If two sperms enter one egg, a condition called polyspermy occurs. This leads to severe problems during cell division.
The Consequences of Polyspermy
When multiple sperm fertilize a single egg, several critical issues arise, primarily concerning the chromosomes. Here's a breakdown:
- Faulty Mitotic Spindles: The presence of multiple sperm results in the formation of multipolar or extra mitotic spindles. This disrupts the normal process of cell division.
- Normal cell division relies on a bipolar spindle that ensures each daughter cell receives an identical set of chromosomes.
- Extra spindles cause chaotic chromosome segregation, resulting in unequal distribution of genetic material.
- Production of Nondiploid Cells: The outcome of faulty chromosome segregation is the creation of nondiploid cells.
- Diploid cells contain the correct number of chromosomes (one set from each parent).
- Nondiploid cells have an incorrect number of chromosomes, leading to severe developmental problems.
- Developmental Arrest: The most common result of polyspermy is the cessation of development.
- The improperly organized cells and incorrect chromosome numbers make normal development impossible.
- The embryo usually cannot survive.
Mechanisms to Prevent Polyspermy
The body has mechanisms to ensure that only one sperm enters an egg, to avoid polyspermy:
- Fast Block to Polyspermy: This is an immediate change in the egg's membrane potential that prevents further sperm from fusing.
- Slow Block to Polyspermy: A change in the egg's outer covering, called the zona pellucida, after initial sperm entry, which hardens the outer layer to prevent further sperm entry.
| Feature | Result of Polyspermy | Prevention Mechanism |
|---|---|---|
| Mitotic Spindles | Multipolar/Extra Spindles | Fast & Slow Block Mechanisms at the egg membrane & zona pellucida |
| Chromosome Segregation | Faulty, Unequal Distribution | |
| Cell Ploidy | Nondiploid Cells | |
| Development | Typically Stops |
In summary, while multiple sperm may attempt fertilization, mechanisms are in place to avoid polyspermy to ensure normal embryonic development. When these fail and polyspermy happens, the results are severe, usually ending in the cessation of development due to the improper segregation of chromosomes and nondiploid cell production, as outlined in our reference.
