Final answer:
The oocyte employs a fast block and a slow block to prevent polyspermy. The fast block changes sodium ion permeability, while the slow block establishes a fertilization membrane. These mechanisms ensure that only one sperm fertilizes the egg, crucial for viable zygote formation.
Step-by-step explanation:
Prevention of Polyspermy
During fertilization, the oocyte must ensure that only one sperm penetrates. The polyspermy prevention mechanisms activated upon the first sperm entry are crucial for the viability of the resulting zygote. Initially, the fast block to polyspermy occurs almost immediately after the first sperm binds, altering the sodium ion permeability and depolarizing the oocyte's plasma membrane which prevents further sperm fusion. This is followed by an influx of calcium ions that lead to the slow block, or cortical reaction, where cortical granules release zonal inhibiting proteins and mucopolysaccharides that build an impenetrable barrier known as the fertilization membrane, ensuring monospermic fertilization.
Failure of these mechanisms results in polyspermy, where multiple sperm penetrate the egg, causing the embryo to be genetically non-viable. It's essential for a single sperm to fertilize the egg to avoid a triploid organism which is not compatible with life. The sperm's entry triggers changes in the egg including the release of proteins that prevent the binding and fusion of additional sperms, highlighting the intricate strategies an oocyte employs to secure successful reproduction.