Male fertility: why are so many sperm needed?


The count is a measurement of how many sperm there are per milliliter, there should be at least 20 million. And yet a man might have a volume of 4ml, which would equal 80 million sperm in that one sample. As it only takes one sperm to fertilise the egg, and indeed nature tries to prevent any other sperm entering once one has penetrated the inner surface, why are millions produced in each ejaculate? As with other aspects of nature, abundance is the rule. Many more seeds are produced from a plant than will actually get a chance to germinate. It is the same with frog spawn where, from that enormous mass, hundreds of tadpoles can emerge but only a small proportion will eventually become frogs. Nature always works on the principle of 'survival of the fittest', basing her calculations on the fact that, from the huge number produced, be it seeds or sperm, many are going to die off en route.
It is estimated that only a small fraction of sperm will actually reach the egg (as few as 100), as the sperm need to swim up the vagina, through the cervix and up the fallopian tubes. Normally only one egg is released and this egg will travel down one fallopian tube, so half of the sperm that are left after the long journey could be traveling up a tube with no egg in it.
When they finally meet the egg, a number of sperm will surround it. On the front of the sperm's head is the acrosomal head cap which contains certain degradative enzymes to help dissolve the cumulous cells surrounding the egg. The combined action of a number of sperm helps with this dissolving process but only one sperm actually gets through the next layer, the zona pellucida. As soon as the egg is penetrated, rapid changes take place in its outer layer and no other sperm can get through.

Intriguing facts about sperm
• A sperm lashes its tail 800 times to travel 1 millimeter.
• Sperm reach the woman's fallopian tubes 30-60 minutes after ejaculation.
• Sperm are produced at an average of 1,500 per second from each testicle.
• In the right conditions, sperm can live up to five days.
• One sperm can swim 3 millimeters per second.


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