Sperm find their way to the egg because it is slightly warmer than the fallopian tube wall. This temperature difference, along with chemical attractants, serves as a guide for the sperm.
The path to the destination
Sperm enter the fallopian tube through the uterus. There, they first attach themselves to the fallopian tube wall, where they mature to a fertilizable state. They then detach from this “intermediate station” again.
If ovulation has occurred in the previous 24 hours and a fertilizable egg has detached from the ovary, the matured sperm makes its journey through the fallopian tube to reach the site of fertilization.
Control by attractants
It was known earlier that the egg uses a chemical substance to guide the sperm. Oocytes release chemical “attractants” to attract sperm. The sperm orient themselves to the attractant gradient surrounding the egg and are thus able to track down the egg. However, because the chemical guidance – called chemotaxis – is only effective over short distances, this signal could not govern the sperm’s entire journey.
Sperm like heat
Sperm manage to travel the long distance to the egg within a few minutes. By what mechanism are they controlled in this process?
Scientists have found that the site where the sperm matures is about two degrees Celsius colder than the site where fertilization occurs. Male sperm cells apparently orient themselves to this temperature difference in their navigation.
Such a temperature-controlled mechanism was already known in microorganisms and worms. That this also applies to mammals has been shown by laboratory studies with hare sperm: as little as half a degree difference in temperature caused the sperm to move toward the egg.
Conclusion
Clearly, sperm are controlled by temperature during the first part of their journey down the fallopian tube. When they get close to the egg, they are guided by the chemical attractant to their final destination.
