Question
Which of the following mechanisms prevents polyspermy (fertilization of an oocyte by multiple sperm)?
| A. |
Zona reaction
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| B. |
Cortical reaction
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| C. |
Acrosome reaction
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| D. |
Both A & B
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Show Answer
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Correct Answer » D
Explanation
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Mechanism Preventing Polyspermy
Polyspermy refers to fertilization of an oocyte by more than one sperm. In humans, polyspermy is prevented by a sequence of reactions initiated immediately after the first sperm–oocyte fusion.
Cortical Reaction
Following the entry of the first sperm into the oocyte, there is a rapid rise in intracellular calcium within the oocyte. This triggers the cortical reaction, during which cortical granules present beneath the oocyte plasma membrane release their contents into the perivitelline space.
The enzymes released cause biochemical modifications of the zona pellucida, initiating the block to polyspermy.
Zona Reaction
The zona reaction is the structural and functional alteration of the zona pellucida resulting from the cortical reaction. These changes lead to:
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Hardening of the zona pellucida
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Inactivation of sperm-binding receptors
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Prevention of further sperm penetration
Together, these effects form the permanent block to polyspermy.
Acrosome Reaction (For Contrast)
The acrosome reaction occurs in the sperm prior to fertilization and facilitates penetration of the zona pellucida. It plays no role in preventing polyspermy.
Hence Correct Answer is Both A & B
