D3.1.15 Mechanisms to prevent polyspermy (HL) Notes

Polyspermy Prevention Is Critical for Viable Zygote Formation

1. When a sperm successfully fertilizes an egg, it’s crucial to ensure that no additional sperm enter.
2. This is because polyspermy, the fusion of multiple sperm with one egg which leads to an unviable zygote with too many chromosomes.
3. Two key biological mechanisms prevent this: the acrosome reaction and the cortical reaction.

The Acrosome Reaction Enables Sperm Entry

1. The acrosome is a specialized vesicle at the tip of the sperm head.
2. It contains enzymes that help the sperm penetrate the egg’s protective layer, the zona pellucida.

Steps of the Acrosome Reaction

1. Sperm Binding: Sperm binds to specific glycoproteins on the zona pellucida, initiating the reaction.
2. Enzyme Release: The acrosome, a specialized vesicle at the tip of the sperm head, releases enzymes that digest the zona pellucida.
3. 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

1. Species-Specific: The sperm can only bind to the zona pellucida of eggs from the same species.
2. 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

1. 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.

Steps of the Cortical Reaction

1. Activation of the Egg: Fusion of the sperm and egg membranes triggers a surge in calcium ions inside the egg. This calcium wave activates the cortical reaction.
2. Cortical Granule Release: Enzyme-filled vesicles near the egg's plasma membrane, called cortical granules, release their contents into the space between the plasma membrane and the zona pellucida.
3. Zona Hardening: The enzymes modify the zona pellucida glycoproteins, making it impenetrable to other sperm.
4. Prevention of Sperm Binding: Glycoproteins that previously facilitated sperm binding are altered, ensuring that no additional sperm can attach.

Why Preventing Polyspermy is Critical

Polyspermy results in a zygote with more than two sets of chromosomes, leading to developmental failure.

Fast and Slow Blocks to Polyspermy

1. In some species, polyspermy is prevented by two mechanisms working together:
   1. Fast Block: A rapid but temporary change in the egg’s membrane potential occurs immediately upon sperm fusion, preventing additional sperm from fusing.
   2. Slow Block (Cortical Reaction): This permanent mechanism modifies the zona pellucida to prevent other sperm from binding and entering.

Implications for Fertility and Reproductive Technologies

1. Understanding these mechanisms is crucial for advancements in in vitro fertilization (IVF) and other reproductive technologies.
2. In IVF, ensuring that only one sperm fertilizes each egg is vital for producing viable embryos.