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1. Fertilization
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Fertilization is when an
egg and a sperm unite
forming a zygote.
The main function of
fertilization is to produce
a diploid egg.
Egg activation occurs
when the sperm contacts
the egg’s surface and
initiates metabolic
reactions that trigger the
onset of embryonic
development.
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1. Fertilization
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In general, when eggs meet sperm, an
acrosomal reaction is triggered.
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1. Fertilization
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The process begins when a vesicle at
the tip of the sperm called the
acrosome discharges hydrolytic
enzymes.
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1. Fertilization
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These enzymes dissolve the egg’s jelly coat
enabling the acrosomal process to penetrate
it and attach to the egg’s surface.
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1. Fertilization
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The sperm binding receptors on the
egg’s surface are attached to the
vitelline layer.
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1. Fertilization
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This recognition of the egg’s surface receptors
to sperm is what ensures only sperm from the
same species can penetrate the egg.
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1. Fertilization
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The binding of the sperm membrane to
the membrane of the egg changes the
state of the ion channels of the egg.
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Slow Block to Polyspermy
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The fusion of the sperm and egg also triggers
a series of changes in the egg that are more
long-lived.
 Sperm binding activates a signal transduction
pathway which causes calcium ions to be
released from the egg’s ER into the cytosol.
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Slow Block to Polyspermy
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Release of Ca2+ from the ER occurs 1st
at the site of sperm entry and moves
like a wave over the surface of the egg.
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Slow Block to Polyspermy
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The high concentration of calcium initiates
the cortical reaction.
 The reaction triggers the fusion of the egg’s
plasma membrane with numerous vesicles
lying just beneath the membrane.
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Slow Block to Polyspermy
The contents of these vesicles enter the
previtelline space.
 Enzymes in these contents degrade the
proteins holding the vitelline layer to the
plasma membrane.
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Slow Block to Polyspermy
These changes transform the vitelline
layer into the fertilization envelope.
 No more sperm can enter the egg at
this time.
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Slow Block to Polyspermy
This is referred to as the slow block to
polyspermy.
 The sharp rise in Ca2+ also increases
the metabolism of the egg.
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Slow Block to Polyspermy
After about 20 minutes, the egg nucleus
fuses with the sperm nucleus creating a
diploid zygote.
 Fertilization has many common features
among species and many differences.
These differences are mainly with
timing and various stages of meiosis.
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1. Fertilization--Sperm Activation
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In mammals, secretions in the female
reproductive tract to help to activate sperm.
 Additionally, the mammalian egg is cloaked
by follicle cells through which the sperm must
penetrate before reaching the zona pellucida.
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1. Fertilization--Sperm Activation
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The zona pellucida is the
external matrix of the
egg and functions as a
sperm receptor.
 When sperm binds to
this, it induces many of
the same reactions as
seen in the sea urchin.
 One difference is that
there is no fast block to
polyspermy in mammals.
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1. Fertilization--Sperm Activation
After the egg and sperm membranes
fuse the whole sperm is taken into the
egg.
 The egg lacks a centrosome which the
sperm has.
 The centrosome will duplicate and
assist in the creation of the mitotic
spindle which will be used for the first
cell division.
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1. Fertilization--Sperm Activation
In mammals, the nuclei do not
immediately fuse. Instead, the 2 sets of
c-somes share a common spindle
apparatus during the 1st mitotic
division.
 Only after the first division, as diploid
nuclei form in the 2 daughter cells, do
the chromosomes from the 2 parents
come together in a common nucleus.
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1. Fertilization
Fertilization is quite slow in mammals.
 Once fertilization is complete, rapid cell
division ensues and the cells proceed
through the M and S phases of the cell
cycle virtually skipping G1 and G2.
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1. Fertilization
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Thus, the embryo doesn’t enlarge
much. Rather, it becomes an
aggregate of blastomeres called a
blastocyst (blastula).
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Copyright ©2005 Pearson Education, Inc. Publishing as Pearson Benjamin Cummings. All rights reserved.
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