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32
Animal Reproduction
Concept 32.1 Reproduction Can Be Sexual or Asexual
Asexual reproduction:
 requires no mating
 does not result in genetic diversity
Asexually reproducing species are mostly
invertebrates, sessile, and live in constant
environments.
Three types of asexual reproduction are
budding, regeneration, and
parthenogenesis.
Concept 32.1 Reproduction Can Be Sexual or Asexual
Budding produces new individuals that
form from the bodies of older animals.
A bud grows by mitotic cell division—cells
differentiate before the bud breaks away.
The bud is genetically identical to the
parent.
Figure 32.1 Three Forms of Asexual Reproduction (Part 1)
Concept 32.1 Reproduction Can Be Sexual or Asexual
Regeneration can replace damaged tissue
or form a complete individual.
Example: Echinoderms.
Figure 32.1 Three Forms of Asexual Reproduction (Part 2)
Concept 32.1 Reproduction Can Be Sexual or Asexual
Parthenogenesis is the development of
offspring from unfertilized eggs.
Parthenogenesis may determine the sex of
the offspring.
In some species, females can act as males
depending on cyclic states of estrogen and
progesterone.
Figure 32.1 Three Forms of Asexual Reproduction (Part 3)
Concept 32.1 Reproduction Can Be Sexual or Asexual
Sexual Reproduction:
• two haploid cells, gametes, form a diploid
individual
Three fundamental steps of sexual
reproduction:
• Gametogenesis—making haploid gametes
• Spawning or mating—getting gametes
together
• Fertilization—fusing gametes to form a
diploid
Concept 32.1 Reproduction Can Be Sexual or Asexual
Sexual reproduction has a big advantage—
the generation of genetic diversity.
Meiosis allows genetic diversity through
crossing over between homologous
chromosomes and independent
assortment.
Concept 32.2 Gametogenesis Produces Haploid Gametes
Gametogenesis occurs in the gonads—the
testes in males and ovaries in females
Male gametes, the sperm, move by flagella.
The larger gametes of females are the ova,
or eggs, and are nonmotile.
Gametes are produced from germ cells—
present early in development and distinct
from other (somatic) cells of the body.
Concept 32.2 Gametogenesis Produces Haploid Gametes
Germ cells migrate to the gonads when they
begin to form.
Embryonic germ cells divide by mitosis to
form diploid spermatogonia in males and
oogonia in females.
These multiply by mitosis, producing
primary spermatocytes and primary
oocytes—these enter meiosis and
produce haploid gametes, sperm and ova.
Concept 32.2 Gametogenesis Produces Haploid Gametes
The production of sperm is
spermatogenesis—of ova is oogenesis.
The first meiotic division of a primary
spermatocyte results in two secondary
spermatocytes.
The second division produces four haploid
spermatids, which will each become a
sperm.
Figure 32.2 Gametogenesis (Part 1)
Concept 32.2 Gametogenesis Produces Haploid Gametes
A man produces over 100 million sperm per
day.
Concept 32.2 Gametogenesis Produces Haploid Gametes
In hermaphroditic species such as
earthworms, a single individual may
produce sperm and ova simultaneously.
An anemone fish produces sperm and ova
sequentially and may function as a male or
a female at different times.
Concept 32.3 Fertilization Is the Union of Sperm and Ovum
Fertilization is the union of a haploid sperm
and a haploid egg.
It creates a single diploid cell, called a
zygote, which will develop into an embryo.
Fertilization involves a complex series of
events.
Concept 32.3 Fertilization Is the Union of Sperm and Ovum
Steps in fertilization:
• Recognition and binding of sperm and
ovum
• Activation of sperm
• Plasma membranes fuse
• Additional sperm entry blocked
• Activation of ovum
• Ovum and sperm nuclei fuse
Concept 32.3 Fertilization Is the Union of Sperm and Ovum
Aquatic animals bring gametes together
through spawning—gamete release
before external fertilization occurs.
Internal fertilization occurs when sperm is
released directly into the female
reproductive tract.
