Human sperm seen propelling selves by four different swim strokes

It’s a new day for sexual surveillance: A team of scientists has developed an imaging technique that allows three-dimensional tracking of the movements of 1,500 human sperm at one time.

Because of the limitations of microscopes and other traditional tools for imaging moving objects such as sperm, scientists typically have been unable to watch the behavior of cells over time as they move. The new technique gets around this by imaging the shadows of sperm using light of two different wavelengths produced from two different angles at the same time. As a result, the researchers could simultaneously track hundreds of sperm as they moved through liquid.

The researchers, from UCLA, discovered that sperm swim in roughly four different ways. The first, and by far the most common, is what you might expect -- a head-forward dash, presumably toward an egg. The second is far stranger. At any given time, about 4% or 5% of sperm are swimming in curved, helical tracks, as if they were moving along an invisible slinky. A smaller percentage seemed to move about willy-nilly, and an even smaller percentage appeared to combine willy-nilly motion with the helical slinky movements of the second group.

Things got really interesting when the researchers used the new technique to watch sperm move over time. It turns out that each sperm doesn’t just use one of these motions all the time. Instead, sperm switch from one movement to another.

And the likelihood of sperm swimming in a helix versus simply swimming straight appears to be dependent on whether they are in the fluid that surrounds semen in the testes or the fluid that is present in the fallopian tubes, raising the possibility that sperm swim differently during different parts of their journey to the egg.

The study was published online Monday in Proceedings of the National Academy of Sciences; you can read a summary here.

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