How sperm get turned on

Scientists have discovered the mechanism that starts sperm swimming once they exit the male...

Most people regard sperm as tiny swimming cells that vigorously dash about in search of eggs. But, in reality, they only begin behaving like this once they enter a female. In the male they remain quiescent so as not to wear themselves out before the "great egg race". How this was achieved though, no one knew.

Now researchers at the University of California San Francisco have found that an ion channel (a specialised pore) in the cell membrane allows protons - hydrogen ions - to flood out of the sperm at the right moment, and this is the trigger that starts them swimming.

Polina Lishko and her colleagues made the discovery using a technique called "patch clamping". A tiny pipette was applied to the membranes of individual sperm cells to measure the electrical currents flowing into and out of the cells under different conditions.

The work, published in the journal Cell, showed that, in response to alkaline conditions, a drop in zinc concentration, or the presence of one of the body's own cannabis-like chemicals - a substance called anandamide - the ion channel - called Hv1 - opened. The exit of hydrogen ions through the channel raises the pH inside the sperm, activating other metabolic processes including switching on the sperm flagellum, which it uses to swim.

The discovery sheds new light on an old mystery but may also hold the key to treatments for some forms of infertility and even the next generation of contraceptives: by blocking the proton pore with a drug scientists are hoping they can switch off sperm to induce a state of reversible infertility; it may also be possible to give the motility of sperm a boost in people with poor sperm function.