Fertilization
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Structure of Gametes
External vs Internal Fertilization
Sperm Attraction
Sperm Binding to Egg and subsequent fusion
Activation of Egg
Fusion of Nuclear material
What is the purpose of Fertilization?
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Transmit genes from Parent to offspring
Initiate functions in egg cytoplasm that permits
development to occur
Common Elements of Fertilization
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Contact and Recognition between Sperm and Egg
Regulation of sperm entry
Fusion of Genetic Material
Activation of egg metabolism to start development
Sperm Cell
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First observed by
Leeuwenhoek (Sperma,
Semen= seed)
Homunculus
Believed to contain all
information for embryo
Work demonstrated that
Semen required sperm cells
for fertilization of egg
1876, Hertwig and Fol
demonstrate sperm entering
egg and fusion of their
genetic material. Studies in
Sea Urchin
Spermatogenesis
Sperm Structure
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Haploid Nucleus and
Acrosome.
Flagellum- axoneme
“9+2” arrangement
Dynein-Force generator!
Mitochondria
Egg Cell- Oocyte
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All material needed for beginning of growth and
development.
Large cytoplasm
Synthesis and absorption of proteins (yolk)
Other cytoplasmic factors:
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Nutritive proteins
Ribosomes and tRNAs
Accumulated mRNA
Morphogenic factors (transcription and paracrine factors)
Protective elements against UV, give egg bad taste, and yolk
contains antibodies.
Oocyte Structure
Fertilization patterns
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External
Internal
Overview of External: Sea Urchin
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Many Marine animals use this method
Chemotaxis main targeting mechanism.
Species specific sperm attraction and species specific
sperm activation.
Overview of Internal: Mammals
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Difficult to study
Sperm Heterogeneous at first.
Both gametes use biochemical and mechanical means to
reach site of Fertilization(Ampula).
Translocation
Capacitation
External Sperm Attraction to Egg
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Species specific
Resact (14 a.a.)
Regulates Calcium influx
Increases ATP formation from Mitochondria: increase
flagellar motive force.
Internal: Sperm attraction to Egg
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Regionally specific molecules-motility and capacitation
During capacitation, flagellum hyperactive
Directional cues: Heat and Chemotaxis (progesterone).
Capacitation of Mammalian Sperm
External: Recognition/fusion
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Initial binding to Egg Jelly,
Bindin
Species specific
EBR-1 interacts with Bindin.
Acrosome reaction
Once membranes fuse
(Bindin), polymerization of
actin in Egg: Fertilization
cone.
Actin from gametes form a
connection, sperm nucleus
and tail pass into egg cell.
All regions of Egg can
interact with sperm.
Internal: Recognition/fusion
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Sperm binds to
glycoprotein matrix of
Zona Pellucida (ZP).
Second interaction occurs
with ZP via SED1 protein
of sperm
Final interaction between
proteins (ADAM3,
ADAM2) interact with
ZP3. Crosslinking of
galactosyltransferasesCa
influx and acrosome
reaction
Internal Recognition/Fusion
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Interaction slightly
different
Acrosomal fusion:
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proteolytic enzymes
Secondary contacts:
Zonadhesion
Other inner acrosomal
proteins bind to ZP2.
Fusion not clear. Possible
interaction with CD9 of
oocyte.
External: Block of Polyspermy
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Effects of Polyspermy
Fast block
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Membrane potential
switches from negative to
positive
Transient (1 min)
Slow block
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Shortly after fast block
Cortical Granules
Increase in Calcium, from
internal stores.
Fertilization envelope
Slow-block for Polyspermy
Internal: Block of Polyspermy
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No observed fast block
Slow block- similar to Sea Urchin
N-acetylglucosaminidase cleave NAG from ZP3.
Other proteins cleave proteins from cell surface.
External and Internal: Activation of Egg
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Calcium-major signal.
Releases inhibitors from stored
mRNAs-Maskin protein
IP3 created by phospholipase
C, DAG also created
G proteins and Src induce PLC
activity
IP3 causes Ca release from ER
Ca required for Na/H pump
that regulates Egg internal pH.
Mammals similar…multiple
waves of Ca release.
External: Fusion of Genetic material.
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Nucleus/centriole separate
from mitochondria and tail.
Nuclear envelope vesiculates
into small packets. Expose
chromatin
Phosphorylation of nuclear
lamin and histones-starts
early
Pronucleus rotates 180o,
placing centriole between
sperm and egg pronuclei
Zygote nucleus formed.
Internal: Fusion of Genetic material.
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Takes longer in mammals
Sperm DNA bound by
Protamines (disulfide bonds
hold complex together)
Glutathione reduces
protamine-relax chromatin
Ca release in waves.
As pronuclei meet, DNA
condenses into
chromosomes and orient on
common mitotic spindle.
True zygote nucleus: 2 cell
stage.