Internal fertilization requires accessory sex
organs, such as penis and vagina.
Copulation is the joining of the male and
female accessory organs.
Concept 32.3 Fertilization Is the Union of Sperm and Ovum
Species-specific sperm and ovum
interactions are controlled by specific
recognition molecules.
Ova of aquatic species release chemical
attractants to cause sperm to swim toward
the ovum.
Sperm must go through two protective
layers to reach an ovum—a jelly coat and
the vitelline envelope.
Concept 32.3 Fertilization Is the Union of Sperm and Ovum
The acrosome is a membrane-enclosed
structure on the sperm head.
Egg and sperm contact causes substances
in the jelly coat to trigger an acrosomal
reaction.
Membranes in the sperm head and
acrosome break down; enzymes are
released and digest the jelly coat.
Figure 32.3 Fertilization of Sea Urchin Egg (Part 2)
Concept 32.3 Fertilization Is the Union of Sperm and Ovum
An acrosomal process extends from the
head of the sperm.
The acrosomal process is coated with
bindin—specific recognition molecules.
Bindin acts on bindin receptors in the
vitelline envelope.
Sperm and egg plasma membranes fuse to
form a fertilization cone.
Concept 32.3 Fertilization Is the Union of Sperm and Ovum
Internal fertilization involves species-specific
mating behaviors and ovum-sperm
recognition mechanisms.
The ovum is surrounded by the cumulus,
cells in a gelatinous matrix.
The zona pellucida, or zona, is a
glycoprotein envelope beneath the
cumulus.
Concept 32.3 Fertilization Is the Union of Sperm and Ovum
When sperm make contact with the zona,
species-specific glycoproteins bind to
recognition molecules on the sperm.
Binding triggers the acrosomal reaction, and
enzymes digest the zona pellucida.
When sperm reaches ovum membrane
other proteins facilitate membrane fusion.
Figure 32.3 Fertilization of Sea Urchin Egg (Part 1)
Figure 32.4 Barriers to Mammalian Sperm
Concept 32.3 Fertilization Is the Union of Sperm and Ovum
Fusion and entry of a sperm into the ovum
lead to:
Blocks to polyspermy—mechanisms to
prevent more than one sperm from
entering an ovum
Concept 32.3 Fertilization Is the Union of Sperm and Ovum
Fast block to polyspermy:
• Transient (die quickly)
• Caused by change in membrane potential
as sodium ions (Na+) enter plasma
membrane of ovum after contact with a
sperm
Concept 32.4 Human Reproduction Is Hormonally Controlled
Sperm are produced in the paired male
gonads, or testes.
The testes are located in the scrotum,
outside of the body, to maintain optimal
temperature for sperm production.
Semen is made up of sperm and other
fluids and molecules.
Figure 32.5 Reproductive Organs of the Human Male (Part 1)
Concept 32.4 Human Reproduction Is Hormonally Controlled
Spermatogenesis occurs in the
seminiferous tubules in each testis.
Between the tubules are Leydig cells, which
produce testosterone.
Spermatogonia reside in the outermost
regions of the tubules, near Sertoli cells,
which provide nutrients.
Figure 32.6 Spermatogenesis Takes Place in the Seminiferous Tubules (Part 1)
Figure 32.6 Spermatogenesis Takes Place in the Seminiferous Tubules (Part 2)
Figure 32.6 Spermatogenesis Takes Place in the Seminiferous Tubules (Part 3)
Concept 32.4 Human Reproduction Is Hormonally Controlled
Immature sperm cells are shed in the lumen
of the seminiferous tubule.
They move into the epididymis, mature, and
become motile.
Sperm travel in the vas deferens which joins
with the semen-carrying ejaculatory duct.
This joins the urethra, the common final
duct, at the base of the penis and opens to
the outside at the tip of the penis.
Figure 32.5 Reproductive Organs of the Human Male (Part 2)
Concept 32.4 Human Reproduction Is Hormonally Controlled
Besides sperm, semen contains seminal
fluids—the products of several accessory
glands:
• The paired seminal vesicles, the prostate
gland, and the bulbourethral glands
The prostate gland produces a fluid that is
alkaline and reduces acidity in male and
female reproductive tracts.
