Human sperm swim more like otters than eels, study finds
For more than 300 years, most scientists have assumed that sperm "swim" through fluids by wriggling their tails back and forth like eels to propel themselves forward. But according to a new paper in Science Advances, this is actually an optical illusion—the result of viewing the creatures from above with 2D microscopes. New observations with 3D microscopy have revealed that human sperm actually roll as they swim, like otters, essentially corkscrewing themselves forward.
"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," said co-author Hermes Gadelha from the University of Bristol.
The honor of directly observing the first sperm rests with Antonie van Leeuwenhoek, a 17th-century Dutch draper with a side interest in science—specifically, building microscopes and coming up with innovative manufacturing methods to make better lenses for said microscopes. Only a few of his microscopes have survived, but they are capable of magnifying small objects up to 275 times, and historians believe some of his instruments could have achieved magnifications as high as 500 times.Further ReadingThe explosive physics of pooping penguins: they can shoot poo over four feet
Van Leeuwenhoek studied lots of substances under his microscopes, including fluids like lake water, blood, milk, spit, and tears, for example, as well as plaque scraped from his own teeth. He was shocked to discover tiny living creatures under his microscopes, which he dubbed "animalcules." Small wonder he is widely considered to be the father of microbiology.
It was only a matter of time before van Leeuwenhoek turned his attention to studying semen samples, although he was initially reluctant to do so, fearing it would be unseemly. But in 1677, he relented. Using a sample of his sperm (following sexual relations with his wife, who must have been a long-suffering woman), he found the sample teeming with animalcules. He described the tiny creatures at length in a letter to the Royal Society: blunt heads, long tails, propelling themselves, he wrote, by lashing their tails "with a snakelike movement, like an eel swimming in water." He went on to observe sperm in semen samples collected from other animals, such as rabbits and dogs.
But van Leeuwenhoek's seminal (ahem) observation turns out to just be an optical trick. Gadelha and his colleagues recorded sperm swimming freely with a high-speed camera, capturing more than 55,000 frames per second. They combined this with a microscopic stage that moved up and down, resulting in a 3D reconstruction of the sperm tail's movement. They observed that the sperm tail only wriggles on one side, which would typically make the sperm swim in circles. So the sperm have adapted to correct for this one-sided stroke: they corkscrew as they swim.
"Our discovery shows sperm have developed a swimming technique to compensate for their lop-sidedness and in doing so have ingeniously solved a mathematical puzzle at a microscopic scale: by creating symmetry out of asymmetry," said Gadelha. "The otter-like spinning of human sperm is, however, complex: the sperm head spins at the same time that the sperm tail rotates around the swimming direction. This is known in physics as precession, much like when the orbits of Earth and Mars precess around the sun."
This makes sense, given what we know about the fluid dynamics involved with the movement of microorganisms. Such creatures live in environments with low so-called Reynold numbers—a number that predicts how a fluid will behave based on the variables viscosity, length, and speed. Named after the 19th-century physicist Osborne Reynolds, the concept is especially useful for predicting when a fluid will transition to turbulent flow. Many years ago, physics writer Aatish Bhatia wrote one of the best popular explanations to date of how this movement-in-fluid stuff works. (It's well worth reading in full.) In practical terms, it means that inertial forces (e.g., pushing against the water to propel yourself forward while swimming) are largely irrelevant at very low Reynolds numbers, where viscous forces dominate instead.
Back in 1977, physicist Edward Purcell did some calculations that showed how animals of different sizes would swim at different Reynolds numbers. The number would be very high for a whale, for instance, which is able to coast a good distance with a single flap of its tail. According to Purcell's calculations, however, bacteria swim at low Reynolds numbers, so they can barely coast any distance at all if you push them to set them in motion. It's akin to a human trying to swim in molasses and moving their arms at slow speeds on par with the movement of a clock's hands. So eels and sperm (or bacteria) would adopt very different swimming strategies by necessity because they are dealing with different Reynolds numbers.
Symmetry and asymmetry
One such strategy might be to break the symmetry of the stroke to create more drag on the power stroke than on the recovery stroke. A creature could do this by changing the shape of the "paddle"—for example, the cilia that cells use to propel themselves forward. Bacteria and sperm, in contrast, have helical tails they can use as a corkscrew-like propeller. This latest study sheds some intriguing light on precisely how this works for human sperm. (Of course, as Bhatia pointed out, "Don't expect to see human swimmers doing 'the corkscrew' anytime soon. This strategy works only at low Reynolds number, where water 'feels' as thick as cork, so you can push against it effectively.")
Fertility clinics today still rely on 2D views when examining sperm movement, so this new work provides a better understanding of how the sperm tail moves, which could in turn lead to better diagnostic tools. "This discovery will revolutionize our understanding of sperm motility and its impact on natural fertilization," said co-author Alberto Darszon from the Universidad Nacional Autonoma de Mexico, who pioneered the 3D microscopy technique with his colleague and co-author Gabriel Corkidi. "So little is known about the intricate environment inside the female reproductive tract and how sperm swimming impinge on fertilization. These new tools open our eyes to the amazing capabilities sperm have."
DOI: Science Advances, 2020. 10.1126/sciadv.aba5168 (About DOIs).
"Every sperm is Sacred" from Monty Python’s The Meaning of Life.Listing image by Polymaths-Lab.com