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Abstract:
Sperm are propelled by bending waves traveling along their flagellum. For steering in
gradients of sensory cues, sperm adjust the flagellar waveform. Symmetric and asymmetric
waveforms result in straight and curved swimming paths, respectively. Two mechanisms
causing spatially asymmetric waveforms have been proposed: an average flagellar curvature
and buckling. We image flagella of human sperm tethered with the head to a surface. The
waveform is characterized by a fundamental beat frequency and its second harmonic. The
superposition of harmonics breaks the beat symmetry temporally rather than spatially. As a
result, sperm rotate around the tethering point. The rotation velocity is determined by
the second-harmonic amplitude and phase. Stimulation with the female sex hormone
progesterone enhances the second-harmonic contribution and, thereby, modulates sperm
rotation. Higher beat frequency components exist in other flagellated cells; therefore, this
steering mechanism might be widespread and could inspire the design of synthetic
microswimmers.
