves

Fertilization - union of sperm and egg, forms a diploid zygote
Hermaphrodites

Hermaphrodites produce both sperm and eggs - still
engage in sex, two individuals exchange sperm


Earthworms and snails
Some hermaphrodites fertilize their own eggs



Tapeworms and some snails are relatively immobile and find
themselves isolated from others of the species
Self-fertilization is an advantage
For species with two separate sexes and hermaphrodites that
cannot self-fertilize, successful reproduction requires that sperm
and eggs from different animals join for fertilization
Hermaphroditic Earthworms
Exchange Sperm
External Fertilization

Occurs outside the parents’ bodies

Spawning - sperm and eggs are released into water and the
sperm swim to the eggs

Spawning animals must synchronize their reproductive
behaviors both temporally and spatially

Synchronization is achieved through environmental cues like
chemical signals, courtship behaviors, or some combination
 Seasonal changes in day length stimulate the physiological
changes required for breeding

More precise synchrony is required to coordinate the actual
release of sperm and eggs

Coral species of Australia’s Great Barrier Reef synchronize
spawning by the phase of the moon, simultaneously releasing
sperm and eggs
Coral Spawning
It is about Chemistry…

Some animals communicate sexual readiness by
sending visual, acoustic, or chemical signals
 Female simultaneously releases eggs and a sex
pheromone into the water
 Nearby male detect the sex pheromone and release
sperm

In some animals, the sperm are also lured by a chemical
attractant produced by the eggs
Courtship Rituals

Synchronized timing alone does not guarantee efficient
reproduction

In mobile animals, both temporal and spatial synchrony can be ensured
by mating behaviors

Most fish have some form of courtship ritual in which the male and
female come close together and release their gametes in the same
place, at the same time

Frogs and toads assume a characteristic mating pose called amplexus
 In shallow water male mounts female and prods the sides of her
abdomen
 This stimulates her to extrude her eggs, which he fertilizes by
releasing sperm from above
Courtship Rituals Synchronize Mating
Beta mating Ritual
Internal Fertilization

Sperm are placed within the female’s reproductive tract,
where the egg is fertilized

An important adaptation to terrestrial life because sperm
quickly die if they dry

In aquatic environments internal fertilization may
increase the likelihood of success because sperm and
eggs are confined to a small space rather than dispersed
in a large volume of water
Copulation & Spermatophores

Occurs by copulation –
male deposits sperm directly
into the female’s reproductive
tract
 Some
males package their
sperm in a container called a spermatophore
 Scorpions, grasshoppers, salamanders
 The male drops a spermatophore on the ground, and if
a female finds it, she fertilizes herself by inserting it
into her reproductive cavity, where the sperm are
released
Sperm and Eggs are Shortlived

In most animals, neither sperm nor eggs live very long

Successful internal fertilization requires that ovulation, the release of a
mature egg cell from the ovary of the female, occurs shortly before or
soon after sperm are deposited in the female’s reproductive tract

Most mammals copulate when the female signals readiness to mate,
usually occurs about the same time as ovulation


In rabbits, courtship and mating stimulate ovulation, so new healthy sperm
and eggs are released
Some female snails and insects have short-lived eggs, but males
produce sperm that remain functional for days to months

The female stores the sperm in a special sac inside her body and releases
sperm whenever she produces eggs
Humans and Mammals

Have separate sexes, copulate, fertilize eggs internally

Although most mammals reproduce only during certain seasons
of the year, and produce sperm and eggs only at that time,
human reproduction is not restricted by season


Men produce sperm continuously, and women ovulate monthly
The ability to reproduce begins at puberty

Sexual maturation in humans occurs at puberty, a stage of
development characterized by rapid growth and appearance of
secondary sexual characteristics

Generally begins in the early teens, it can start as early as 8 or as
late as 15
Puberty and Hormones

Brain maturation causes the hypothalamus to release
gonadotropin-releasing hormone (GnRH), which
stimulates the anterior pituitary to produce luteinizing
hormone (LH) and follicle-stimulating hormone (FSH)
 LH and FSH derive their names from functions in
females, but equally essential in males
 LH and FSH stimulate testes to produce
testosterone and ovaries to produce estrogen
Testosterone

