How does the human sperm flagellum beat in 4D?
University of Bristol, UK
The waving motion of human sperm flagella is the archetype of self-organized cell swimming in low Reynolds number in nature. However, our understanding of the flagellar movement thus far has been limited to planar microscope projections. Here we employ a state-of-the-art, high-speed 3D microscope imaging capture with bespoke mathematical imaging processing capable of resolving the human sperm flagellar movement in 4D (3D+time) with high-resolution. This allows novel exploration of the flagellar swimming kinematics in inertialess environment in 3D, from simple head trajectories to three-dimensional curvature and torsion with a microscopic resolution. This revealed a novel complex regulatory mechanism coupling cell swimming, bending and torsional travelling waves along the flagellum. We observe for the first time that flagellar beat in 3D is dynamically modulated by non-slip solid boundaries. The kinematic torsion is singularly distributed along the flagellum, and capable of inducing a novel flagellar helical ‘perversion’ phenomenon. The latter is characterized by flagellar sections of opposite chirality that travels during the beat. This may lead to artefacts on the sperm swimming predictions if only examined with 2D microscopy. The lack of conserved chirality is stabilized by spiralling traveling waves along the flagellum and may be critical for the sperm ascension in convoluted geometries within the reproductive tract.