Human sperm swim upstream 'in a shoal'

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Human sperm cells are very good at swimming upstream against a strong current to reach their ultimate goal - the egg - scientists have discovered.

Rather than swim in a straight line, they spiral along to move to where the flow is slowest, the Massachusetts Institute of Technology team observed.

Their laboratory studies also hint that sperm may work together as a team rather than in competition.

Swimming as a shoal may boost the odds of fertilisation, the scientists say.

Of the hundreds of millions of sperm cells that begin the journey up the oviducts, only a few 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.

To study how sperm deal with this challenge, Prof Jorn Dunkel, working with a team at Cambridge University, set up a mini "sperm assault course" in their laboratory using a series of different-sized tubes and channels.

They discovered that, at certain flow speeds, the sperm cells were able to swim very efficiently upstream for several minutes.

'Co-operation'

The sperm tended to avoid the centre of the tube where the flow was fastest and, instead, wiggled along the walls of the channel in a spiralling motion.

The researchers suspect sperm cells do the same in the fallopian tube when swimming towards the egg.

What is more, sperm group together, probably in order to swim faster, the scientists told the journal eLife.

Prof Dunkel said: "It is a commonly held belief that there is competition between sperm cells, with the fittest reaching the egg first.

"But recent studies by our team and others show that sperm practically always accumulate at the surface of a tube, and you can end up with a high local concentration of sperm cells, so there could actually be co-operation among these cells that allows them to swim faster collectively."

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