In response to increases in testosterone, males develop
secondary sexual characteristics




The penis and testes enlarge
Pubic, underarm, and facial hair appears
The larynx enlarges, deepening the voice
Muscular development increases
Estrogen

In response to estrogen and other hormones that surge
at puberty, secondary sexual characteristics develop





The breasts enlarge
The hips become wider
Pubic and underarm hair appears
Menstruation begins
Courtship behaviors start to appear in both sexes
Male Reproductive System

The testes and accessory structures


The testes produce testosterone and sperm
Several glands and ducts secrete substances that activate,
nourish, store, and conduct sperm to the female reproductive
system
The Human Male Reproductive
System
pubic bone
urinary bladder
ureter
rectum
seminal
vesicle
prostate
gland
urethra
penis
vas deferens
epididymis
testis
urethral
opening
scrotum
bulbourethral
gland
Animation: Male Reproductive Anatomy
Sperm is produced in the testes



Scrotum - pouch that hangs outside the body cavity
 This keeps the testes about 1º - 6º F cooler than the
core of the body, depending
 Cooler temperatures promote sperm development
Coiled, hollow seminiferous tubules are where sperm
are produced, fill each testis
Interstitial cells synthesize testosterone, located in the
spaces between the tubules
The Anatomy of the Testes
vas deferens
epididymis
uncoiled
seminiferous
tubule
testis
(a) A section through the testis
spermatogonia
daughter cells
sperm
Sertoli
cells
interstitial
cell
(b) Cross-section of a seminiferous tubule
Inside the Seminiferous Tubules

Inside the seminiferous tubule lie cells - spermatogonia
that give rise to sperm, and larger Sertoli cells that
nourish the developing sperm and regulate their growth

Diploid cells undergo mitosis to form two daughter cells
 One daughter cell remains a spermatogonium,
ensuring a steady supply throughout life
 The other daughter cell undergoes spermatogenesis,
a developmental process that produces haploid sperm
Spermatogenesis






The daughter cell differentiates into a primary spermatocyte
- a large diploid cell that undergoes meiosis
At the end of meiosis I, each primary spermatocyte gives rise
to two haploid secondary spermatocytes
Each secondary spermatocyte undergoes meiosis II,
producing two spermatids; each primary spermatocyte
generates four spermatids
Spermatids differentiate into sperm without further cell division
Spermatogonia, spermatocytes, spermatids are enfolded in
Sertoli cells
As spermatogenesis proceeds, developing sperm migrate to
the central cavity of the seminiferous tubule, and are released
Animation: Sperm Maturation
Spermatogenesis
occurs continuously starting
at puberty
spermatogonium
primary
spermatocyte
secondary
spermatocytes
spermatids
Mitosis
Meiosis I
Meiosis II
sperm
Differentiation
Human Sperm

Very little cytoplasm, the nucleus fills the sperm’s head

Acrosome – atop the nucleus - a specialized lysosome
containing enzymes that dissolve layers around the egg

Midpiece - packed with mitochondria

Mitochondria provide the energy needed to move the tail

Whiplike movements propel the sperm
A Human Sperm Cell
acrosome
nucleus
tail sheath
mitochondria
flagellum
Tail
Head
Midpiece
Hormones that Regulate
Spermatogenesis

GnRH from the hypothalamus stimulates the anterior
pituitary to produce LH and FSH


LH stimulates the interstitial cells to produce testosterone
In combination with FSH, testosterone stimulates the Sertoli cells
and promotes spermatogenesis
Testicular function is regulated by
negative feedback

Testosterone inhibits the release of
GnRH, LH, and FSH

The Sertoli cells are stimulated by
FSH and testosterone to secrete
inhibin, which inhibits FSH
production by the anterior pituitary

This complex feedback process
maintains constant levels of
testosterone and sperm production
Accessory Structures

Seminiferous tubules form the epididymis, a long, continuous, folded
tube

Epididymis leads to the vas deferens, a tubule that carries sperm out
of the scrotum

