Fertilization is one of the most important processes in biology, ensuring the union of a sperm and an egg to form a zygote. But what if more than one sperm enters the egg during fertilization.? This condition, called polyspermy, leads to abnormal chromosome numbers (polyploidy). This usually results in a defective or non-viable embryo. To avoid this, nature has evolved special defense mechanisms inside the egg to ensure only a single sperm fertilizes it — a process known as monospermy.In this article, we will explore how the egg prevents polyspermy through two key mechanisms: fast block to polyspermy and slow block (cortical reaction) to polyspermy
Polyspermy Definition
In simpler words, polyspermy is:
👉 “The condition where multiple sperm fertilize a single egg, disrupting normal development.”
How is Polyspermy Prevented?
The egg prevents polyspermy through two ion-driven mechanisms:
- Fast Block (Na⁺ influx): As soon as the first sperm fuses, sodium ions rush in, causing membrane depolarization. This creates an immediate but temporary electrical barrier that stops other sperm from fusing.
- Slow Block (Ca²⁺ wave): The sperm entry triggers a calcium surge that releases cortical granules. These granules modify the egg’s outer covering, forming a permanent mechanical and chemical barrier.
Na⁺ (Fast Block): Immediate depolarization → sperm fusion impossible.
Ca²⁺ (Slow Block): Wave release → cortical granules → hardened barrier.
Integration: Na⁺ gives a temporary shield, Ca²⁺ builds a permanent wall.
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Molecular Mechanisms of Fast and Slow Blocks to Polyspermy
1. Fast Block to Polyspermy (Na⁺-Mediated Electrical Block)
Type: Electrical Barrier.
- Trigger: As soon as the first sperm fuses with the egg, the plasma membrane responds within seconds.
- Ion Movement: Rapid influx of sodium ions (Na⁺) depolarizes the egg membrane (from –70 mV → +20 mV).
- Effect: Other sperm cannot fuse with a depolarized membrane because their fusion proteins fail to bind.
- Timeframe: Immediate but temporary (few seconds to minutes).
- Organisms: Seen in amphibians, sea urchins, and some invertebrates; less prominent in mammals.
2. Slow Block to Polyspermy (Ca²⁺-Mediated Cortical Reaction)
Type: Electrical Barrier
- Trigger: Sperm entry activates a calcium wave across the egg cytoplasm.
- Molecular Pathway:
- Sperm introduces PLCζ enzyme.
- PLCζ hydrolyzes PIP₂ → IP₃ + DAG.
- IP₃ binds ER receptors → Ca²⁺ released in waves across the egg.
- Effect of Ca²⁺ Rise:
- Stimulates cortical granule exocytosis.
- Enzymes modify & harden the egg covering (zona pellucida in mammals, vitelline layer in sea urchins).
- Fertilization membrane forms → permanent barrier to sperm.
- Additional Role: Ca²⁺ also activates the egg → meiosis II completion + metabolism restart.
- Timeframe: Begins 1–2 minutes after fertilization; permanent.
- Organisms: Universal in animals; primary block in mammals.
Comparative Table: Fast vs Slow Block
| Feature | Fast Block | Slow Block (Cortical Reaction) |
|---|---|---|
| Mechanism | Na⁺ influx → depolarization | Ca²⁺ wave → cortical granule release |
| Type | Electrical | Mechanical + Chemical |
| Time of Action | Immediate (seconds) | Delayed but permanent (1–2 min) |
| Duration | Temporary | Permanent |
| Organisms | Amphibians, sea urchins | All animals (main in mammals) |
| Barrier Type | Electrical repulsion | Fertilization membrane, hardened coat |
Prevention of Polyspermy (General View)
Across animals:
- Sea urchins & amphibians rely heavily on both fast & slow blocks.
- Mammals depend mostly on the slow Ca²⁺-dependent cortical reaction.
This flexibility ensures evolutionary success in diverse environments.
Why is Blocking Polyspermy Important?
Blocking polyspermy is critical because:
- It maintains genetic stability (diploid chromosome number).
- It ensures the development of a healthy zygote.
- It prevents early embryonic death caused by polyploidy.
Without these blocks, normal animal reproduction would not be possible.
Conclusion
The egg uses a two-step defense system to prevent polyspermy:
- Fast Na⁺-dependent electrical block – immediate but temporary.
- Slow Ca²⁺-dependent cortical reaction – delayed but permanent.
Together, these processes protect the zygote, ensuring normal development and the continuation of life.
| Block Type | Ion Involved | Mechanism | Outcome |
|---|---|---|---|
| Fast Block | Na⁺ influx | Depolarizes egg membrane | Immediate electrical barrier |
| Slow Block | Ca²⁺ release (IP₃ pathway) | Triggers cortical granule exocytosis | Permanent chemical + mechanical barrier |
Also Read : Spermatogenesis: Definition, Stages, Process, and Spermiation