Fertilization is the process during which fusion of a haploid male gamete (spermatozoan) and a haploid female gamete (ovum) takes place so as form diploid zygote.
Fertilization in human being is internal. It takes place usually in the ampulla of the pallopian tube of women.
Event of fertilization :
The discharge of semen in vagina during copulation is called insemination. Sperms swim from vagina to fallopian tube through uterus. Out of millions of sperm, only few sperms reach fallopian tube. The sperms in the female genital tract are made capable of fertilizing the egg by section of ovidual epithelium. Activation of sperms is carried out by viscous fluid secreted by internal lining of Fallopian tube.
The secondary oocyte released from the mature Graafian follicle during ovulation is received by fallopian tube. Secondary oocyte (egg) gets fertilized within 6 hours of fertilization. The oocyte secretes a glycoprotein called fertilizin. Surface of sperms head produces anti-fertilizin. Adhesion of sperm with surface of the oocyte is due to fertilizin anti-fertilizin reaction. It is highly species specific. If copulation occurs at appropriate time then the egg is surrounded by many sperms. Hyaluronidase enzyme of acrosome hydrolyses the hyaluronic acid of follicular cells. Zona lysine or acrosin digests zona pellucida. Now, the sperm reaches plasma membrane of the oocyte.
When sperm touches the vitelline membrane, it undergoes physicochemical changes and gets converted into fertilization membrane. This prevents entry of other sperms to avoid polyspermy.
Amphimixis (karyogamy) :
The entry of sperm stimulates secondary oocyte to complete meiosis-II. This is the end of oogenesis and second polar body is released. Now, nucleus of secondary oocyte becomes female pronucleus. At this time, head of the sperm gets detached from middle piece and tail. Now the sperm nucleus is called, male pronuclei. Mixing of chromosomes of male and female pronucleus takes place. It is called amphimixis or karyogamy. Genetic information of two parent is mixed. The ovum is now diploid and termed as zygote (zygon= yolked together). Zygote or synkaryon is the first cell of new life. The mother is now said to be conceived.
Significance of Fertilization :
1) After completion of meiosis II secondary oocyte concludes the process of oogenesis.
2) Fertilization restores diploid number of chromosomes in the zygote.
3) In combines the characters of two parents. This leads to variation and has significance in evolution.
4) It determines the sex of young one.
5) Fertilization introduces centrioles which are missing in the ovum.