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Animal Development
Chapter 53
Fertilization
In all sexually-reproducing animals, the first
step is fertilization – union of male and
female gametes
Fertilization itself consists of three events:
-Sperm penetration and membrane fusion
-Egg activation
-Fusion of nuclei
2
Fertilization
Sperm penetration and membrane fusion
-Protective layers of egg include the jelly
layer and vitelline envelope in sea urchins,
and the zona pellucida in mammals
-The acrosome of sperm contains digestive
enzymes that enable the sperm to tunnel its
way through to the egg’s cell membrane
-Membrane fusion permit sperm nucleus
to enter directly into egg’s cytoplasm
3
Fertilization
Egg activation
-Membrane fusion triggers egg activation by
the release of Ca2+ which initiates changes
in the egg
-A block to polyspermy occurs
-Changes in egg’s membrane potential
-Alteration of egg’s exterior coats
-Enzymes from cortical granules
remove sperm receptors
4
Sperm
Jelly layer
Granulosa cell
Sperm
Zona pellucida
Plasma
membrane
Plasma
membrane
Vitelline
envelope
First
polar
body
Nucleus
Cytoplasm of egg
Cortical granules
a.
Cortical granules
Cytoplasm
b.
5
Fertilization
Egg activation
-Sperm penetration has three other effects
1. Triggers the egg to complete meiosis
2. Triggers a cytoplasmic rearrangement
3. Causes a sharp increase in protein
synthesis and metabolic activity in
general
6
Fertilization
Primary Oocyte
First Metaphase of Meiosis Second Metaphase of Meiosis
Diploid nucleus
Meiosis Complete
Polar
bodies
Polar
body
Female
pronucleus
(haploid)
• Roundworms (Ascaris)
• Polychaete worms (Myzostoma)
• Clam worms (Nereis)
• Clams (Spisula)
• Nemertean worms (Cerebratulus)
• Polychaete worms (Chaetopterus)
• Mollusks (Dentalium)
• Many insects
• Sea stars
• Lancelets (Branchiostoma)
• Amphibians
• Mammals
• Fish
• Cnidarians
• Sea urchins
7
Fertilization
Fusion of nuclei
-The haploid sperm and haploid egg nuclei
migrate toward each other along a microtubule based aster
-They then fuse, forming the diploid
nucleus of the zygote
8
Fertilization
1. Sperm penetrates
2. Some of the zona
4. The sperm nucleus
3. Sperm and egg
between granulosa
pellucida is degraded
dissociates and
plasma membranes
cells.
by acrosomal enzymes.
enters cytoplasm.
fuse.
Plasma
membrane
Granulosa
cells
Zona
pellucida
Cortical
granules
6. Additional sperm
can no longer penetrate
the zona pellucida.
5. Cortical granules
release enzymes that
harden zona pellucida
and strip it of sperm
receptors. Hyalin
attracts water by osmosis.
7. Sperm and egg
pronuclei are
enclosed in a
nuclear envelope.
9
Cleavage
Cleavage is the rapid division of the zygote
into a larger and larger number of smaller
and smaller cells called blastomeres
-It is not accompanied by an increase in the
overall size of the embryo
In many animals, the two embryo ends are:
-Animal pole = Forms external tissues
-Vegetal pole = Forms internal tissues
10
Cleavage
The outermost blastomeres in the ball of cells
become joined by tight junctions
Innermost blastomeres pump Na+ into the
intracellular spaces
-Create osmotic gradient, which draws water
The result is a hollow ball of cells, the
blastula, containing a fluid-filled cavity, the
blastocoel
11
Cleavage Patterns
Cleavage patterns are highly diverse
-Influenced by amount of yolk in the egg
Sea Urchin
Frog
Chicken
Animal pole
Nucleus
Cytoplasm
Cytoplasm
Shell
Nucleus
Air
bubble
Nucleus
Plasma
membrane
Albumen
Yolk
Yolk
Vegetal pole
a.
b.
Yolk
c.
