In the attempt to fertilize an egg, human spermatazoa must travel quite a distance for their size; if a sperm were a human, it would traverse the equivalent of several kilometers. Plus, it must compete with hundreds of millions of other sperm for its 1 percent chance to reach an egg. The journey, which requires multiple sensory abilities, is arduous.

Until now, researchers knew only that sperm locate the fallopian tube by following two key sensory systems: they “feel” the heat of the tube, which may only be the tiniest degree warmer, and they “taste” chemical signals given off by the egg. A new study published in the journal Scientific Reports by a team at the Weizmann Institute of Science, in Israel, now shows that sperm may also “see” their way to the egg by using the proteins of optical sensors typically found in the visual systems of animals. Researchers believe these three sensory systems exist in case one or both of the others fail. 

Sperm are extraordinarily heat sensitive, a mechanism known as thermotaxis. From a distance of 46 microns—the length of a single sperm—they can sense differences in temperature as small as .00006°C across a wide temperature range, from 29°C to 41°C. The Weizmann team set out to discover how sperm sense the heat of the fallopian tube.

“I asked myself how such a shallow temperature gradient can be sensed by a cell, any cell, with the known thermosensors in mammals—ion channels,” Michael Eisenbach, a co-author of the study and a professor of biological chemistry at the Weizmann Institute, tells mental_floss. “It was obvious to me that such high sensitivity over such a wide temperature range cannot be achieved by a single channel or protein, but rather a family of thermosensors.” 

To identify this family of proteins, his team identified molecular components involved in thermotaxis and deduced the signaling pathway. “Since each pathway is associated with a known family of receptors, knowing the pathway enabled us to deduce the identity,” he says. The family they honed in on is called GPCR (G-protein-coupled-receptor). The team further deduced that the sub-family of proteins they were looking for were opsins. These proteins are most often found in the eyes, especially rhodopsins, which serve as photoreceptors in the cells of the retina. In fruit fly larvae, rhodopsin has been shown to act as a thermosensor for thermotaxis. In other words, the rhodopsin enables the cells of the eye to sense heat, which can allow the fly to pick an environment that is comfortable. 

The presence of these proteins doesn't mean that sperm "see," of course. However, says Eisenbach, it does show that "these proteins have dual functions, and that the function they fulfill—photosensors or thermosensors—depends on the context and the tissue.”

The next step of the research is to study what happens to the opsin protein to change it to be temperature sensitive rather than light sensitive, and also to investigate how opsins confer such high-temperature sensitivity on sperm. “Both questions are currently challenging enigmas,” he says.

Answering these questions could help identify unexplained cases of infertility. “The process of thermotaxis can, in principle, be used to select sperm ripe for fertilization and use them in intra-uterus insemination,” he says. Preliminary tests for feasibility will soon be carried out.