How the Sperm and Egg Meet - and What happens Next. In vivo fertilisation
What happens to the sperm once they enter the woman's vagina?
What happens to the egg when conception occurs?
What happens when the egg and sperm meet?
The sperm's odyssey in the female reproductive tract
A million spermatozoa,
All of them alive;
Out of their cataclysm but one poor Noah
Dare hope to survive.
-- Aldous Huxley
When a man and woman have sexual intercourse, the man places his erect penis inside the woman's vagina. Here it releases millions of sperm when ejaculation occurs. Once the sperm have been deposited here 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. Some sperms die in the acidic vaginal fluid; and some enter the cervical mucus and cervical crypts. They are stored here and can remain alive here for as long as 48 to 72 hours.
During this time, the sperms are released in small numbers and these continue to swim towards the fallopian tubes. This is why you don't need to have sex every day to get pregnant even though the egg remains alive for only 24 hours.
Sperms in the female reproductive tract swim under their own steam - as a result of the whip- like activity of their tail which propels them on. Of the millions of sperms released in an ejaculate, only a few hundred will make the arduous trip upto 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.
What about the other partner in this mating dance, the egg? Remember that a mature egg is released from the ovary (this process is called ovulation) only once during the menstrual cycle. This is the "fertile time", during which a pregnancy can occur.
How does the egg reach the tube? When ovulation occurs, the mature egg is released from the follicle in the ovary. This process of follicular rupture looks a bit like a small volcano erupting on the ovarian surface. At this time, the tubal fimbria, like tentacles, sweep over the surface of the ovary, and actually "swallow" the egg.
The egg has a shell, called the zona pellucida, which looks like the ring around Saturn. It is surrounded by a cluster of nest cells called the corona cells which serve to nurture the egg. They form the cumulus oophorus which is a sticky gel which protects the egg and also helps the beating of the hair-like cilia of the fallopian tube to propel the egg towards the uterus - like a conveyor-belt. The egg must now wait in the protective confines of the fallopian tube, for a sperm to swim up and reach it. An egg remains alive for about 24 hours, and if fertilization does not occur, it dies.
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 mother's chromosomes (in the egg) and the father'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. We have now learnt quite a lot about fertilization thanks to in vitro fertilization (IVF) - and it is truly one of Nature's miracles.
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 a 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. A chemical is released here by the sperms 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 sperm from entering it.
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.
Fig 6. How an egg is fertilised