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Fertilization and Activation
Chapter 8
Principles of Development
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Contact and Recognition
Between Egg and Sperm
Fig. 8.4
Cortical Reaction
• 1) Thousands of enzyme-rich cortical granules
below the egg membrane fuse with the
membrane.
• 2) The cortical granules release contents
between the membrane and vitelline envelope.
• 3) This lifts the envelope and forms a moat.
• 4) One cortical granule enzyme causes the
vitelline envelope to harden, becoming the
fertilization membrane.
Contact and Recognition Between Egg and Sperm
• Marine organisms release enormous numbers of
sperm in the ocean to fertilize eggs.
• Many marine eggs release a chemotactic molecule
to attract sperm of the same species.
• Sea urchin sperm first penetrate the jelly layer
before contacting the vitelline envelope.
• Egg-recognition proteins on the acrosomal process
bind to species-specific sperm receptors on the
vitelline envelope.
• In the marine environment, many species may be
spawning at the same time.
• Similar recognition proteins are found on sperm of
vertebrate species.
Prevention of Polyspermy
• A fertilization cone forms where
the sperm contacts the vitelline
membrane.
• Important changes in the egg
surface block entrance to any
additional sperm.
• In the sea urchin, an electrical
potential rapidly spreads
across the membrane; this is
the “fast block.”
• This is followed by the cortical
reaction.
Fig. 8.6
Fusion of Pronuclei and Egg Activation
• After sperm and egg membranes have fused,
the sperm disconnects from its flagellum.
• The enlarged sperm nucleus is the male
pronucleus and migrates inward to contact the
female pronucleus.
• Fusion forms a diploid nucleus.
– Nuclear fission takes 12 minutes in sea urchins; about
12 hours in mammals.
– The fertilized egg is now properly called a zygote.
• Fertilization initiates reorganization of cytoplasm
and repositions determinants that begin
development and cleavage.
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Cleavage and Early Development
Blastomeres
• The embryo undergoes cleavage to convert the
large cytoplasmic mass into small maneuverable
cells.
• No cell growth occurs, only subdivision until cells
reach regular somatic cell size.
• At the end of cleavage, polychaete worms have
1000 cells, amphioxus has 9000, and frogs have
700,000.
• Polarity—a polar axis—establishes the direction of
cleavage and differentiation.
Types of Cleavage
• Isolecithal yolk describes eggs
with very little yolk and the yolk is
distributed evenly.
• In such eggs, cleavage is
holoblastic.
• The cleavage furrow extends
completely through the egg.
• Isolecithal eggs are widespread
and seen in: echinoderms,
tunicates, cephalochordates,
molluscs and mammals.
• Cleavage is slowed in the yolkrich vegetal pole.
Types of Cleavage
• Telolecithal eggs have
much yolk concentrated at
the vegetal pole.
– Actively dividing cytoplasm is
confined to a narrow discshaped mass on the yolk.
– Cleavage is partial or
meroblastic; the furrow does
not cut through the heavy
yolk.
– Birds, reptiles, most fishes
and a few amphibians have
telolecithal eggs.
Patterns of Cleavage
• The pattern of
cleavage is affected
by
– quantity and
distribution of yolk
present, and
– genes controlling the
symmetry of cleavage.
• There are four
principal types of
cleavage.
Types of Cleavage
• Mesolecithal eggs have a
moderate amount of yolk
concentrated in the vegetal pole.
– The animal pole is opposite the
vegetal pole and contains cytoplasm
and very little yolk.
– These eggs cleave holoblastically, but
cleavage is retarded in yolk-rich
vegetal
– The cleavage furrow progresses much
more slowly through the vegetal pole;
thus cleavage is faster in the animal
region.
– Amphibians have mesolecithal eggs.
Types of Cleavage
• Centrolecithal eggs have a
large mass of centrally located
yolk.
– Cytoplasmic cleavage is limited
to a surface layer of yolk-free
cytoplasm; yolk-rich inner
cytoplasm is uncleaved.
– They have meroblastic cleavage.
– Insects and many other
arthropods have centrolecithal
eggs.
– Yolk is therefore an impediment
to cleavage.
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Amount of Yolk Affects
Developmental Mode
Types of Development
•
•
•
In most animals, a mother does not
directly nourish embryonic development
but has provisioned the egg with yolk.
The amount of yolk is related not only to
cleavage pattern, but also to whether a
larval stage occurs during development.
