Human sperm don’t swim the way that anyone had thought

Sperm have long fooled scientists. Instead of swimming straight by twirling their tails like propellers, human sperm flick their tails lopsidedly and roll to balance out the off-center strokes.

Over 300 years ago, microscopy pioneer Antonie van Leeuwenhoek described sperm tails swaying in a symmetric pattern, like “that of a snake or an eel.” The prevailing view that sperm tails move in a balanced way, however, doesn’t capture what actually happens in three dimensions, researchers report July 31 in Science Advances.

High-speed 3-D microscopy of human sperm swimming freely in the lab revealed that the cells corkscrew as they move, consistent with previous studies. The sperm almost seemed to be drilling into the surrounding fluid, says Hermes Gadêlha, a mathematician at the University of Bristol in England.

Contrary to what people have thought, sperm tails don’t beat symmetrically. High-speed 3-D microscopy and mathematical analyses reveal that the tails wiggle to only one side as the cells roll. The combination of movements keeps sperm swimming straight ahead.

Using automated tracking of swimming sperm and mathematical analyses of position data, Gadêlha and colleagues broke sperm tail movement down into two components. Surprisingly, one was a wiggle to only one side of the cell. It’s like someone swimming using just one side of the body, Gadêlha says. By itself, such a lopsided stroke would lead to swimming in circles.

But a second component of tail movement causes the sperm to rotate, balancing out the lopsided strokes. From above, the sperm tail looks like it is beating symmetrically, as has been described historically. But a more complex, 3-D movement keeps the sperm swimming straight ahead.

The new 3-D measurements are a big step forward in understanding sperm movement, says Allan Pacey, a male fertility specialist at the University of Sheffield in England. Additional investigation is needed, though, to know if sperm move the same way in the female reproductive tract, where they must contend with fluid movement and narrow passages to reach the egg (SN: 2/13/19). Such research may inform diagnosis and treatment of human infertility, Pacey says.

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