Abstract
The eukaryotic flagellum propels sperm cells and simultaneously detects physical and chemical cues that modulate the waveform of the flagellar beat. Most previous studies have characterized the flagellar beat and swimming trajectories in two space dimensions (2D) at a water/glass interface. Here, using refined holographic imaging methods, we report high-quality recordings of three-dimensional (3D) flagellar bending waves. As predicted by theory, we observed that an asymmetric and planar flagellar beat results in a circular swimming path, whereas a symmetric and non-planar flagellar beat results in a twisted-ribbon swimming path. During swimming in 3D, human sperm flagella exhibit torsion waves characterized by maxima at the low curvature regions of the flagellar wave. We suggest that these torsion waves are common in nature and that they are an intrinsic property of beating axonemes. We discuss how 3D beat patterns result in twisted-ribbon swimming paths. This study provides new insight into the axoneme dynamics, the 3D flagellar beat, and the resulting swimming behavior.Graphic abstract
Highlights
Sperm use the flagellum for sensing and self-propulsion to achieve fertilization [1,2,3,4,5,6]
Using digital inline holographic microscopy (DIHM), we report the 3D flagellar beat pattern underlying swimming behaviors of invertebrate and mammalian sperm near a boundary surface
We identified characteristic differences in the trajectory and three-dimensional flagellar waveform of sea urchin and human sperm swimming near a boundary surface
Summary
Sperm use the flagellum for sensing and self-propulsion to achieve fertilization [1,2,3,4,5,6]. Aquatic animals such as cnidarians, echinoderms, fishes, and amphibians [7] use external fertilization. Sperm and eggs are spawned into the water, i.e., eggs are fertilized outside the female body. Animals with external fertilization are referred to as external fertilizers. Fertilization in animals such as all mammals, birds, and reptiles occurs in the genital tract of the female body [8]. These animals are referred to as internal fertilizers
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