For millions of sperm it is the end of the road. Scientists have found evidence that the female reproductive tract is shaped in such a way that stops poor swimmers from reaching their goal.
Researchers used small-scale models and computer simulations to show that pinch points that behave like gates along the sperm’s arduous path from cervix to egg allow only the fastest ones through.
Tests with sperm from men and bulls revealed that the strongest swimmers were most likely to make it through the tight spots, known as “strictures”, while weaker ones were caught in oncoming currents that propelled them backwards when they got too close.
“The overall effect of these strictures is to prevent slow sperm from making it through and to select for sperm with highest motility,” said Alireza Abbaspourrad, a chemist and lead author on the study at Cornell University in New York.
Natural fertilisation is a brutal game. In humans and other mammals, the race begins with the sudden onrush of more than 60m sperm. Each is intent on fusing with the egg, but for a sperm to have a chance it must outswim all rivals and endure hazards from acid baths to immune attack.
The swimming skills of sperm have been studied before, but scientists at Cornell looked specifically at how sperm fared when they reached narrow parts in the female reproductive tract, such as the small opening from the uterus to the fallopian tubes. These pose a particular challenge, not least because the sperm are swimming upstream, meaning they must battle through fluid that is flowing towards them.
“If you look at the anatomy of the reproductive system in mammals, you can see that the dimensions of the canal that leads to the egg is not constant,” said Abbaspourrad. “At some points it is extremely narrow so that only a few sperm can pass while others fail.”
To see how sperm behaved at the strictures, Abbaspourrad and his colleagues built a small “microfluidic” device that mimicked the tight spots the sperm had to navigate. The device had three little eye-shaped compartments, each separated by a pinch point.
The scientists arranged the device so that sperm injected into it had to swim upstream against a moving a fluid to reach the strictures. Writing in the journal Science Advances they describe how some swam fast enough to make it through the pinch points, but most were caught in the oncoming current. Video of the sperm showed them swimming up to the stricture, being propelled backwards, and then having another bash at it.
Both human and bull sperm behaved the same way when stuck at the entrance to a stricture. They moved in a sideways figure-of-eight, or butterfly-shaped, pattern heading towards the opening down one wall of the compartment, before being swept backwards onto the opposite wall, and then swimming back towards the opening, only to be swept back again.
“The most surprising part for us was the way sperm swim on this butterfly-shaped path,” said Abbaspourrad. “The shape of the path leads to an accumulation of the sperm such that the faster sperm stay closer to the stricture and to each other while the slowest sperm are swept back by the flow and spread further apart.” The sperm that finally make it through are again the better swimmers.
In one experiment, a single sperm swimming at 84.2 micrometres per second battled its way through one of the pinch points, while its competitors got caught in currents that cast them back every time they tried to pass through. The poorest swimmers were swept backwards the farthest, leaving the stronger ones with a better chance of success on future attempts.
“The results show that only the fastest, and therefore assumed best, sperm can pass through these narrowings against a fluid flow,” said Allan Pacey, professor of andrology at the University of Sheffield. “It makes perfect biological sense and would help to explain how the female reproductive tract is able to make sure the best sperm reach the egg.”