Vas deferens joins the urethra, which conducts both urine and
sperm
 The roughly hundred million sperm produced by a human male
each day are stored in the vas deferens and epididymis
Semen

Semen is ejaculated from the penis - 5% sperm, mixed
with secretions that empty into the vas deferens and
urethra:



The seminal vesicles
The prostate gland
The bulbourethral glands
Seminal Vesicles

Paired seminal vesicles make up 60% of the semen
 Rich
in fructose, that provides energy for the sperm
 Slightly
alkaline pH protects the sperm from the
acidity of urine in the man’s urethra and the woman’s
vagina
 It
also contains prostaglandins, which stimulate
uterine contractions to transport the sperm up the
female reproductive tract
Prostate Gland

Produces an alkaline, nutrient-rich secretion, about 30%
of the semen volume
 Includes enzymes that increase the fluidity of the
semen after it is released into the vagina, allowing
sperm to swim more freely
Bulbourethral Glands

Paired bulbourethral glands secrete a small amount of
alkaline mucus in the urethra, neutralizing remaining
traces of acidic urine
Female Reproductive System

The ovaries and accessory structures


Almost entirely contained within the abdominal cavity
It consists of the ovaries and structures that accept sperm,
conduct the sperm to the egg, and nourish the developing
embryo
Animation: Human Female Reproductive Anatomy
The Human Female Reproductive
System
fimbria
ovary
uterine
tube
myometrium
endometrium
urinary bladder
pubic bone
cervix
urethra
vagina
clitoris
labia
rectum
anus
uterus
Oogensis

Oogenesis - formation of egg cells

Begins in the developing ovaries before birth



Starts with the formation of diploid cells - oogonia - as early as
the 6th week of embryonic development
From the 9th - 20th weeks, oogonia enlarge and differentiate,
becoming primary oocytes
So a woman is born with a lifetime’s supply of primary oocytes—
1 -2 million— no new are generated
 Many die each day, but 400,000 remain at puberty
 This is plenty, only a few oocytes resume meiotic cell division
during each month of a woman’s reproductive span, from
puberty to menopause
Oogenesis
Follicle

Surrounding each oocyte is a layer of accessory cells
and together, they make up a follicle




Every month after puberty, the hormonal changes of the
menstrual cycle stimulate the development of a dozen follicles
The small follicle cells multiply, providing nourishment for the
developing oocyte
In response to hormones secreted by the anterior pituitary, they
also release estrogen into the bloodstream
Only one follicle matures during each menstrual cycle


As the follicle develops, its primary oocyte completes meiosis I,
dividing into a single secondary oocyte and a polar body - a
small cell that is little more than a discarded set of chromosomes
Meiosis II will not occur unless the egg is fertilized
Mature Follicle




As the follicle matures, it becomes larger and fills with fluid
Ovulation occurs when the follicle erupts through the
surface of the ovary, releasing its secondary oocyte
 From this point, it is an egg
Some of the follicle cells accompany the egg, but most of
them remain in the ovary where they enlarge, forming a
temporary gland called the corpus luteum
The corpus luteum secretes estrogen and progesterone


These hormones stimulate development of the uterine lining and
play a role in controlling the menstrual cycle
If fertilization does not occur, the corpus luteum degenerates
Animation: Steps in Ovulation
Follicle Development
uterine tube
4 Ovulated
secondary
oocyte (egg
5 Ruptured
follicle
6 Corpus luteum
7 Degenerating
corpus luteum
3 Mature
follicle with
a secondary
oocyte
ovary
ovulating
egg
2 Developing
follicles
ovary
1 New follicle
containing a
primary oocyte
Accessory Structures

Uterine tubes, uterus, and vagina





The ovary nestles within the open end of a uterine tube (oviduct
or Fallopian tube), which is fringed with ciliated fimbria that
nearly surround the ovary
Cilia create a current that sweeps the newly ovulated egg into
the uterine tube
The egg may encounter sperm and be fertilized
Cilia lining the uterine tube, sweeping the fertilized egg down the
tube and into the uterus
The wall of the uterus has two layers that correspond to its
functions of nourishing the developing embryo and delivering a
child
Uterine Layers