The fluid also contains enzymes to thicken
semen and later to dissolve it.
Concept 32.4 Human Reproduction Is Hormonally Controlled
The bulbourethral glands produce an
alkaline secretion that:
• Neutralizes acidity in the urethra
• Provides lubrication and facilitates sperm
movement during climax
These secretions precede climax yet carry
residual sperm capable of fertilization.
Pregnancy can occur even when the penis
is withdrawn prior to ejaculation (coitus
interruptus).
Concept 32.4 Human Reproduction Is Hormonally Controlled
Male copulatory organ is the penis.
The sensitive tip of the penis, the glans
penis, is covered by the foreskin—removal
of the foreskin is circumcision.
Sexual stimulation triggers the nervous
system to produce penile erection.
Nitric oxide (NO) acts on blood vessels by
stimulating production of cGMP.
cGMP causes dilation of blood vessels so
that spongy erectile tissue fills with blood.
Concept 32.4 Human Reproduction Is Hormonally Controlled
At the climax of copulation, semen is
ejaculated through the vasa deferentia and
urethra.
Ejaculation is accompanied by feelings of
intense pleasure called orgasm.
After ejaculation NO release decreases and
enzymes break down cGMP—blood
vessels are no longer compressed, and
erection declines.
Concept 32.4 Human Reproduction Is Hormonally Controlled
Hormones control male sexual function:
GnRH (gonadotropin-releasing hormone)—
released by the hypothalamus at puberty
GnRH increases the release of LH
(luteinizing hormone) and FSH (folliclestimulating hormone) by the anterior
pituitary.
Figure 32.7 Male Reproductive Hormones
Concept 32.4 Human Reproduction Is Hormonally Controlled
LH increases testosterone:
• Increases growth rate and starts
development of secondary sexual
characteristics
FSH and testosterone control
spermatogenesis in the Sertoli cells.
Sertoli cells also produce inhibin, which
exerts negative feedback on cells that
produce and secrete FSH.
And now here’s Alex with the Reproductive
system!
Concept 32.2 Gametogenesis Produces Haploid Gametes
Oogenesis:
A primary oocyte immediately begins prophase I of
meiosis. Here, development stops in many
species.
The primary oocyte grows larger and acquires
nutrients.
When meiosis resumes, the nucleus of the oocyte
divides into two daughter cells of unequal sizes.
The cell with more cytoplasm is the secondary
oocyte—the smaller one is the first polar body.
Concept 32.2 Gametogenesis Produces Haploid Gametes
The second meiotic division forms daughter
cells of unequal sizes.
One is a large ootid, which differentiates to
become a mature ovum.
The other forms the second polar body.
Figure 32.2 Gametogenesis (Part 2)
Concept 32.2 Gametogenesis Produces Haploid Gametes
Few primary oocytes complete all meiotic
stages—females produce far fewer
gametes than males.
The average woman has about 450
menstrual cycles and releases on ovum
each time, until menopause—the end of
fertility.
Concept 32.3 Fertilization Is the Union of Sperm and Ovum
Fertilized ova may be released into the
environment or retained:
Oviparity—egg laying:
• Oviparous animals lay eggs in the
environment, and embryos develop
outside the mother’s body.
Viviparity—live bearing:
• Viviparous animals retain the embryo in
the mother’s body during early
development.
Concept 32.3 Fertilization Is the Union of Sperm and Ovum
Viviparity differs in mammals as they have a
specialized female reproductive tract:
• Uterus (or womb)—holds the embryo
• Placenta—develops in the uterus and
enables exchange of nutrients and waste
Concept 32.4 Human Reproduction Is Hormonally Controlled
In females, the ovary releases an ovum into
one of the oviducts, or Fallopian tubes,
where it may be fertilized.
The ovum is propelled towards the uterus
where it will develop if fertilized.
The bottom of the uterus is the narrow
cervix, which opens into the vagina.
