Model Answer
0 min readIntroduction
Polyspermy, the fertilization of an egg by more than one sperm, is a lethal event for diploid organisms, leading to an abnormal number of chromosomes and typically resulting in a non-viable zygote. To ensure proper embryonic development and the maintenance of species-specific chromosome numbers, organisms have evolved sophisticated mechanisms to prevent polyspermy. These protective barriers are collectively known as the "block to polyspermy," acting rapidly and effectively to allow only a single sperm to fertilize the egg.
The block to polyspermy involves two primary mechanisms, often described as fast and slow blocks, which work in tandem to prevent the entry of supernumerary sperm.
1. Fast Block to Polyspermy
- This is an immediate, transient electrical block.
- Upon the first sperm's fusion with the egg plasma membrane, there is a rapid influx of sodium ions (Na+) into the egg cell.
- This influx causes a depolarization of the egg membrane potential, changing it from negative to positive.
- The depolarized membrane repels other incoming sperm, physically preventing their fusion with the egg.
- While well-characterized in marine invertebrates like sea urchins, its role in mammals is less understood, with some sources suggesting it's less significant or absent in mammalian fertilization.
2. Slow Block to Polyspermy (Cortical Reaction)
- This is a slower, but permanent, chemical and mechanical block.
- Triggered by the sperm's entry, an intracellular wave of calcium ions (Ca2+) is released from the egg's endoplasmic reticulum.
- This calcium surge causes cortical granules, vesicles located just beneath the egg's plasma membrane, to fuse with the membrane and release their contents into the perivitelline space.
- These contents, including enzymes like proteases, modify the extracellular matrix of the egg:
- In sea urchins, they lead to the formation of a rigid 'fertilization envelope'.
- In mammals, they modify the zona pellucida (e.g., cleaving ZP2 and ZP3 receptors), making it impenetrable to other sperm (known as the 'zona reaction') and preventing further sperm binding and penetration.
- The slow block is considered the main mechanism for preventing polyspermy in humans, typically occurring within 5-8 minutes after sperm entry.
Together, these blocks ensure monospermy, which is crucial for the formation of a viable zygote with the correct diploid chromosome number, thereby safeguarding embryonic development.
Conclusion
The block to polyspermy is a critical evolutionary adaptation ensuring successful reproduction by preventing the catastrophic genetic consequences of fertilization by multiple sperm. Through a rapid electrical depolarization and a subsequent, more sustained cortical reaction, the egg effectively creates an impenetrable barrier, maintaining the species' genetic integrity. Understanding these intricate mechanisms is fundamental to reproductive biology and has implications for fields like assisted reproductive technologies, where monitoring and preventing polyspermy is paramount for viable embryo formation.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.