The inner lining or endometrium, is supplied with blood vessels and
glands that secrete carbohydrates, lipids, proteins


Will form the placenta - the structure that transfers oxygen, carbon
dioxide, nutrients, and wastes between mother and embryo
The outer muscular wall or myometrium, contracts during childbirth,
expelling the infant out of the uterus

The outer end of the uterus is closed off by the cervix, a ring of
connective tissue that encircles a tiny opening
 The cervix holds the developing baby in the uterus and expands during
labor, permitting passage of the child

Beyond is the vagina, which opens to the outside

Vaginal lining is acidic, reducing the likelihood of infections
 Serves as a receptacle for the penis and sperm during intercourse and
as then as the birth canal
Hormones

Developing follicles secrete estrogen, which stimulates
the endometrium to become thicker and grow a network
of blood vessels and glands

After ovulation, estrogen and progesterone released by the
corpus luteum further stimulate the endometrium
 If an egg is fertilized, it encounters a rich environment for
growth
 If the egg is not fertilized, the corpus luteum disintegrates,
estrogen and progesterone levels fall, and the overgrown
endometrium disintegrates
 The uterus then contracts (menstrual cramps) and
squeezes out excess endometrial tissue, causes the flow
of tissue and blood called menstruation
Control of the Menstrual Cycle

The menstrual cycle actually is two cycles:
 The
ovarian cycle - interactions of hormones
produced by the hypothalamus, anterior pituitary
gland, and ovaries, drive the development of follicles,
the maturation of oocytes, and conversion of follicle
cells after ovulation into the corpus luteum
 The
uterine cycle - estrogen and progesterone
produced by the ovaries drive the development of the
endometrium of the uterus
The Ovarian Cycle

In the 28-day menstrual cycle, the beginning of
menstruation is designated as day 1, because it is easily
observed, even though the hormonal events that drive
the cycle begin a day or two earlier
Hypothalamic hormone
level
1
3
9
7
GnRH
Pituitary hormone levels
LH
4
1
FSH
7
Structures in the ovary
ovulation
follicle develops
5
2
corpus
luteum
forms and
matures
corpus
luteum
degenerates
8
Ovarian hormone levels
3
6
estrogen
progesterone
2
(a) Ovarian cycle
Uterine lining
1 menstruation
3
development of
the endometrium
(b) Uterine cycle
6
continued endometrial
development
9
1.
The hypothalamus releases GnRH, which stimulates the
anterior pituitary to release FSH and LH

2.
FSH stimulates the development of follicles within ovary




3.
At this time the endometrium of the uterus is still being shed
Follicle cells surrounding the oocyte secrete a small amount
of estrogen
Under the influence of FSH and LH from anterior pituitary
and estrogen from the follicles, follicles grow
The primary oocyte within each follicle enlarges, storing
food and other substances
One follicle completes development each month
The maturing follicle secretes increasing amounts of
estrogen, which has three effects:



First, it promotes the continued development of the follicle and the
primary oocyte
Second, it stimulates the growth of the endometrium
Third, estrogen stimulates the hypothalamus to release more
GnRH
4.
Increased GnRH stimulates a surge of LH at the 13th or
14th day of the cycle


5.
Second, the LH surge causes ovulation

6.
7.
8.
The increased LH has three important consequences:
First, it triggers the resumption of meiosis I in the oocyte,
producing the secondary oocyte and the first polar body
Third, it transforms the follicle remnants into the corpus luteum
The corpus luteum secretes estrogen and progesterone,
which stimulate the growth of the endometrium
Estrogen and progesterone inhibit GnRH production,
reducing the release of FSH and LH, and preventing
development of more follicles
If the egg is not fertilized, the corpus luteum starts to
disintegrate 12 days after ovulation


The corpus luteum cannot survive without LH stimulation
Because estrogen and progesterone secreted by the corpus
luteum shut down LH production, it causes its own destruction
9.
With the corpus luteum gone, estrogen and
progesterone levels drop, and the
endometerium disintegrates



