Fertilization means the process of union of the male gamete or sperm with the female gamete or ovum.
The process is irrespective/independent of the relative structure, behavior, and nature of the gametes.
The ovum carries the maternal hereditary information and it is small and passive. On the other hand the sperm bears the paternal genetic information and is large and active.
The single-cell formed after the fusion of sperm and ovum after fertilization is called the zygote.
During this process, the haploid male gamete (with ‘n’ number of chromosomes) combines with the haploid female gamete (with ‘n’ number of chromosomes) resulting in the formation of diploid (having 2n number of chromosomes) zygote.
The zygote undergoes several mitotic divisions to produce a new organism with a normal (2n) set of chromosomes in its somatic cells.
Strictly speaking, the word fertilization denotes the process of making the egg fruitful, i.e., to activate the egg by sperm contact.
The fertilization occurs in the aquatic media such as sea-water, fresh-water, or intra-somatic (body) fluid of maternal individuals.
Types of Fertilization
Fertilization is of various types, based on the occurrence and numbers of sperms involved.
- External Fertilization: It occurs outside of the body of an organism. For example: in Frog
- Internal Fertilization: It occurs inside the body of an organism. This type of fertilization is seen in those animals that possess specialized sex organs for receiving and transmitting sperm. For example: in Rabbit and Man
- Monospermic Fertilization: It occurs when only one sperm enters the egg. For example: in the frog, mammals, etc.
- Polyspermic Fertilization: This type occurs when many sperm enters the egg. For example: in reptiles and birds (due to large yolky eggs).
Why union of Gametes is very specific?
The process of fertilization is very specific. The sperm of one particular species fertilize the ova of the same species.
This type of specificity of male and female gametes is of utmost biological importance. It is achieved with the help of certain chemical substances.
Previously, it was thought that fertilization was possible due to attraction between the male and female gametes. But now it is presumed that the process is purely accidental.
It is found that the egg contains a chemical substance called fertilizing. The sperm has another chemical substance called antifertilizin present on its surface.
The fertilizin interacts with antifertilizin and makes the sperm adhere to the egg membrane.
In other words, the eggs attract the sperm that contain a particular type of antifertilizin.
We also got to know that the egg fertilizin of any species reacts efficiently with the sperm antifertilizin of the same species.
Process of Fertilization
The process of fertilization includes two major successive steps such as:
- Activation of eggs.
- Amphimixis or fusion of gamete nuclei.
Activation of Eggs
The process of activation of the egg is completed in the following stages:
1. Movement of the sperm towards the egg
Fertilization always takes place in a fluid medium. The sperm swim towards the egg at random and collide with the egg by chance.
The fertilizin and antifertilizin become active after the chance collision of the sperms with the ova.
The egg fertilizin usually occurs in the jelly surrounding the egg. It gradually dissolves in the surrounding water of the egg and forms the so-called egg water.
2. Penetration of Sperms into the ovum
It is accomplished by the (a) activation of sperms and (b) the activation of egg and insemination.
a. Activation of Sperm
When a sperm with a specific antifertilizin comes in contact with the egg water of its own species, then certain significant changes occur in the acrosome of sperm.
The peripheral portion of the acrosome of the sperm collapses and its enzyme, the lysin is released out and dissolve in the water.
The central portion of the acrosome elongates to form a long, thin, and rigid tube known as acrosomal filament.
When the sperm acquires such acrosomal filament, it is said to be activated and becomes ready for penetration in the unfertilized egg.
When the activated sperm reaches the egg, the acrosomal filament penetrates the egg jelly and vitelline membranes by the dissolving action of the lysin enzymes.
As soon as the tip of the acrosomal filament touches the egg membrane, various important morphological and physiological changes occur in the egg.
b. Activation of egg and insemination:
When acrosomal filament touches the egg surface, the cytoplasm of the egg bulges out to form a cone-like precess, known as the fertilization cone at the point of contact.
The fertilization cone engulfs the sperm and the sperm moves inwards. Only the head and middle piece of the sperm enter but not the tail.
The entry of the sperm into the egg is known as insemination.
Just after insemination the vitelline membrane increases in size and separates from the plasma membrane forming a fertilization membrane.
This membrane formed around the plasma membrane of the egg prevents the entry of further sperms. The space between the fertilization membrane and plasma membrane is called perivitelline space.
The route through which the sperm moves into the cytoplasm of the egg is known as the penetration path.
Then the sperm changes its direction to reach the female pronucleus. This changed route is known as the copulation path.
On entry of the sperm, the egg completes the second maturation division and gives off a second polar body. Under the vitelline membrane, the first polar body is also seen.
Now the nucleus of the sperm is known as male pronucleus and similarly, the nucleus of the egg is known as female pronucleus.
After extrusion of the second polar body, the female pronucleus approaches the male pronucleus.
The sperm rotates in such a way so that the middle piece containing centrosome and mitochondria remains in between male and female pronuclei.
Now the haploid number of chromosomes appears in each pronucleus and nuclear membranes of both the pronuclei dissolve.
Then centrosome divides into two, forming asters around each centriole. The maternal and paternal chromosomes now become arranged around the equator of the spindle.
In this way, the diploid number of chromosomes is restored, forming a zygote nucleus. This spindle represents the first cleavage spindle.
The zygote now becomes ready for cleavage and formation of the embryo.
Importance of Fertilization
- The egg is activated by the entry of sperm and gets stimulus to undergo second maturation division releasing the second polar body.
- As a result of fertilization, the zygote is formed in which a diploid number of chromosomes is restored.
- During fertilization, the centriole is introduced into the egg by the sperm. This centriole is absent in the mature egg and it initiates the division of nucleus of zygote.
- The entry of the sperm brings about the formation of fertilization membrane which prevents further entry of the sperms.
- Fertilization provides a new genetic constitution and variations in individuals and this is due to the mixing of chromosomes of two gametes. The zygote containing maternal and paternal chromosomes develops into a new organism.
- It enhances the metabolic activities and rate of protein synthesis of the cell and establishes definite polarity in the egg.
Significance of Fertilization
The process of fertilization brings out the following significant results:
- The gametes from the only physical bridge between successive generations. These gametes possess a haploid (n) number of chromosomes. Fertilization restores the specific parental diploid (2n) set of chromosomes.
- Fertilization brings together the chromosomes and genes from two different lines of ancestry and results in new genetic recombinations. This ensures improved adaptations to ever-changing environments.