How Sperm ‘Swim’ May Be Nothing But an Optical Illusion

An international group of scientists say their latest research will upturn one of the earliest scientific discoveries ever made about human sperm. In a new paper out Friday, they seem to show that sperm cells don’t propel themselves by simply flicking their tails back and forth, as is commonly believed. Rather, sperm perform a complex routine of three-dimensional rolling and spinning motions that only make them look like they’re lashing their tails when viewed through a typical microscope.

In 1677, Anton Van Leeuwenhoek took his recently invented compound microscope out for a spin and looked at his own semen (which he collected from his wife after sex). When he did so, he was the first to see a teeming world of sperm cells, which he named “animalcules.” He described these sperm cells as “moving like a snake or like an eel swimming in water.”

The scientists of Van Leeuwenhoek’s day made a lot of assumptions about sperm that turned out to be wrong. For one, many theorized that sperm actually contained the miniature but fully formed version of a person, with the mother and her egg only serving as a means to nourish this mini-person into a baby. But Van Leeuwenhoek’s initial descriptions of how sperm moved have stood the test of time.

Hermes Gadelha, a mathematician at the University of Bristol in the UK, and his team worked with researchers in Mexico to study sperm motions in 3D. They developed a camera capable of taking pictures on a microscopic level at over 55,000 frames per second. They also created a fluid environment with low friction for the sperm to swim in. This novel imaging technique effectively allowed the researchers to scan the sperm cells up and down as they swam. From these scans, Gadelha and his team used complex mathematical formulas to create a model of their movement.

“Human sperm can beat between 20 to 30 swimming strokes in less than one second. So we needed a camera that is so fast that the sperm don’t have the chance to move before the next picture is taken,” Gadelha said.

In the end, they determined that Van Leeuwenhoek had really been the first to see an optical illusion of sperm motion. On a 2D plane, a sperm cell’s movement looks largely symmetrical, its tail flicking back and forth as it moves forward. But the team concluded that sperm cells are actually spinning their tails asymmetrically in one direction while also rotating their heads at the same time. The tail spinning and head rotating balance each other out, propelling the sperm forward. Gadelha likens the sperm’s complicated motions to an otter playfully corkscrewing through water.

The team’s findings were published in Science Advances.

Other scientists had noticed that the observed movements of human sperm didn’t quite make sense under the assumption they only moved like eels, according to the researchers. Others had also shown that the heads of sperm from many animals, including humans, roll around as they move. But Gadelha said that their research is the first to reconcile these observations and clearly demonstrate the ballet of sperm.

However, according to Donner Babcock, a now-retired biophysicist who has long studied the movement of sperm, the team’s conclusions may be plausible but are still far from proven.

“I would say that their claim of a link between the rolling of the sperm head and the movement of the flagella [tail] is too strongly worded,” said Babcock, who is not affiliated with the new study. One way to help settle the question, he said, would be to attach a tracker to a sperm cell, then follow the tracker’s movements as the sperm swims.

Gadelha credits the leaps in mathematics and science over the centuries since Van Leeuwenhoek’s time for making the new research possible, including the microscope technology that his colleagues in Mexico pioneered a decade earlier. He notes that human nature likely causes us to misinterpret what our eyes perceive.

“We often believe in what we see. And that’s a problem, because what we see is always going to be limited to the precision of the instruments that we use to see,” he said.

Revealing the true nature of sperm movement is likely to have important medical implications. One indicator of sperm quality is their motility, otherwise known as their ability to move quickly on their own. So knowing how sperm truly move should help us better understand why some can’t do a good job of it. The team hopes their techniques can be used to peer into other nooks and crannies of the microscopic world, including the movement of other species’ sperm cells.

For Gadelha, there’s also an existential element to their research.

“The motion of sperm underpins the reason why we are here today. I’m here talking to you because a sperm met the egg,” he said. “And at this basic level, if we don’t understand how that works, then how can we hope to solve all the more serious problems, including infertility?”