Development determined by zygote genome
& distribution of maternal substance
(cytoplasmic determinants) in egg
 Location of cells affects early development
 Selective gene expression leads to cell
differentiation, the specialization of cells in
structure & function
 Morphogenesis produces body shape
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“After fertilization, embryonic development
proceeds through cleavage, gastrulation,
& organogenesis”
 “Morphogenesis in animals involves specific
changes in cell shape, position, &
adhesion”
 “The development fate of cells depends on
their history & on inductive signals”
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Activates egg & brings together nuclei of sperm
& egg, triggering embryonic development
Combines haploid sets of chromosomes
Acrosomal reaction releases hydrolytic enzymes
that digest material around egg
Gamete contact/fusion depolarizes egg cell
membrane & sets up fast block to polyspermy
Membrane fusion initiates cortical reaction,
where rise in calcium ions stimulates cortical
granules to release contents outside egg
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Forms fertilization envelope that functions as slow
block to polyspermy
Rapid cell divisions where embryo becomes
partitioned into many small cells (blastomeres)
 Creates multicellular ball (blastula), which
contains fluid-filled cavity (blastocoel)
 Meroblastic cleavage- incomplete division of
egg in animals w/ yolk-rich eggs (i.e. birds &
reptiles)
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› Produces cap of cells (blastoderm) on top of large,
undivided yolk
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Holoblastic cleavage- complete division of
eggs w/ little amounts of yolk (i.e. mammals,
frogs, sea urchins)
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Changes in cell motility, shape, &
adhesion, resulting in 3-layered embryo
(gastrula)
› 3 embryonic germ layers: outer ectoderm,
middle mesoderm, & inner endoderm lining
embryonic digestive tract
› Has digestive cavity (archenteron)
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Organs develop from
embryonic germ layers
› Vertebrates: formation of
notochord from
condensation of dorsal
mesoderm, neural tube from
folding of ectodermal neural
plate (will develop central
nervous system), & coelom
from splitting of lateral
mesoderm
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Amniotes (i.e. reptiles & mammals) create
aqueous environment necessary for embryonic
development
› Develop in fluid-filled sac contained in shell or uterus
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Germ layers give rise to 4 extraembryonic
membranes: amnion, chorion, yolk sac, &
allantois
Fertilization occurs in oviduct, & embryonic
development begins on way to uterus
 Eggs small & store few nutrients
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› Cleavage is holoblastic & show no obvious polarity
Gastrulation & organogenesis similar to birds & reptiles
 Blastocyst implants uterus after fertilization & cleavage in
oviduct
 Trophoblast initiates formation of fetal portion of placenta
& embryo develops from single layer of cell (epiblast) in
blastocyst
 4 extraembryonic membranes homologous to birds &
reptiles
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Changes in cell shape & position involve
reorganization of cytoskeleton
 Convergent extension- cells of tissue layer
crawl between each other, causing sheet
of cells to become narrower & longer
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Fibers of ECM provide anchorage for
crawling cells & tracts that direct
movement of migrating cells
› Cell adhesion molecules help regulate
movement & tissue building by holding
cells together
 Cadherins are important adhesion molecules
Developed by Vogt, fate maps of embryos
have shown that specific regions of zygote
or blastula develop into specific parts of
older embryos
 “Founder cells” identified that will generate
specific tissues
 Cell’s developmental potential restricts as
development proceeds
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In nonamniotic vertebrates, major axes
established in egg or at fertilization
› Unevenly distributed cytoplasmic determinants in egg
important in formation of body axes & differences of
blastomeres
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In amniotes, polarity not established until later
› Environment differences play influence initial
differences between cells & later body axes
› As embryonic development continues, potency of
cells becomes limited
Cells influence development of other cells w/
signals that switch on or off sets of genes
 Cells & offspring differ based on positional infomolecular cues indicating position
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› Info in form of signal molecules in “organizer” regions
of embryo
 (i.e.) Dorsal lip of blastopore in amphibian gastrula (shown
above), AER & ZPA of vertebrate limb bud
 Influence gene expression in cells that receive them,
leading to differentiation