8. The major function of cortical granules in cytoplasm of egg is to
(1) Early block to polyspermy
(2) Late block to polyspermy
(3) Allowing meiosis to complete
(4) Helping in reorganization of sperm

The major function of cortical granules in the cytoplasm of the egg is to prevent polyspermy by establishing the slow block after fertilization. Upon sperm entry, these granules undergo exocytosis, releasing their contents into the space outside the egg. The secretions modify the egg’s outer extracellular matrix (such as the zona pellucida in mammals) to create a physical and chemical barrier that prevents additional sperm from penetrating and fertilizing the egg. This is known as the late block to polyspermy.

Therefore, the correct answer is:

(2) Late block to polyspermy

Introduction to Cortical Granules

Cortical granules are specialized secretory vesicles located just beneath the plasma membrane of the egg’s cytoplasm. They are especially crucial during fertilization, where their exocytosis plays a central role in ensuring only one sperm fertilizes the egg, preventing the detrimental condition known as polyspermy.

The Cortical Reaction and Polyspermy Prevention

Polyspermy, the fertilization of an egg by multiple sperm, leads to an abnormal number of chromosomes and is lethal to the developing embryo. To prevent this, eggs have evolved multiple blocks against polyspermy:

• The fast block, occurring immediately after sperm fusion, involves electrical changes in the egg’s membrane.

• The slow block, mediated by the cortical reaction, involves the secretion of cortical granules.

Upon the fusion of the sperm with the egg’s plasma membrane, a calcium ion wave triggers the cortical granules to fuse with the egg membrane and expel their contents into the perivitelline space between the egg membrane and surrounding envelopes.

What Happens After Cortical Granule Exocytosis?

The substances released by the cortical granules induce several modifications to the egg’s outer layers:

• Hardening of the zona pellucida (or vitelline envelope in non-mammals) by enzymatic cross-linking.

• Cleavage of sperm-binding proteins on the egg coat to block further sperm adherence.

• Osmotic swelling of the perivitelline space to physically lift the fertilization envelope away from the egg membrane.

• Formation of a fertilization envelope, a tougher barrier preventing additional sperm entry.

These changes constitute a permanent, biochemical block to additional sperm, hence a late block to polyspermy.

Other Functions and Biological Context

Beyond blocking polyspermy, cortical granule components may play roles in:

• Protecting the fertilized egg.

• Supporting early embryonic development.

• Participating in the egg’s cytoplasmic reorganization after fertilization.

However, their defining and primary role remains focused on polyspermy prevention.

Why Other Options Are Incorrect

• Early block to polyspermy is mediated by fast electrical changes, not cortical granules.

• Allowing meiosis to complete is triggered by calcium signals from fertilization but not directly caused by cortical granules.

• Helping in the reorganization of sperm is unrelated; cortical granules affect the egg’s environment post-fertilization and not sperm organization.

Conclusion

Cortical granules are essential cytoplasmic organelles in the egg that, upon fertilization, release their contents to modify the egg’s extracellular matrix, establishing the slow, late block to polyspermy. This function safeguards the integrity of zygote formation by preventing multiple sperm from fertilizing the egg.

Hence, the correct statement is:

(2) Late block to polyspermy

This detailed overview explains the role of cortical granules in fertilization and their crucial function in blocking polyspermy to ensure successful embryonic development.