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Chapter 27
Reproduction and Embryonic Development
Figure 27.5A_2
2n
Diploid cell
Differentiation and
onset of meiosis I
2n
Primary
spermatocyte
(diploid; in prophase
of meiosis I)
Meiosis I completed
n
Secondary
spermatocyte
n
(haploid)
Meiosis II
Developing
sperm cells
n
Sperm cells
(haploid)
n
Mature sperm released into center of
seminiferous tubule
n
n
n
Differentiation
n
n
n
27.5 The formation of sperm and egg cells
requires meiosis
 Oogenesis and spermatogenesis are
– alike in that both produce haploid gametes but
– different in that
– oogenesis produces only one mature egg and polar bodies that
degenerate and
– spermatogenesis produces four mature gametes.
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27.6 Hormones synchronize cyclic changes in the
ovary and uterus
 About every 28 days
– the hypothalamus signals the anterior pituitary to secrete
follicle-stimulating hormone (FSH) and luteinizing
hormone (LH),
– which trigger the growth of a follicle and ovulation, the
release of an egg.
© 2012 Pearson Education, Inc.
Table 27.6
Figure 27.6_2
0
5
10
Pituitary hormones
in blood
Days
14 15
20
LH peak triggers
ovulation and
corpus luteum
formation
LH
FSH
FSH stimulates
follicle to grow
LH surge
triggers ovulation
25
28
Figure 27.6_3
0
10
5
Days
14 15
20
25
28
Ovarian cycle
Growing
follicle
Mature
follicle
Ovulation
Pre-ovulatory phase
Estrogen secreted
by growing follicle
Corpus Degenerating
corpus
luteum
luteum
Post-ovulatory phase
Progesterone and
estrogen secreted by
remnant of follicle
Figure 27.6_4
0
5
10
Days
14 15
20
25
28
Peak causes
LH surge
Ovarian
hormones
in blood
Estrogen
Progesterone
Low levels of
estrogen trigger
menstruation
Progesterone and
estrogen promote
thickening of
endometrium
Figure 27.6_5
Menstrual cycle
Endometrium
0
5
Menstruation
10
14 15
Days
20
25
28
PRINCIPLES OF
EMBRYONIC DEVELOPMENT
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27.9 Fertilization results in a zygote and triggers
embryonic development
 Embryonic development begins with fertilization,
– the union of sperm and egg,
– to form a diploid zygote.
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27.9 Fertilization results in a zygote and triggers
embryonic development
 Sperm are adapted to reach and fertilize an egg.
Sperm have
– a streamlined shape, which moves easily through fluids,
– many mitochondria, which provide ATP for tail
movements, and
– a head that contains a haploid nucleus and is tipped with
an acrosome containing enzymes that help it penetrate
the egg.
© 2012 Pearson Education, Inc.
Figure 27.9C_1
A sperm touches
the egg’s jelly
coat, and its
acrosome
releases enzyme
molecules.
Sperm
The sperm’s
acrosomal
enzymes
digest the
egg’s jelly
coat.
Proteins on the
sperm head bind
to egg receptors.
Acrosomal
enzymes
Plasma
membrane
Nucleus
Acrosome
Jelly
coat
Plasma
membrane
Vitelline
layer
Receptor protein
molecules
Figure 27.9C_2
The plasma membranes
of sperm and egg fuse.
The sperm nucleus enters
the egg cytoplasm.
The vitelline
layer
separates
and becomes
impenetrable.
n Sperm
nucleus
Figure 27.9C_3
n Sperm
nucleus
n
Egg
nucleus
n
n
The nuclei of sperm
and egg fuse.
2n
Zygote nucleus
27.10 Cleavage produces a ball of cells from the
zygote
 Cleavage is a rapid series of cell divisions that
produces
– more cells,
– smaller cells, and
– a fluid-filled embryo called a blastula.
Video: Sea Urchin Embryonic Development
© 2012 Pearson Education, Inc.
Figure 27.10_s4
Zygote
2 cells
4 cells
8 cells
Many cells (solid ball)
Blastocoel
Blastula Cross section
(hollow ball) of blastula
27.11 Gastrulation produces a three-layered
embryo
 During gastrulation
– cells migrate to new locations,
– a rudimentary digestive cavity forms, and
– the basic body plan of three layers is established with
– ectoderm outside—becomes skin and nervous systems,
– endoderm inside—becomes digestive tract,
– mesoderm in the middle—becomes muscle and bone.
© 2012 Pearson Education, Inc.
Figure 27.11_s3
Blastula
(end of cleavage)
Animal pole
Blastocoel
Vegetal pole
Gastrulation
(cell migration)
Blastocoel
shrinking
Gastrula
(end of gastrulation)
Simple
digestive
cavity
Formation of a
simple digestive
cavity
Blastopore
Ectoderm
Mesoderm
Endoderm
Table 27.11
27.12 Organs start to form after gastrulation
 Organs develop from the three embryonic layers.
– The stiff notochord forms the main axis of the body and
is later replaced by the vertebral column in most
chordates.
– The neural tube develops above the notochord and will
become the
– brain and
– spinal cord.
Video: Frog Embryo Development
© 2012 Pearson Education, Inc.
