Sperm find their way to the egg, because it is a bit warmer than the fallopian tube wall. This temperature difference as well as chemical attractants serve the sperm as a guide.
The way to the goal
Sperm enter the fallopian tube via the uterus. There they first attach to the fallopian tube wall, where they mature to the fertile state. Then they detach themselves again from this "intermediate station".
If an ovulation has taken place in the previous 24 hours and a fertile egg cell has been released from the ovary, the mature sperm makes its way through the fallopian tube to reach the place of fertilization.
Control by attractants
Earlier it was known that the egg cell shows sperm the way with the help of a chemical substance. Eggs release chemical "attractants" to attract sperm. The spermatozoa orientate themselves to the attractant gradient that surrounds the egg and are thus able to detect the egg. However, since chemical control - called chemotaxis - is only effective over short distances, this signal could not be decisive for the entire journey of the sperm.
Sperm like heat
Sperm manage within a few minutes to overcome the great distance to the egg. Which mechanism does it control?
Scientists have found that the spot where the sperm matures is about two degrees Celsius colder than the place where fertilization takes place. Male sperm cells are obviously guided by this temperature difference during their navigation.
Such a temperature-controlled mechanism has already been known in microorganisms and worms. This also applies to mammals have revealed laboratory studies with rabbit semen: Already half a degree difference in temperature caused the sperm to move towards the egg.
Obviously, the sperm are controlled by the temperature on the first part of their journey through the fallopian tube. When they get close to the egg cell, they are directed from the chemical attractant to the final target.