FRIDAY, Jan. 31, 2003 (HealthDayNews) -- Like moths to the flame, sperm are guided to the egg by heat, new research says.
A study from Israel has found the site where sperm pause in their journey through the female genital tract is cooler than the site where the egg lies.
Sperm use this temperature difference as a navigational tool, heading from their cool hangout where they are waylaid for a brief maturation period to the hotter site of the egg, says Michael Eisenbach, lead author of the study and a professor of biochemistry at the Weizmann Institute in Rehovot, Israel.
The finding appears in the Jan. 31 issue of Nature Medicine.
Eisenbach and his colleagues hatched their theory about the role of temperature differences in fertilization when they came across previous studies that noted a two-degree difference in the genital tracts of rabbits and a 0.9-degree difference in pigs.
However, it was unclear whether the egg heats up or the sperm storage site cools down.
In their own experiments in rabbits, Eisenbach and his colleagues determined the temperature around the egg was closest to normal body temperature, meaning it's the sperm storage site that gets chilly.
In the lab, researchers then created a special chamber that mimics the temperature difference in the genital tract. They watched rabbit sperm swim, and sure enough, the sperm made a beeline from cool temperatures to the warmer climes.
Sperm, acting like little heat-guided missiles, sensed temperature differences as small as 0.5 degrees, the smallest difference tested in the study.
"This gives us important new understandings of the mechanism of fertilization," Eisenbach says.
Researchers later placed human sperm in a similar chamber and saw the same reaction.
When sperm enters the female genital tract, they embark on a long, complicated journey that's fraught with hazards.
After passing the uterus, sperm cells enter the woman's fallopian tubes. Once inside a tube, sperm attach themselves to the tube's wall, where they pause for "storage" during a maturation process that prepares them for penetrating the egg.
The maturation process can take about 20 minutes, Eisenbach says.
From there, the sperm detaches themselves from the wall and leave the storage site. If ovulation has occurred in the preceding 24 hours, releasing an egg ready for fertilization, the mature sperm head for it.
Previous research by Eisenbach found the egg calls upon the sperm by releasing a chemical substance. However, the chemical signal can attract the sperm only across a short distance, Eisenbach says.
Since the tube normally moves in a wavelike motion, the chemical cannot spread effectively through the entire tube and therefore cannot call to the sperm over long distances.
The distance between the vagina and the egg fertilization site is about 7 centimeters to 10 centimeters, says Dr. James Grifo, director of the Division for Reproductive Endocrinology and Infertility at New York University Medical Center. The distance between the sperm storage site and the egg is a few centimeters.
"It's been know that there's a chemical attraction from sperm to egg," Grifo says. "People had suspected other factors could play a role. These researchers have found temperature could also contribute. From a basic scientific perspective, it's a very interesting finding."
Grifo says the new knowledge will probably not improve in vitro fertilization, because the egg and the sperm are placed so close together the sperm hardly need to navigate at all. In some cases, the sperm is actually implanted into the egg.
More information
The American Medical Association's Atlas of the Body has anatomical drawings of the female reproductive system. To read more about how the sperm finds the egg, check out HowStuffWorks.
