Stage 2 of fertilization:
Penetration of the zona pellucida around the oocyte:
- Acrosomal enzymes: esterases, acrosin, and neuraminidase cause lysis of the zona pellucida
Once sperm penetrates zona pellucida, the zona reaction occurs:
- This reaction makes the zona pellucida impermeable to other sperms.
- When more than one sperm manages to enter the ovum (dispermy = 2; triploidy = 3), the fetus nearly always aborts.