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Chapter 27
Reproduction and Embryonic Development
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Baby Bonanza
• The increased use of fertility drugs
– Has caused an increase in the number of
multiple births in the United States
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Fertility drugs
– Are sometimes too effective
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ASEXUAL AND SEXUAL REPRODUCTION
27.1 Sexual and asexual reproduction are both
common among animals
• In asexual reproduction
– One parent produces genetically
identical offspring
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• Asexual reproduction
– Enables an individual to produce many
offspring rapidly
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• Reproduction in some animals is accomplished
through external fertilization
– In which the parents release their
gametes into the environment where
fertilization occurs
Eggs
Figure 27.1E
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• Sexual reproduction
– May enhance reproductive success in
changing environments
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HUMAN REPRODUCTION
27.2 Reproductive anatomy of the human female
• Both sexes in humans
– Have a set of gonads where gametes are
produced
– Have ducts for delivery of the gonads
and structures for copulation
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• A woman’s ovaries
– Contain follicles that nurture eggs and
produce sex hormones
Ovaries
Oviduct
Follicles
Corpus luteum
Wall of uterus
Uterus
Endometrium
(lining of uterus)
Cervix
(“neck” of uterus)
Vagina
Figure 27.2A
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• Oviducts convey eggs to the uterus
– Where the eggs develop
Ovary
Figure 27.2B
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LM 200
Egg
cell
• The uterus opens into the vagina
– Which receives the penis during
intercourse and forms the birth canal
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• Other structures of the female reproductive
system include
– Labia, the clitoris, Bartholin’s glands, and
the cervix
Oviduct
Ovary
Uterus
Rectum
(digestive system)
Urinary bladder
(excretory system)
Pubic bone
Cervix
Urethra
(excretory system)
Vagina
Shaft
Glans
Bartholin’s gland
Clitoris
Prepuce
Labia minora
Anus
(digestive system)
Labia majora
Vaginal opening
Figure 27.2C
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27.3 Reproductive anatomy of the human male
• A man’s testes
–
Produce sperm
Rectum
(digestive system)
Urinary bladder
(excretory
system)
Seminal vesicle
Vas deferens
Ejaculatory
duct
Pubic bone
Erectile
tissue of
penis
Prostate gland
Bulbourethral gland
Vas deferens
Urethra
(excretory
system)
Epididymis
Glans of penis
Testis
Scrotum
Prepuce
Figure 27.3A
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Penis
• Several glands
– Contribute to the formation of fluid that
nourishes and protects sperm
Urinary bladder
(excretory
system)
Seminal
vesicle
(behind
bladder)
Prostate gland
Bulbourethral
gland
Erectile tissue
of penis
Urethra
Vas deferens
Scrotum
Epididymis
Testis
Figure 27.3B
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Glans of
penis
• During ejaculation
– Sperm and the nourishing fluid, called
semen, are expelled through the penis
Contractions
of vas deferens
Sphincter
contracts
Urinary
bladder
Urethra region here
expands and fills
with semen
Contractions
of seminal
vesicle
Contractions
of prostate
gland
Sphincter
contracts
Contractions
of epididymis
First stage
Sphincter remains
contracted
Semen expelled
Contractions
of muscles
around base
of penis
Sphincter
relaxes
Figure 27.3C
Second stage
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Contractions
of urethra
• A negative feedback system of hormones
– Controls sperm production
Stimuli from other
areas in the brain
Releasing
hormone
Anterior
pituitary
LH
FSH
Testis
Androgen
production
Sperm
production
Figure 27.3D
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Negative feedback
Hypothalamus
• Primary spermatocytes, which are diploid, are made
continuously in the testes
–
And undergo meiosis to produce haploid sperm
Epididymis
Testis
Scrotum
2n
Diploid cell
Differentiation and
onset of Meiosis I
2n
Primary spermatocyte
Penis
Testis
Seminiferous tubule
Cross section of
seminiferous
tubule
(in prophase of Meiosis I)
Meiosis I completed
n
Secondary spermatocyte
n
(haploid; double chromatids)
Meiosis II
n
n
n
n
Developing sperm cells
(haploid; single chromatids)
Differentiation
n
n
n
n
Sperm cells
(haploid)
Figure 27.4A
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Center of
seminiferous tubule
• Each month one primary oocyte
–
Matures to form a secondary oocyte, which can be
fertilized
–
Completes meiosis and becomes a haploid ovum
Diploid cell
In embryo
2n
Differentiation and
onset of Meiosis I
Primary oocyte
(arrested in prophase
of Meiosis I)
2n
Present at birth
Completion of Meiosis I
and onset of Meiosis II
Secondary oocyte
n
(arrested at metaphase of Meiosis II;
released from ovary)
n
First
polar body
Entry of sperm triggers
completion of Meiosis II
Ovum
(haploid)
Figure 27.4B
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n
n
Second
polar body
• The development of an ovarian follicle
– Involves many different processes
Degenerating
corpus luteum
Start:
Corpus luteum
Primary oocyte
within follicle
Growing
follicles
Mature follicle
Ovary
Secondary
oocyte
Ruptured follicle
Ovulation
Figure 27.4C
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27.5 Hormones synchronize cyclic changes in
