When Mom and Dad have sexual intercourse, my Dad places his erect penis inside my mom's vagina. He releases millions of sperm when ejaculation occurs. And the sperm have been deposited they have a long and arduous journey ahead of them, like salmon entering the mouth of a river to swim upstream to spawn.
Some of the sperm swim straight up into the fallopian tubes through the cervix and uterus. - And some of them are so fast, that sperms have been found in the tubes in as little as a few minutes after ejaculation. I’m one of them joining the rides. Some sperms die in the acidic vaginal fluid; and some enter the cervical mucus and cervical crypts. During this time, the sperms are released in small numbers and these continue to swim towards the fallopian tubes.
Over the millions of sperms released in an ejaculate, only a few hundred of us will make the arduous trip unto the egg successfully. Perhaps this is why so many millions of sperms are produced in the first place even though only one is needed to fertilize the egg - because the wastage is so prodigal.
During the time 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 sperms 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 sperms) they become hyperactivated - they start beating their tails in frenzy. This is useful because it provides the mechanical energy the sperm head needs to burrow its way through the outer shell of the egg called the zona.
The sperms disperse the cumulus oophorus (and so far it's a team effort) and when they reach the egg, they first bind to the zona. 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 sperm from entering it.
And over the few hundred sperm which reach the egg, only one will successfully fertilize it. I’m so lucky because I am the one! 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 then continue to divide rapidly, producing a ball of cells - the embryo. The embryo then travels through the fallopian tube (which nurtures it and 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 is born.
Finally, the champion baby was born although many trials and battle she faced and still she is the champion, I am the Champion!