Human sperm swim in corkscrew motions to compensate for the asymmetry in their tails. That is according to researchers in the UK and Mexico who have used high-speed 3D microscopy to determine how sperm maintain their forward swimming direction. The results correct a long-standing misconception about the motions of sperm and could enable biologists to better understand human infertility.
In the 17th century, pioneering Dutch microbiologist Antoine van Leeuwenhoek became the first person to view human sperm cells under the microscope. He described their motions as being “like eels in water”, propelling themselves forward by lashing their tails, or flagella, from side to side in a seemingly symmetric way. Centuries since van Leeuwenhoek’s work, observations with 2D microscopes have been unable to challenge his conclusions.
Nature has shown us that there are many ways to swim in a straight line
Hermes Gadêlha
In the latest study – led by mathematician Hermes Gadêlha at the University of Bristol – the team used a 3D camera capable of taking images at over 55,000 frames per second to view sperm as they swam in a low-viscosity fluid. The resulting images and mathematical analysis let the researchers describe the beating of the sperm’s flagella from a frame of reference which moved and rotated with the cells in all three directions as it moved.
The results reveal that sperm cells swim in corkscrew motions that were governed by two separate controls. One is an asymmetric travelling wave along the flagella that results in a one-sided, or asymmetric, stroke. With this movement alone, the sperm would swim in circles – like a one-handed breast stroke. However, the team found that pulsating standing waves along the tail rotate the entire sperm around their swimming directions. The overall movement resembles a precessing spinning top: as the head of the sperm spins – drilling into the surrounding fluid – its tail rotates about a central axis at the same rate.
Gadêlha and colleagues suggest that the lop-sidedness in the sperm’s swimming beat is down to various asymmetries in the molecular structure of the sperm’s flagella, which is then compensated for in the rolling motion to allow the sperm to swim forward. “Nature has shown us that there are many ways to swim in a straight line,” says Gadêlha. Indeed, their observations show that the 2D side-to-side motion of sperm as first seen by van Leeuwenhoek is simply an optical illusion. Therefore, resolving a centuries-long misconception about how the cells propel themselves forward.
Move with the beat
David Smith from the University of Birmingham, who studies the mathematics of swimming sperm and was not involved in the work, told Physics World that the results are “technically very impressive”. “There has not been a huge amount of 3D imaging data on human sperm, and the concept of one-sided beating combined with rolling is intriguing and will undoubtedly lead to a lot of interest from modellers,” he says.
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Smith adds that human sperm show a lot of different behaviours so further work will be needed with larger samples and in different environments. Indeed, in the female reproductive tract, sperm encounter a fluid that is more viscous than the one used in the current work and it has been shown that human sperm swim in a much smoother, less chaotic way in higher-viscosity fluids.
Yet the researchers hope that their findings could lead to new diagnostic tools for identifying unhealthy sperm and subsequent factors that could cause male infertility. The current gold standard for sperm examination during infertility investigations is computer-aided sperm analysis, which tracks several parameters of sperm including head shape, size and swimming ability. Gadêlha says that the new results point to other important parameters that could now be considered. “What this work shows is that the rotation of the sperm is also critical,” adds Gadêlha.
The research is published in Science Advances.
