Polyspermy Prevention Is Critical for Viable Zygote Formation
- When a sperm successfully fertilizes an egg, it’s crucial to ensure that no additional sperm enter.
- This is because polyspermy, the fusion of multiple sperm with one egg which leads to an unviable zygote with too many chromosomes.
- Two key biological mechanisms prevent this: the acrosome reaction and the cortical reaction.
Definition
Polyspermy
Polyspermy refers to the fertilization of an egg by more than one sperm.
The Acrosome Reaction Enables Sperm Entry
Definition
Acrosome reaction
The acrosome reaction is the first step in fertilization. It allows a sperm to penetrate the egg's protective glycoprotein layer, called the zona pellucida, to reach the plasma membrane of the egg.
- The acrosome is a specialized vesicle at the tip of the sperm head.
- It contains enzymes that help the sperm penetrate the egg’s protective layer, the zona pellucida.
The zona pellucida is a glycoprotein-rich layer surrounding the egg, acting as a barrier to sperm entry.
Steps of the Acrosome Reaction
- Sperm Binding: Sperm binds to specific glycoproteins on the zona pellucida, initiating the reaction.
- Enzyme Release: The acrosome, a specialized vesicle at the tip of the sperm head, releases enzymes that digest the zona pellucida.
- Zona Penetration: The sperm's tail propels it forward, allowing it to break through the now-weakened zona pellucida and reach the egg’s plasma membrane.
Key Features of the Acrosome Reaction
- Species-Specific: The sperm can only bind to the zona pellucida of eggs from the same species.
- One-Way Mechanism: Once the acrosome reaction occurs, the sperm becomes "committed" and cannot bind to another egg.
The Cortical Reaction Secures the Egg Against Polyspermy
Definition
Cortical Reaction
The cortical reaction is a process initiated during fertilization that prevents polyspermy, the fusion of multiple sperm with one egg.
- Once a sperm successfully penetrates the zona pellucida and fuses with the egg’s plasma membrane, the cortical reaction prevents any additional sperm from entering.
