Twirling Tails

Scientists shatter the accepted view of how sperm swim — in 3D

Models of a sperm's eye view upend centuries of fertility science

When considering how sperm move, the word "swimmers" comes to mind. The classic microscopic image is of a tiny cell swishing its tail from side to side as it propels forward.

A new 3D model upends that premise and presents a sense of movement that's more twirl than shimmy.

When sperm cells "swim," they are actually spinning as they move forward. This finding was reported Friday in the journal Science Advances.

Naturally lopsided, sperm tails curve to one side. They do wiggle – but they move to one side only. From the lens of a microscope positioned above a sample, as the curved line spins around, it looks like a swishing motion.

A 3D model based on this finding overturns 350 years of understanding about how human sperm move.

Seen from above, sperm create the illusion of swishing their tails back and forth.

Mathematician Hermes Gadȇlha works at the University of Bristol. He's the study's first author and has worked on modeling sperm motion for years.

Using microscopes, the research team determined that sperm must be rotating. They observed that head of a sperm "blinks" as the cell moves, suggesting it is turning. The next step was to model exactly how a sperm moves through fluid.

"This is a shape — and the best language to describe a shape is really mathematics," Gadȇlha tells Inverse.

"You look at the same movement, but from the perspective of the sperm."

When Gadȇlha's team set out to build a computer model, they were expected to find a symmetrical cell that moved like a corkscrew. Instead, they discovered that the sperm cells wiggle only in one direction.

"At no time, the sperm is tail moving from side to side," Gadȇlha says. "The tail is actually rolling around the middle point."

Gadȇlha compares it to a lap swimmer using only one arm. Naturally, they would move in a circle. To move forward instead, they would have to counteract that asymmetry by rotating in the opposite direction. In the same way, sperm are able to average out their own asymmetry by twirling.

Here's the head-on view:

Sperm wiggle asymmetrically, toward one side only, causing them to roll as they move forward.

The best way to get a sense of this motion, Gadȇlha says, would be to attach a tiny GoPro to the head of a sperm. Unfortunately, that's not an option, for now, so the next best option is math. This study design allowed scientists to shift from an outsider's stance, watching sperm swim from above, to a sperm's eye view.

"You look at the same movement, but from the perspective of the sperm," Gadȇlha says.

Researchers explain how sperm "fooled" scientists for centuries.

Fertility findings — Sperm science has come a long way, and yet, these new findings challenge fundamental principles of how human sperm move.

The first scientist to record observations of sperm under a microscope was Antonie van Leeuwenhoek. That was in 1677, and van Leeuwenhoek thought the cells were tiny, fully formed people. The intervening centuries have corrected his hypotheses. The new finding is yet another chapter in fertility science.

That tracks with how Gadȇlha sees approaching these questions.

"We're always wrong," as he puts it. At a given time, "you are less wrong than you were before — but you're definitely more wrong than you'll be in the future."

Understanding how sperm moves forward is key information potentially underlying complex problems like infertility, Gadȇlha says. In the future, the new information could lead to revisiting old experiments through the new lens of sperm rotation, for instance.

Exactly how spinning sperm will inform fertility science remains to be seen, Gadȇlha says: "These are the scenes of the next chapters."

Abstract: Flagellar beating drives sperm through the female reproductive tract and is vital for reproduction. Flagellar waves are generated by thousands of asymmetric molecular components; yet, paradoxically, forward swimming arises via symmetric side-to-side flagellar movement. This led to the preponderance of symmetric flagellar control hypotheses. However, molecular asymmetries must still dictate the flagellum and be manifested in the beat. Here, we reconcile molecular and microscopic observations, reconnecting structure to function, by showing that human sperm uses asymmetric and anisotropic controls to swim. High-speed three-dimensional (3D) microscopy revealed two coactive transversal controls: An asymmetric traveling wave creates a one-sided stroke, and a pulsating standing wave rotates the sperm to move equally on all sides. Symmetry is thus achieved through asymmetry, creating the optical illusion of bilateral symmetry in 2D microscopy. This shows that the sperm flagellum is asymmetrically controlled and anisotropically regularized by fast-signal transduction. This enables the sperm to swim forward.
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