A sperm cell, much like an expensive robotic vacuum cleaner, is a minimally intelligent body on a mission. Both the Roomba and the male gamete have to navigate a walled space without much idea where they're going or why. And although it won't clean your floors on the way, the sperm cell uses some of the same strategy as the robot vacuum.
To discover the set of rules that sperm cells steer by, researchers used--what else?--sperm mazes. Led by fluid dynamics researcher Petr Denissenko at the University of Warwick, a group of scientists in the United Kingdom built hair-thin tunnels in various shapes. Then they sent human sperm into the curving or zigzagging tunnels. A camera watched through a glass wall on each channel to see what paths the tiny explorers took.
In a narrow tunnel, frantically swimming sperm soon come up against a wall. Then, the camera showed, they follow that wall, seeming to keep their heads against it as they swim. (This same trick will get you out of a corn maze if you're lost, though you might want to keep a hand on the wall instead of your head.)
Wall-following is also one of the rules used by a Roomba. In the case of the robot, it ensures that the edges of the room and the base of the sofa get clean. In the case of sperm, wall-following keeps them moving in one direction as they trace the twists and folds of a fallopian tube.
But sperm aren't experts. When the wall takes a sharp turn away from them, sperm often don't notice; they simply shoot off in the direction they were already swimming. Luckily, they'll find another wall soon. "There are no large open spaces in the reproductive tract," Denissenko says.
Not all sperm are equally spacey about following walls. When the path bends, some follow it better than others. If future research finds a connection between wall-following skill and sperm success--are better navigators also better fertilizers?--then fertility doctors might be able to sort out the best sperm using mazes.
Knowing the rules that sperm swim by also means doctors can coax all of them to travel in the same direction. Denissenko and his coauthors built another maze, shaped like a wreath of grapes, that herds sperm into U-turns until they're all swimming one way.
Roombas use other rules that sperm don't. For example, a Roomba knows to avoid cliffs, a hazard human sperm are unlikely to encounter since there are no staircases inside a human.
Sperm have their own rule too: When they collide with each other, they swim off in different directions. Is this a trick for getting out of traffic? And how do sperm cells know they've hit a fellow swimmer, rather than a wall? Scientists aren't sure yet. "Understanding the role of collisions is really on my to-do list now," Denissenko says.
Like cat-harassing robots, humans' own little automatons rely on a few simple algorithms to do their job. It's nice to see that these seemingly clueless cells know a thing or two. Now if only they'd take on some household chores.
Denissenko, P., Kantsler, V., Smith, D., & Kirkman-Brown, J. (2012). Human spermatozoa migration in microchannels reveals boundary-following navigation Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1202934109