12
Cleavage Patterns
Eggs with moderate to little yolk undergo
holoblastic (complete) cleavage
-In sea urchins, a symmetrical blastula is
produced, surrounding spherical blastocoel
-In amphibians, an asymmetrical blastula is
produced, with a displaced blastocoel
-Because egg contains much more yolk
in one hemisphere than the other
13
14
Cleavage Patterns
Eggs with large amounts of yolk undergo
meroblastic (incomplete) cleavage
-In eggs of reptiles and birds, the clear
cytoplasm is concentrated at one pole called
the blastodisc
-Cleavage is restricted to this area
-Resulting embryo is not spherical
15
Cleavage Patterns
16
Cleavage Patterns
Mammalian eggs contain very little yolk, and
so undergo holoblastic cleavage
-Form a blastocyst, which is composed of:
-Trophoblast = Outer layer of cells
-Contributes to the placenta
-Blastocoel = Central fluid-filled cavity
-Inner cell mass = Located at one pole
-Forms the developing embryo
17
Cleavage Patterns
ICM
Blastocoel
Blastodisc
Yolk
Trophoblast
18
Fate of Blastomeres
In mammals, early blastomeres do not appear
to be committed to a particular fate
-The earliest patterning events occur at the
eight-cell stage
-Outer surfaces of blastomeres flatten
against each other in a process called
compaction
-Produces polarized blastomeres,
which then divide asymmetrically
19
Gastrulation
Gastrulation is a process involving a complex
series of cell shape changes and cell
movements that occurs in the blastula
-It establishes the basic body plan and
creates the three primary germ layers
-Ectoderm – Exterior
-Mesoderm – Middle
-Endoderm – Inner
20
Gastrulation
21
Gastrulation
Cells move during gastrulation using a variety
of cell shape changes
-Cells that are tightly attached to each other
via junctions will move as cell sheets
-Invagination – Cell sheet dents inward
-Involution – Cell sheet rolls inward
-Delamination – Cell sheet splits in two
-Ingression – Cells break away from cell
sheet and migrate as individual cells 22
Gastrulation Patterns
Also vary according to the amount of yolk
Gastrulation in sea urchins
-Begins with formation of vegetal plate and
ingression of primary mesenchyme cells
(future mesoderm cells) into blastocoel
-Remaining cells of vegetal plate invaginate
into blastocoel forming the endoderm
-Archenteron (future digestive gut)
-Cells staying at surface form ectoderm 23
Animal pole
Ectoderm
Future
ectoderm
Ectoderm
Blastocoel
Primary
mesenchyme
cells (PMC’s)
Vegetal pole
a.
Filopodia
Archenteron
PMC
Future
endoderm
Blastopore
b.
Anus
c.
24
Gastrulation Patterns
Gastrulation in frogs
-Cells from the animal pole involute over the
dorsal lip of blastopore into the blastocoel
-Cells eventually press against far wall
-Eliminate blastocoel, producing the
archenteron with yolk plug
-These movements create two layers
-Outer ectoderm and inner endoderm
25
-Mesoderm forms later in between
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Animal pole
Dorsal lip
Ectoderm
Ectoderm
Mesoderm
Archenteron
Endoderm
Ectoderm
Archenteron
Blastocoel
Blastocoel
Mesoderm
Vegetal pole
a.
Blastocoel
Yolk plug
Dorsal lip of
blastopore
Ventral lip
b.
c.
Neural plate Neural fold
Neural plate
26
d.
e.
Gastrulation Patterns
Gastrulation in birds
-Avian blastula consists of a disc of cells,
the blastoderm, sitting atop large yolk mass
-First, blastoderm delaminates into two
layers, with blastocoel cavity in between
-The upper layer produces all 3 germ layers
-Cells that migrate through primitive
streak form endoderm or mesoderm
-Cells that remain form ectoderm
27
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Blastoderm
Yolk
Blastocoel
Yolk
Primitive streak
Mesoderm
Ectoderm
Endoderm
Yolk
28
Gastrulation Patterns
Gastrulation in mammals
-Proceeds similarly to that in birds
-Embryo develops from inner cell mass
-ICM flattens and delaminates into 2 layers
-A primitive streak forms
-Cell movements through it give rise to
the three primary germ layers
29
Gastrulation Patterns
Inner cell mass
Primitive streak
Amniotic cavity
Ectoderm
Ectoderm
Mesoderm
Formation of
yolk sac
Trophoblast
a.
b.
Endoderm
c.
Endoderm
d.
