The Process of Fertilization
The Process of Fertilization
Of the few hundred sperm which reach the egg, only one will successfully fertilize it. The process” of fertilization is truly the primeval mating dance—the fertilization tango—when the female’s chromosomes (in the egg) and the male’s chromosomes (in the sperm) fuse together to create a new life—one which is totally different from all others, because of its unique genetic composition. As a result of advances in medical research, we have now learnt quite a lot about fertilization and this is truly one of nature’s spectacular miracles.
During the time that the sperm spend in the female reproductive tract, while swimming towards the egg, they acquire the capacity to fertilize it—a process called capacitation. When the sperm reach the corona cells ( only a few hundred successfully make the trip, guided by chemicals produced by the egg which serve as “guiding beacons” to the sperm ) they become hyperactivated—they start beating their tails in a frenzy. This hyperactivation is useful because it provides the mechanical energy that the sperm head needs to burrow its way through the outer shell of the egg (called the zona). The sperm disperse the cumulus oophorus (and so far it’s a team effort) and when they reach the egg, they first bind themselves to the zona. A chemical-is released here by the sperm in a process called the acrosomal reaction in which the acrosome (which sits like a cap on the head of the sperm and behaves much like a battering ram) is removed. The acrosomal enzymes dissolve the zona pellucida by making a tiny hole in it, so that one sperm can swim through and reach the surface of the egg. At this time, the egg transforms the zona to an impenetrable barrier; thus preventing other sperms from entering.
The genetic material of the sperm (the male pronucleus) and the genetic material of the egg (the female pronucleus) then fuse—to form an embryo, which then divides into 2 cells. These cells, in turn, thereafter continue to divide rapidly, producing a ball of cells—now called the blastocyst. The embryo next travels through the fallopian tube (which nurtures it; and, at the appropriate time, propels it) into the uterus—a journey which takes about 3 to 5 days. The embryo must then break through its zona (this is called embryo hatching); and then attach itself to the lining of the uterus in a process called implantation, and, in 9 months, if all goes well, a baby will be born.