(Inside Science) -- When Antonie van Leeuwenhoek, the "father of microbiology," peered through a microscope at a human sperm in the 1670s, it seemed clear what was going on. The cell's movement was powered by the symmetrical thrashing of its long tail, with waves undulating equally down both sides. In the ensuing centuries, countless scientists and microscope hobbyists have seen the same thing.
But their eyes were lying.
Using advanced microscopy techniques and mathematical analysis, researchers have for the first time described how human sperm move in 3D.
"When we saw the asymmetry, it shocked us. We didn't believe it at first," said Hermes Gadelha, a mathematician at Bristol University in the U.K., who conducted the research with colleagues at the National Autonomous University of Mexico. The findings were published today in the journal Science Advances.
Researchers have long known that sperm spin as they swim, essentially drilling into the fluid in front of them. This spinning is visible in the 2D view one gets looking down at a microscope slide, since the sperm's flattened head appears to blink with each rotation, said Gadelha. But the 2D view also creates an optical illusion of symmetrical tail movement.
In fact, said Gadelha, if you could attach a tiny camera to the head of a sperm and point it down toward the tail, you would see that from the sperm's point of view, the tail only beats on one side. But because the sperm also spins around, the tail ends up beating in a circle around the direction of travel, averaging out the lopsided strokes to propel the sperm forward.
After making their discoveries, the researchers searched the published literature for clues, and learned that the internal structure of a sperm cell is also highly asymmetrical. Many of the components that power and control movement are unevenly distributed -- so much so that it's hard to imagine how they could produce symmetrical waves. But until now, said Gadelha, no one had suggested that a sperm's movements might be as lopsided as its innards.
"Then we feel, like, this warm feeling inside: 'Yes, finally, this makes sense,'" said Gadelha. "The sperm is as asymmetric as it should be, because of its asymmetric heart."