Definition:
The process of union of a haploid male gamete or sperm with a haploid female gamete or ovum to form a diploid cell, the zygote, is called fertilization.
Site of Fertilization:
In man fertilization is internal (external in case of frog) as in other mammals. It takes place usually in the ampulla of the fallopian tube.
Process of Fertilization:
Male discharges the semen into the vagina of the female during copulation (coitus). From the vagina, the sperms reach the ampulla partly by the movement of their tails and partly by the action of uterus. The sperms present in the semen travel a long way from vagina through the uterus into the fallopian tube.
Sperms may reach fallopian tube within five minutes. The sperm can survive in the female’s reproductive tract for 1 to 3 days and it can fertilize the ovum in 12 to 24 hours following ovulation. During sexual intercourse, nearly 300 million sperms are introduced into the vagina, but only few hundreds of them reach near the ovum.
Events of Fertilization:
1. Activation of sperm and ovum:
The sperms can fertilize an ovum only they are able to secrete the chemical hyaluronidase and possess a surface protein called antifertilizin (composed of acidic amino acid). The ovum secretes a chemical named fertilizin (composed of glycoprotein = mono saccharides + amino acids). It mixes with the water to form egg water which attracts the sperms of its own species.
2. Penetration of sperm:
The fertilizin of an egg interacts with the anti fertilizin of sperm of the same species. This attraction between fertilizin and antifertilizin makes the sperms stick to the egg surface. The process of acquiring the capacity to fertilize the egg by the sperm is called capacitation. In this process, the membrane surrounding the acrosome of the sperm breaks and releases its contents, the sperm lysin. It is the chemical substance present in the sperm’s acrosome.
The ovum is surrounded by three membranes such as corona radiata, zona pellucida and the vitelline membranes. At first the sperm passes through corona radiata to reach zona pellucida. There it releases the enzyme hyaluronidase or sperm lysin from its acrosome (Fig. 3(B).8). This enzyme dissolves zona pellucida as a result of which the sperm reaches the plasma membrane of the egg. The above changes on the sperm head are called acrosome reaction.
At the point of contact with the sperm, the egg forms a projection, termed the cone of reception or fertilization cone which receives the sperm. Once one sperm has entered the egg (ovum) the vitelline membrane thickens and is converted into fertilization membrane. This membrane is rigid and never allows other sperms to pass through this membrane. Penetration of the sperm initiates a second maturation division of the ovum and a second polar body is given off.
3. Amphimixis:
A sperm consists of three parts: head, middle piece and tail. Shortly before or after entering the egg, the sperm loses its tail (Fig. 3(B).9). After the sperm enters the egg, the membrane of head and middle piece dissolves, liberating nucleus, centrosome and mitochondria. Now the sperm nucleus enlarges to form the male pronuclear and the nucleus of the ovum becomes female pronuclear.
The male pronuclear moves inwards and then changes its direction to meet the egg nucleus. The initial path is known as penetration path and the second path is known as copulation path. The chromosomes (haploid set) of (he sperm and the chromosomes (haploid set) of the egg or ovum are set free by the breakdown of their nuclear envelops.
Mixing up of the chromosomes of a sperm and an ovum resulting in a diploid zygote nucleus is known as amphimixis or karyogamy. The mother is now said to be pregnant. The centrosome form asters and spindle fibres. The paternal and maternal chromosomes move to lie in the equator of the spindle and the zygote is ready for division by cleavage.
Significance:
Fertilization has the following significance:
1. Fertilization restores the diploid number of chromosomes, i.e. 46 in human being.
2. It provides stimulus for the ovum to complete its maturation.
3. Fertilization combines the characters of two parents. This brings about recombination of genes and introduces variations.
4. It determines the sex of the embryo in humans.
5. Fertilization introduces the centrioies which are absent in ovum.
6. Fertilization membrane formed after the entry of the sperm prevents the entry of additional sperms.