Update 5/31/21: The research article “Human sperm uses asymmetric and anisotropic flagellar controls to regulate swimming symmetry and cell steering” was retracted on May 19, 2021. In the retraction, the study authors say that their conclusion that sperm moved in a corkscrew motion (contradicting the historically held belief that sperm locomotion is eel-like) was not sufficiently supported by their existing evidence.
In human reproduction, intercourse is really only half the battle. Once sperm is in the female it has a long way to go before reaching the egg, so to boost their chances these mobile gametes are fitted with a wiggly tail. We once believed that the sperm's tail, known as the flagellum, moved in a way comparable to a snake or eel, but new research published in the journal Science Advances has revealed that they actually corkscrew their way to victory. Fetch your biology books, folks. It’s time for a rewrite.
Antonie van Leeuwenhoek was the first scientist to get a good look at sperm locomotion more than 300 years ago. He used one of the earliest microscopes to gaze upon its glory and described it as having a "tail, which, when swimming, lashes with a snakelike movement, like eels in water.”
Nowadays the kit has had somewhat of a glow up, and researchers from the University of Bristol and the Universidad Nacional Autonoma de Mexico got their hands on some state-of-the-art 3D microscopy and mathematics equipment to get a better look. They set up a microscope with a device that could rapidly move a sperm sample up and down and recorded the whole thing on a high-speed camera clocking 55,000 frames per second to make a 3D map of the tail's movement.
The resulting artwork revealed that a sperm’s flagellum is actually wonky, only wiggling on one side. The predicted effect of such a structure would be that it swam in circles, somewhat of a handicap in the race for life, but sperm have adapted a countermove to keep them on the straight and narrow. The sperm can correct their lop-sided tails by rolling as they swim much like “playful otters corkscrewing through water,” said Dr Hermes Gadelha, head of the Polymaths Laboratory at Bristol's Department of Engineering Mathematics and an expert in the mathematics of fertility.
So why did Leeuwenhoek think they moved liked eels, and how has nobody noticed he was wrong before now? "The sperms' rapid and highly synchronized spinning causes an illusion when seen from above with 2D microscopes – the tail appears to have a side-to-side symmetric movement,” Gadelha explained. In research, sperm has always been viewed on a 2D plane and so the invention of 3D microscopy for sperm swimming was crucial to the discovery.
Beyond proving poor Leeuwenhoek and his ancient microscope wrong, the discovery has the potential to improve computer-assisted semen analysis systems used in fertility clinics, which currently still rely on 2D views of sperm. By overcoming the optical illusion that inspired sperm's eel-like comparison, improved semen quality assessments could provide new hope for effective treatments for men, a research area of significant importance given over half of infertility is caused by male factors.
"This discovery will revolutionize our understanding of sperm motility and its impact on natural fertilization,” said Dr Alberto Darszon from the Universidad Nacional Autonoma de Mexico who co-pioneered the 3D microscopy for sperm swimming. “So little is known about the intricate environment inside the female reproductive tract and how sperm swimming impinges on fertilization. These new tools open our eyes to the amazing capabilities sperm have.”