How do sperm find their egg?

Sperm find their way to the egg because it is slightly warmer than the wall of the fallopian tube. This temperature difference and chemical attractants serve as a guide for the sperm.

The way to the goal

Sperm enter the fallopian tube through the uterus. There they first attach themselves to the fallopian tube wall, where they mature into a fertile state. They then detach themselves from this “intermediate station”.

If ovulation has taken place in the previous 24 hours and a fertilizable egg has detached itself from the ovary, the mature sperm sets off on its journey through the fallopian tube to reach the site of fertilization.

Control by attractants

It was already known in the past that the egg cell shows the sperm the way with the help of a chemical substance. Egg cells release chemical “attractants” to attract sperm. The sperm orientate themselves on the attractant gradient that surrounds the ovum and are thus able to track down the ovum. However, since the chemical control – called chemotaxis – is only effective over short distances, this signal could not be authoritative for the sperm’s entire journey.

Sperm like warmth

Sperm manage to cover the great distance to the egg cell within a few minutes. By what mechanism are they controlled?

Scientists have found that the place where the sperm matures is about two degrees Celsius colder than the place where fertilization takes place. Male sperm cells apparently use this temperature difference as a guide when navigating.

Such a temperature-controlled mechanism was already known in microorganisms and  worms  . Laboratory tests with rabbit sperm have shown that this also applies to mammals: Even a temperature difference of half a degree caused the sperm to move towards the egg cell.

Conclusion

Apparently, sperm are temperature controlled during the first part of their journey down the fallopian tube. When they get close to the egg cell, the chemoattractants guide them to their final destination.

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