Concept 32.4 Human Reproduction Is Hormonally Controlled
The female reproductive cycle is about 28
days and consists of two linked cycles:
• The ovarian cycle that produces mature
ova and hormones
• The uterine, or menstrual, cycle that
prepares the uterus for the arrival of an
embryo.
Concept 32.4 Human Reproduction Is Hormonally Controlled
Each primary oocyte and its surrounding
ovarian cells constitute a follicle.
At the beginning of a cycle, the anterior
pituitary increases FSH and LH.
6 to 12 follicles grow in the first two weeks
of the cycle—the follicular phase.
Follicles increase estrogen production until
the largest follicle matures completely and
others die (atresia).
Figure 32.9 The Ovarian Cycle (Part 1)
Concept 32.4 Human Reproduction Is Hormonally Controlled
Estrogen exerts negative feedback on the
pituitary early in the ovarian cycle.
During days 12–14 estrogen becomes a
positive feedback signal and causes a
surge of LH and FSH.
The LH surge triggers ovulation—the follicle
ruptures and the oocyte is released from
the ovary.
Concept 32.4 Human Reproduction Is Hormonally Controlled
Follicle has two types of cells:
• Granulosa cells surround the developing
oocyte and are stimulated by FSH.
• Thecal cells enclose the whole follicle,
produce androgens when stimulated by
LH.
Levels of circulating estrogen increase,
which feeds back negatively to the
hypothalamus and anterior pituitary.
FSH and LH levels then fall.
Figure 32.10 The Ovarian and Uterine Cycles (Part 1)
Figure 32.10 The Ovarian and Uterine Cycles (Part 2)
Figure 32.10 The Ovarian and Uterine Cycles (Part 3)
Figure 32.10 The Ovarian and Uterine Cycles (Part 4)
Concept 32.4 Human Reproduction Is Hormonally Controlled
The follicle with the most FSH receptors
survives.
The other follicular cells form the corpus
luteum, which remains in the ovary.
The corpus luteum functions as an
endocrine gland and produces estrogen
and progesterone for about two weeks—
the luteal phase.
Progesterone causes the uterine lining, the
endometrium, to thicken.
Concept 32.4 Human Reproduction Is Hormonally Controlled
If an embryo does not arrive within 2 weeks
after ovulation, then endometrium breaks
down.
Menstruation is the sloughing off of the
endometrium through the vagina.
In other mammals, estrus, or sexual
receptivity, corresponds to ovulation.
Most other mammals do not menstruate; the
uterine lining is reabsorbed.
Concept 32.4 Human Reproduction Is Hormonally Controlled
Different hormones control pregnancy.
After fertilization, the zygote divides and
moves toward the uterus.
It attaches to the endometrium as a
blastocyst and burrows in—implantation.
A new cover layer of cells secretes human
chorionic gonadatropin (hCG).
hCG causes the corpus luteum to continue
to produce estrogen and progesterone,
and endometrium is maintained.
Concept 32.4 Human Reproduction Is Hormonally Controlled
Pregnancy tests use an antibody to detect
hCG in urine.
The placenta forms from combined
maternal and embryonic tissues and
secretes progesterone and estrogen.
Both hormones prevent the pituitary from
releasing gonadotropins, so the ovarian
cycle ceases during pregnancy.
Concept 32.4 Human Reproduction Is Hormonally Controlled
The onset of labor is triggered by hormonal
and mechanical stimuli:
• Progesterone—inhibits uterine
contractions
• Estrogen—stimulates contractions
The ratio shifts in favor of estrogen near the
end of pregnancy.
The onset of labor is marked by an increase
in oxytocin—a powerful stimulant of
uterine contraction—by mother and fetus.
Concept 32.4 Human Reproduction Is Hormonally Controlled
Mechanical stimuli:
• Uterine stretching by fully grown fetus
• Pressure on the cervix by the head
These stimuli create a positive feedback
loop—cause release of oxytocin which
increases contractions and puts more
pressure on the cervix.
