Question

Describe the chemical and physical events of fertilization in mammals.

Answer 1

• Fertilization is the process of union of an ovum with the sperm. The site of fertilization is a fallopian tube (ampullary-isthmic junction).
  
• Fertilization performs two basic functions in the process of embryonic development:
  

(a) Upon fertilization, the secondary oocyte completes meiotic division and produces a second polar body, which degenerates.

(b) A diploid zygote is formed, which undergoes cleavage to form a morula and then a blastocyst, which implants in the endometrium.

• Chemical events that occur during fertilization are as follow:

1. Approximation of sperm and ovum: This can be done by fertilizin-antifertilizin compatibility reaction. Fertilizin which is a glycoprotein of egg interacts with the antifertilizin i.e., composed of acidic amino acids of sperm of the same species which results in making the sperms stick to the egg surface.
   
2. Zona lysins are proteolytic enzymes that are capable of degrading the zona pellucida, perhaps easing the passage of sperm cells through to the ovum.
   

• Physical events that occur during fertilization are as follows:

1. The acrosomal membrane fuses with the plasma membrane of the sperm cell.
   
2. The enzymes within the acrosome are released into the milieu surrounding the sperm and egg.
   
3. Activation of ovum: As sperm enters the ovum (actually a secondary oocyte) it gets activated and undergoes the second meiotic division. As a result of this, the oocyte produces an ovum and a secondary polar body. The polar bodies ultimately degenerate and the ovum contains the haploid number of chromosomes. As the sperm cells approach the egg, the acrosome reaction occurs to prepare the sperm to fertilize the ovum (called capacitation).