Prevention of polyspermy?

Polyspermy is the fertilization of an egg by multiple sperm cells, which can lead to abnormal development and potential embryo death. To prevent polyspermy, several mechanisms are in place:

1. Fast block to polyspermy: Immediately after the sperm penetrates the egg's outer layer, the egg undergoes a rapid change in its electrical charge, known as the fast block. This change makes the egg's outer membrane temporarily impenetrable to other sperm, preventing additional sperm from entering.

2. Slow block to polyspermy: Following the fast block, the egg initiates the slow block to polyspermy, also known as the cortical reaction. This process involves the release of cortical granules from the egg's cytoplasm into the perivitelline space (the space between the egg's plasma membrane and the zona pellucida). The cortical granules release enzymes that modify the zona pellucida, making it hard and impenetrable to other sperm.

3. Zona reaction: The zona reaction is another mechanism that prevents polyspermy. It involves the modification of the zona pellucida by the egg to prevent further sperm binding. This modification includes the removal of sperm-binding proteins from the zona pellucida, making it less receptive to additional sperm.

4. Sperm competition: In some species, sperm competition occurs, where the sperm cells compete with each other to fertilize the egg. This competition can involve various mechanisms, such as the release of enzymes by the first sperm to enter the egg, which creates a barrier preventing other sperm from entering.

These mechanisms work together to prevent polyspermy and ensure that only one sperm successfully fertilizes the egg, allowing for normal embryo development.