Female bodies block slow sperm, revealing research finds
Female bodies block slow sperm, revealing research finds

Secret to why so many sperms fail

When it comes to making babies, it's all about a need for speed.

It's long been said that "strong swimmers" win the day but new research shows just how important being fast is in the race to fertilisation. That's because zippy sperms are likely to be the only ones quick enough to wriggle past the bottlenecks of the female reproductive tract.

Scientists used small-scale models and computer simulations to "investigate sperm locomotion" in research that could one day be used to boost male fertility. But in the meantime, it has produced a lovely video of semen struggling to swim upstream (say that five time quickly).

With a single ejaculate, 100 million sperm can swim up the female reproductive tract but there are bottlenecks on the route from cervix to egg known as "strictures". These can change the mucus flow and impact how the little guys try to battle against the current.

The researchers at Cornell University built a device to mimic a fallopian tube and used human and bull semen to test it and observe how the little swimmers navigate these difficult bottlenecks.

They found that sperm congregate beneath the opening of the bottleneck and the fastest ones that lead the pack only compete among themselves, giving them a greater shot at the finish line, while the slower swimmers get caught in oncoming currents that disperse and propel them backwards.

"Accumulation below the stricture occurs in a hierarchical manner so that dense concentrations of sperm with higher velocities remain closer to the stricture, with more sparsely distributed arrays of lower-velocity sperm lagging behind," researchers wrote.

John Amory, a professor and fertility expert at the University of Washington, who was not involved with the study said it confirms a key question about the traits of winning sperm.

"Clinically, we've always suspected that it's the highly motile sperm that are responsible for fertilisation, and this paper suggests that's the correct way of looking at things," he said. "It's a very cool little model."

The research was published this week in the journal Science Advances.

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