30
Extraembryonic Membranes
As an adaptation to life on dry land, amniotic
species developed several extraembryonic
membranes
-Nourish and protect the developing embryo
These membranes are formed from
embryonic cells
31
Extraembryonic Membranes
1. Amnion = Encloses amniotic fluid
2. Chorion = Located near eggshell in birds
-Contributes to the placenta in mammals
3. Yolk sac = Food source in bird embryos
-Found in mammals, but it is not nutritive
4. Allantois = Unites with chorion in birds,
forming a structure used for gas exchange
-In mammals, it contributes blood vessels
32
to the developing umbilical cord
Extraembryonic Membranes
Chick Embryo
Mammal Embryo
Chorion
Amnion
Chorion
Yolk sac
Amnion
Umbilical
blood
vessels
Yolk sac
Villus of chorion
frondosum
Allantois
Maternal blood
a.
b.
33
Organogenesis
Organogenesis is the formation of organs in
their proper locations
-Occurs by interaction of cells within and
between the three germ layers
-Thus, it follows rapidly on the heels
of gastrulation
-Indeed, in many animals it begins
before gastrulation is complete
34
Organogenesis
To a large degree, a cell’s location in the
developing embryo determines its fate
At some stage, every cell’s ultimate fate
becomes fixed – cell determination
A cell’s fate can be established in two ways:
1. Inheritance of cytoplasmic determinants
2. Interactions with neighboring cells
-Cell induction
35
Organogenesis in Drosophila
Salivary gland development
-The sex combs reduced (scr) gene is a
homeotic gene in the Antennapedia complex
-Prior to organogenesis, it is expressed
in an anterior band of cells
-At the same time, Decapentaplegic
protein (Dpp) is released from dorsal cells
-Forms a gradient in the dorsal-ventral
direction
36
Organogenesis in Drosophila
Salivary gland development
-Dpp inhibits formation of salivary gland
rudiments
-Thus, during organogenesis, salivary
glands develop in areas where Scr is
expressed and Dpp is absent
37
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Prior to Organogenesis
Dpp
a.
During Organogenesis
Salivary
gland
Labium
b.
38
Organogenesis in Vertebrates
Organogenesis in vertebrates begins with the
formation of two structures unique to
chordates
-Notochord
-Dorsal nerve cord
-Its development is called neurulation
39
Development of Neural Tube
The notochord forms from mesoderm
-Region of dorsal ectodermal cells situated
above notochord thickens to form the
neural plate
-Cells of the neural plate fold together to
form a long hollow cylinder, the neural
tube
-Will become brain and spinal cord
40
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Neural plate
Amniotic
cavity
Ectoderm
Mesoderm
Notochord
Endoderm
Yolk sac
a.
Neural groove
Neural fold
Ectoderm
Notochord
Mesoderm
Endoderm
b.
Neural tube
Ectoderm
Neural crest
Mesoderm
Endoderm
Somite
41
c.
Generation of Somites
Mesoderm sheets on either side of notochord
separate into rounded regions called
somitomeres
-These separate into segmented blocks
called somites
-Form in an anterior-posterior wave with
a regular periodicity
-Ultimately give rise to skeleton,
muscles and connective tissues
42
Generation of Somites
Mesoderm in the head region remains
connected as somitomeres
-Form muscles of the face, jaws and throat
Some body organs develop within a strip of
mesoderm lateral to each row of somites
-Remainder of mesoderm moves out to
surround the endoderm completely
-Mesoderm separates into two layers
-Coelom forms in between
43
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Chordamesoderm
Notochord
Intermediate
mesoderm
Kidney
Gonads
Circulatory
system
Lateral plate
mesoderm
Linings of
body cavities
Extraembryonic
Head
Paraxial
mesoderm
Somite
Cartilage
Skeletal
muscle
Dermis
44
Neural Crest Cells
Neurulation occurs in all chordates
However, in vertebrates it is accompanied by
an additional step
-Just before the neural groove closes to
form the neural tube, its edges pinch off,
forming a small cluster of cells called the
neural crest
-These cells migrate to colonize many
different regions of developing embryo
45
Neural Crest Cells
Neural crest cells migrate in three pathways
-Cranial neural crest cells are anterior cells
that migrate into the head and neck
-Trunk neural crest cells are posterior cells
that migrate in two pathways
-Ventral pathway cells differentiate into
sensory neurons and Schwann cells
-Lateral pathway cells differentiate into
melanocytes of the skin
46
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Epidermis
Neural tube
Posterior
Lateral Pathway
Cells take a
dorsolateral
route between
the epidermis
and somites
Neural
crest cells
Anterior Aorta Notochord
a.
