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  1. During copulation, semen containing millions of sperms are deposited in the vagina of the female.
  2. Only a few hundred sperms reach the site of fertilization, i.e. the fallopian tube.
  3. Usually, only a single sperm penetrates the ovum (egg).
  4. During the penetration process, the sperm produced anti-fertilizin.
  5. The egg is surrounded by layers of follicular cells called Corona radiata.
  6. The cells of the corona radiate are joined to each other by a mucopolysaccharide, i.e. hyaluronic acid.
  7. Thus, the corona radiata acts as a barrier for the entry of sperms.
  8. To overcome this barrier, the sperm releases hyaluronidase (sperm lysins) and proteases.
  9. Due to the activity of this enzyme, the cells of the corona radiata, at the point of contact with the sperm, get digested.
  10. The sperm then achieves contact with the zona pellucida.
  11. The acrosome secretes another enzyme called zona lysine so that an aperture is created in the zona pellucida.
  12. Sperm then achieves contact with the plasma membrane of the ovum.
  13. At the point of contact, the plasma membrane of the ovum and the sperm get dissolved and only the nucleus of the sperm enters the ovum.
  14. The remains of the acrosome, tail and mitochondria of the middle piece of the sperm remain outside and do not enter the egg.
  15. A signal is transmitted to the egg surface, once a sperm enters the ovum.
  16. This prevents the entry of other sperms.
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