The process in which union of male and female gametes (formed by gametogenesis) and fusion of pronuclei of sperm and ovum takes place thus diploid zygote is formed, is called fertilization.
Fertilization has following processes :- The union of male and female gametes is called Syngamy, where as 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.
(A) INTERNAL FERTILIZATION - Fertilization in the body (i.e., genital organs of animal) is called internal fertilization. In this type of fertilization, sperms are discharged by male directly into the genital tract of female after coitus. - Whole process of fertilization takes place within the body of female. This is the most common adaptation in terrestrial animals.
Examples :- Aschelminthes, reptiles, birds and mammals. (B) EXTERNAL
FERTILIZATION - External fertilization takes place outside the body of females i.e, in water.
Examples :- In most of the invertebrates, some protochordates, amphibia and most of the fishes.
(a) Self fertilization - This process takes place in the body of single animal i.e, fusion of male and female gametes produced by male and female organs of the same animal. This is called self-fertilization. This is possible only in bisexual or hermaphrodite animals.
Examples :- Taenia solium
(b) Cross-Fertilization - Fertilization takes place between two (male & female) different animals of same species.
This is called cross fertilization. - This process is found in all unisexual animals. These animals are also called dioecious animals - Cross- fertilization is also found in most of the bisexual or hermaphrodite animals because in these animals male genital organs develop first. This condition is called protandrous condition. In some of the species female organs develop first, this condition is called protogynous condition e.g. sponges.
MECHANISM - We can understand the process of fertilization in following steps.
1. Movement of sperms towards ovum.
2. Entrance of sperms in the ovum.
3. Activation of ovum.
Approach of sperm towards the egg - It is a chance factor, so sperms perform random (directionless) movement. To increase the chances of approach of sperm towards egg there are mainly two adaptation
(a) Number of sperms is very high :- e.g. In man 20 to 120 million sperms are present per cubic mm of semen.
(b) Egg is 1000 times larger than sperm - Some special proteins are found on the surface of egg and sperm to help in fertilization.
According toLillie, chemicals named as "fertilizins" are found on the surface of egg. Fertilizins are glycoproteins or acid mucopolysaccharides. According toBallinsky, an acidic protein named as Antifertilizin is present on the surface of sperms. "Fertilin" proteins are also present on sperm surface.
- Both the proteins are specific for a particular species. Antifertilizin present on sperm of a particular species will react with fertilizin of present on egg of the same species of animals.
- If we place some eggs of sea-urchin in sea-water, this sea water becomes viscous, this is called egg-water.
When some sperms come in contact with this egg water, sperms adhere with each other. It is called agglutination.
Here the reaction of fertilizin (dissolved in water from egg) and antifertilizin of sperm is observed clearly.
- Fertilizins behave like lock and antifertilizins behave like keys.
- Fertilization is always intraspecific.
- According to Washerman and Sailing (1989) a specific pair of protein molecules is found on the surface of mammalian sperm, which can recognise specific carbohydrates and proteins in ZP3 region of zona pellucida.
Thebindinprotein of sperm reacts with these molecules to initiate the changes in acrosome. A specific sugar galactose remains attached with ZP3 glycoprotein. The sperm fails to recognise the ovum of its own species, if this sugar is removed from zona pellucida. - In addition to these glycoproteins, there are some hormones also, which help in fertilization.
The hormones present at the surface of sperm are called androgamones. These are of two types. Androgamone first & androgamone second.
- Androgamones I help in the energy conservation of sperms.
- Androgamones II dissolve the gelatinous covering present all over the egg.
- Hormones present at the surface of egg are called gyanogamones these are of 2 types.
(a) Gyanogamones I - this hormone neutralizes Androgamone I and activates sperm to move
(b)Gyanogamones II - It makes sperm head sticky.
Enzyme of acrosome (Hyaluronidase and sperm lysins) dissolve the egg membrane. This is called acrosomal reaction. As the result sperm head make the contact with the plasma membrane of egg, now inner membrane of acrosome evaginates outside and form rigid tube is called acrosomal filament. Acrosomal filament provide stimulus to plasma membrane of egg and due to stimulus of sperm, egg is induce for fertlization. Mammalian sperms do not form this type of filament becouse mammalian sperms are highly active and provide stimulus to plasma-membrane of egg without any filament. Mammalian sperms acquire activity at two places. First-epididymis and second-vagina. Vaginal secretion make the sperm highly active and sperm acquire capacity of fertilization is calledcapacitation.
Activation of egg : Due to stimulus of sperm an enzyme is induced in plasma-membrane of egg it is called adenyl cyclase enzyme and function of this enzyme is to catalyze C-AMP in egg cytoplasm.
C-AMP is the second messanger. Cyclic AMP receives stimulus from plasma membrane of egg and transfers it in egg cytoplasm and induces all the response of egg for sperm.
All the response of egg for sperm are collectively calledgyanogenesis.
Due to stimulus of sperm, permeability of plasma menbrane of egg increases specially for k+ and Ca2+ ions.
Function of Ca2+ions is to inactivate the cytostatic factors in egg. As a result egg is now ready for cleavage (In egg cytoplasm special type of protein called cytostatic factor are present these factors prevent the cleavage in unfertilized egg) Due to stimulus of sperm, H+– Na+ pump activates and induces the plasma-membrane of egg. Function of this pump is to continuously influx H+ ions and outflux Na+ions. As a result concentration of H+ion increases in egg cytoplasm and develops an acidic medium. In acidic medium, proteolytic enzyme become active and liberate the m-RNA from informosome. These m-RNA become active and rapidly synthesize different types of protein and enzymes . Due to more availability of protein and enzymes metabolic activity of egg increases.
(1) Due to stimulus of sperm, meiosis-II i