Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
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]