Human sperm swim faster and straighter when aligned close to a surface than when there is no surface nearby, reports a study in Nature Communications. This may reflect a strategy adapted to a confined reproductive system.
Sperm swim by beating a long tail, called a flagellum, in a helical pattern. In aquatic animals, this propels sperm through the water, but internal reproductive systems have a lot of surface area to contend with. How the presence of a surface affects the behaviour of sperm is not known.
In order to examine this in detail, David Sinton and colleagues filmed human sperm swimming within a micron of a glass surface, and compared them to sperm swimming in bulk solution. They find that sperm swimming on a surface adopt a unique slither swimming mode, which results in faster and straighter swimming of the sperm along the surface. Slither swimming was observed more often in viscous liquids designed to mimic the human reproductive tract. Bull sperm, on the other hand, swim more slowly when aligned to a surface.
The human fallopian tube, where fertilization takes place, is a confined and highly-viscous environment. The bull fallopian tube is considerably larger and surface interactions may be less frequent. Therefore the slither swimming mode adopted by human sperm appears to be well-suited to the narrow confines of the human reproductive system.