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Chapter
47
Animal Development
Embryonic development/fertilization
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Preformation: until 18th century; miniature
infant in sperm or egg
Epigenesis: proposed by Aristotle; animal
emerges/develops from an unformed egg
Fertilization in a Sea Urchin
At fertilization/conception in a sea urchin:
1. Contact: sperm contacts egg’s jelly coat
2. Acrosomal reaction: hydrolytic enzymes in acrosome
make a hole in the jelly; actin filaments lengthen
from the sperm head - the acrosomal process
3. Growth of acrosomal process: process attaches to
receptors on the vitelline layer; enzymes digest a hole
into the layer
4. Fusion of membranes of sperm and egg
5. Fast block to polyspermy: membrane depolarization
prevents multiple fertilizations; causes the cortical
reaction
6. Cortical reaction: release of Ca2+ causes the cortical
granules to release enzymes that separates the
vitelline from the plasma membrane. The vitelline
layer becomes the fertilization envelope, further
preventing more sperm from entering the fertilized
egg.
Slow block to polyspermy: the fertilization envelope
Egg activation: the rise in Ca2+ increases metabolic activity and
protein synthesis
Fertilization in mammals
Fertilization in Mammals:
1. Sperm migrates past follicle cells and binds to
receptors in the zona pellucida of the egg.
2. The acrosomal reaction: release of hydrolytic
enzymes that break through the zona.
3. Sperm reaches the plasma membrane and bind
to receptors on the egg membrane.
4. The membranes fuse and sperm contents
spill into the egg.
5. The cortical reaction: harden the zona
pelucida (block of polyspermy).
The Fertilized Egg & Cleavage
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Cleavage: succession of rapid cell
divisions.
Blastomeres: resultant cells
of cleavage by mitosis
 No G1 or G2 phase in
mitosis
 The blastomeres will
contain cytoplasm from
the original, large cell
 Therefore, the blastomeres will
end up with different cytoplasmic
contents. This is very important as it
sets the stage for major developmental
events.
Most animals have eggs and zygotes with
definite polarity:
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Yolk: nutrients stored in
the egg at the
vegetal pole.
The lowest concentration
of yolk is at the
animal pole.
The animal pole in
many animals becomes
the anterior (head)
part of the body.
In frog eggs, the animal
pole is dark grey in color
due to melanin granules
the yolk is yellow.
At fertilization, the grey
cortex slides over to
reveal a non-pigmented
region called the “grey
crescent,” which will become the dorsal side of the embryo.
Cleavage in a frog embryo
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1 cell  2 cell  4 cell  8 cell 
Morula: 16-64 cell stage; solid ball of cells
Blastula: at least 128 cells; hollow ball stage
of development
Blastocoel: fluid-filled cavity in
late morula and blastula
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Cleavage
Meroblastic cleavage: in eggs of birds, reptiles,
fishes, and insects; yolk-rich egg with most cell
division taking place in the small disc at the animal
pole of egg.
Holoblastic cleavage: in eggs of urchins and
frogs where there is little yolk.
Gastrulation in sea urchins
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Gastrulation:
rearrangement of cellls
of the blastula to form a
triploblastic gastrula
1. Mesenchyme cells migrate
into the blastocoel
2. Vegetal pole invaginates
(blastopore becomes ?)
3. Endoderm  Archenteron
 Digestive tube
4. Archenteron fuses with
the ectoderm
5. Gastrulation is complete
Gastrulation in frogs
1. Blastocoel off-center;
wall is more than
one-cell thick
2. Dorsal lip of the
blastopore forms
on side of blastula.
Cells of endo/mesoderm
move inward (“involution”).
Ecotoderm spreads over
the surface of embryo.
3. The three germ layers
continue to move inward,
filling in the space of
the blastocoel.
The lip becomes circular.
4. Blastopore filled = yolk plug
Organogenesis: formation of organs
Germ Layer
Organs and Tissues in
the Adult
Ectoderm
Skin, glands, nails, epithelial lining of
mouth and rectum, sense receptors in
epidermis, cornea and lens of eye,
nervous system, adrenal medulla, tooth
enamel, epithelium of pineal and pituitary
gland
Endoderm
Epithelial lining of digestive tract (except
mouth and rectum), epithelial lining of
respiratory system; liver, pancreas,
thyroid, parathyroids, thymus; lining of
urethra, urinary bladder, and reproductive
system.
Mesoderm
Notochord, skeletal system, muscular
system, circulatory and lymphatic system,
excretory system, reproductive system
(except germ cells); dermis of skin, lining
of body cavity, adrenal cortex.
Organogenesis starts with folds,
splits, and clustering of cells
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The organs that first develop in all frog and
chordate embryos are the
– Neural tube: forms from the ectoderm
just above the archenteron; becomes the
brain and spinal chord
– Notochord: forms from the dorsal
mesoderm
– Somites: forms from the mesoderm just
lateral to the notochord; arranged on both
sides along the notochord; becomes the
vertebrae of backbone and skeletal
muscles.
Amniote Development
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Two solutions to reproducing on land:
1. Shelled egg (amniotic egg)
Amniotes
2. Uterus of placental animals
 Both provide a watery/fluid environment
that surrounds the embryo
 The fluid and embryo are surrounded by a
membrane called the amnion.