Posterior
somite
Anterior somite
Ventral Pathway
Cells travel
ventrally
through the
anterior half
of each somite
Ventral Pathway Cell Fates Lateral Pathway Cell Fates
Dorsal root
ganglia
Ventral root
Schwann
cells
Melanocytes
Sympathetic
ganglia
Adrenal
medulla
b.
47
Neural Crest Cells
A mutation in a gene that promotes survival of
neural crest cells produces white spotting
on ventral surfaces of human babies & mice
48
Neural Crest Cells
Many of the unique vertebrate adaptations
that contribute to their varied ecological
roles involve neural crest derivatives
-For example gill chambers provided a
greatly improved means of gas exchange
-Allowed transition from filter feeding to
active predation (higher metabolic rate)
-Other changes = Better prey detection, and
rapid response to sensory information
49
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Chordates
Vertebrates
Zygote
Pharynx
Lining of
respiratory
tract
Lining of
digestive
tract
Endoderm
Blastula
Gastrula
Ectoderm
Neural
crest
Gill arches,
sensory ganglia,
Schwann cells,
adrenal medulla
Liver
Mesoderm
Outer covering
of internal
organs
Lining of
thoracic and
abdominal
cavities
Dorsal
nerve cord
Epidermis, skin, hair,
epithelium, inner
ear, lens of eye
Major
glands
Pancreas
Brain,
spinal cord,
spinal nerves
Notochord
Circulatory
system
Integuments
Blood
Heart
Vessels
Somites
Gonads
Kidney
Dermis
Skeleton
Striated
muscles
50
Vertebrate Axis Formation
Hans Spemann & Hilde Mangold transplanted
cells of the dorsal lip of one embryo into the
future belly region of another
-Some of the embryos developed two
notochords: a normal dorsal one, and a
second one along the belly
-Moreover, a complete set of dorsal axial
structures formed at the ventral
transplantation site in most embryos
51
Vertebrate Axis Formation
Donor embryo
Recipient embryo
Primary
neural fold
Primary notochord, somites,
and neural development
Dorsal lip
Secondary
neural fold
Secondary
notochord, somites,
and neural
development
Primary embryo
Secondary embryo
52
Organizers
An organizer is a cluster of cells that release
diffusible signal molecules, which convey
positional information to other cells
-The closer a cell is to an organizer, the
higher the concentration of the signal
molecule (morphogen) it experiences
-Different morphogen concentrations
stimulate development of different organs
53
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Organ A
Concentration
of morphogen
Organizer cells
secreting morphogen
Organ B
Organ C
Distance from secretion site
Embryo
Decreasing
morphogen
concentration
gradient
54
Organizers
Creation of the Spemann organizer
-In frogs, as in fruit flies, the process starts
during oogenesis in the mother
-Maternally-encoded dorsal determinants
are localized at the vegetal pole of the
unfertilized egg
-At fertilization, rearrangements in the
cytoplasm cause this determinant to shift
to the future dorsal side of the egg
55
Animal pole
Pigmented
cortical cytoplasm
Microtubule
array
Inner
cytoplasm
Microtubules
Diffuse black
pigment
Clear cortical
cytoplasm
Dorsal determinants
Vegetal pole
a.
Point of
sperm entry
Gray crescent
Shifted dorsal
determinants
b.
Organizer
Dorsal mesoderminducing signal
Mesoderminducing signals
(TGF-b family proteins)
c.
