Sperm on the Egghunt


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Left to right: Ph.D. students Anna Gakamsky and Anat Bahat, Prof. Michael Eisenbach. Obstacle course
Sperm have much in common with the heat-guided missiles used to track down planes and other military targets, a new Weizmann Institute study suggests.
The site where the egg lies is slightly warmer than the place where sperm pause during their journey through the female reproductive tract. This temperature difference apparently guides sperm in their navigation. The study, headed by Prof. Michael Eisenbach of the Institute’s Biological Chemistry Department and reported in Nature Medicine, may improve future IVF treatments for couples finding it difficult to conceive.
After passing through the womb, sperm cells enter the fallopian tubes. Once inside a tube, they attach themselves to the tube’s wall and pause for “storage,” during which they go through a maturation process that prepares them for penetrating the egg. A sperm cell that has completed this maturation process detaches itself from the wall and leaves the storage site.
If ovulation has taken place in the preceding 24 hours, releasing an egg ready to be fertilized, the mature sperm embarks on a long, complicated journey through the tube to the site of potential fertilization.
Egg and sperm meet in the middle of the fallopian tube
How does the sperm steer a course through the fallopian tube? In earlier studies, Prof. Eisenbach discovered that the egg “calls” 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 spread effectively through the entire tube and therefore cannot signal the sperm over longer distances. This chemical attraction mechanism, known as chemotaxis, cannot therefore explain the sperm’s entire journey.

Some like it hot


Guided by the knowledge that the sperm storage site is about 2 degrees C cooler than the site of fertilization, Eisenbach and his team – Ph.D. students Anat Bahat and Anna Gakamsky, Dr. Ilan Tur-Kaspa from the Barzilai Medical Center in Ashkelon and visiting Argentinean scientist Dr. Laura C. Giojalas – hypothesized that sperm may be attracted to the fertilization site by a difference in temperature. The technical term for such attraction is thermotaxis.
To test this theory, the team built a model of the fertilization process consisting of the sperm storage site, the area where fertilization takes place and the tube in between. They then heated the sperm storage site to a temperature of 37 degrees C (98.6 degrees F) and the fertilization site to the slightly warmer temperature of 39 degrees C (102.2 degrees F), and checked the effect of this setup on the behavior of rabbit sperm. 
The findings were clear: The rabbit sperm were indeed sensitive to heat and moved quickly to the warmer fertilization area. On gradually reducing the difference in temperature, the scientists found that even a half-degree difference was enough to attract the sperm. Moreover, they found that only mature sperm – those most likely to penetrate the egg – are heat sensitive.
“Apparently, sperm are guided by temperature in their travels through most of the fallopian tube,” says Eisenbach. “Only when they near the fertilization site do they navigate by tuning in to the egg’s chemical call.”
The team’s findings were replicated in further research with human sperm, conducted in collaboration with Prof. Haim Breitbart of Bar-Ilan University.
Prof. Michael Eisenbach is the incumbent of the Jack and Simon Djanogly Chair of Carbohydrate Biochemistry.