Transcript Fertilization, Acrosomal reaction

Fertilization - fusion of 2 gametes
ensure encounter of sperm & oocyte/egg
External fertilization
# of gametes
stimulus for release
Internal fertilization
copulatory organs
receptors PH-30 humans
activation of metabolic dormant oocyte/egg
- different stages of oogenesis
fusion of 2 haploid nuclei - syngamy
to produce diploid embryo
The Acrosomal Reaction.
Acrosomal reaction: when exposed to the jelly coat the sperm’s acrosome
discharges it contents by exocytosis.
Hydrolytic enzymes enable the acrosomal process to penetrate the
egg’s jelly coat.
.
B locks to Polyspermy
1. Fast Block - short-lived
Na+ channels in the egg’s plasma membrane opens.
Na+ flows into the egg and the membrane depolarizes
2. The Cortical Reaction - slow, long-lasting
Ca2+ from the eggs ER is released into the cytosol and stimulates cortical granules to
fuse with the plasma membrane and release their contents into the perivitelline space.
The vitelline layer separates from the plasma membrane.
An osmotic gradient draws water into the perivitelline space, swelling it and pushing it
away from the plasma membrane.
The vitelline layer hardens into the fertilization envelope: a component of the slow
block to polyspermy.
The plasma membrane returns to normal and the fast block to polyspermy no longer
functions.
Activation of the Egg,
High concentrations of Ca2+ in the egg stimulates an
increase in the rates of cellular respiration and proteins
synthesis.
The sperm nucleus swells and merges with the egg nucleus
 diploid nucleus of the zygote.
DNA synthesis begins and the first cell division
occurs.
In amphibians a rearrangement of the egg cytoplasm occurs at the
time of fertilization.
The plasma membrane
and cortex rotate
toward the point
of sperm entry.
The gray crescent
is exposed and marks
the dorsal surface
of the embryo.
Fertilization in Mammals.
Capacitation, a function of the female reproductive system
removes inhibitory enzymes on the sperm head which allows the acrosomal
reaction to occur.
A capacitated
sperm migrates
through a layer
of follicle cells
before it reaches
the zona pellucida.
Binding of
the sperm cell
induces an
acrosomal
reaction.
Voltage Clamps and Fast Block to Polyspermy
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unfertilized
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sperm
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60 sec
90 sec
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membrane
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Cleavage follows fertilization.
Functions of cleavage:
Multicellular for differentiation
The zygote is partitioned into blastomeres.
Each blastomere contains different regions of the undivided
cytoplasm and thus different cytoplasmic determinants.
Restores Somatic Nuclear to Cytoplasmic Ratio
1:500
-> ->->
1:6
sea urchin fertilized egg
at end of cleavage - somatic cell
Controls on # of cleavage divisions?
2n = 6 divisions
4n = ?
n=?
first two cleavages are vertical.
The third division is horizontal.
The result is an eight-celled embryo with two tiers of four cells Yolk is most concentrated at the vegetal
pole and least concentrated at the animal pole.
In animals with less yolk there is complete division of the egg: holoblastic cleavage.
Holoblastic equal
Holoblastic unequal where there
is more yolk at the vegetal pole.
Cleavage occurs more
rapidly in the animal
pole than in the
vegetal pole.
A blastocoel forms within the morula  blastula
In birds the yolk is so plentiful that it restricts cleavage to the
animal pole: meroblastic cleavage.
• Superficial cleavage in insects
• Holoblastic equal in mammals
What Controls Cleavage?
Information in Cytoplasm – Source?
Spiral Cleavage in Snails
Rt = Dextral (dominant)
Left= recessive
Female dd X male DD
F1 genotype vs. phenotype
F1 cross
dD X dD
Prediction? Genotype
Phenotype
Gastrulation rearranges the
blastula to form a three-layered
embryo with a primitive gut
 Gastrulation rearranges the embryo into a
triploblastic gastrula.
– The embryonic germ layers are the ectoderm,
mesoderm, and endoderm.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fate map – vital stain
Invagination
Involution
Epiboly
Avian Development.
Cleavage is meroblastic, or incomplete.
Cell division is restricted to a small cap of cytoplasm at the animal pole.
Produces a blastodisc, which becomes arranged into the epiblast and
hypoblast that
bound the
blastocoel, the
avian version
of a blastula.
During gastrulation some cells of the epiblast migrate (arrows) towards the
interior of the embryo through the primitive streak
Ingression
(immigration)
Once again, the embryonic membranes – homologous
with those of shelled eggs.
Chorion: completely surrounds the embryo and
other embryonic membranes.
Amnion: encloses the embryo in a fluid-filled
amniotic cavity.
Yolk sac: found below the developing embryo.
Develops from the hypoblast.
Site of early formation of blood cells
which later migrate to the embryo.
Allantois: develops as an outpocketing of the
embryo’s rudimentary gut.
Incorporated into the umbilical cord, where it
forms blood vessels.
Activation of embryonic genome
Mexican axolotl o-mutant strain
The “o” gene is a recessive gene
“O” gene is the normal, dominant gene
In embryos obtained from female axolotls homozygous for
gene “o”, development is always arrested during gastrulation.
WHY?
“O” protein is necessary to activate the embryonic genome.
PROOF?
The eggs can be rescued by injecting eggs of o/o
females with cytoplasm from normal eggs.
Where is the "corrective component" (O protein)
produced?
Rescue Experiments
Cytoplasm from Egg
vs.
Primary Oocytes Cytoplasm
vs.
Primary Oocyte fluid from germinal vesicle
What is the “O” protein doing?
activation of the embryonic genome.
How can we prove it?
When is mammalian embryonic genome activated?
Two-cell stage determined by radioactive labeling experiments
Imprinting of mammalian gamete’s nucleus
Experiments combining:
Female pronucleus combined with Female pronucleus
Male pronucleus combined with Male pronucleus