Polyspermy, the fertilization of an egg by multiple sperm, is prevented by several mechanisms. The zona pellucida, a glycoprotein layer surrounding the egg, acts as a barrier to sperm penetration. Upon fertilization, the egg undergoes a process called the cortical reaction, which releases enzymes that modify the zona pellucida, making it impermeable to additional sperm. Additionally, the sperm’s plasma membrane undergoes a transformation known as the acrosome reaction, releasing enzymes that help the sperm penetrate the zona pellucida. Lastly, the egg’s pronucleus fuses with the sperm’s pronucleus, forming a zygote, and the egg’s nucleus undergoes meiosis II, which blocks polyspermy by preventing the entry of additional sperm nuclei.
How Is Polyspermy Prevented?
Polyspermy, the fertilization of an egg by more than one sperm, is prevented by a series of mechanisms that act at different stages of the fertilization process. These mechanisms include:
1. Physical Barriers
- Zona pellucida: A transparent glycoprotein layer that surrounds the egg and prevents sperm from penetrating.
2. Chemical Changes in the Egg Membrane
- Cortical reaction: A rapid series of exocytotic reactions that occur after the first sperm fertilizes the egg. These reactions release enzymes that modify the zona pellucida, making it impermeable to other sperm.
3. Electrical Changes in the Egg Membrane
- Depolarization: The rapid reversal of the electrical potential across the egg membrane, which makes it less attractive to sperm.
4. Sperm Inactivation
- Acrosome reaction: The release of enzymes from the sperm’s acrosome that help it penetrate the zona pellucida. Once the acrosome has reacted, the sperm is no longer capable of fertilizing another egg.
5. Intracellular Signaling Pathways
- Calcium influx: The increase in intracellular calcium concentration after fertilization triggers a cascade of events that prevent polyspermy.
Table: Summary of Polyspermy Prevention Mechanisms
| Mechanism | Type | Stage of Fertilization |
|---|---|---|
| Zona pellucida | Physical barrier | Before fertilization |
| Cortical reaction | Chemical change | After fertilization |
| Depolarization | Electrical change | After fertilization |
| Sperm inactivation | Sperm-mediated | After fertilization |
| Calcium influx | Intracellular signaling | After fertilization |
Question 1:
How is polyspermy prevented?
Answer:
- Polyspermy is prevented through several mechanisms that ensure only one sperm fertilizes the egg.
- A thick outer layer called the zona pellucida surrounds the egg, preventing multiple sperm from penetrating.
- After fertilization, a fast block to polyspermy occurs when the egg’s membrane depolarizes, preventing further sperm entry.
- A slow block to polyspermy occurs when the egg’s cortical granules release enzymes that alter the zona pellucida, making it impenetrable to additional sperm.
Question 2:
What role does the zona pellucida play in preventing polyspermy?
Answer:
- The zona pellucida, a protective glycoprotein layer surrounding the egg, serves as a physical barrier against polyspermy.
- It prevents multiple sperm from reaching the egg’s plasma membrane, thereby reducing the chances of multiple fertilizations.
- The zona pellucida undergoes changes after fertilization, contributing to the slow block to polyspermy.
Question 3:
How does the fast block to polyspermy differ from the slow block?
Answer:
- The fast block to polyspermy occurs immediately upon fertilization when the egg’s membrane undergoes depolarization.
- This prevents the entry of additional sperm by creating an electrical barrier around the egg.
- In contrast, the slow block to polyspermy occurs gradually after fertilization when the egg’s cortical granules release enzymes.
- The enzymes modify the zona pellucida, making it impermeable to sperm, and contribute to the prevention of polyspermy.
And that’s how the delicate dance between egg and sperm is regulated, preventing the chaotic scenario of multiple fathers. Thanks for sticking around and learning about this fascinating biological mechanism. If you’re curious about other scientific wonders, feel free to swing by again. We’ve got plenty more mind-boggling topics to keep you entertained and informed!