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Animal Develepment
Chapter 53
Sila and Kharee
Fertilization
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Fertilization, the union of male and female gametes, is
the first step in reproduction.
Sperm must penetrate to the plasma membrane
Saclike organelle named the acrosome is positioned
between the plasma membrane and nucleus of sperm,
digestive enzymes within create a hole in eggs
membrane
The egg is protected by coats
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zona pellucide (mammals)
Jelly layer (urchin, frogs)
Chorion (insects)
Membrane Fusion Activates the Egg
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The egg remain dormant until the sperm activates it
Polyspermy: response to sperm fusion of additional
sperm
Sperm penetration of egg can also cause:
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Many eggs did not complete meiosis, therefore not a haploid
yet, sperm penetration activates
Triggers movement of egg cytoplasm
Sharp increase of protein synthesis and metabolic activity in
general
The Fusion of Nuclei
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Final Stage of Fertilization
The haploid male nuclei and the haploid female
form the diploid zygote
Two nuclei migrate toward each other along a
aster
Clevage and Blastula Stage
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Cleavage: The rapid division of the zygote into
a larger and larger number of smaller cells.
Blastomere: Each individual cell
Animal Pole and Vegetal Pole
Blastula
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The blastula is a hollow mass of cells
Outermost blastomeres join together through
protein belt which create a seal that isolates the
interior cell mass
Cleavage patterns are diverse in every animal cell
Cleavage Patterns
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Eggs with moderate or little yolk go through
Hoboblastic Cleavage (Complete)
Echinoderms
 Annelids, Mollusks, and Flatworms
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Eggs with Dense Yolk go through Meroblastic
Cleavage (incomplete)
Fish, Reptiles, Birds
 Insects
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Cleavage in Mammals
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Contain little yolk
Holoblastic, forms a structure called a blastocyst
A single layer of cells surrounds a blastocoel
Inner cell mass (ICM) located at one pole of of
blastocoel
Trophoblast, the outer cells, part of them enter
the maternal uterus lining and forms placenta
Gastrulation
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Gastrulation: Cells of blastula rearrange
themselves to form the body plan
Forms the three germ bilayers
Converts the blastula into a bilaterally
symmetrical embryo with a gut, visible anterior
and posterior, and dorsal ventral axis.
How the Cells Move
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Lamellipdoa: crawl over neighbor cells
Filopodia: feel out the surfaces of other cells and
pull the cell forward
Tightly attached cells move in sheets
Gastrulation in Chordates
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The movement of surface cells into the interior,
by folding as a sheet ("invagination“)
The cells of vegetal plate move inward to the
blastocoel cavity
The cells produce a tube (primitive gut) called
archenteron and the opening (anus)
Gastrulation in Amphibians
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By rolling as a sheet ("involution")
Cells from animal pole move to vegetal pole
When on the dorsal lip, cells involute into
interior
Movement of the cells forms a new internal
cavity
Organogenesis then begins (explained later)
Amphibian Gastrulation
Gastrulation in Reptiles /Birds/
and Mammals
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The Gastrulation of all are similar by
internalization/separation of cells from an
epithelium ("ingression "),
The embryo develops from flattened collection
of cells forming two layers (endoderm and
ectoderm) lower cells migrate out to line cavity
to form yolk (ectoderm) mesoderm cells migrate
to the interior
Three Germ Layers
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Endoderm: formed by cells that move into embryo to
form the tube of a primitive gut
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Ectoderm: formed by the cells that remain on the
exterior
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Epidermis of skin, nervous system, sense organs
Skeleton, muscles, blood vessels, heart, blood, gonads, kidneys,
dermis of skin
Mesoderm: formed by the cells that move into the
space between endoderm and ectoderm
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Lining of digestive and respiratory tracts, liver, pancreas, thymus,
thyroid