Step one:
Contact between
the sperm
and the eggshell
Step two:
Contact between
the sperm
and the egg’s
plasma
membrane
(ECM)
Fertilization:
a multistep
process
Sperm-eggshell contact triggers
the acrosome reaction
Example 1-- the sea urchin
Species specificity
(equivalent to
zona pellucida)
enzymes
attracts/activates sperm
Figure 7.8
Sperm-eggshell
contact triggers
the acrosome
reaction
Example 2:
Mammals
Sperm have
ZP3 receptor
Figure 7.8
BLOCKING POLYSPERMY
1. Fast/transient: prevent
sperm fusion
2. Slow/permanent: removal
of other bound sperm
Slow block
20-60 seconds
permanent
Fast block
1-3 seconds
lasts 1 min
How do we know this?
We can measure these events directly!
(sperm can’t fuse with egg)
+20 mV
resting -70
(sperm can fuse with egg)
Figure 7.17 Membrane potential of sea urchin eggs before and
After fertilization.
Blocking polyspermy: SLOW BLOCK
Figure 7.6
Cortical
granules:
poised for
release
15,000 cortical granules in a sea urchin egg
• contain enzymes that clip the egg’s bindin receptor
and any attached sperm
• alter the vitelline envelope (zona pellucida in
mammals)
Sperm-eggshell
contact triggers
the acrosome
reaction
Example 2:
Mammals
Sperm have
ZP3 receptor
A wave of increased calcium can be visualized
moving across the egg from the point of sperm entry
Figure 7.20
Slow block
20-60 seconds
permanent
Fast block
1-3 seconds
lasts 1 min
pH
10
-5
7.4
Ca
10
2+
-6
7.2
intr acel lul ar
intr acel lul ar pH
calcium
10
-7
7.0
10
-8
6.8
200
400
600
t ime (s econds)
Cleavage and Gastrulation
Fig. 8.20
Gastrulation
Fig. 8.15
Many embryos fail to successfully complete
early cleavage divisions or gastrulation
CLEAVAGE
GASTRULATION
EARLY CLEAVAGE
 rapid increase in the number of cells
after fertilization
blastomeres
 egg contents distributed to cells, often
asymmetrically
The cell cycle is different during cleavage
Early Blastomeres
Normal Cells
mitosis
DNA synthesis
Figure 5.1
Much shorter cell cycles during early development-no growth phases and cells rely on stored material
Activation of MPF controls start of cleavage
Mitosis-promoting
factor (MPF)
Figure 5.1
Mitosis is dependent on presence of Cyclin B
CLEAVAGE and
the cytoskeleton
Fig. 5.2
mitotic spindle
contractile ring
actin
tubulin
Different organisms have distinct cleavage patterns
II. MEROBLASTIC (INCOMPLETE CLEAVAGE)
A. Telolecithal (Dense yolk throughout most of cell)
Fig. 5.3
What controls the pattern of
cleavage?
1. Amount and distribution of yolk in the egg
2. Factors in the egg that influence the angle
and timing of mitotic spindle formation
Sea urchin egg
yolk-rich
Fig. 5.6
Different organisms have distinct cleavage patterns
II. MEROBLASTIC (INCOMPLETE CLEAVAGE)
A. Telolecithal (Dense yolk throughout most of cell)
Fig. 5.3
Discoidal cleavage
in fish and birds
 creating a cellular region above the dense yolk
zebrafish
Figure 7.40
What controls the pattern of
cleavage?
1. Amount and distribution of yolk in the egg
2. Factors in the egg that influence the angle
and timing of mitotic spindle formation
Radial cleavage
in sea urchins:
simple planes of cleavage
a) Placement of mitotic spindles
Synchronous vs. asynchronous cleavage
1248
even cell stages
odd numbered cell stages
are frequent
b) Timing of mitotic spindle formation
c) Symmetric vs.
asymmetric cell divisions
Fig. 5.6
Mammalian cleavage
is unique
Rotational cleavage
in mammals
meridional
meridional
(radial cleavage)
equatorial
Figure 8.16
Synchronous vs. asynchronous cleavage
1248
even cell stages
odd numbered cell stages
are frequent
Compaction during mammalian development:
A sudden change in cell adhesion
8-cell embryos: uncompacted
compacted
Compaction requires
the cell adhesion
protein E-cadherin
Control embryos
Embryos treated with
antibodies blocking E-cadherin
Development of a Human Embryo
From Fertilization to Implantation
8-cell stage
compaction
Fig. 8.15
Early mammalian development follows
(in function) the pattern seen in other animals
morula
Development of a Human Embryo
From Fertilization to Implantation
hatch from zona pellucida
Fig. 8.20
Fig. 8.15
End of cleavage =
mid-blastula transition
controlled by nuclear/cytoplasmic
(N/C) ratio
The Mid-Blastula Transition
• Growth phases added to Cell Cycle
• Embryo starts transcribing its own RNA
• Cells start moving around in preparation for gastrulation
Mid-blastula transition
and the N/C ratio
Lets say you have an organism which normally
undergoes 4 cell divisions before the MBT
pre-cleavage: N/C ratio = 1/16 = 0.0625 (close to 0)
after 4 cell divisions (1 > 2 > 4 > 8 > 16): 16/16 = 1