the ovary and uterus
• The ovarian cycle includes
– Changes in the ovary that occur about
every 28 days
• The menstrual cycle
– Involves changes that occur in the uterus
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An Overview of the Ovarian and Menstrual
Cycles
• Events in the menstrual cycle
– Are synchronized with the ovarian cycle,
which occurs about every 28 days
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• Uterine bleeding, called menstruation
– Includes the breakdown of the
endometrial lining
– Usually persists for 3–5 days
• After menstruation
– The endometrium, the lining of the
uterus, regrows
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• Five hormones
–
Synchronize the events in the ovarian cycle
Table 27.5
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Hormonal Events Before Ovulation
• Approximately every 28 days
– The hypothalamus signals the anterior
pituitary to secrete FSH and LH
• FSH and LH
– Trigger the growth of a follicle
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• As the follicle grows, it secretes estrogen
– Which causes a burst in FSH and LH
levels, leading to ovulation
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Hormonal Events at Ovulation and After
• After ovulation
– The follicle becomes the corpus luteum
• The corpus luteum secretes both estrogen and
progesterone
– Which exert negative feedback on the
hypothalamus and pituitary, causing a
decline in FSH and LH levels
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• As FSH and LH levels drop
– The hypothalamus can once again
stimulate the pituitary to secrete more
FSH and LH, and a new cycle begins
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Control of the Menstrual Cycle
• The menstrual cycle
– Is directly controlled by estrogen and
progesterone
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• If fertilization of an egg occurs
– A hormone from the embryo maintains
the uterine lining and prevents
menstruation
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• The ovarian and menstrual
cycles
A Control by hypothalamus
Inhibited by combination of
estrogen and progesterone
Stimulated by high levels
Releasing hormone of estrogen
Hypothalamus
Anterior pituitary
1
FSH
LH
B Pituitary hormones
in blood
4
LH peak triggers
ovulation and
corpus luteum
formation
6
LH
FSH
2
FSH
LH
C Ovarian cycle
5
Corpus Degenerating
luteum corpus luteum
Mature Ovulation
follicle
Pre-ovulatory phase
Post-ovulatory phase
Growing follicle
Progesterone
and estrogen
Estrogen
D Ovarian hormones
in blood
3
7
8
Estrogen
Progesterone
Progesterone
and estrogen
Estrogen
E Menstrual cycle
Endometrium
0
Figure 27.5
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5
Menstruation
10
14 15
Days
20
25
28
27.6 The human sexual response occurs in four
phases
• The excitement phase
–
Prepares the sexual organs for coitus
• The plateau phase
–
Is marked by increases in breathing and
heart rate
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• Orgasm follows
– Characterized by rhythmic contractions
of the reproductive structures
• The resolution phase
– Completes the cycle and reverses the
previous responses
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CONNECTION
27.7 Sexual activity can transmit disease
• Sexual intercourse
– Carries the risk of exposure to sexually
transmitted diseases (STDs)
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• STDs common in the United States
Table 27.7
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CONNECTION
27.8 Contraception can prevent unwanted
pregnancy
• Contraception
– Is the deliberate prevention of pregnancy
Table 27.8
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• Contraception can be accomplished
– Through various methods
Skin patch
Condom
Diaphragm
Spermicide
Birth control pills
Figure 27.8
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PRINCIPLES OF EMBRYONIC DEVELOPMENT
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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The Properties of Sperm Cells
• Only one sperm
Figure 27.9A
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Colorized SEM 500
– Fertilizes an egg
• During fertilization
– A sperm releases enzymes from the
acrosome that pierce the egg’s coat
Plasma membrane
Middle
piece
Neck
Head
Tail
Mitochondrion
(spiral shape)
Nucleus
Acrosome
Figure 27.9B
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The Process of Fertilization
• Sperm surface proteins bind to egg receptor
proteins
– Sperm and egg plasma membranes
fuse, and the two nuclei unite
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• Changes in the egg membrane
– Prevent entry of additional sperm
• The fertilized egg (zygote)
– Develops into an embryo
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• The process of fertilization
1 The sperm
2 The sperm’s
approaches
the egg
Sperm
acrosomal enzymes
digest the egg’s jelly 3 Proteins on the
sperm head bind to
coat
egg receptors
4 The plasma
membranes of sperm
and egg fuse
Nucleus
Sperm
head
Acrosome
Plasma
membrane
5 The sperm nucleus
enters the egg
cytoplasm
Acrosomal
enzymes
6 A fertilization
envelope forms
Receptor protein
molecules
Plasma
membrane
Vitelline
layer
Cytoplasm
Sperm
nucleus
Egg
nucleus
Jelly
coat
Egg cell
7 The nuclei of
sperm and egg
fuse
Zygote nucleus
Figure 27.9C
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27.10 Cleavage produces a ball of cells from the zygote
• Cleavage is a rapid series of cell divisions
