What are the various steps in fertilization?

What are the various steps in fertilization?

The act of fusion of the sperm and the method of approach, entry and eventual fusion of the male and female nuclei constitutes the mechanism of fertilization. This is believed to take place in the following stages.

1. Movement of the sperm towards the egg.

2. Capacitation and contact.

3. Penetration of sperm into ovum.

4. Cortical reaction.

5. Activation of the ovum.

6. Fusion of male and female pronuclei (amphimixis).

1. Movement of the sperm towards the egg:

This is the first step which brings the sperm in physical contact with the ovum. As the male and female gametes are produced in different individu­als, there are various mechanisms to bring them nearer. The initial attrac­tion of the sperms towards the egg is supposed to be chmotactic. The sperms swim towards the egg collides with it. Usually several sperms at­tach themselves to the egg.

In individuals with external fertilization large number of eggs and sperms are released outside so that they have a favourable chance of meeting with each other. In individuals with internal fertilization the sperms are dis­charged into the genital tract of the females by the male individual. From here the sperms move upward and reach the egg present in the uterus.

2. Capacitation and contact:

When a large number of sperms approach the egg for contact, the fertilizin and antifertilizin reaction ensures that only a few spermatozoa are allowed to reach the ovum. The initial attachment of the sperm to the egg is be­lieved to be due to the chemical bonding of fertilizin and antifertilizin.

3. Penetration of sperm into ovum:

The acrosome portions of the sperm produces some lytic enzymes called sperm lycines which have the ability to dissolved the egg membrane al­lowing the entry of the sperm into the egg cytoplasm. Usually only the head and the middle piece of the sperm enter the egg while the tial is left outside.

4. Cortical reaction:

The entry of the sperm head into the egg brings about several changes in the egg surface. These are the cortical changes and the development of fertilization membrane. In some echinoderm eggs some fine granules are visible in the ooplasm after the entry of the sperm head.

The vitelline membrane starts lifting itself up from the point of sperm entry and a perivitelline space is formed between it and the egg surface. The cortical granules from the egg cortex release their contents into the privitelline space. These contents attach themselves to the inner surface of the vitelline membrane forming what is known as a fertilization mem­brane. The development of the fertilization membrane prevents the entry of other sperms into the egg.

5. Activation of the ovum:

The mature egg will be generally in a hibernating condition with very low rates of metabolism and inactive nucleus. The penetration of the sperm triggers the egg into activity. The metabolic rates increase allowing for entry of water and other particles. Metabolically the rate of protein syn­thesis goes up as these are needed for further cell divisions. At this stage the nucleus (pronucleus) of the egg which has remained in the metaphse II stage (of the meiotic II division) becomes active completing its second division and releases the second polar body.

6. Fusion of male and female pronuclei (amphimixis):

In this process there is fusion of the male and female nuclei. Initially the two nuclei remain close together and at the point of contact the nuclear membranes disappear and the chromosomes come to lie on the equator. Finally the nuclear fusion is completed and it becomes a zygote nucleus. The egg is said to have been fertilized and it becomes a zygote. It is now ready to undergo cleavage to develop into the embryo.