Researchers from the University of Cambridge and Massachusetts Institute of Technology (MIT) have identified the physical mechanisms which may allow sperm to navigate inside the human body and stay on course through a variety of environments.
The research may help us to understand how some sperm travel such long distances, through difficult terrain, to reach and fertilise an egg, researchers said.
Of the hundreds of millions of sperm cells that begin the journey up the Fallopian tube, only a few hardy travellers will ever reach their destination.
Not only do the cells have to swim in the right direction over distances that are around 1,000 times their own length, but they are exposed to different chemicals and currents along the way.
While we know that sperm cells can 'smell' chemicals given off by the egg once they get very close to it, this does not explain how they navigate for the majority of their journey, said Dr Jorn Dunkel of MIT, who conducted the research while a postdoctoral researcher at the University of Cambridge.
"We wanted to know which physical mechanisms could be responsible for navigation," said Dunkel.
"If you think of salmon for example, they can swim against the stream, and the question was whether something similar could really be confirmed for human sperm cells," said Dunkel.
"There have been qualitative reports for over 100 years that sperm cells can swim upstream, but it is only recently that the role of surfaces bounding the flow has been established," said Goldstein.
To understand what the cells are capable of, the researchers built a series of artificial micro-channels of different sizes and shapes, into which they inserted the sperm.
They modified the flow of fluid through the tubes, to investigate how sperm responded to different current speeds.
They discovered that at certain flow speeds, the sperm cells were able to swim very efficiently upstream.
"We found that if you create the right flow velocities, you can observe them swimming upstream for several minutes. They're very robust," Dunkel said.
Researchers observed that the sperm were not swimming in a straight line upstream, but in a spiralling motion, along the walls of the channel.
The cells react to the difference in the speed of current near the walls of the chamber - where the fluid is attracted to the surface and is therefore at its slowest and the free-flowing centre of the tube.
The study was published in the journal Life.