Sperm Don’t Swim Anything Like We Thought They Did

September 13, 2020

Under a microscope, human sperm appear to swim like wriggling eels, their tails wagging back and forth as they search for an egg to fertilize.

But now, new 3D microscopes and high-speed video show that sperm don’t swim in this simple, symmetrical motion at all. Instead, they compensate for the fact that their tails actually only beat to one side with a scalding, rotating motion.

“It’s almost like if you’re a swimmer, but you can only swing your legs to one side,” says study author Hermes Gadêlha, a mathematician at the University of Bristol in the United Kingdom.” If you do this in a swimming pool and you only do this to one side, you will always be swimming in circles…. Nature, in its wisdom, has come up with a very complex and ingenious way to move forward.”

Strange Swimmer.
The first person to see human sperm up close was the Dutch scientist Anthony van Leeuwenhoek, known as the father of microbiology. van Leeuwenhoek aimed his newly developed microscope at his own semen in 1677 and saw for the first time that it was filled with tiny, wriggling cells.

Under the two-dimensional microscope, it was clear that the sperm was propelled by the tail, which appeared to wiggle around as the sperm head rotated. For the next 343 years, this was the understanding of how human sperm moved.

“Any scientist has speculated that there is probably a very important 3D element to how the sperm tail moves, but so far we haven’t had the technology to reliably make such measurements,” says Allan Pacey, a professor of radiology at the University of Sheffield in the U.K., who was not involved in the study.

The new study is therefore “an important step forward,” Pacey wrote in an email to Live Science.

Gadêlha and his colleagues at the Universidad Nacional Autónoma de México began the study out of “blue sky exploration,” Gadêlha said. Using microscopy technology that allows three-dimensional imaging and a high-speed camera that can record 55,000 frames per second, they recorded the swimming process of human sperm on microscope slides.

“What we found was something completely surprising because it completely broke our belief system,” Gadêlha told Live Science.

The sperm’s tails don’t wiggle around, like a whip. Instead, they only bounce in one direction. To wring forward motion out of this asymmetrical tail motion, the sperm head rotates in a jiggling fashion at the same time the tail rotates. Head rotation and tail rotation are actually two separate movements, controlled by two different cellular mechanisms, Gadêlha says. But when they’re combined, the result is like a spinning otter or a spinning drill. During a 360-degree rotation, one side of the tail moves evenly, adding up to forward motion.

“The sperm aren’t even swimming, the sperm are drilling into the liquid,” says Gadêlha.

The researchers published their findings today (July 31) in the journal Science Advances.

Asymmetry and fertility
Technically, the way a sperm moves is called precession, which means that it rotates around an axis, but that axis of rotation is variable. Planets do this on their rotational journey around the sun, but a more familiar example might be a spinning top that wobbles and dances across the floor as it spins on its tip.

“It’s important to note that on their journey to the egg, the sperm will swim through a much more complex environment than the drop of fluid observed in this study,” Pacey said.” In women, they would have to swim through narrow passages of very viscous fluid in the cervix, the undulating cell walls of the fallopian tubes, as well as having to deal with muscle contractions and fluid being pushed around (the floating tips of cells called cilia) in the opposite direction of where they want to go. However, if they do manage to burrow forward, I can now see more clearly how the sperm respond to this route of attack in order to reach the egg and be able to enter the interior of the egg,” Pacey said.

Gadêlha says that sperm motility, or the ability to move, is one of the key indicators fertility doctors look for when assessing a man’s fertility. Sperm head roll is not currently considered in these metrics, but further research has the potential to reveal certain defects that disrupt this rotation and thus hinder sperm movement.

Pacey said fertility clinics use 2D microscopes and more work is needed to understand whether 3D microscopy could benefit their analyses.

“Of course, any 3D method would have to be fast, cheap and automated to have any clinical value,” he said.” But in any case, this paper is certainly a step in the right direction.”