Transcript Animal Development

Animal
Development
Chapter 47
The Miracle of Life
Human embryo
Fertilization
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Function to bring the haploid nuclei of sperm and egg
together to form a diploid zygote
Contact of the sperm with the egg’s surface
 Initiates metabolic reactions within the egg that
trigger the onset of embryonic development
Studied in Sea Urchins
Very few minor differences in mammlian system
Made up of three reactions
 Acrosomal
 Cortical
 Activation of the egg
The Acrosomal Reaction
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The acrosomal reaction
Is triggered when the sperm meets the
egg
 Releases hydrolytic enzymes that digest
material surrounding the egg
 Sperm membrane adhere to receptor
proteins on the egg plasma membrane
 Egg and sperm fuse with in seconds
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Fusion of egg and sperm
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Causes ion channels to open and sodium
ions to flow into the egg
Depolarizes the egg cell membrane
Prevents other sperm from entering
sets up a fast block to polyspermy
Polyspermy  multiple sperm entering
the egg resulting in an aberrant # of
Chromosmes
The Acrosomal and Cortical
Reactions
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1 Contact. The
sperm cell
contacts the
egg’s jelly coat,
triggering
exocytosis from the
sperm’s acrosome.
2 Acrosomal reaction. Hydrolytic
enzymes released from the
acrosome make a hole in the
jelly coat, while growing actin
filaments form the acrosomal
process. This structure protrudes
from the sperm head and
penetrates the jelly coat, binding
to receptors in the egg cell
membrane that extend through
the vitelline layer.
3 Contact and fusion of sperm
and egg membranes. A hole
is made in the vitelline layer,
allowing contact and fusion of
the gamete plasma membranes.
The membrane becomes
depolarized, resulting in the
fast block to polyspermy.
4 Entry of
sperm nucleus.
The acrosomal reaction
Sperm plasma
membrane
5 Cortical reaction. Fusion of the
gamete membranes triggers an
increase of Ca2+ in the egg’s
cytosol, causing cortical granules
in the egg to fuse with the plasma
membrane and discharge their
contents. This leads to swelling of the
perivitelline space, hardening of the
vitelline layer, and clipping of
sperm-binding receptors. The resulting
fertilization envelope is the slow block
to polyspermy.
Sperm
nucleus
Acrosomal
process
Basal body
(centriole)
Fertilization
envelope
Sperm
head
Actin
Acrosome
Jelly coat
Sperm-binding
receptors
Figure 47.3
Fused plasma
Cortical membranes
granule
Perivitelline
Hydrolytic enzymes
space
Cortical granule
membrane
Vitelline layer
Egg plasma
membrane
EGG CYTOPLASM
The Cortical Reaction
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Fusion of egg and sperm plasma membranes triggers a
signal-transduction pathway.
Ca2+ from the eggs ER is released into the cytosol and
propagates as a wave across the fertilized egg  IP3
and DAG are produced.
 IP3 opens ligand-gated channels in the ER and the
Ca2+ released stimulates the opening of other
channels
High concentrations of Ca2+ cause cortical granules to
fuse with the plasma membrane and release their
contents into the perivitelline space.
The vitelline layer hardens into the fertilization
envelope: a component of the slow block to
polyspermy.
The plasma membrane returns to normal and the fast
block to polyspermy no longer functions.
Activation of the Egg
High concentrations of Ca2+ in the egg
stimulates an increase in the rates of
cellular respiration and proteins
synthesis.
 In the meantime, back at the sperm
nucleus...
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 The
sperm nucleus swells and merges with
the egg nucleus  diploid nucleus of the
zygote.
 DNA synthesis begins and the first cell
division occurs.
Fertilization in Mammals
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Capacitation, a function of
the female reproductive
system, enhances sperm
function.
A capacitated
sperm migrates through a
layer of follicle cells before
it reaches the zona
pellucida.
The envelopes of both the
egg and sperm nuclei
disperse.
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The chromosomes from the
two gametes share a common
spindle apparatus during the
first mitotic division of the
zygote
Building the Body of the Animal
Cleavage
 Gastrulation
 Organogenesis
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Cleavage
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Cleavage follows fertilization.
The zygote is partitioned into blastomeres.
Each blastomere contains different regions of
the undivided cytoplasm and thus different
cytoplasmic determinants.
Gastrulation
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Rearranges the cells of a blastula into a threelayered embryo, called a gastrula, that has a primitive
gut
The three layers produced by gastrulation
 Are
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called embryonic germ layers
The ectoderm
 Forms
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The endoderm
 Lines
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the outer layer of the gastrula
the embryonic digestive tract
The mesoderm
 Partly
fills the space between the endoderm and
ectoderm
ECTODERM
• Epidermis of skin and its
derivatives (including sweat
glands, hair follicles)
• Epithelial lining of mouth
and rectum
• Sense receptors in
epidermis
• Cornea and lens of eye
• Nervous system
• Adrenal medulla
• Tooth enamel
• Epithelium or pineal and
pituitary glands
Figure 47.16
MESODERM
• Notochord
• Skeletal system
• Muscular system
• Muscular layer of
stomach, intestine, etc.
• Excretory system
• Circulatory and lymphatic
systems
• Reproductive system
(except germ cells)
• Dermis of skin
• Lining of body cavity
• Adrenal cortex
ENDODERM
• Epithelial lining of
digestive tract
• Epithelial lining of
respiratory system
• Lining of urethra, urinary
bladder, and reproductive
system
• Liver
• Pancreas
• Thymus
• Thyroid and parathyroid
glands
Organogenesis
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Various regions of the three embryonic
germ layers develop into the
rudiments of organs during the process
of organogenesis
Notochord Stiff dorsal rod
characteristic of all chordates
develops from the mesoderm
Neural Plate forms from the
ectoderm. Curves inwardrolling into the
Neural Tube Becomes the CNS
Birds & Reptiles Amniotes
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The four extraembryonic membranes are the yolk
sac, amnion, chorion, and allantois.
Cells of the yolk sac digest yolk providing
nutrients to the embryo.
The amnion encloses the embryo in a fluidfilled amniotic sac which protects the embryo
from drying out.
The chorion cushions the embryo against
mechanical shocks.
The allantois functions as a disposal sac for
uric acid.
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The four extraembryonic membranes are the
yolk sac, amnion, chorion, and allantois.
 Cells of the yolk sac digest yolk providing
nutrients to the embryo.
 The amnion encloses the embryo in a fluidfilled amniotic sac which protects the
embryo from drying out.
 The chorion cushions the embryo against
mechanical shocks.
 The allantois functions as a disposal sac for
uric acid.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 47.14
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Mammalian Development
Blastocyst mammalian version of the
baastula
 Inner cell mass develops into the
embryo embryonic stem cells
 Trophoblast fetal portion of the
placenta
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