The research was conducted by experts from University of Warwick, Massachusetts Institute of Technology, ITMO University, St. Petersburg Academic University and The Institute for Analytical Instrumentation. It was discovered that all sperm tails (flagella) rotate in a counter-clockwise motion. It means that sperm should only be able to move in a leftwards direction, but the researchers observed that approximately 50% of the sperm moved to the right.

Sperm cells consist of a head, mid connecting piece and the flagella. According to experiments, they were distorting their bodies at the mid-piece to counteract the physical forces that would cause them to turn left. The differences between the actions necessary for a sperm to turn left or right means that all could be able to turn in both directions or only one; indicating that there could be two physiologically distinct spermatozoa subpopulations.

“Details of the spermatozoa motion have just started to be understood. We recently have learned that the sperm swim primarily near surfaces, which turned to be crucial for understanding why they navigate against a fluid flow. However, the reason why the sperm cells deviate from strait trajectories by going left or right has been unknown. Analyzing the details of 3D a sperm’s flagella beat, we have realized that all the cells rotate their flagella anti-clockwise, which would make them to turn left only. At the same time we have noticed the right moving sperm also have a right bent in the mid-piece section of the flagella providing a force, which would counteract the left-turning,” said Vasily Kantsler, researcher of University of Warwick’s Department of Physics and head of the research project.

 The research has also proved by experiment, which aimed to understand sperm cell behavior under peristaltic conditions, such as the flow caused by the contraction and relaxation of muscles. Something similar happens in a human fertility tract after insemination, where peristaltic contractions move the fluid back and forth providing an advantage for sperm propagation.

By doing so the researchers observed ‘heart’ shape trajectories of sperm cells in the experiments, showing the sperm turning left or right against the flow to form half of a heart shape depending on the direction they took. This finding motivated the researchers to deeply analyze the 3D motion of spermatozoa flagella at high spatial and temporal resolution, something which had never been done before leading to the discovery of a mechanism distinguishing two different modes of the spermatozoa motion.

“Sperm cells are carriers of genetic material. It would be interesting to research how their packing affects asymmetry of cells` motion. It may result than future boys turn left while girls turn to the right. However this issue requires further research, and above all, biological,” noted Igor Kukhtevich, researcher of Chair of Nanotechnologies and Materials Science.

The full name of the article: “Bimodal rheotactic behavior reflects flagellar beat asymmetry in human sperm cells,” Anton Bukatin, Igor Kukhtevich, Norbert Stoop, Jörn Dunkel, Vasily Kantsler. PNAS.