Concept 32.4 Human Reproduction Is Hormonally Controlled
Between contractions in early labor,
hormonal and mechanical stimuli cause
the cervix to dilate.
It becomes large enough for the baby to
pass through, about 10 cm.
The head enters the vagina and passage is
assisted by mother bearing down and
“pushing” with abdominal muscles.
Concept 32.4 Human Reproduction Is Hormonally Controlled
If the baby suckles at the breast
immediately after birth, additional oxytocin
is secreted.
This causes the uterus to continue to
contract and reduce in size and helps stop
bleeding.
Oxytocin also promotes bonding between
mother and infant.
Concept 32.5 Humans Use a Variety of Methods to Control
Fertility
Ways to prevent fertilization or implantation
(conception) are referred to as
contraception.
The only failure-proof methods of preventing
pregnancy are complete abstinence from
sexual activity or gonad removal.
Other methods vary in their failure rate.
Table 32.1 Methods of Contraception (Part 1)
Table 32.1 Methods of Contraception (Part 2)
Table 32.1 Methods of Contraception (Part 3)
Concept 32.5 Humans Use a Variety of Methods to Control
Fertility
An abortion is a termination of the
pregnancy once the fertilized egg is
implanted in the uterus.
A spontaneous abortion is a miscarriage,
and may occur before the pregnancy is
known.
Abortions through medical intervention may
be for therapeutic reasons or for fertility
control—the embryo and some of the
endometrium are removed.
Concept 32.5 Humans Use a Variety of Methods to Control
Fertility
Infertility is the inability of a couple to
conceive a child—several treatments exist.
Artificial insemination—physician
positions sperm in woman’s reproductive
tract
Assisted reproductive technologies
(ARTs)—remove unfertilized eggs,
combine them with sperm outside the
body, and return them for implantation
• In vitro fertilization (IVF)—the first ART.
Concept 33.1 Fertilization Activates Development
In an unfertilized frog egg:
• Vegetal hemisphere—the lower half of
the egg, where nutrients are concentrated
• Animal hemisphere—the opposite end of
the egg, contains the haploid nucleus
Cytoplasmic movement after fertilization is
visible because of pigments.
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
Cleavage—a rapid series of cell division,
but no cell growth. Embryo becomes a ball
of small cells.
Blastocoel—a central fluid-filled cavity that
forms in the ball.
The embryo becomes a blastula and its
cells are called blastomeres.
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
Complete cleavage occurs in eggs with
little yolk.
Cleavage furrows divide the egg
completely—blastomeres are of similar
size.
Figure 33.3 Some Patterns of Cleavage (Part 1)
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
Incomplete cleavage occurs in eggs with a
lot of yolk when cleavage furrows do not
penetrate.
Discoidal cleavage is common in eggs with
a dense yolk—the embryo forms as a
blastodisc that sits on top of the yolk.
Figure 33.3 Some Patterns of Cleavage (Part 2)
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
Superficial cleavage is a type of
incomplete cleavage.
A syncytium, a cell with many nuclei, forms.
The plasma membrane grows inward
around nuclei and forms cells.
Figure 33.3 Some Patterns of Cleavage (Part 3)
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
Mammalian cleavage is slow. During the
fourth division, cells separate into two
groups:
• Inner cell mass—becomes the embryo—
cells are pluripotent and in culture are
embryonic stem cells (ESCs)
• Trophoblast—a sac that forms from the
outer cells—secretes fluid and creates the
blastocoel, with inner cell mass at one end
Embryo is now called a blastocyst.
Figure 33.4 A Human Blastocyst at Implantation (Part 1)
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
Blastomeres become determined—
committed to specific development—at
different times.
In mosaic development each blastomere
contributes certain aspects to the adult
animal.
In regulative development, cells
compensate for any lost cells.
Figure 33.5 Fate Map of a Frog Blastula
Concept 33.3 Gastrulation Creates Three Tissue Layers
The blastula is transformed into an embryo
during gastrulation, through movement of
cells.
The embryo has multiple tissue layers and
distinct body axes.