Avian Development
1. Meroblastic
“incomplete” cleavage:
cleavage atop a large
yolk mass. A blastodisc
with two layers (epiblast
and hypoblast) is formed.
2. Gastrulation: Some cells
of the epiblast migrate
into the interior by the
primitive streak. Some
cells move laterally to form
the mesoderm, and others
move downward to form
the endoderm.
3. Early organogenesis:
Archenteron forms when
endoderm pinches
upward. The embryo
will remain attached to
the yolk by the yolk stalk
which formed from the
hypoblast.
The neural tube, somites,
and notochord form the
same way as in the frog.
Extraembryonic membranes in a chick
1. Yolk sac: membrane over
the yolk; blood vessels in
sac will deliver nutrients to
embryo.
2. Amnion: membrane
around the embryo.
3. Chorion: cushions the
embryo against
mechanical shock.
4. Allantois: disposal sac for
uric acid. It will expand,
pushing the chorion
against the vitelline
membrane. It will also
serve as a respiratory
organ for the embryo,
delivering oxygen to the
embryo.
Mammalian Development
1. 7 Days: Blastocyst
containing inner mass of
cells reaches uterus. The
outer layer of cells is
called the trophoblast.
It will form the fetal
portion of the placenta.
2. Trophoblast secretes
enzymes that help it
penetrate the endometrium.
Trophoblast thickens and
produces fingerlike
projections into the
maternal tissue.
-Epiblast  embryo
-Hypoblast  yolk sac
Mammalian Development
3. Extraembryonic membranes
develop:
-Trophoblast  Chorion
-Epiblast  Amnion and
placenta
4. Gastrulation: cells from the
epiblast move inward
through the primitive streak
(as seen in the chick).
The three germ layers are
formed and are surrounded
by mesodermal extraembryonic membranes.
Late in the second week of human gestation,
the embryo has two cell layers, an
epiblast and a hypoblast.
The following website has some images of human embryo development.
http://www.med.unc.edu/embryo_images/unit-welcome/welcome_htms/contents.htm
Epiblast cells invaginate at the primitive streak. They
will form the mesoderm cells.
The Cellular and Molecular Basis for
Morphogenesis
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Reorganization of the cytoskeleton will change
a cell’s shape. Example: Formation of the
neural tube is do to microtubule elongation
and microfilament contraction.
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Movement of cells via
lamellipodia (flat sheets
of cells moving) or filopodia
(spikes of cells moving).
Example: Gastrulation –
invagination of cells by
filopodia.
Convergent extension is when cells merge together
to become narrower (convergent) and longer
(extension).
-Example: Archenteron elongation
-ECM fibers may direct cell movement
-Cell adhesion molecules (CAMs): located
on cell surfaces bind to CAMs on other cells.
-Cadherins: cell-to-cell adhesion molecules;
require Calcium to function; very important
in the formation of blastula.
The Fate of Cells Depend on:
1.Cytoplasmic determinants
2.Cell-cell induction
- chemical signals
- cell-surface interactions affect gene
expression
Fatemaps
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Fatemaps: cell lineage is determined
-Embryologist W. Vogt found that you can
determine which parts of the embryo will be
derived from regions of the blastula.
Cytoplasmic Determinants
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Location melanin and yolk in frog eggs
determine the animal and vegetal poles, which
in turn determine the anterior-posterior axis.
Entry of sperm into
egg triggers formation
of grey crescent, which
determines the dorsalventral axis.
Axis of cleavage:
-1st cleavage in frog
embryo is equal.
-Uneven cleavage may
produce abnormal
embryo.
Induction: cells influence on
another cell’s fate
Hans Spemann and
Hilde Mangold (1920s)
discovered that the
dorsal lip of the
blastopore signals a
series of inductions
that result in the
formation of the
neural tube and other
organs.
 Dorsal lip =
primary organizer
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Pattern Formation in Vertebrate Limbs
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Pattern formation: the development of an
animal’s spatial organization; the arrangement
of organs and tissue.
Positional information: molecules that tell
cells its own location and what it will become.
Pattern formation of limbs determined by
two areas:
1. AER (apical ectodermal ridge): a limb-bud
organizer; made up of mesoderm and very
thick layer of ectoderm; it is responsible
the distal-proximal (outward) growth of
limbs).
-AER release FGF (fibroblast growth
factors), which are protein signals for
distal-proximal limb growth.
2. ZPA (zone of polarizing activity): located at
the posterior side of limb bud. It is
necessary for the posterior-anterior growth
of the limb.
- ZPA secretes a protein, sonic hedghog.
Fertilization in a Sea Urchin
Fertilization in mammals
Blastula
Gastrula
Ectoderm
Endoderm
Embryonic gut
Epidermis
associated with
structure (nails,
hair, skin, etc.)
Brain and
nervous sys.
Mesoderm
Inner lining
of digestive
tract
Glands
(including
pancreas/liver)
Notochord
Mesenchyme
(migratory cells)
Somites
Muscle
Excretory
System
Dermis
(inner layer)
Circulatory
System
Inner lining
of respiratory
tract
Bones &
Cartilage
Outer covering
of organs
Gonads