Like heat-seeking missiles drawn to the warmth of a targeted engine, sperm steer toward an egg using temperature sensors, researchers in Israel report. The site where fertilization occurs is warmer than other sites in the female genital tract, according to scientists from the Weizmann Institute in Rehovot. And because they can sense this temperature difference, sperm cells can find their way to the egg, according to a study published in the February issue of the journal Nature Medicine.

The study, which scrutinized the details of rabbit reproduction, found that rabbit sperm wriggle toward warmth. But a researcher in human reproduction who was not involved in the research cautioned that the study's results might not apply to human beings.

Rabbit reproductive anatomy is somewhat different from that of human beings, Dr. Carmen Williams, an assistant professor at the Center for Research on Reproduction and Women's Health at the University of Pennsylvania, said in an interview with Reuters Health.

Researchers have wondered for a long time how it is that sperm find their way from the uterus up the fallopian tubes to the egg, according to the study's lead author, Dr. Michael Eisenbach.

" One of the puzzles in mammalian fertilization is how sperm cells navigate within the female genital tract toward the egg,"

Eisenbach and his colleagues note in their report. The authors point out that the egg sends out a chemical message, but that can only be sensed when the sperm are relatively close to the egg.

" Another potential cue for sperm guidance," according to the report, "is the ovulation-dependent temperature difference within the female genital tract." In the new study, Eisenbach and his colleagues measured temperature at several spots in the rabbits' reproductive organs just after the animals ovulated. They found that the temperature at the site where eggs would normally be fertilized is approximately 2 degrees Celsius warmer than other spots along the path that sperm must traverse to meet up with an egg.

To check to see if this was the cue being used by rabbit sperm, the researchers ran a test in the lab. They designed a device that had two dishes connected by a narrow bridge. In one part of the experiment, the researchers kept the temperature in the dishes the same. For comparison, in a second experiment, the researchers made the temperature in one dish 2 degrees Celsius warmer than the other.

Eisenbach and his colleagues found that the sperm were more likely to move from dish to dish if they were put in the cooler dish and allowed to navigate their way to the warmer one. While the results are interesting, they may not apply to human sperm, according to Williams, the University of Pennsylvania expert.

She pointed out that studies in pigs have shown that there is no temperature difference after ovulation. No one knows whether such a temperature difference exists in the human reproductive system after ovulation, Williams said.

Reuters Health,· SOURCE: Nature Medicine 2003;9.