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When a sperm cell encounters an egg
of the same species, components of the jelly coat bind to
specific "egg receptors" in the plasma membrane. This
triggers a signal
transduction
cascaderesulting in a
series of events that facilitate fertilization.
First,
an influx of
calciuminduces the fusion
of the plasma membrane with the
membrane surrounding the
acrosome, releasing a set of hydrolytic
enzymeswhich
digest a channel in the jelly
coat. This also results in the front of the sperm becoming
covered by what had previously been the inside of the
acrosome. This exposes a new "egg receptor".
Actin
filaments form at the tip of the sperm head extending the
acrosomal process, a
long proboscis that penetrates through the jelly to contact
the vitelline membrane of the
egg. The tip of the acrosomal process binds to the egg
plasma membrane.
D. Egg meets
sperm47.2
1. Receptor binding/ signal
transduction
a. Na + influx
depolarizes(Fast block)
b. Wave of cytoplasmic
"free" Ca++47.3, #4
c. Cortical granule
fusion#5
d. Elevation of
fertilization envelope (Slow
block)#6
e. Rise in intracellular pH
activated metabolism
f. Nuclei fuse
The egg
also has specific "sperm receptors" which extend
through both the plasma
membrane of the egg and the vitelline membrane. They bind to
newly exposed surface proteins that had previously been
inside the acrosome. This selects for sperm that have
undergone the acrosome reaction. Therefore, the two
binding events have to occur
sequentiallyand help to
ensure species-specificity.
The
binding of sperm to the receptor triggers a second signal
transduction cascade resulting in 1) the opening of Na+
channelscausing a
transient depolarization; 2) a
wave of "free" Ca++and
3) a rise in intracellular pH.
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