Where does fertilization occur in humans? Explain the events that occur during this process.
Fertilisation in humans occurs in the fallopian tube of the female reproductive system.
It is the process of fusion of a haploid sperm and a haploid ovum to form a diploid zygote. The process of fertilisation is completed under the following major steps:
- Attraction: During copulation (intercourse of coitus), millions of sperms are inserted into the vagina of the female through the penis. The sperms swim in the fluid mucous lining of the female genital tract at the rate of 1.5−3.0 mm per minute to reach the ovum in the upper part of the fallopian tube where fertilisation takes place. The ova are formed in the ovaries, which are released, into the abdominal cavity through a process called ovulation. One mature egg is released from the ovary on the 14–15-day of the menstrual cycle and that is picked off by the fimbriae of the ampulla of the fallopian tube. The ovum enters the fallopian tube and moves in it by the muscular contractions and ciliary action of the epithelium of the fallopian tube. The ovum secretes a chemical substance called fertilizin glycoprotein or mucopolysaccharide to attract the sperms. The sperms produce a chemical substance antifertilizin (protein). Each species produces a specific type of fertilizin and antifertilizin, and the reactions between them bring about the process of fertilisation.
- Penetration of sperm into ovum: The human ovum is a rounded and nonmotile structure surrounded by the vitelline membrane, zona pellucida and corona radiata. The radially arranged follicle cells of the corona radiata are attached together by a complex organic substance called hyaluronic acid (a mucopolysaccharide) which acts as barrier for the entry of sperms. The human sperm undergoes several changes so that it may be able to fertilise the ovum. This is called capacitation of sperm. The sperm attaches to the surface of the ovum near the animal pole and starts penetrating the various membranes of the egg. The acrosome of the sperm bursts and secretes sperm lysins containing an enzyme hyaluronidase to dissolve the adhesive substance and to disperse the cells of the corona radiata. Ultimately, with the help of sperm lysin, one sperm penetrates the layers of the corona radiata and zona pellucida in about 30 minutes.
- Activation of the ovum: After the penetration of sperm, a series of changes are brought about in the egg cortex. The dark cortical granules appear below the cell membrane in the cortex which migrates through the plasma membrane. These granules get attached along the inner surface of vitelline membrane and make it thick. This thickened vitelline membrane is called the fertilisation membrane which prevents polyspermy by inhibiting the entry of other sperms.
- Fusion of sperm and ovum nuclei: The sperm entry stimulates the ovum to undergo a second meiotic division for the removal of the last polar body. Usually the sperm head and middle piece enter the ovum through a definite path called the copulation path. The sperm nucleus acts as a male gamete and the egg nucleus as the female gamete. The centrioles of the middle piece of sperm form the spindle and nuclear membranes of the sperm and the ovum breaks down. This process of mixing of the haploid sperm nucleus with the haploid egg nucleus is called amphimixis. The fusion product of sperm and egg pronuclei results in the formation of the diploid zygote which initiates pregnancy in females.
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