Strength in numbers: sperm swim upstream 'in a shoal'

Sperm cells swim very efficiently when moving upstream against a current and may cooperate whilst doing so, scientists have observed.

Researchers found that when sperm cells are faced with a current, they swim in a spiralling motion, rather than in a straight line, and move towards where the current is slowest. This causes the cells to accumulate in groups, swimming in shoals.

This challenges the commonly held belief that sperm cells compete to reach the egg cell first. 'There could actually be cooperation among these cells that allows them to swim faster collectively', said Dr Jörn Dunkel, lead researcher on the study.

Hundreds of millions of sperm cells enter the oviduct, or Fallopian tube, however only a select few will reach the egg. They have to cover distances more than 1,000 times their length and navigate complex terrain. To try to understand how sperm cells manage this, researchers set up an artificial 'sperm assault course' in the lab. They used a variety of sizes of channels and forced liquid through them at different speeds. They then measured how sperm cells, which they inserted into the tubes, dealt with these challenges.

Apart from determining that sperm cells group together at the tops of the tubes, a further finding from the study, published in the journal eLife, was that at certain flow speeds, the sperm were very efficient at swimming upstream.  'We found that if you create the right flow velocities, you can observe them swimming upstream for several minutes', said Dr Dunkel. 'The mechanism is very robust'.

The authors believe that similar current speeds to those seen in the lab could be found within the oviduct. The researchers, from MIT in the USA and the University of Cambridge in the UK, claim that if this is the case, this study could help design better artificial insemination techniques.

'The idea would be to fine-tune the properties of the fluid medium that the sperm cells are contained in, before you insert it into the body, so that you know the cells can achieve optimal upstream swimming', said Dr Dunkel.