Most sperm, it turns out, aren't straight swimmers: They tend to follow a curved path. A new imaging technique small enough to fit on a computer chip has yielded never-before-seen insights into the swimming patterns of the human sperm. High-powered microscopes can show cellular structures in exquisite detail, but those instruments' field of view is too small to follow the paths of swiftly moving microorganisms. So an engineering team from the University of California, Los Angeles, has sidestepped that obstacle by getting rid of the lens. Using their chip-based, holographic imaging method, the researchers tracked the swimming patterns of some 1500 human sperm cells over several hours. Reporting today in the Proceedings of the National Academy of Sciences, the researchers found that about 90% of sperm cells swim along a slightly curved axis. Another 4% to 5% follow a corkscrew path (see video). And for some reason, the vast majority of these helical swimmers (90%) like to form a helix that curves to the right. These right-handers, though, only began to appear when the sperm were extracted from the semen they typically swam in and placed into a laboratory fluid used for imaging. It's possible, the authors write, that for reasons unknown, human semen suppresses the sperm cells' more unusual swimming patterns. The new technique, the researchers say, isn't just useful for observing sperm locomotion—it can also open a window into the twists and turns taken by bacteria and other microorganisms.
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