Nieuwkoop center
56
Organizers
The maternally-encoded dorsal determinants
are mRNAs for proteins that function in the
intracellular Wnt signaling pathway
-Wnt genes encode a large family of cellsignaling proteins
-Affect the development of a number of
structures in both vertebrates and
invertebrates
57
Organizers
Function of the Spemann organizer
-Dorsal lip cells do not directly activate
dorsal development
-Instead, dorsal mesoderm development
is a result of the inhibition of ventral
development
58
Organizers
The bone morphogenetic protein 4 (BMP4)
is expressed in all marginal zone cells (the
prospective mesoderm) of a frog embryo
-BMP4 is a morphogen that at high levels
specifies ventral mesoderm cell fates
The Spemann organizer functions by
secreting BMP4 antagonists
-Bind to BMP4 and prevents its binding to
its receptor
59
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Animal pole
Mesoderm
Epidermal ectoderm
Neural ectoderm
Endoderm
Ventral
Dorsal
Organizer molecules:
Chordin, Noggin, and others
60
Vegetal pole
Organizers
Evidence indicates that organizers are
present in all vertebrates
-In chicks, a group of cells anterior to the
primitive streak called Hensen’s node
functions like the Spemann organizer
-Secrete molecules that inhibit ventral
development
-Same as those in frog embryos
61
Induction
Primary induction occurs between the three
primary germ layers
-Example: Differentiation of the central
nervous system during neurulation
Secondary induction occurs between
tissues that have already been specified to
develop along a particular pathway
-Example: Development of the lens of the
vertebrate eye
62
Induction
Wall of forebrain
Ectoderm
Optic cup
Lens
vesicle
Neural
cavity
Optic stalk
Lens
invagination
Lens
Optic nerve
Lens
Sensory
layer
Pigment
layer
63
Retina
Human Development
Human development from fertilization to birth
takes an average of 266 days, or about 9
months
-This time is commonly divided into three
periods called trimesters
64
First Trimester
First month
-The zygote undergoes its first cleavage
about 30 hr after fertilization
-By the time the embryo reaches the
uterus, 6-7 days after fertilization, it has
differentiated into a blastocyst
-Trophoblast cells digest their way
into the endometrium in the
process known as implantation
65
First Trimester
First month
-During the second week, the developing
chorion and mother’s endometrium engage
to form the placenta
-Mom and baby’s blood come into close
proximity, but do not mix
-Gases are exchanged, however
66
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Chorion
Amnion
Yolk sac
Umbilical
cord
Chorionic
frondosum
(fetal)
Decidua
basalis
(maternal)
Placenta
Umbilical artery
Umbilical vein
Uterine wall
a.
67
First Trimester
First month
-One hormone released by the placenta is
human chorionic gonadotropin (hCG)
-Maintains mother’s corpus luteum
-Gastrulation occurs in the second week
-Neurulation occurs in the third week
-Organogenesis begins in the fourth week
-Embryo is 5 mm in length
68
First Trimester
Second month
-Miniature limbs assume adult shape
-Major organs within abdominal cavity
become evident
-Embryo grows to about 25 mm in length
-Weighs about 1 gm, and looks distinctly
human
69
First Trimester
Third month
-The ninth week marks the transition from
embryo to fetus
-Nervous system develops
-Limbs start to move
-Secretion of hCG by the placenta declines,
and so corpus luteum degenerates
-Placenta takes over hormone secretion
70
Increasing Hormone Concentration
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hCG
Estrogen
Progesterone
0
1
2
3
4
5
6
Months of Pregnancy
7
8
9
71
Second Trimester
The basic body plan develops further
-Bones actively enlarge in fourth month
-Rapid fetal heartbeat can be heard by a
stethoscope
By the end of the sixth month, fetus is over
30 cm long, and weighs 600 gm
72
Third Trimester
A period of growth and organ maturation
Weight of the fetus doubles several times
Most of the major nerve tracts in the brain
are formed
-Brain continues to develop and produce
neurons for months after birth
73
Birth
Estrogen stimulates mother’s uterus to
release prostaglandins, and produce
more oxytocin receptors
-Prostaglandins begin uterine contractions
-Sensory feedback from uterus stimulates
oxytocin release from posterior pituitary
-Oxytocin and prostaglandins further
stimulate uterine contractions
74
Birth
Strong contractions, aided by the mother’s
voluntary pushing, expel the fetus
-Now called a newborn baby, or neonate
After birth, continuing uterine contractions
expel the placenta and associated
membranes
-Collectively called the afterbirth
75
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Intestine
Placenta
Umbilical
cord
Wall of
uterus
Cervix
Vagina
76
Nursing
Milk production (lactation) occurs in alveoli
of mammary glands when stimulated by
the anterior pituitary hormone prolactin
-Milk is secreted into alveolar ducts
During pregnancy, progesterone stimulates
development of mammary alveoli
-And estrogen stimulates development of
alveolar ducts
77
Nursing
After birth, anterior pituitary secretes prolactin
-Sensory impulses associated with baby’s
suckling trigger the posterior pituitary to
release oxytocin
-Stimulates contraction of smooth
muscles surrounding alveolar ducts
-Milk is ejected (milk let-down reflex)
The first milk produced after birth, colostrum,
is rich in nutrients & maternal antibodies 78
Postnatal Development
Growth of the infant continues rapidly after
birth
-Babies typically double their birth weight
within 2 months
Different components grow at different rates
-Allometric growth
79
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Infant
Child
Adult
Human
Chimpanzee
Fetus
80