We know what human sperm do, but it seems we may be wrong about one aspect of how they do it.
Research by fertility scientists in the UK and Mexico challenges the accepted view of how sperm “swim”, suggesting that it is an optical illusion.
More than three centuries ago, Antonie van Leeuwenhoek, using one of the earliest microscopes, described human sperm as having a tail which, “when swimming, lashes with a snakelike movement, like eels in water”.
But modern 3D microscopy and mathematics suggest otherwise, say Hermes Gadelha from the University of Bristol, UK, and Gabriel Corkidi and Alberto Darszon from the Universidad Nacional Autonoma de Mexico.
The tail is in fact wonky and only wiggles on one side, they write in a paper in the journal Science Advances. Sperm don’t just swim in circles, however, because they have found a clever way to adapt.
“Human sperm figured out if they roll as they swim, much like playful otters corkscrewing through water, their one-sided stoke would average itself out, and they would swim forwards,” says Gadelha.
“The sperms’ rapid and highly synchronised spinning causes an illusion when seen from above with 2D microscopes: the tail appears to have a side-to-side symmetric movement, ‘like eels in water’, as described by Leeuwenhoek in the 17th century.
“However, 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,” says 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.”
Corkidi and Darszon pioneered the 3D microscopy. They used a high-speed camera capable of recording over 55,000 frames a second and a microscope stage with a piezoelectric device to move a sample up and down at an incredibly high rate, allowing them to scan the sperm swimming freely in 3D.
In contrast, computer-assisted semen analysis systems in use today, both in clinics and for research, still use 2D views to look at sperm movement, they say.
“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,” Gadelha says.
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