Which process prevents polyspermy during fertilization?

The cortical reaction is the key process that prevents polyspermy during fertilization. When a sperm successfully fertilizes an egg, calcium ions are released, triggering the cortical granules within the egg to fuse with the egg's plasma membrane. This fusion causes the contents of the granules to be released into the space outside the egg, leading to changes in the structure of the zona pellucida, the protective layer surrounding the egg.

These changes modify the zona pellucida, which creates a barrier that is impenetrable to additional sperm. This instant transformation effectively ensures that only one sperm can fertilize the egg, preventing polyspermy, which could lead to abnormal development and complications for the zygote.

The other processes mentioned do not serve the primary function of preventing polyspermy. The acrosome reaction occurs when the sperm binds to the egg, providing enzymes that help digest the zona pellucida and facilitate entry into the egg. The formation of the fertilization envelope is a later reaction that contributes to embryo protection but is secondary to the immediate prevention of polyspermy. Cytoplasmic streaming is a cellular movement involving the distribution of cytoplasm within the egg, which does not have any role in preventing multiple sperm from entering the egg