Slower sperm are beaten by stronger and faster sperm when competing to fertilise an egg due to restrictive bottlenecks in the female reproductive tract.

The findings could help design new and improved ways to screen for speedier sperm for use in IVF, to improve a woman's chance of conceiving.

Investigating sperm motion, researchers at Cornell University in Ithaca and New York City, New York used a new type of microfluidic device that features narrow channels too difficult for weak sperm to pass through.

'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,' Dr Alireza Abbaspourrad, the study's lead author, told The Guardian. 'At some points it is extremely narrow so that only a few sperm can pass while others fail.'

To accurately mimic the shape of the swimming channel that sperm encounter in vivo, the research team created a sectioned device with bottlenecks and pinch points. When reaching narrower channels, sperm accumulated in a hierarchy – with the faster, stronger sperm appearing at the tip of the pack. Slower sperm were washed back by the flow of genital fluid and did not get past the strictures.

Dr Abbaspourrad and his team also found that slower sperm adopted a butterfly shaped path as they continued trying to access the narrower points. If cells were pushed back from the opening on the left-hand side they would cross over and try again on the right, making a sideways figure of eight.

Previous studies have only used straight channels or simple obstacles to measure sperm swimming ability. During this experiment, however, bottlenecks demonstrated how a change in mucus flow determines how sperm swims or steers upstream.

The paper, published in the journal Science Advances, claims the observations quantitatively show how the shape of the female reproductive tract hierarchically filters competing sperm. The strongest, highly motile swimmers were made to compete more directly with one another, rather than with weaker sperm behind.

'Clinically, we've always suspected that it's the highly motile sperm that are responsible for fertilisation,' Professor John Amory at the University of Washington in Seattle, who was not involved in the study, told The Verge. 'This paper suggests that's the correct way of looking at things.'

More clinical research will help determine if these sperm help create healthier embryos.

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Reproduced from BioNews with permission, a web- and email-based source of news, information and comment on assisted reproduction and human genetics, published by Progress Educational Trust.