BLOCKS OF POLYSPERMY (DEVELOPMENTAL BIOLOGY )

Two mechanisms are used by animals to ensure that only one sperm will fertilize the egg to prevent the polyspermy condition .
1.FAST BLOCK - ( In sea urchins and Frogs) No fast block polyspermy are identified in mammals .

•  (It happens immediately after fertilization )Depolarization of membrane so it is the change in potential of the plasma membrane. Sperm binding causes Na+ influx. 
• STEPS-   a. Opening of Na+ channels in the egg plasma membrane .

                          b.Flow of Na+ into the egg cell (Na+ influx) .

                          c.Depolarization of the membrane .

                          d. As a result of that it prevents the additional sperm entry to the egg plasma membrane .

                          e. The egg plasma membrane restored to its normal -70 Mv resting potential within minutes of fusion as the Na+ channels  close  and Na+ is pumped out .

          (IF DEPOLARIZATION IS PREVENTED IT WOULD RESULTS IN POLYSPERMY )

2.SLOW BLOCK -(CHEMICAL AND MECHANICAL BLOCK)
Upon sperm entry the cortical granules fuse with the egg plasma membrane and  exocytosis of cortical granules occurs which  releases zonal inhibiting proteins like  serine proteases (which digest the connection between vitelline membrane and plasma membrane .) ,mucopolysaccharides ( It produce osmotic gradient and water rushes to perivitelline space), peroxidases ( oxidizes and cross links tyrosine )going to harden the fertilization envelope and in case of Humans it is zona Pellucida ) and hyaline which provides support during cleavage by formimg a coat around the egg called hyaline layer  .
STEPS - (In most of the species)

• Inositol triphosphate -(IP3) causes the release of  Ca2+ from intracellular stores in the egg Endoplasmic Reticulum .
• At the site of sperm entry ca2+ is released and it passes to the egg .
• This ca2+ results in the fusion of cortical vesices with the egg plasma membrane releasing their contents into the space surrounding the egg called the perivitelline space. which is presnt in between cell membrane and vitelline envelope .
• This raises the vitelline envelope and inactivates or destroys  the bindin receptor.
• So, any additional sperm cannot bind which in turn prevents polyspermy .  Image source credit -http://slideplayer.com/slide/6364295/

NOTE -SOME ORGANISMS DO NOT BLOCK POLYSPERMY MANY SPERM ENTER ONLY ONE SURVIVE AND THE REST ARE DEGRADED.


What would be the reason to block polyspermy ?
Ans.       It leads to polyploidy and eventually death ..

Thank you for visiting my  blog. Please share your comments on this

article.  Subscribe and follow me in Google + to get notified with new articles.