fertilization polyspermy prevention

an

d

in the sea urchinThe fast block is found in the sea urchin but is probably not used in mammals. It is achieved

by changing the electric potential of the egg cell membrane. The surrounding seawater has

a high sodium ion (Na+) concentration while the egg cytoplasm contains relatively little Na+.

The egg has a resting membrane potential of -70mV based on this concetration. Sperm can

fuse with the membrane at this negative potential. Within 1 to 3 seconds after the

binding of the first sperm, Na+ ions flow into the egg. This causes the membrane potential

to shift to about +20mV. Sperm cannot bind to membranes having a positive membrane

potential. No more sperm are able to fuse to the egg after this change. Though the fast block

is effective, it only lasts for about 1 minute. Another mechanism is needed to permanently

prevent polyspermy.

step one the fast block

seawaterhigh Na+ concentration

egglow Na+ concentration

Na+

Na+

Na+Na+

Na+

Na+

Na+

seconds

mV

0 20 40 60 80

sperm added

change in electric potential after addition of sperm

100

50

0

-50

-100

influx of Na+ ions

[before] negative sperm can bind“Fertilization is not a moment or an event,

but a process of carefully orchestrated events. It is a process whereby two cells, each at the verge of death, unite to create a new orgagnism that will have numerous cell types and organs. It is just the beginning.” [Scott F. Gilbert]

Fertilization is the process in which the sperm and egg fuse together to begin the

creation of a new individual. Both the egg and the sperm are haploid cells containing only

half of the genetic material needed to begin an organism. Once the two cells combine, the

full amount of genetic material is restored, marking the beginning of a new life.

The entrance of more than one sperm, polyspermy, leads to disastrous consequences

in most organisms. Because of excess genetic material, such cells will either die or develop

abnormally. A mechanism is needed to prevent this polyspermy. The sea urchin egg has

two main mechanisms to ensure the entrance of only one sperm.

two steps ensuring the entrance of only one sperm

[1] An unfertilized sea urchin egg (transmission electron micrograph). The circular cortical granules are visible. [2] The surface of a sea urchin egg (scanning electron micrograph). The cell membrane is exposed where the vitelline envelope has been torn. [3] Sea urchin sperm bound to the vitelline envelope of an egg (scanning electron micrograph). [4] Formation of the fertilization envelope in a sea urchin egg. The envelope is forming around the egg, starting at the point of sperm entry.

figures

[Developmental Biology: Eight Edition, Scott F. Gilbert] [http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=dbio&part=A1388] [http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=dbio&part=A1425] [http://en.wikipedia.org/wiki/Polyspermy] [http://worms.zoology.wisc.edu/urchins/nfert.html] [http://www.youtube.com/watch?v=Rfc684v3ZEs&feature=related]

[after] postive sperm cannot bind

the “cortical granule reaction”step two the slow blockThis block is found in many animals, including most mammals. It is

a block that becomes completely effective about 60 seconds after

the first successful sperm-egg fusion. By the end of this

mechanism, a rigid fertilization envelope is formed that is able to

permanently prevent polyspermy. To begin with,15,000 organelles

called cortical granules are found right beneath the egg cell

membrane. These cortical granules contain various substances

necessary for the slow block. Upon sperm entry, the cortical

granules fuse with the egg cell membrane and release their

contents into the space between the cell membrane and the mat

of the vitelline envelope proteins. The cortical granunles then

release 4 substances:

h2oh2o

[a] cortical granule serine proteaseThe substances of the cortical granules fuse

with the vitelline envelope to form the start

of the fertilization envelope. Cortical granule

serine protease is released. This enzyme

dissolves the protein posts that connect the

cell membrane to the vitelline envelope. It

also clips off any sperm attached to the egg

cell membrane.

[b] mucopolysaccharidesThese are sticky com-

pounds that force water to

rush ino the space

between the cell mem-

brane and the fertilization

envelope. This causes the

envelope to expand and

move away from the egg.

[c] peroxidase enzymeThis enzyme

hardens the

fertilization

envelope by

crosslinking

tyrosine residues

on adjacent

proteins.

[d] hyalin and other proteins

These proteins form a coating

around the egg, completing

the fertilization envelope!

These 4 processes start about

20 seconds after sperm

fusion and are complete after

about 60 seconds.

jelly layer

vitelline envelope

egg cell membrane

protein

posts

cortical granule

protein

h2o

fertilization envelope

Now, polyspermy can permanently be prevented.

[4]

the sperm

the egg

acrosomecontains enzymes that enable passage through outer coverings of the egg

nucleuscontains the genetic material

jelly layera glycoprotein meshwork used to either attract or activate sperm (the egg is a cell specialized for receiving sperm and initiating development)

vitelline envelopeessential for the species-specific binding of sperm

cell membranecapable of fusing with the sperm cell membrane and regulates the flow of certain ions during fertilization

nucleuscontains the genetic material

cortical granulescontain enzymes that help prevent polyspermy

tailsperm’s method of movement

10 seconds 25 seconds

finished35 seconds

[4][3][2][1]