Sperm from different animals must find their way to the egg in quite different environments. For instance, sperm from sea urchins swim freely in the ocean, whereas sperm from humans must find the egg in the intricate and narrow passages of the female genital tract. Whereas sperm from sea urchins swim on helices or circles, human sperm swim in straighter paths while rolling around their own axis. For swimming, sperm use their tail, the. flagellum. How can different swimming “styles” arise from the beating of a flagellum? A detailed analysis of swimming patterns and the flagellar beat is challenging because conventional microscopy can only image a plane. Additionally, the beat of the flagellum is fast, thus scanning microscopes cannot capture the flagellar motion. As a result, most studies provided only 2D or semiquantitative 3D information.
A new study by caesar researchers now provides novel insight into the dynamics of flagellar beating and swimming behavior. This report shows high-quality recordings of three-dimensional flagellar bending waves from different sperm species. Sea urchin sperm feature an almost planar beat pattern that results in swimming along helical paths. By contrast, mammalian sperm display a pronounced non-planar beat pattern that results in rolling and swimming along elongated curvilinear paths. In addition, the study shows that the flagellar bending waves are not only characterized by curvature waves, as previously reported, but also by torsion waves. The location of torsion peaks is consistent with the activity of the molecular motors that power the flagellar motion. The authors speculate that such torsion waves are widespread in nature.