Perhaps it's no surprise that sperm often swim in circles. They must rely on a tail smaller than the width of a human hair to conquer an obstacle course with 200-million-to-one odds. Until now, scientists blamed sperms' circular locomotion on their erratic, asymmetric tail propulsion. But where they swim might be more important than how they swim, according to a study published online tomorrow in the Journal of the Royal Society Interface. Researchers simulated sperm cell migration and found that if the surrounding fluids (such as semen or vaginal secretions) are viscous enough, the sperm tails will buckle in the current, trapping the cells into a circular loop they can't escape even if the cells swim perfectly. Knowing how to trap sperm in the right places could improve the effectiveness of in vitro fertilization and other reproductive technologies, experts say.
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