Generally, two sperm do not fertilize one egg. When this rare event occurs, it's called dispermy, and it usually leads to non-viable embryos.
The Normal Fertilization Process
Normally, only one sperm fertilizes an egg due to specific mechanisms that prevent polyspermy (fertilization by more than one sperm).
- Zona Pellucida Reaction: According to the provided reference, when a single sperm makes contact with the zona pellucida layer (the outer layer of the egg), changes occur in the egg's membrane. These changes block the entry of any further sperm.
Dispermy: When Two Sperm Fertilize an Egg
Dispermy is a rare event where two sperm manage to penetrate and fertilize a single egg. This typically happens when the mechanisms preventing polyspermy fail.
What Happens in Dispermy?
- Abnormal Chromosome Number: A normal egg has 23 chromosomes, and a normal sperm has 23 chromosomes. After normal fertilization, the resulting embryo has 46 chromosomes (23 pairs). When two sperm fertilize one egg, the resulting embryo has 69 chromosomes (triploidy).
- Non-Viable Embryos: Embryos with an abnormal number of chromosomes, like those resulting from dispermy, usually do not survive. These pregnancies often end in early miscarriage.
Why is Dispermy Rare?
The egg has several mechanisms to prevent more than one sperm from entering:
- Fast Block to Polyspermy: An immediate change in the egg's membrane potential prevents other sperm from binding.
- Cortical Reaction: This slower, more permanent block involves the release of cortical granules that alter the zona pellucida, preventing further sperm penetration (as mentioned in the reference relating to changes to the membrane when one sperm contacts it).
Conditions Where Dispermy Might Occur
While rare, dispermy can occur in these scenarios:
- In Vitro Fertilization (IVF): Although IVF techniques are highly controlled, there is a slightly increased risk of polyspermy compared to natural conception.
- Defects in Egg Activation: If the egg's activation processes are not functioning correctly, it might not trigger the necessary blocks to polyspermy.
| Feature | Normal Fertilization | Dispermy (Two Sperm) |
|---|---|---|
| Sperm Involved | One | Two |
| Chromosome Count | 46 | 69 |
| Viability | Usually Viable | Usually Non-Viable |