–
That results in a blastula, a ball of cells
Zygote
2 cells
4 cells
8 cells
Blastocoel
Many cells
(solid ball)
Blastula Cross section
(hollow ball)
of blastula
Figure 27.10
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27.13 Changes in cell shape, cell migration, and
programmed cell death give form to the
developing animal
Ectoderm
• Cells of the ectoderm
–
Fold inward during
neural tube formation
Figure 27.13A
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• Programmed cell death, or apoptosis
– Is a key developmental process in which
cells die
Apoptosis
Dead cell
engulfed and
digested by
adjacent cell
Figure 27.13B
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27.14 Embryonic induction initiates organ
formation
• In a process called induction
– Adjacent cells and cell layers influence
each other’s differentiation via chemical
signals
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• Induction during eye development
Lens ectoderm
Optic cup
Cornea
Future
brain
Lens
Optic
vesicle
1
Optic
stalk
Figure 27.14
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Future
retina
2
3
4
27.15 Pattern formation organizes the animal
body
• Pattern formation
– Is the emergence of the parts of a
structure in their correct relative positions
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HUMAN DEVELOPMENT
27.16 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
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An Overview of Developmental Events
• Human development
– Begins with fertilization in the oviduct
Cleavage starts
Fertilization
of ovum
Ovary
Oviduct
Secondary
oocyte
Ovulation
Blastocyst
(implanted)
Endometrium
Uterus
Figure 27.16A
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• Cleavage produces a blastocyst
– Whose inner cell mass becomes the
embryo
Endometrium
Inner cell mass
Cavity
Trophoblast
Figure 27.16B
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• The blastocyst’s outer layer, the trophoblast
– Implants in the uterine wall
Endometrium
Blood vessel
(maternal)
Future embryo
Multiplying cells
of trophoblast
Future
yolk sac
Trophoblast
Uterine cavity
Figure 27.16C
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• Meanwhile, the four extraembryonic
membranes develop
– The amnion, the chorion, the yolk sac,
and the allantois
Amniotic
cavity
Amnion
Chorion
Chorionic villi
Amnion
Embryo:
Ectoderm
Mesoderm
Endoderm
Allantois
Mesoderm
cells
Chorion
Yolk sac
Figure 27.16D, E
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Yolk sac
Roles of the Extraembryonic Membranes
• About a month after conception
–
The extraembryonic membranes are fully formed
Placenta
Mother’s blood
vessels
Allantois
Yolk sac
Amniotic
cavity
Amnion
Embryo
Chorion
Chorionic
villi
Figure 27.16F
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• The embryo floats in a fluid-filled amniotic
cavity
– Surrounded by the amnion
• The chorion and mesoderm cells from the yolk
sac
– Form the embryo’s part of the placenta
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• The allantois
– Forms part of the umbilical cord
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The Placenta
• The placenta’s chorionic villi
– Absorb food and oxygen from the
mother’s blood to nourish the embryo
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27.17 Human development from conception to
birth is divided into three trimesters
• Human development is divided into three
trimesters
– Each about three months in length
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The First Trimester
• The most rapid changes
– Occur during the first trimester
Figure 27.17A
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• By 9 weeks
– The embryo is called a fetus
Figure 27.17B
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Second Trimester
• The second trimester
–
Involves an increase in size and general
refinement of the human features
Figure 27.17C
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• At 20 weeks
– The fetus is about 19 centimeters long,
and weighs half a kilogram
Figure 27.17D
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Third Trimester
• The third trimester
– Is a time of rapid growth, which ends in
birth
Figure 27.17E
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27.18 Childbirth is hormonally induced and
occurs in three stages
• The birth of a child
– Is brought about by a series of strong,
rhythmic contractions of the uterus,
called labor
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• Estrogen makes the uterus more sensitive to oxytocin
Which acts with prostaglandins to initiate labor
Estrogen
from
ovaries
Oxytocin
from fetus
and pituitary
Induces oxytocin
receptors on uterus
Stimulates uterus
to contract
Stimulates
placenta to make
Prostaglandins
Stimulate more
contractions
of uterus
Figure 27.18A
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Positive feedback
–
• Labor occurs in three stages
Placenta
Umbilical
cord
Uterus
– Dilation, expulsion,
and delivery of the
placenta
Cervix
1
Dilation of the cervix
2
Expulsion: delivery of the infant
Uterus
Placenta
(detaching)
Umbilical
cord
3 Delivery of the placenta
Figure 27.18B
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CONNECTION
27.19 Reproductive technology increases our
reproductive options
• New techniques
– Can provide help to infertile couples
Figure 27.19
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• Some of these methods
– Raise important ethical and legal
questions
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