Fertilisation is the fusion of a haploid male gamete (sperm) and haploid female gamete (ovum) to form a diploid zygote. In human internal fertilisation take place. Fertilisation is a process that: (i) Restore diploid number of chromosomes (ii) Determine sex of the new organism (iii) Activation of zygote to start a series of mitotic divisions called cleavage.
During copulation (coitus) semen is released by the penis into the vagina, it is known as insemination. The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the ampullary region of the fallopian tube. So fertilisation take place in ampulla of fallopian tube. The ovum released by the ovary is also transported to the ampullary region where fertilisation take place. Fertilisation can only occur if the ovum and sperm are transported simultaneously to the ampullary region. This is the reason why not all copulations lead to fertilisation and pregnancy.
Events of Fertilisation
The process of fusion of a sperm with an ovum is called fertilisation. During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. Thus it enusres that only one sperm can fertilise an ovum. The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane. This induces the completion of the meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid). Soon the haploid nucleus of the sperms and that of the ovum fuse together to form a diploid zygote. All this process can be grouped into following reproductive events:
(1) Acrosomal Reaction
(2) Cortical Reaction
(3) Sperm Entry
(5) Activation of Egg
Significance of Fertilisation
- Fertilisation provides stimulus to the egg to complete its maturation.
- Fertilisation activates the ovum to develop into a new individual by repeated mitotic divisions.
- Fertilisation restores the diploid number of chromosomes, characteristic of the species (46 in humans), in the zygote.
- Fertilisation combines the characters of two parents. This introduces variations, which make the offsprings better equipped for the struggle for existence and contribute to evolution of the race.
- Fertilisation determines the sex of the young one to be developed from the zygote in humans.
- Fertilisation membrane developed after the entry of the sperm prevents the entry of additional sperms into the ovum.
- Fertilisation introduces centrioles which are missing in the ovum.
Cleavage is the rapid mitotic division in the fertilised egg. Cleavage begins almost immediately after fertilisation and continues during its passage down the fallopian tube to the uterus. During cleavage the embryo does not increase in size. The cells produced do not grow after division. The embryo with 8-16 solid ball of cells is called morula. Cells of morula is called blastomeres.
The morula continues to divide and transforms into blastocyst. The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass. The inner cell mass is the source of embryonic stem cells from which all body structures are formed. The inner cell mass looks like a knob at one pole. It is called embryonal knob. The cells of trophoblast which are in contact with the inner cell mass (embryonal knob) are known as cells of Rauber. Blastocyst contain a fluid filled cavity, known as Blastocoel.
Role of zona pellucida
After the formation of blastocyst, zona pellucida becomes thinner and finally disappears. The function of the zona pellucida is to prevent the implantation of the blastocyst at an abnormal site. It does not expose the sticky and phagocytic trophoblast cells till the blastocyst reaches the proper implantation site.
Implantation is the process by which the developing attaches itself to the wall of the mother's uterus and stimulates the development of the placenta. At the time of implantation embryo is present in form blastocyst. So Implantation is the attachment of blastocyst to the uterine wall (in endometrium). It take place about 7 days after fertilisation. With implantation, pregnancy is established. During implantation trophoblast layer gets attached to the endometrium and the inner cell mass gets differentiated as the embryo. After attachment, the uterine cells divide rapidly and covers the blastocyst. As a result, the blastocyst becomes embedded in the endometrium of the uterus. This is completion of implantation.