Blood and tissue are shed, the menstrual flow
The reduced levels of estrogen and progesterone no
longer inhibit the hypothalamus, so spontaneous
release of GnRH resumes
GnRH stimulates the release of FSH and LH,
initiating the development of a new set of follicles
and restarting the cycle
Animation: Hormonal Control of the Menstrual
Cycle
Hormonal Control of the Menstrual
Cycle
Hypothalamic hormone
level
1
3
9
7
GnRH
Pituitary hormone levels
LH
4
1
FSH
7
Structures in the ovary
ovulation
follicle develops
5
2
corpus
luteum
forms and
matures
corpus
luteum
degenerates
8
Ovarian hormone levels
3
6
estrogen
progesterone
2
(a) Ovarian cycle
Uterine lining
1 menstruation
3
development of
the endometrium
(b) Uterine cycle
6
continued endometrial
development
9
The menstrual cycle includes
Positive and Negative Feedback




During the first half of the cycle, FSH and LH stimulate estrogen
production by the follicle
High levels of estrogen stimulate the midcycle surge of FSH and LH
release
This early positive feedback causes hormone concentrations to
reach high levels
During the second half of the cycle, negative feedback dominates

Estrogen and progesterone from the corpus luteum inhibit the release of
GnRH, FSH, and LH
 Without LH to keep it alive, the corpus luteum dies, shutting down
progesterone production and reducing estrogen production
The Embryo Sustains Its Own
Pregnancy

If fertilization occurs, the embryo prevents the negative
feedback that would otherwise end the menstrual cycle





Shortly after the ball of cells formed by the dividing fertilized egg
implants in the endometrium, it secretes an LH-like hormone
called chorionic gonadotropin (CG)
This hormone travels in the blood stream to the ovary, where it
keeps the corpus luteum alive
The corpus luteum continues to secrete estrogen and
progesterone for a few months
These hormones stimulate the development of the endometrium,
nourishing the embryo and sustaining the pregnancy
Some CG is excreted in the mother’s urine, where it can be
detected to confirm pregnancy
Male role in Copulation



The penis is flaccid - smooth muscles surrounding the arterioles that
supply it are contracted, allowing little blood flow into the penis
Under psychological and physical stimulation, the nervous system
releases nitric oxide (a gas) onto the muscles of the arterioles
Nitric oxide activates an enzyme that synthesizes cyclic guanosine
monophosphate (cyclic GMP), a chemical relative of cyclic amp
 Cyclic GMP is an intracellular second messenger that causes
smooth muscles to relax
 Arterioles dilate, blood flows into tissue spaces within the penis
 As tissues swell, they squeeze off the veins that drain the penis
 Blood pressure increases, causing an erection
Changes in Blood Flow Cause Erection
veins
(open)
arteriole
(constricted)
skin
central
arteriole
(constricted)
connective
tissue
(a) Relaxed
little blood
in tissues
veins
(squeezed
shut)
erectile
tissue
urethra
arteriole
(open)
central
arteriole
(open)
erectile
tissue
blood-filled
tissue
(b) Erect
urethra
(squeezed shut)


After the penis is inserted into the vagina, movement
stimulates touch receptors on the penis, triggering
ejaculation
Muscles encircling the epididymis, vas deferens, and
urethra contract, forcing semen out through the penis
and into the vagina

A typical ejaculation consists of about 2 to 5 mL of semen
containing about 100 - 400 million sperm
The Female Role in Copulation


Sexual arousal causes increased blood flow to the
vagina, labia, and clitoris
Stimulation by the penis may result in orgasm, a series
of rhythmic contractions of the vagina and uterus
accompanied by intensely pleasurable sensations
 Female orgasm is not necessary for fertilization, but
the contractions of the vagina and uterus help move
sperm up toward the uterine tubes
Fertilization