Animals in which the zygote is telolecithal generally
have direct development.
In direct development is characteristic of animals
where the larval stage is between embryo and
adult.
•
–
–
–
–
•
Cleavage Affected by Different
Inherited Patterns
• Different cleavage patterns are
characteristic of different phylogenetic
lineages.
• Isolecithal eggs demonstrate four major
patterns:
– Radial Cleavage
– Spiral Cleavage
– Bilateral Cleavage
– Rotational Cleavage
Spiral Cleavage
• Spiral cleavage proceeds in a sequence oblique to
the animal-vegetal axis.
• Cells produced pack tightly in the adjacent furrows,
like soap bubbles.
• Spiral cleavage is found in annelids, nemerteans,
turbellarians, all molluscs except cephalopods,
some brachiopods, echiurans and some other
Prostomia.
Metamorphosis is a change from larval to adult body
form.
Mammalian zygotes bypass the larval stage.
A placental attachment to the mother provides ongoing
nourishment.
Direct development can occur when there is enough yolk
to support growth as juveniles; this occurs in reptiles and
birds.
Species with isolecithal or mesolecithal zygotes
generally have indirect development.
Radial Cleavage
• Embryonic cells are arranged in a radial symmetry around
the animal-vegetal axis.
• In sea stars, cleavage begins by two identical daughter cells
cut through the animal vegetal axis.
• The next cleavage runs parallel to the animal vegetal axis
and cuts the blastomeres in half.
• The next cleavage is perpendicular to the animal vegetal axis
and forms two tiers of 4 cells each.
• Amphibian embryos have similar cleavage with slower
furrowing in the yolk region.
• Radial cleavage is characteristic of the Deuterostomia,
including echinoderms, hemichordates and chordates.
Bilateral Cleavage
• Prior to fertilization, the egg is defined by unequal
cytoplasmic components.
• The first cleavage furrow passes through the animal
vegetal axis dividing the asymmetrically divided
cytoplasm between the two blastomeres.
• This first cleavage pattern determines the future right
and left side.
• The half-embryo formed on one side is the mirror image
of the half embryo on the other side.
• Ascidians (tunicates) demonstrate this cleavage pattern.
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Discoidal Cleavage
• Telolecithal eggs divide by discoidal cleavage.
• There is a large mass of yolk in each egg;
cleavage is confined to a small disc of
cytoplasm.
• Early cleavage furrows carve the disc into a
single layer of cells called the blastoderm.
Blastulation
• The blastula is the resulting cluster
of cells regardless of cleavage
pattern.
• In mammals, this is called the
blastocyst.
• Often, cells arrange themselves
around a central fluid-filled
blastocoel.
• At this stage, cell number ranges
from a few hundred to several
thousand.
• The embryo has not increased in
size beyond the size of the zygote,
but each nucleus has a full set of
DNA.
Sea Star Gastrulation
• Gastrulation begins with vegetal area flattening to
form the vegetal plate.
• Invagination is a bending inward of the vegetal
plate one-third into the blastocoel.
• The archenteron is the new cavity formed by this
invagination.
• The archenteron is the primitive gut; the
blastopore is the opening to the outside.
• In deuterostomia, the blastopore becomes the
anus, and the mouth forms secondarily.
• The archenteron elongates toward the animal
pole and expands into two pouch-like coelomic
vesicles
Rotational Cleavage
• The first cleavage plane is aligned with the animal vegetal axis.
• However, in the second cleavage, one blastomere divides
meridionally while the other divides equatorially, rotated 90 degrees
to the first.
• Early divisions may be asynchronous and possess odd numbers of
cells below the 2-4-8-16... series that would occur with synchronous
division.
• After the third division, cells form a tightly packed cluster stabilized
by outer cells with tight junctions, the trophoblast.
• The trophoblast will form the embryonic portion of the placenta.
• Cells that give rise to the embryo are the inner cell mass.
• This type of cleavage is present in mammals and is slower than in
other animal groups.
Gastrulation and the Formation of Germ Layers
• Gastrulation converts the spherical blastula
into a complex structure with three layers.
– Ectoderm covers the embryo.
– Mesoderm and endoderm are on the interior.
– The new positions and cell neighbors establish
the embryonic body plan.
• Patterns of gastrulation vary enormously
depending on the amount of yolk.
– The yolk impedes gastrulation.
– Gastrulation is simple in non-yolky embryos,
complex in yolk-laden eggs.
