Fertilization Notes | Study Biology Class 12 - NEET

What is Fertilization?

• The process in which union of male and female gametes (formed by gametogenesis) and fusion of pronuclei of sperm and ovum take place thus diploid zygote is formed, which is called fertilization.
• Fertilization has the following processes:- The union of male and female gametes is called Syngamy, whereas intermixing of their cytoplasm is called plasmogamy. 
• The fusion of pronuclei of sperm and ovum is called karyogamy. 
• The intermingling of their chromosomes is called amphimixis. 
• Due to fertilization, a diploid zygote is formed, by the union of two different types of gametes.

Fusion of gametes

Mechanism of fertilization:

We can understand the process of fertilization in the following steps.

1. Movement of sperms towards the secondary oocyte.

2. Penetration of corona radiata by the hyaluronidase enzyme.

3. Adherence of sperm to the ZP3 receptors/region on zona pellucida, the glycoprotein layer surrounding the oocyte.

4. Sperm binds to a sperm receptor on the zona and this leads to the initiation of the acrosomal reaction. Various enzymes are released. eg. Acrosin (Zona lysin). 

5. Acrosin facilitates the penetration of sperm through zona pellucida.

6. Fusion of sperm and membrane of secondary oocyte. (Syngamy)

7. Phagocytosis of sperm by the secondary oocyte.

8. Completion of meiosis-II of the secondary oocyte during phagocytosis to form ovum and simultaneously it releases 2nd polar body.

9. Structural changes in zona pellucida through cortical reaction and discharge of cortical granules in perivitelline space forms fertilization membrane, which prevents polyspermy.

10. In the event of fertilization complete sperm enters inside the ovum. (By phagocytosis)

11. It is followed by plasmogamy, karyogamy and amphimixis i.e. completion of fertilization.

Sperms entering the Ovum

The fate of sperm in the egg:

• 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 the egg. All the structures of sperm dissolve in the egg cytoplasm except the sperm nucleus proximal centriole.
• The centriole of the egg itself degenerates at the time of second maturation division. So proximal centriole of sperm starts division, it divides into 2 daughter centrioles, which migrate towards the opposite pole and start forming spindles.

The fate of sperm nucleus:

• 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 fertilization paths.

Significance of Fertilization

1. Secondary oocyte completes its second maturation division on coming in contact with the sperm.

2. 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.

5. The peripheral changes occurring in the egg prevent the further entry of sperm into the ovum, thus checking polyspermy.