Question

Which of the following is true of the slow block to polyspermy?
1. It is directly produced by the depolarization of the membrane.
2. It involves the release of cortical granule contents (exocytosis).
3. It is a slow forming, transient mechanism to prevent polyspermy.
4. It involves the propagation of a Ca2+ wave.
5. It is initiated when the sperm binds to the zona pellucida.

A. 1, 3
B. 2, 3
C. 2, 4
D. 2, 5
E. 2, 3, 4

Answer 1

Answer will be option (c).

2.As,the slow block to polyspermy starts within 10 sec of the fusion of the sperm and egg plasma membrane (not zona pellucida).It involves formation of a physical barrier to prevent further penetration of egg from other sperms.A fertilisation membrane/envelope is formed by the exocytosis of cortical granules by lifting of the vitelline membrane away from the egg plama cortical granules also contain some extracellular matrix protein which get deposited to the exterior of the fertilised egg making it impenetrable to the sperm entry.

4.There is a compound named inositol triphosphate (IP3) which causes the release of intracellular ca+2 stored in the endoplasmic reticulum of the egg.At the beginning ca+2 is first released at the site of sperm entry and after that a wave of free ca+2 passes through the egg.These ca+2 involves releasing of cortical contents by the fusion of cortical vesicles with the egg plasma membrane.

However,the fast block to polyspermy involves opening of Na+2 channels by which Na+2 ions flows into the egg membrane resulting in the depolarisation of the membrane.The depolarisation prevents fusion of the egg from multiple sperms.Also the fast block to polyspermy is transient,since the potential of membrane changes after few minutes.