During gastrulation three germ layers
form—also called cell layers or tissue
layers
Concept 33.3 Gastrulation Creates Three Tissue Layers
• Endoderm—innermost layer; becomes
the lining of the digestive and respiratory
tracts, pancreas, and liver
• Ectoderm—outer germ layer; becomes
the nervous system, the eyes and ears,
and the skin
• Mesoderm—middle layer; contains cells
that migrate between the other layers;
forms organs, blood vessels, muscle, and
bones
Concept 33.3 Gastrulation Creates Three Tissue Layers
During gastrulation:
• Vegetal hemisphere flattens as cells
change shape
• Vegetal pole bulges inward, invaginates;
cells become endoderm and form the
archenteron, or gut
• Some cells migrate into the central cavity
and become mesenchyme—cells of the
mesoderm layer
Concept 33.3 Gastrulation Creates Three Tissue Layers
Filopodia form and adhere to the ectoderm;
pull the archenteron by contracting
The mouth forms where the archenteron
meets the ectoderm.
The blastopore is the opening of the
invagination of the vegetal pole and
becomes the anus.
Figure 33.6 Gastrulation in Sea Urchins (Part 1)
Concept 33.3 Gastrulation Creates Three Tissue Layers
.
Involution occurs as bottle cells move
inward and create the dorsal lip.
Cells from the animal hemisphere move
toward the site of involution—epiboly.
At end of gastrulation, embryo has three
germ layers and dorsal-ventral and
anterior-posterior organization
Figure 33.7 Gastrulation in the Frog Embryo (Part 1)
Figure 33.7 Gastrulation in the Frog Embryo (Part 3)
Concept 33.3 Gastrulation Creates Three Tissue Layers
Reptiles and birds:
Gastrulation occurs in a flat disk of cells
called the blastodisc.
Some cells enter a fluid space between the
blastodisc and the yolk and form the
hypoblast—a continuous layer that will
contribute to extraembryonic membranes.
Overlying cells form the epiblast—
becomes the embryo.
Concept 33.3 Gastrulation Creates Three Tissue Layers
Epiblast cells move toward the midline and
form a ridge called the primitive streak.
The primitive groove develops along the
primitive streak—cells migrate through it
and become endoderm and mesoderm.
Hensen’s node is the equivalent of the
amphibian dorsal lip and contains many
signaling molecules.
Concept 33.4 Neurulation Creates the Nervous System
Gastrulation produces an embryo with three
germ layers that will influence each other
during development.
During organogenesis, organs and organ
systems develop simultaneously.
Neurulation is the initiation of the nervous
system—occurs in early organogenesis.
Concept 33.5 Extraembryonic Membranes Nourish the Growing
Embryo
The amniote egg, with its contained water
supply, frees development from requiring
an external water supply.
Extraembryonic membranes surround
embryos in amniote eggs.
They function in nutrition, gas exchange,
and waste removal.
Concept 33.5 Extraembryonic Membranes Nourish the Growing
Embryo
In the chick, four membranes form:
• Yolk sac—encloses yolk within the egg
and passes nutrients to the embryo
• Allantoic membrane—a sac for waste
storage
• Amnion—secretes fluid for protection
• Chorion—reduces water loss and
exchanges gases
Figure 33.15 The Extraembryonic Membranes of Amniotes (Part
1)
Figure 33.15 The Extraembryonic Membranes of Amniotes (Part
2)
Concept 33.5 Extraembryonic Membranes Nourish the Growing
Embryo
Human gestation is divided into trimesters of
about 12 weeks each.
In the first trimester the embryo is very
sensitive to damage from radiation, drugs,
and chemicals.
Gastrulation occurs, tissues differentiate,
and the placenta forms.
By the end of the first trimester, most organs
have started to form and the embryo
becomes a fetus.
Concept 33.5 Extraembryonic Membranes Nourish the Growing
Embryo
During the second and third trimesters the
fetus grows rapidly.
Toward the end of the third trimester the
organ systems mature.
Birth occurs when the last of its critical
organs—the lungs—matures.
The end!