Polyspermy is the fertilization of an egg by multiple sperm, which is lethal in diploid organisms due to complications such as competition for chromosomes. Sea urchins have developed two primary mechanisms to prevent polyspermy: the fast block, which occurs within 1/10th of a second and involves depolarization of the egg's membrane, and the slow block, which happens 10-60 seconds later and involves structural changes to the egg's membranes to prevent further fertilization. These mechanisms effectively safeguard the zygote from the adverse effects of polyspermy.

1. Blocks to polyspermy
By Miss Iram

2. Polyspermy
 Polyspermy describes an egg that has been fertilized by more
than one sperm. Diploid organisms normally contain two copies
of each chromosome, one from each parent. The cell resulting
from polyspermy

3. Block to polyspermy
 The first issue that an egg and a sperm of any organism type face in
successfully producing an embryo is the possibility of polyspermy. Polyspermy
is the fertilization of an egg by multiple sperm, and the results of such unions
are lethal.
 If multiple sperm fertilize an egg, the embryo inherits multiple paternal
centrioles. This causes competition for extra chromosomes and results in the
disruption of the creation of the cleavage furrow, thus causing the zygote to
die. As an important model organism in the study of fertilization and
embryonic development, polyspermy in sea urchins has been studied in
detail. The sea urchin’s methods of polyspermy prevention have been broken
down into two main pathways. These two primary pathways are known as
the fast block and the slow block to polyspermy

4. Fast block
 After the sperm’s receptors come into contact with the egg’s jelly layer and the
acrosomal enzymes are released and break down the jelly layer, the sperm head
comes into contact with the vitelline and plasma membranes of the egg. When
the two plasma membranes contact one another, signals in the egg are initiated.
 First, Na+ channels in the egg open, allowing Na+ to flood into the egg. This
causes a depolarization of the egg from it’s normal resting potential of -70 mV.
 While depolarization is occurring, the remainder of the jelly layer is
dissolving. With the dissolution of the jelly layer and the depolarization of the
plasma membrane, the first block to preventing fertilization by multiple sperm
is put into place.
 These two simple changes are part of the first block to polyspermy, known as
the fast block. Within 1/10th of a second of contact, the fast block to
polyspermy is initiated.

5. Slow block
 While the first fast block is relatively simple and happens quickly, the second
block to polyspermy, the slow block, takes place 10-60 seconds after the initial
penetration of the egg’s plasma membrane by the sperm. In this pathway, the
physical layout of the membranes change.
 Once the sperm has made its way through the jelly layer, receptors in the
vitelline layer confirm that it is indeed a sea urchin sperm. As fertilization
happens entirely externally in sea urchin species, this step is to confirm that
another organism’s sperm is not fertilizing the sea urchin egg. Once the sperm
is confirmed to be compatible, the plasma membranes of the sperm and egg
fuse.
 Fusion of the two membranes causes a release of inosital triphosphate. The
release of inosital triphosphate, or IP3, causes Ca2+ to be released into the egg’s
cytoplasm.

6.  Waiting in the cytoplasm are cortical vescicles, which are allowed to fuse with
the egg’s plasma membrane and release their contents when the appropriate
signal is sent. Ca2+ released is that necessary signal, and the vesicles fuse with
the membrane. The cortical vescicles contain enzymes which cause a number of
changes to occur in the egg.
 1. The egg’s sperm receptors are removed
 2. H2O is absorbed, causing the membrane to swell and push the vitelline
membrane away from the plasma membrane.
 3. The vitelline membrane hardens.
 These changes ensure that fertilization of the egg by additional sperm is
prevented. They are part of the slow block to polyspermy.

7. Block to polyspermy