Polyspermy, the fertilization of an egg by more than one sperm, is a significant challenge in the reproductive process for many sexually reproducing organisms. This event is fatal for the developing embryo because it leads to an incorrect number of chromosomes, disrupting proper cell division and development. To counteract this challenge and ensure offspring survival, organisms have evolved defense mechanisms. These mechanisms allow only a single sperm to fertilize the egg, maintaining the correct chromosomal balance.
The Depolarization Event
The fast block to polyspermy involves a rapid electrical change in the egg’s plasma membrane. An unfertilized egg maintains a negative electrical charge (resting potential). When the first sperm fuses with the egg’s membrane, it triggers the opening of channels, allowing sodium ions to flow into the egg.
This rapid influx of sodium ions causes the egg’s membrane potential to shift from negative to positive. This electrical alteration, called depolarization, occurs within seconds, acting as an immediate electrical barrier. Imagine it like an electric fence suddenly switching on: the sudden positive charge at the egg’s surface repels any other approaching sperm, preventing them from fusing with the now-fertilized egg.
The Role of the Fast Block Across Species
The fast block to polyspermy is a well-documented defense strategy, especially in marine invertebrates with external fertilization. The sea urchin is a classic example where this electrical block is observed. In these species, where many sperm encounter an egg simultaneously, a rapid response prevents multiple fertilizations. It is also present in amphibians like frogs, which undergo external fertilization.
However, the fast block is not a significant mechanism in mammals, including humans. Mammalian eggs rely on other processes to prevent polyspermy. These strategies involve physical and biochemical changes to the zona pellucida, the outer protective layer of the mammalian egg, rather than electrical repulsion.
A Temporary but Important Solution
The fast block to polyspermy provides an immediate, temporary defense against multiple sperm entry. This electrical barrier is short-lived, lasting for about a minute in species like the sea urchin. While brief, this window of protection is sufficient to provide time for a more permanent and robust solution to be established.
This longer-lasting defense is known as the slow block to polyspermy, also referred to as the cortical reaction. The slow block involves the release of contents from thousands of small sacs called cortical granules, located just beneath the egg’s plasma membrane. These granules release enzymes and other substances into the space between the egg membrane and the vitelline layer, causing the vitelline layer to lift away and harden. This creates a tough, impenetrable fertilization envelope that physically blocks any further sperm from reaching the egg, forming a coordinated, two-step defense system in the species that employ them.