Germ Layers
• The outer ectoderm will give rise to epithelium and the
nervous system.
• The endoderm gives rise to the epithelial lining of the
digestive tube.
• The mesoderm will form the muscular system, reproductive
system, peritoneum and the sea star’s endoskeleton and
water-vascular system.
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Gastrulation in Prostomia
• The blastopore becomes
the mouth and the anus
forms secondarily.
• The mesoderm forms
differently, arising from the
lip of the blastopore and
proliferating between the
walls of the archenteron
and outer body wall.
• These mesodermal
precursors arise from the
large 4d cell at the 29 to 64
cell stage.
Fig. 8.13
Two versus Three Germ Layers
• Cnidaria and Ctenophora have only two
germ layers (endoderm and ectoderm)
and are diploblastic.
• Other metazoa have three germ layers
and are triploblastic.
Formation of the Coelom
The coelom is a true body cavity that contains the
viscera; it is formed in one of two methods.
– Schizocoelous formation forms the coelom from splitting
of mesodermal bands originating from blastopore region
and growth between ectoderm and endoderm.
– Enterocoelous formation forms the coelom from pouches
of the archenteron.
– Protostomes develop by the schizocoelous method.
– Deuterostomes, except for vertebrates, follow the
enterocoelous plan.
– Vertebrates form a coelom by schizocoelous formation;
this evolved anew to accommodate large stores of yolk.
Development of Systems and Organs
Germ Layers
• Germ layers should not be confused with germ cells
(eggs and sperm).
• Germ layers do not alone determine differentiation but
rather the position of embryonic cells.
Derivatives of Ectoderm:
Nervous System and Nerve Growth
– Just above the notochord, the ectoderm
thickens to form a neural plate.
Edges of the neural plate fold up to create
an elongated, hollow neural tube.
– The anterior end of neural tube enlarges
and forms the brain and cranial nerves.
– The posterior end forms the spinal cord and
spinal motor nerves.
– Neural crest cells pinch off from the neural
tube.
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Nervous System (continued)
Nervous Tissue development
Neural crest cells form many structures.
• In 1907, Ross Harrison
cultured nerve cells;
each axon grows from
one cell.
• Additional research
revealed that a nerve
axon grows in response
to guidance molecules
secreted into its path.
– They become portions of cranial nerves,
pigment cells, cartilage, bone, ganglia of
the autonomic system, medulla of the
adrenal gland, and parts of other
endocrine glands.
– It is unique to vertebrates and was
important in evolution of the vertebrate
head and jaws.
Derivatives of Endoderm:
Digestive Tube and Survival of Gill Arches
• During gastrulation, the archenteron forms as the
primitive gut.
• This endodermal cavity eventually produces the
digestive tract, lining of pharynx and lungs, most of
the liver and pancreas, thyroid and parathyroid
glands and thymus.
• The alimentary canal develops from primitive gut;
ends are lined with ectoderm.
• Lungs, liver and pancreas arise from the foregut.
Fig. 8.28
Endoderm derivatives
During development, endodermally-lined
pharyngeal pouches interact with overlying
ectoderm to form gill arches.
– In fish, gill arches become gills.
• Gill arches remain as necessary primordia
for a variety of other structures in
terrestrial vertebrates.
– The first arch and its endoderm-lined pouch
form the upper and lower jaws.
– Second, third and fourth gill pouches become
the tonsils, parathyroids and thymus.
Derivatives of Mesoderm:
Support, Movement and the Beating Heart
• With an increase in size and complexity,
mesodermally derived structures take up a greater
proportion.
• Muscles arise from mesoderm along each side of
the neural tube.
– The mesoderm divides into a linear series of somites (38
in humans).
– The splitting, fusion and migration of somites produce the:
• axial skeleton,
• dermis of dorsal skin, and
• muscles of the back, body wall and limbs.
• Limbs begin as buds from the side of the body;
projections become fingers and toes.
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Mesoderm gives rise to the embryonic heart
• Guided by underlying endoderm, two
clusters of precardiac mesodermal cells
move to either side of the gut.
• These clusters differentiate into a pair of
double-walled tubes that fuse into a single
thin tube.
• The primitive heart begins beating on the
second day of the 21-day incubation period;
there are no blood or vessels at this time.
• Twitching becomes rhythmical as the
ventricle and atrium develops; each
chamber has a faster intrinsic beat.
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