Question

Define fertilization and write the list of events occuring in human fertilization.

Answer 1

• Fertilization is the process by which the male gamete or the sperm fuses with the female gamete or the ovum.
• The product of fertilization is a cell called a zygote.
• The steps involved in human fertilization are:-

1) Preparation of the Sperm:-

• Ejaculated sperm are not ready to fertilize an egg when they enter the vagina.
• Dilution of semen occurs in the vagina and they also undergo several changes, which are collectively known as capacitation.

2) Sperm- Zona pellucida recognition and binding:-

• Binding of sperm to the zona pellucida ( egg's tough membrane) is a receptor-ligand interaction.
  

3) Acrosome Reaction:-

• The sperm penetrates the zona pellucida to get to the oocyte.
  
• As the acrosome reaction progresses, the sperm passes through the zona pellucida, most of the plasma membrane and acrosomal contents are lost.
  

4) Sperm-Oocyte Binding:-

• Once a sperm penetrates the zona pellucida, it binds to and fuses with the plasma membrane of the oocyte.

5) Egg Activation and the Cortical Reaction:-

• Prior to fertilization, the egg is arrested in metaphase of the second meiotic division.
• Binding of a sperm causes the egg to rapidly undergoes a number of metabolic and physical changes called egg activation.
• The cortical reaction is the massive exocytosis of cortical granules seen shortly after sperm-oocyte fusion.