The female reproductive tract is an obstacle course that prefers agile sperm. Narrow straits in parts of the tract act like gates and prevent slow-moving sperm from ever reaching an egg, as a study shows.

Using a device that mimics the variable width of the tract, the researchers studied sperm behavior at a narrow point where the reproductive cells were exposed to strong direct fluid flows. The faster, stronger swimmers moved along a butterfly-shaped path, keeping them close to the narrow spot, increasing the chances of getting through. Meanwhile, slower, weaker swimmers were carried away, reports the team on February 13 online science advances,

"Narrow nodes in the tract can be a barrier for weak swimmers," says co-author Alireza Abbaspourrad, a biophysicist at Cornell University. The findings suggest that women choose the healthiest sperm in this way, he says.

Get through

When the sperm approaches a narrow part of the female reproductive canal, the rapid flow of fluid can push it back (red arrows). The faster swimmers reach the wall, swim along the wall (blue arrows) and try again to pass the narrow space. The researchers say that multiple passes appear as a butterfly pattern.

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Sperm migrate through the reproductive canal – the vagina, cervix, uterus, and fallopian tubes – by swimming upstream against the liquid flowing through the tract, which moves at different speeds along the way. Previous studies have shown that sperm tend to follow the walls of the tract to "direct" the egg. However, it was not investigated what impact the tight spots on the migration could have.

Through computer simulations and sperm tests, Abbaspourrad and his colleagues found that the fastest sperm, when stopped by the flow at a narrow point, could return to a wall, swim along it, and try again. Repeatedly, this movement led to a butterfly pattern. Whether or not a swimmer could get through was ultimately dependent on the speed of the fluid through the bottleneck and the speed of a sperm.

Most studies have tended to use devices with long, straight swimming channels that "effectively test the endurance" of sperm, says David Sinton, a mechanical engineer at the University of Toronto who was not involved in the research. However, in the narrow areas, the fluid flows faster than at the wider areas, so the sperm will essentially have to sprint to get past it. He looked at how sperm move in a device like the researchers, using "endurance and sprinting tests" and "I assume both abilities are needed" in the female reproductive tract.

JUST KEEP SWIMMING Using a device that mimics tight spots in the female reproductive tract, the researchers studied how sperm control the obstacle. Faced with a stronger current at this point, more agile sperm swam in a butterfly pattern, increasing their chances.

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