The sperm and egg nuclei unite
Neither lives very long on its own. Sperm may live for 2 - 4 days
inside the female reproductive tract and an unfertilized egg remains
viable for a day or so.
During intercourse, penis releases sperm into the vagina
 The sperm move through the cervix, into the uterus, and enter
the uterine tubes
If copulation occurs within a day or two of ovulation, sperm may
meet an egg in a uterine tube
When it leaves the ovary, the egg is surrounded by follicle cells
 These cells, now called the corona radiata, form a barrier
between the sperm and the egg
 A second barrier, the jelly-like zona pellucida (“clear area”), lies
between the corona radiata and the egg
The Secondary Oocyte and Fertilization
The Sperm Reach the Egg

In the uterine tube, hundreds of sperm reach the egg
and encircle the corona radiata

Each sperm releases enzymes from its acrosome



The enzymes weaken the corona radiata and the zona pellucida,
allowing the sperm to wiggle through to the egg
If there aren’t enough sperm, not enough enzyme is released,
and no sperm reaches the egg
When the first sperm contacts the egg’s surface, the
plasma membrane of egg and sperm fuse, and the
sperm’s head is drawn into the egg cytoplasm
What happens when the sperm
enters the egg?

As the sperm enters, it triggers two changes:
 First,
vesicles near the surface of the egg release
chemicals into the zona pellucida that reinforce it and
prevent additional sperm from entering
 Second,
the egg undergoes meiosis II, and fertilization
occurs as the haploid nuclei of sperm and egg fuse,
forming a diploid nucleus
Animation: Steps in Fertilization
Defects in the Reproductive
System

Defects can prevent fertilization



A blocked uterine tube can prevent sperm from reaching the egg
A man with a low sperm count may be unable to impregnate a
woman through sexual intercourse because too few sperm reach
the egg
Many couples who have difficulty conceiving a child seek
help through artificial insemination or in vitro fertilization
How Can People Limit Fertility?

During human evolution child mortality was high,
and natural selection favored people who produced
enough children to offset this high mortality rate
 Today
most people do not need to have as many
children to ensure that a few will survive to adulthood
 We still retain reproductive drives that are better
suited to a more precarious existence
 As a result, each week sees nearly 1.5 million new
people added to our increasingly crowded planet
Controlling Birth Rates

Controlling birth rates is an environmental necessity

Individually, birth control allows people to plan their families and
provide the best opportunities for themselves and their children

Historically, limiting fertility has not been easy
 In the past, women in some cultures have tried inventive
techniques such as swallowing froth from the mouth of a
camel or placing crocodile dung in the vagina

Since the 1970s, several effective techniques have been
developed for contraception—the prevention of pregnancy
Sterilization is permanent contraception

The most foolproof and effortless method is sterilization, pathways
through which sperm and eggs travel are interrupted

In men, the vas deferens from each testis is severed and the ends
tied, clamped, or sealed in an operation called a vasectomy

Sperm are still produced, they cannot leave the epididymis, they die
 Phagocytic white blood cells remove the debris


Tubal ligation renders a woman infertile by clamping or cutting her
uterine tubes, and tying or sealing the cut ends
 Ovulation still occurs, but sperm cannot travel to the egg
An alternative method is to insert tiny springlike structures into each
uterine tube through the vagina and uterus

The coil causes scar tissue that blocks passage of sperm and eggs
Sterilization
The uterine tube
is severed and its
ends are sealed
The vas
deferens is
severed and
its ends are
sealed
ovary
testis
uterus
scrotum
(a) Vasectomy
(b) Tubal ligation
Sterilization can often be reversed

If a sterilized woman or man wishes to reverse the operation, a
surgeon can attempt to reconnect the uterine tubes or vas deferens


70% to 90% of young women are able to become pregnant after their
uterine tubes have been reconnected by a skilled, experienced surgeon
Pregnancy rates vary dramatically according to:


The age of the woman—older women have a lower success rate
The method of the original tubal ligation—higher success rates
occur if the uterine tubes were clamped rather than cut or
cauterized

Reversing a vasectomy is more difficult
 The vas deferens can usually be reconnected, and
sperm reappear in the ejaculate in 70% to
98%,depending on the skill of the surgeon
 However, pregnancy rate is lower, 30% - 75%

