Write short notes in 150 words:
Block to polyspermy
Block to Polyspermy
The block to polyspermy prevents multiple sperm from fertilizing a single egg, ensuring the formation of a healthy diploid zygote. It occurs in two steps: the fast block and the slow block. The fast block acts immediately after sperm entry, changing the egg’s membrane potential. In sea urchins, sodium ions rapidly enter, depolarizing the membrane and temporarily preventing additional sperm from fusing.
The slow block, or cortical reaction, follows. Calcium ions flood the egg, causing cortical granules to release enzymes. In mammals, these enzymes harden the zona pellucida, forming a barrier against more sperm. In sea urchins, the vitelline envelope elevates, creating a fertilization membrane.
These mechanisms ensure only one sperm’s DNA merges with the egg’s, preventing abnormal ploidy and developmental defects. This two-step process is essential for proper embryogenesis, highlighting an evolutionary adaptation that safeguards successful fertilization.
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