Where does Fertilization Occur in Humans? Explain the events that Occur During this Process.
Fertilization in humans occurs in 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 fertilization is completed under the following major steps:
During copulation (intercourse of coitus), the millions of sperms are inserted into the “vagina” of the female through the “penis”. The sperms swim in the fluid mucus lining of the female genital tract at the rate of 1.5 to 3.0mm per minute to reach the ovum in the upper part of fallopian tube where fertilization takes place.
The ova are formed in the ovaries, which are released, into abdominal cavity through a process called ovulation.
One mature egg is released from the ovary on 14-15 day of the menstrual cycle and that is picked of by the fimbriae of the ampulla of fallopian tube. The ovum enters the fallopian tube and moves in it by the muscular contractions and ciliary action of the epithelium of fallopian tube.
The ovum secretes a chemical substance called fertilizin glycoprotein or mocopolyacchride to attract the sperms. The sperms produces a chemical substance antifertilizin (protein). Each species produces specific type of fertilizing and antifertilizin and the reactions between them bring about the process of fertilization.
Penetration of sperm into ovum:
The human ovum is a rounded and non-motile structure surrounded by vitelline membrane, zona pellucid and corona radiata. The radially arranged follicle cells of corona radiate are glued together by a complex organic substance called hyaluronic acid( a mucopolysaccharide) which act as barrier for the entry of sperms.
The human sperm undergoes several changes so that it may be able to fertilize the ovum. This is called as capaciation 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 a enzyme hyaluronidase to dissolve the adhesive substance as well as to disperse the cells of corona radiate. Ultimately, with the help of sperm lysin, one sperm penetrates the layers of corona radiata and zona pellucida in about 30 minutes.
Activation of ovum:
Consequent to penetration of sperm, the series of changes are brought about in the egg cortex. The dark cortical granules appear below the cell membrane in the cortex which migrate 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 as fertilization 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 second meiotic division for removal of last polar body. Usually the sperm head and middle piece enters the ovum through a definite path called copulation path. The sperm nucleus acts as a male gamete and egg nucleus as female gamete.
The centrioles of the middle piece of sperm form the spindle and nuclear membranes of the sperm and ovum breaks down.
This process of mixing up of haploid sperm nucleus with haploid egg nucleus is called amphimixes. The fusion product of sperm and egg pronuclei results in the formation of diploid zygote which initiates the pregnancy in females.