Introduction to Gamete Fusion and Prevention of Polyspermy in Mammals

The process of gamete fusion in mammals involves a series of intricate steps, starting from the initial contact between the sperm and egg, to the critical mechanisms that ensure successful fertilization. Key proteins like Izumo and Juno play essential roles in sperm-egg binding and fusion. Additionally, the prevention of polyspermy is crucial to avoid multiple sperm from fertilizing the egg, which is achieved through mechanisms such as ZP2 clipping and the release of Juno protein.

1. Gamete Fusion in Mammals

• Contact Point:
  • Unlike sea urchins, where sperm enters perpendicularly, in mammals, the side of the sperm head makes contact with the egg.
  • Acrosome reaction expels enzymatic contents and exposes the inner acrosomal membrane.
  • Membrane fusion begins at the equatorial region where the inner acrosomal membrane meets the sperm cell membrane.
  • Actin polymerization extends microvilli from the egg to the sperm.

2. Mechanism of Mammalian Gamete Fusion

• Izumo Protein:
  • Originally found in the acrosomal granule membrane.
  • After acrosome reaction, redistributes to the equatorial section on the surface of acrosome-reacted sperm.
  • Critical for sperm-egg binding and fusion.
  • Antibodies against Izumo prevent sperm-egg fusion in humans.
• Juno Protein:
  • Oocyte protein.
  • Binds to Izumo on the sperm.
  • Recruits CD9 protein to the sperm-egg adhesion area.
  • CD9 is essential for fusion and is also involved in muscle cell fusion.

3. Prevention of Polyspermy

• ZP2 Clipping:
  • Cortical granule enzymes modify zona pellucida sperm receptor proteins.
  • Ovastacin protease clips ZP2, preventing sperm binding.
  • Ovastacin is released during cortical granule fusion.
  • Mutant ZP2 leads to increased polyspermy.
• Juno Release:
  • Juno protein released during sperm-egg membrane fusion.
  • Soluble Juno binds sperm in the perivitelline space.
  • Prevents further sperm binding by removing docking sites.

Conclusion

Mammalian fertilization involves intricate steps, including gamete fusion and polyspermy prevention mechanisms, ensuring successful reproduction .