• Fusion of sperm nucleus with egg nucleus to form a diploid zygote cell
• Occurs in upper parts of fallopian tube ie. Ampulla

Process of Fertilization

1. Ejaculation in vagina

2. Capacitation – removal of glycoproteins to make sperm active and identify ovum

3. 1st contact – Acrosome reaction

• Contact of head of sperm with zona granulosa cells
• Acrosome reaction – Acrosome burst by exocytosis
• Releases hydrolytic enzymes

4. Break down of first barrier – Corona radiata

• Enzymes digest the sticky substances that holds the follicle cells together
• Cause cells to break away from zona pellucida

5. Break down of 2nd barrier – Zona pellucida

• Another acrosomal enzyme then digests the zona pellucida to create path for entry of the sperm cell

6. Contact with third barrier – Oocyte membrane

Formation of fertilization membrane:

• Head of 1st sperm makes contact with oocyte membrane
• Oocyte membrane become permeable to calcium ions
• Calcium ions rush into oocyte by diffusion
• Calcium ions stimulate rapid exocytosis of cortical granules, which release chemical substances
• Cortical reaction – release of cortical granules
• A new membrane forms, called fertilization membrane
• Fertilization membrane prevents polyspermy (entry of many sperm cells)

Q) How does fertilization membrane prevent polyspermy?

Acrosomal enzymes released by other sperms cannot digest the changed zona pellucida/fertilization membrane

7. Release of sperm nucleus into the secondary oocyte

8.  This stimulates 2ry oocyte to complete 2nd meiotic division and form a true ovum

9. Fusion of male and female pronucleus (enlarged nucleus) to form a zygote