FERTILIZATION

The process of fusion of sperm(male gamete) with an ovum(female gamete) is called fertilization. Fertilization can only occur if sperm and ovum are transported simultaneously to the ampullary region of the female reproductive organ.


All copulation(Sexual Intercourse) does not lead to fertilization. During fertilization, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms.
The process in which the union of male and female gametes (formed by gametogenesis) and fusion of pronuclei of sperm and ovum takes place thus diploid zygote is formed, which is called fertilization. Thus, it ensures that only one sperm can fertilize an ovum.

Fertilization has the following processes

  • SYNGAMY:- Fusion of plasma membrane of male and female gametes is called Syngamy.
  • PLASMOGAMY:- Intermixing of their cytoplasm is called plasmogamy.
  • KARYOGAMY:-The fusion of pronuclei of sperm and ovum is called karyogamy. 
  • AMPHIMIXIS:- The intermingling of their chromosomes is called amphimixis.

Steps of Fertilization


  • Movements of sperms towards the secondary oocyte.
  • Release of acrosomal enzymes (sperm lysin)
  • Hyaluronidase present in the acrosome dissolves the hyaluronic acid present between corona radiata cells

  • Corona penetrating enzymes dissolve cells of corona radiata.
  • Adherence of sperm to the ZP3 receptor on zona pellucida, the glycoprotein layer surrounding the oocyte.
  • Recognition of the sperm by ZP3 receptor present on the zona pellucida
  • Sperm bind to a sperm receptor on the zona and this leads to the initiation of the acrosomal reaction. Release of enzyme Acrosin(zona lysin)
  • Acrosin facilitates the penetration of sperms through zona pellucida.
  • Acrosin breaks the zona pellucida from point of contact  
  • Fusion of sperm and membrane of the secondary oocyte (Syngamy)
  •  Completion of meiosis-II of the secondary oocyte during phagocytosis
  • Formation of ovum and a second polar body.
  • Structural change in zona pellucida through cortical reaction
  • Discharge of cortical granules in perivitelline space forms Fertilization membrane
  •  
  • Fertilization membrane – Prevents polyspermy  
  • In the majority of animals, only the head and middle piece enter inside the egg, and the tail is left outside.
  • (In mammals, whole sperm enters in the egg)

     

     
  • All the structures of sperm dissolve in the egg cytoplasm except the sperm nucleus and proximal centriole.
  • Intermixing of sperm and egg cytoplasm (Plasmogamy)
  • Proximal centriole of sperm starts division
  • It divides into 2 daughter centrioles, which migrate towards the opposite poles and start forming spindles.
  • The nucleus of sperm absorbs water from the egg cytoplasm and becomes enlarged. Now it is called male pronucleus.  
  • Male pronucleus and female pronucleus migrate through definite routes and come close to each other.
  • These routes are called the fertilization path
  • Fusion of pronuclei of sperm and ovum (Karyogamy) 
  • Intermixing of their chromosomes (Amphimixis)
  • Formation of diploid zygote
Significance of Fertilization

  1. The secondary oocyte completes its second maturation division by coming in contact with the sperm.
  2. The Amphimixis process leads to the formation of a diploid zygote to restore the normal diploid number of the chromosomes.
  3. The centriole of sperm after entering into the egg induces the egg to undergo cleavage.
  4. The paternal and maternal characters are transmitted to the offspring through the process of fertilization. 



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