HOW DO SPERM swim? A new study highlights that the journey of human sperm toward the egg may not be straightforward.

“To reach the site of fertilization, sperm must traverse narrow and convoluted channels, filled with viscous fluids,” write the authors in the Proceedings of the National Academy of Sciences. So to examine the behaviours of sperm cells, the researchers put human spermatozoa through their paces in artificial micro-channels of various shapes.

And what did they see? The sperm tended to swim against the walls of the channels, but came unstuck when turning corners.

“Once a cell reaches a horizontal wall, it is likely to travel along horizontally while translating until it, by chance, reaches a vertical wall (or vice versa),” write the researchers.

“It then remains trapped by both walls, swimming along their intersection, until it finally reaches a sufficiently sharp change to the curvature of a vertical wall to cause departure.”

Sperm can also crash into the walls and into each other along the way as they migrate through the confined spaces. Sperm following the walls obey simple mechanical rules, according to researcher Dr Petr Denissenko, who admitted some mirth at the antics. “I couldn’t resist a laugh the first time I saw sperm cells persistently swerving on tight turns and crashing head-on into the opposite wall of a micro-channel,” he said. “And if you were wondering why anyone would want to subject sperm to such rigours, it’s to understand more about why some sperm cells make it to the egg.

“In basic terms – how do we find the Usain Bolt among the millions of sperm in an ejaculate,” said researcher Dr Jackson Kirkman-Brown. “Through research like this we are learning how the good sperm navigate by sending them through mini-mazes.”