Transcript Chap 47 - Mid-Pacific Institute

Development
Chemotaxis
• Sea Urchin’s eggs have a chemotatic
molecule called resact.
• This molecule is found in the outer jelly
coat of the egg and attract sperm.
• Sperm have receptors on their surface that
will bind to resact and trigger the sperm tail
to drive in the sperm.
• Fertilization in Mammals.
• Capacitation, a function of the female reproductive
system, enhances sperm function.
– 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 similar
to that seen in the
sea urchin.
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Cortical reaction
forms slow
block
Fig. 47.5
• The Acrosomal Reaction.
– Acrosomal reaction: when exposed to the jelly
coat the sperm’s acrosome discharges
• Enzymes enable the acrosomal process to penetrate the
egg’s jelly coat.
• The tip of the acrosomal process adheres
to the vitelline layer just external to the
egg’s plasma membrane.
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– The sperm and egg plasma membranes fuse and a single
sperm nucleus enter the egg’s cytoplasm.
• Na+ channels in the egg’s plasma membrane opens.
– Na+ flows into the egg and the membrane depolarizes: fast block to
polyspermy.
Na+
Bindin in the
acrosomal process
will bind to the egg’s
protein recptors
ensuring species
recognition
• The Cortical Reaction.
– Fusion of egg and sperm plasma membranes triggers
reaction
• Ca2+ from the eggs ER is released into the cytosol and propagates as
a wave across the fertilized egg – cortical granules release contents
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The Cortical Reaction
• Fusion of egg and sperm also initiates the
cortical reaction
– Inducing a rise in Ca2+ that stimulates cortical
granules to release their contents outside the egg
EXPERIMENT
A fluorescent dye that glows when it binds free Ca2+ was injected into unfertilized sea urchin eggs. After sea urchin
sperm were added, researchers observed the eggs in a fluorescence microscope.
500 m
RESULTS
10 sec after
fertilization
1 sec before
fertilization
Point of
sperm
entry
Figure 47.4
20 sec
30 sec
Spreading wave
of calcium ions
CONCLUSION
The release of Ca2+ from the endoplasmic reticulum into the cytosol at the site of sperm entry triggers the release
of more and more Ca2+ in a wave that spreads to the other side of the cell. The entire process takes about 30 seconds.
• Activation of the Egg,
– High concentrations of Ca2+ in the egg stimulates an
increase in the rates of cellular respiration and
proteins synthesis.
– In sea urchins, DAG activates a protein that
transports H+ out of the egg.
• The reduced pH may be indirectly responsible for the
egg’s metabolic responses to fertilization.
• In the meantime, back at the sperm nucleus...
• The sperm nucleus swells and merges with the egg
nucleus  diploid nucleus of the zygote.
– DNA synthesis begins and the first cell division occurs.
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Actin
filaments
Depolarization provides Fast Block
- blocks multiple fertilizations
Fusion>release of Ca2+ into cystol>IP3 production>ligand-gated Ca2+ channel opens in ER>Cortical
granules fuse with plasma membrane>enzymes from granule released into perivitelline
space>swelling pushes vitillene layer away from plasma membrane and along with enzymes
hardening it >turns it into the fertilization membrane (Slow Block to polyspermy)
3. Cleavage partitions the zygote
into many smaller cells
• Fertilization of the egg by the sperm forms the
zygote
• Cleavage follows fertilization.
• Cleavage is a type of mitosis where the cell divides rapidly with
little or no growth.
– The zygote is partitioned into blastomeres.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 47.6
• Cleavage partitions the cytoplasm of one
large cell
– Into many smaller cells called blastomeres
(a) Fertilized egg. Shown here is the (b) Four-cell stage. Remnants of the (c) Morula. After further cleavage
mitotic spindle can be seen
divisions, the embryo is a
zygote shortly before the first
between
the
two
cells
that
have
multicellular ball that is still
cleavage division, surrounded
just
completed
the
second
surrounded by the fertilization
by the fertilization envelope.
cleavage
division.
envelope. The blastocoel cavity
The nucleus is visible in the
has begun to form.
center.
Figure 47.7a–d
(d) Blastula. A single layer of cells
surrounds a large blastocoel
cavity. Although not visible here,
the fertilization envelope is still
present; the embryo will soon
hatch from it and begin swimming.
• In both sea urchins and frogs first two
cleavages are vertical.
• The third division is horizontal.
• The result is an eight-celled embryo with two
tiers of four cells.
Fig. 47.8a
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• Continued cleavage produces the morula.
Fig. 47.8b
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• A blastocoel forms within the morula 
blastula
Fig. 47.8d
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Yolk impedes
cleavage
establishing
vegetal pole
• Gastrulation in a
sea urchin
– Produces an
embryo with a
primitive gut and
three germ layers
Organogenesis
• Various regions of the three embryonic
germ layers
– Develop into the rudiments of organs during the
process of organogenesis