Fertilization

In all sexually-reproducing animals, the first step is

Fertilization itself consists of three events:

1
The sperm: a stripped down
machine for delivering
DNA

Figure 7.2
Chemical cues from the egg attract sperm

In sea urchins, the cue is resact

Figure 7.9

Sperm chemotaxis
Step one:

Contact between
the sperm
and the eggshell
Step two:

Contact between
the sperm
and the egg’s
plasma
membrane
 Fertilization
Sperm penetration and membrane fusion
-Protective layers of egg include the jelly
layer and vitelline envelope in sea urchins,
and the zona pellucida in mammals
-The acrosome of sperm contains digestive
enzymes that enable the sperm to tunnel its way
through to the egg’s cell membrane
-Membrane fusion permit sperm nucleus to
enter directly into egg’s cytoplasm
6
 Fertilization
Egg activation
-Membrane fusion triggers egg activation by
the release of Ca2+ which initiates changes in
the egg
-A block to polyspermy occurs
-Changes in egg’s membrane potential
-Alteration of egg’s exterior coats
-Enzymes from cortical granules
remove sperm receptors 7
 (zona pellucida=egg shell)

Fertilization:
a multistep
process
 Sperm Granulosa cell
Jelly layer Sperm
Plasma Zona pellucida
membrane
Plasma
Vitelline membrane
envelope

First
polar

body

Nucleus
Cytoplasm of egg

Cortical granules
Cortical granules Cytoplasm

a. b.

9
 Fertilization
Egg activation
-Sperm penetration has three other effects
1. Triggers the egg to complete meiosis
2. Triggers a cytoplasmic rearrangement
3. Causes a sharp increase in protein
synthesis and metabolic activity in
general

10
 Fertilization

Primary Oocyte First Metaphase of Meiosis Second Metaphase of Meiosis Meiosis Complete
Diploid nucleus Polar Polar
bodies
body

Female
pronucleus
(haploid)
• Roundworms (Ascaris) • Nemertean worms (Cerebratulus) • Lancelets (Branchiostoma) • Cnidarians
• Polychaete worms (Myzostoma) • Polychaete worms (Chaetopterus) • Amphibians • Sea urchins
• Clam worms (Nereis) • Mollusks (Dentalium) • Mammals
• Clams (Spisula) • Many insects • Fish
• Sea stars

11
 Fertilization
Fusion of nuclei
-The haploid sperm and haploid egg nuclei
migrate toward each other along a micro-
tubule based aster
-They then fuse, forming the diploid
nucleus of the zygote

12
 Fertilization

6. Additional sperm
can no longer penetrate
1. Sperm penetrates 2. Some of the zona 3. Sperm and egg 4. The sperm nucleus the zona pellucida.
between granulosa pellucida is degraded plasma membranes dissociates and
cells. by acrosomal enzymes. fuse. enters cytoplasm.

5. Cortical granules
release enzymes that
Cortical
Zona harden zona pellucida
Plasma granules
pellucida and strip it of sperm
membrane receptors. Hyalin
Granulosa attracts water by
cells osmosis.
7. Sperm and egg
pronuclei are
enclosed in a
nuclear envelope.

13
 Sperm-eggshell contact triggers
the acrosome reaction
Example 1-- the sea urchin

Species specificity (equivalent to

enzymes
attracts/activates sperm

Figure 7.8
Sperm-eggshell
contact
triggers the
acrosome
reaction
Example 2:
Mammals
Eggshell includes
ZP3/Sperm
have ZP3