Sperm have been tricking us with an optical illusion

Researchers have discovered that sperm have been tricking us with an optical illusion for many years.

Using high-precision 3D video microscopy, Hermes Gadelha from the University of Bristol, and Gabriel Corkidi and Alberto Darszon from the Universidad Nacional Autonoma de Mexico disproved an idea that hadn’t changed since the first observations by Antonie van Leeuwenhoek in the 17th Century, Science Advances reported.

Leeuwenhoek made his own lenses for his microscopes. Upon observing the human sperm, he described it as having a "tail, which, when swimming, lashes with a snakelike movement, like eels in water," Eureka Alert reported.

Since that observation up to the time of this publication, sperm were thought to swim forward by whipping their tail from side to side.

What Gadelha and his team discovered is that human sperm only whip their tail in a single direction, but compensate by rolling as they move forward.

“The rolling motion enables spermatozoa to swim in a straight trajectory despite its biased waveform,” the scientists said, Science Advances reported.

"Human sperm figured out if they roll as they swim, much like playful otters corkscrewing through water, their one-sided stroke would average itself out, and they would swim forward," Gadelha, head of the Polymaths Laboratory at Bristol's Department of Engineering Mathematics, told Eureka Alert.

The spinning of human sperm is complex, Gadelha said, with the sperm tail rotating round the direction of swimming at the same time the sperm head spins.

“This is known in physics as precession, much like when the orbits of Earth and Mars precess around the sun," he told Eureka Alert.

Combining the rolling with the whipping of the flagellum creates an optical illusion when viewed with traditional 2D microscopy, making it appear to move side-to-side.

The discovery may affect all existing research that has been dependent on 2D imagery.

The researchers said human, bull and murine species’ spermatozoa all roll when swimming freely, making them prone to frequency bias in 2D. The asymmetry of the beat can’t be detected through planar observations, which explains why the flagellar bias remained invisible until now, Science Advances reported.

“Computer-assisted semen analysis systems, both in clinics and for research, use 2D views of the sperm’s movement and are potentially prone to errors while assessing beating symmetry,” the scientists said.

"With over half of infertility caused by male factors, understanding the human sperm tail is fundamental to developing future diagnostic tools to identify unhealthy sperm," Gadelha told Eureka Alert.

The researchers hope their results will motivate other theoretical and empirical investigations, including the study of microorganisms such as Chlamydomonas, a genus of 325 species all unicellular flagellates, and sperm flagella of the subfamily murinae, which includes the house mouse and the brown rat.

"This was an incredible surprise, and we believe our state-of-the-art 3D microscope will unveil many more hidden secrets in nature. One day this technology will become available to clinical centres," Corkidi told Eureka Alert.