Q.
Polyspermy is prevented by all the events, except
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NTA AbhyasNTA Abhyas 2020Human Reproduction
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Solution:
Although many sperm attaches to the egg, it is only one sperm enters into the egg. There are few mechanisms the animals adopt to ensure to avoid polyspermy. Among them the fast block to polyspermy is best studied.
Fast block to polyspermy occurs within a tenth of a second of fusion and involves the opening of Na+ channels in the plasma membrane of the egg. Na+ flows into the egg, by depolarizing the membrane and prevents additional sperm from fusing to the egg.
The slow block to polyspermy starts within 10 seconds of fusion of the sperm and egg. Inositol triphosphate (IP3) releases Ca++ from the endoplasmic reticulum and a wave of free Ca++ passes through the egg. This Ca++ results in the fusion of cortical vesicles with the egg plasma membrane, releasing their contents into the space surrounding the egg, called the perivitelline space. This raises the vitelline membrane and inactivates bindin receptors on the vitelline membrane. Thus, any additional sperm are released from the vitelline membrane and no more bind.