The sperm are guided to the egg, attracted by its warm temperature.For years, scientists have wondered how the sperm of mammals navigate their way through the female reproductive system directly to the egg they must fertilize. Now, Israeli scientists have discovered that the sperm operate much like guided heat-seeking missiles that sense the heat of a plane’s engine. The sperm are guided to the egg, attracted by its warm temperature.

The breakthrough Weizmann Institute study is published in the February issue of Nature Medicine. The finding may make it possible in the future to make use of temperature guidance to improve in vitro fertilization, said team leader Prof. Michael Eisenbach of the Institute’s Biological Chemistry Department.

According to the study, the site where the egg lies is slightly warmer than the area in which the sperm make a pause in their journey through the female genital tract. Sperm cells are apparently guided by this temperature difference in their navigation. The heat-driven journey was previously known to exist in microorganisms and worms, but the Weizmann study has provided evidence for its existence in mammals for the first time.

“Apparently, the sperm are guided by temperature when they travel through most of the fallopian tube and navigate by tuning in to the egg’s chemical call when they get close to the fertilization site,” said Eisenbach.

The study, which took place on rabbits, said that it contributes to the understanding of fertilization in humans and other mammals. In further research conducted in collaboration with Prof. Haim Breitbart of Bar-Ilan University, the scientists obtained similar findings in human sperm cells.

The study focused on examining closely the way in which sperm steer a course through the fallopian tube. In the past, Prof. Eisenbach discovered that the egg “calls upon” the mature sperm by releasing a chemical substance. However, the chemical signal can attract the sperm only across a short range: since the tube normally moves in a wavelike fashion, the chemical apparently cannot not spread effectively through the entire tube and therefore cannot dispatch a signal to the sperm over longer distances. Therefore, the chemical attraction, known as “chemotaxis,” cannot account for the sperm’s entire journey.

What allows the sperm cells to cover the distance between their storage site and the fertilization site was found to be the temperature.

To prove this, the scientists built a lab installation simulating the storage site, the fertilization site, and the tube in between. They tested the behavior of rabbit sperm in this system and found that the sperm were indeed sensitive to heat: they were attracted from the relatively cool area, with the temperature of 37° C (98.6° F), to the relatively warm area, with the temperature of 39° C (102.2° F). When the scientists gradually reduced the difference in temperature, they found that even a half a degree difference was enough to attract the sperm.