Sperm collide and crawl the walls in chaotic journey to egg

In sex education videos the race of sperm to egg is portrayed like an Olympic swimming final as sperm surge purposefully down the female reproductive tract to the finish line. The reality, however, may be rather less elegant.

A study published in the journal PNAS (Proceedings of the National Academy of Sciences) has shown that instead of swimming through the centre of a channel, sperm crawl along walls, frequently colliding with obstacles and each other.

On a microscopic scale the walls of the female reproductive tract are ridged, and sperm have to swim down narrow, convoluted channels filled with mucus. To model this environment, scientists at the Universities of Birmingham and Warwick in the UK injected sperm into hair-thin micro-channels and observed their behaviour.

Corners posed a particular problem. While the sperm were able to follow a gentle bend, tighter turns caused them to collide as only a few managed to make the turn in time.

Lead author, Dr Petr Denissenko from the University of Warwick, admits: 'I couldn't resist a laugh the first time I saw sperm cells persistently swerving on tight turns and crashing head-on into the opposite wall of a micro-channel'.

How well sperm handle corners and recover from collisions may depend on both the viscosity of the fluid and minute differences in the shape of the sperm. These findings could finally help to answer a central question in reproductive biology — why, of the 200 million sperm released in a single ejaculate, do only around ten make it to the egg?

Understanding what differentiates the good sperm may eventually help scientists to better select sperm for fertility treatments. 'In basic terms', explained co-author Dr Jackson Kirkman-Brown from the University of Birmingham, 'how do we find the 'Usain Bolt' [of sperm]?'

To reach the egg, a sperm must swim through the cervix, the uterus and along the fallopian tubes, travelling about four to five centimetres an hour. While the journey may be chaotic, it's still impressive.

Dr Kirkman-Brown told Postmedia News that the journey is 'the equivalent, in an hour, of me getting halfway up Everest crawling in cold butter. The fact that something so tiny can do it under its own power is pretty incredible'.