Egg and Sperm

How come only one sperm enters the egg? If there is so much sperm, to make sure the egg is fertilized. How come none of the other sperms enter once an egg is fertilized? The only reason i can come up with is that the ovary has a tough shell and only a minute part of it is weak and small enough to alow one sperm to enter. How ever to me it remains in the possibilty that a second sperm can enter the same minute hole. But to my knowledge only one sperm ever enters the egg.
- Rob (age 21)
Salford Uni, Manchester, England

You’re way out our area of expertise, but I’m pretty sure I remember the answer from some course in school. After one sperm enters the egg, the egg reacts to change its surface chemistry, keeping other sperm out. I guess this method must occasionaly fail, like everything in biology, but usually it works.

If you think about the sort of answer you suggested, you’ll see it wouldn’t work well. You’re proposing that the entry of each sperm is independent, but that the probability is low enough to make multiple entries unlikely. The problem with that system is that there’s no way to adjust the probability to both make it very likely that one enters and very unlikely that two or more enter. The probability distribution for this sort of process is well known (its name is ’Poisson’), and the best it can give for the likelihood of exactly one entry is just under 0.4. With a more active process, like the one I remember, that probability can approach 1.

Mike W

(published on 10/22/2007)