Figure 27.12A
Neural Neural
fold
plate
Notochord
Ectoderm
Mesoderm
Endoderm
Neural folds
Figure 27.12B
Neural
fold
Neural plate
Outer layer
of ectoderm
Neural tube
27.12 Organs start to form after gastrulation
 As the embryo elongates, paired somites
– form along the sides of the notochord,
– hollow out to form a coelom, and
– eventually contribute to muscles, bone, and other
connective tissues.
 Other systems develop at the same time.
© 2012 Pearson Education, Inc.
Figure 27.12C
Neural tube
Notochord
Somite
Coelom
Somites
Digestive
cavity
Eye
Tail bud
27.13 Multiple processes give form to the
developing animal
 Through induction, adjacent cells and cell layers
– influence each other’s differentiation
– via chemical signals.
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27.14 EVOLUTION CONNECTION: Pattern
formation during embryonic development is
controlled by ancient genes
 Pattern formation,
– the emergence of the parts of a structure in their correct
relative positions,
– involves the response of genes to spatial variations of
chemicals in the embryo, and
– results in tissues and organs developing in their proper
positions at the correct times.
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HUMAN DEVELOPMENT
© 2012 Pearson Education, Inc.
27.15 The embryo and placenta take shape during
the first month of pregnancy
 Pregnancy, or gestation, is the carrying of
developing young within the female reproductive
tract.
 Human pregnancy
– averages 266 days (38 weeks) from fertilization or
– 40 weeks (9 months) from the start of the last menstrual
period.
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27.15 The embryo and placenta take shape during
the first month of pregnancy
 Human development begins with fertilization in the
oviduct.
 Cleavage produces a blastocyst whose
– inner cell mass becomes the embryo and the
– trophoblast, the outer cell layer, which
– attaches to the uterine wall and
– forms part of the placenta.
 Gastrulation occurs and organs develop from the
three embryonic layers.
© 2012 Pearson Education, Inc.
Figure 27.15A–B
Cleavage starts
Fertilization
of mature
egg
Blastocyst
Trophoblast
Uterine
cavity
Cavity
Oviduct
Ovary
Inner cell mass
Blastocyst
(implanted)
Secondary
oocyte
Ovulation
Endometrium
Uterus
Uterine
cavity
Figure 27.15C
Endometrium
Uterine cavity
Multiplying
cells of
trophoblast
(contribute to
future placenta) Trophoblast
Embryo
Future
yolk sac
Blood vessel
(maternal)
27.15 The embryo and placenta take shape during
the first month of pregnancy
 Four extraembryonic membranes develop.
1. The amnion
– surrounds the embryo and
– forms a fluid-filled amniotic cavity that protects the embryo.
2. The yolk sac,
– in reptiles, stores yolk,
– in humans, does not store yolk but is a source of the first germ
cells and blood cells.
© 2012 Pearson Education, Inc.
27.15 The embryo and placenta take shape during
the first month of pregnancy
3. The allantois
– contributes to the umbilical cord,
– forms part of the urinary bladder, and
– in reptiles, stores embryonic waste.
4. The chorion
– contributes to the placenta and
– secretes human chorionic gonadotropin (HCG), which
prevents menstruation in mammals.
© 2012 Pearson Education, Inc.
Figure 27.15D
Yolk sac
Chorion
Amnion
Amniotic
cavity
Mesoderm
cells
Figure 27.15E
Embryo:
Endoderm
Mesoderm
Ectoderm
Chorionic
villi
Chorion
Amnion
Allantois
Yolk
sac
Figure 27.15F
Placenta
Amnion
Amniotic
cavity
Embryo
Mother’s
blood Allantois
vessels
Yolk
sac
Chorion
Chorionic
villi
Figure 27.17A
Estrogen
from
ovaries
Oxytocin
from fetus and
mother’s pituitary
Induces oxytocin
receptors on uterus
Stimulates
placenta to make
Prostaglandins
Stimulate more
contractions
of uterus
Positive feedback
Stimulates uterus
to contract
27.17 Childbirth is induced by hormones and
other chemical signals
 Labor occurs in three stages:
1. dilation of the cervix,
2. expulsion, delivery of the infant,
3. delivery of the placenta.
© 2012 Pearson Education, Inc.
27.18 CONNECTION: Reproductive technologies
increase our reproductive options
 New techniques can help many infertile couples.
– About 15% of couples wanting children are infertile.
– Drug therapies can help address problems of impotence
(erectile dysfunction) and induce ovulation.
– Assisted reproductive technologies (ART) require
eggs to be harvested from the ovaries, fertilized, and
returned to a woman’s body.
– In vitro fertilization (IVF) is the most common assisted
reproductive technology. Fertilization occurs in a culture
dish and an early embryo is implanted in the uterus.
© 2012 Pearson Education, Inc.
Figure 27.18
Implantation
Zygote
Collected
egg
In vitro fertilization
Collected
sperm
8-cell
embryo
Figure 27.UN01
Oogenesis
2n
Once per
month
Spermatogenesis
Primary
oocyte
Primary
spermatocyte
Continuously
n Polar body
n
Secondary
oocyte
2n
Secondary
spermatocyte
Developing n
sperm cells
n
n
n
n
Sperm n
Fertilization
Polar body n
Mature egg
n
2n
Zygote
n
Figure 27.UN02
Cleavage
Gastrulation
Ectoderm
Mesoderm
Endoderm
Zygote
2-cell
embryo
Many-celled Blastula
(cross
solid ball
section)
Gastrula
(cross
section)
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