What is Fertilization? Definition, Process, Stages and Significance
What is Fertilization?
Fertilization is the fusion of sperm and ovum in the isthmus region of the fallopian tube resulting in the formation of a zygote.
Events of Fertilization
Fertilization involves recognition and interaction between sperm and egg followed by several biological processes.
Capacitation
A species-specific chemical substance called fertilizin is produced by the egg and anti-fertilizin by sperm.
The interaction between fertilizin and anti-fertilizin enables sperm to fertilize the egg of the same species.
Fertilizin and Anti-Fertilizin Reaction
According to F. R. Lillie (1919), fertilizin consists mainly of glycoproteins and mucopolysaccharides.
Each fertilizin molecule contains multiple active groups capable of interacting with several sperm cells.
Acrosomal Reaction and Sperm Penetration
- Ion exchange increases intracellular pH.
- Fusion of acrosomal membrane occurs.
- Hydrolytic enzymes are released.
- Sperm penetrates the egg envelope.
- Fusion with egg membrane takes place.
Prevention of Polyspermy
Fast Block
A temporary electrical change in membrane potential prevents entry of additional sperm.
Slow Block
Calcium release initiates cortical reactions leading to formation of the fertilization membrane.
Cortical Reaction
- Cortical granules fuse with plasma membrane.
- Proteases modify the vitelline membrane.
- Water enters and expands the fertilization membrane.
- Peroxidase enzymes harden the membrane.
- Hyaline layer formation supports blastomeres during cleavage.
Activation of Egg
- Begins after prevention of polyspermy.
- Oxygen consumption increases.
- NAD kinase activity increases.
- Pronuclei formation and migration occur.
- Amphimixis results in zygote formation.
Regulation of Fertilization
| Hormone | Source | Function |
|---|---|---|
| Androgamone I | Sperm | Conserves sperm activity |
| Androgamone II | Sperm | Helps sperm entry |
| Gynogamone I | Ovum | Enhances sperm activity |
| Gynogamone II | Ovum | Facilitates sperm attachment |
Significance of Fertilization
- Restores diploid chromosome number.
- Produces genetic recombination.
- Initiates embryonic development.
- Promotes adaptation and variation.