The longer the interval between vasectomy and
reconnection, the lower the pregnancy rate

The gradual development of an immune response against the
man’s sperm, resulting in damaged sperm
Temporary Birth Control



Temporary methods of birth control prevent pregnancy in the
immediate future, leaving the option of later pregnancies
Perfect abstinence, of course, provides complete protection against
pregnancy and sexually transmitted diseases (STDs)
Fall into three categories:




Preventing ovulation
Preventing sperm and egg from meeting
Preventing implantation in the uterus
Birth control methods do not protect against STDs
unless they prevent physical contact of the penis and
vagina
Synthetic Hormone Birth Control

Synthetic hormones prevent ovulation



Birth control pills contain synthetic versions of estrogen and
progesterone; minipills contain only progesterone
Follicle development is stimulated by FSH, and ovulation is
triggered by a midcycle surge of LH
The estrogen in birth control pills prevents FSH release, so
follicles do not develop



Even if one were to develop, the progesterone in the pill would
suppress the surge in LH needed for ovulation
Progesterone thickens cervical mucus, making it more difficult
for sperm to move from the vagina into the uterus, and slows
down the movement of sperm and egg in the uterine tubes
Contraceptive patches, rings, injections, implants of estrogen
and progesterone are available, and each lasts from weeks to
years
Barrier Methods
 Some barrier devices cover the cervix, preventing the
entry of sperm into the uterus
 A female condom completely lines the vagina
 A male condom prevents sperm from being
deposited in the vagina
 Both
types of condoms offer some protection against
the spread of STDs

Barrier devices are more effective when combined with a spermicide
Unreliable Methods

The rhythm method has a high failure rate because the
menstrual cycle often varies, making ovulation difficult to
predict

Additionally, sperm can survive for days in the female
reproductive tract, so intercourse must be avoided for
days before ovulation

Highly unreliable methods include:


Withdrawal
Douching—attempting to wash sperm out of the vagina before
they have entered the uterus
Other Birth Control Methods

Some birth control methods work through
multiple mechanisms

Simultaneously prevent ovulation, hinder
survival or motility of sperm, or prevent
implantation of the embryo in the uterus

The intrauterine device (IUD)—a copper
or plastic loop, squiggle, or shield inserted
through the cervix and into the uterus—
interferes with sperm motility or survival
and alters the uterine lining, lessening the
likelihood of implantation
The Morning After Pill

The “morning after” pill, contains high doses of the same
hormones as in birth control pills

Acts by delaying or preventing ovulation, but may also interfere
with the development of the corpus luteum and prevent
implantation

Emergency contraceptive pills are usually effective if taken within 72
hours after intercourse, but ideally should be taken within 24 hours
Most methods are for females

Most birth control techniques are designed for use by women

There are three major reasons for this:
 First, the woman, not the man, becomes pregnant, bears the
associated health risks of the pregnancy and childbirth, and
usually plays a larger role in child care than men do

Second, it was relatively simple to design “use it and forget
about it” birth control methods for women, including birth control
pills and intrauterine devices

Third, opinion surveys indicate that a significant number of men,
often more than 25%, claim that they would never use hormonal
birth control methods
Male Methods are Under Development

Researchers in several countries are working on male
contraception

The options under investigation include:




Reversible plugs injected into the vas deferens
Substances administered by injection that block the action of
GnRH or other pituitary hormones, thus preventing sperm from
being produced
Vaccines that cause an immune response against sperm or
other essential components of male reproduction
All of these methods require injections or minor surgery
So far…

Human trials of the hormonal methods generally give
only 80% - 90% effectiveness and include side effects
such as weight gain or changes in blood cholesterol
levels


The development and marketing of male contraceptives
are hindered by:
Difficulties due to lack of male acceptance
 Lower effectiveness than female birth control methods
 Side effects, reversibility issues, and the fact that many require
injection or minor surgery
It appears that the widespread availability of safe, effective, and
convenient male contraception—except for condoms—is years away


Vaccines are unlikely to be 100% effective and may not be
rapidly reversible