The human sperm beats anisotropically and asymmetrically in 3D

Abstract

The canonical beating of the human sperm flagellum is postulated to be symmetric. This is despite the reported asymmetries inherent to the flagellar axonemal structure, from distribution and activation of molecular motors to, even, the localisation of regulatory ion channels. This raises a fundamental question: how symmetric beating is possible within such intrinsically asymmetric flagellar complex? Here, we employ high-speed 3D imaging with mathematical analysis capable of resolving the flagellar movement in 4D (3D+time). This reveals that the human sperm beating is both anisotropic and asymmetric, and composed by a superposition of two transversal waves: an asymmetric travelling wave and a symmetric standing wave. This novel anisotropic travellingpulsation mechanism induces sperm rolling self-organisation and causes a flagellar kinematic illusion, so that the beat appears to be symmetric if observed with 2D microscopy. The 3D beating anisotropy thus regularises the intrinsic flagellar asymmetry to achieve symmetric side-to-side movement and straight-line swimming.

IRONSperm: Sperm-templated soft magnetic microrobots

“…In our case, the cell is not alive. Its flagellum is passive and does not undergo similar anisotropic and asymmetric beating as live cells (51), which is originated by the active bending moment of the flagellum. Therefore, the flagellar waveform of IRONSperm is preserved during rotation and two-dimensional projections can be used to estimate the time-averaged shape, bending amplitude, and the wave propagation speed.…”

Flagellar energetics from high-resolution imaging of beating patterns in tethered mouse sperm

“…To quantitatively analyze beat patterns in a statistically meaningful way, we need to image swimming sperm over several beat cycles. While rapid progress is being made on full three-dimensional tracking ( Muschol et al, 2018 ; Dardikman-Yoffe et al, 2020 ; Gadêlha et al, 2019 ), it is unlikely that sufficient beat cycles can be reliably recorded with freely swimming sperm that can quickly move out of focal plane or the FOV ( Mondal et al, 2020 ). Instead, we image flagella beating freely in the focal plane in cells tethered chemically at their heads to a glass slide.…”

Flagellar energetics from high-resolution imaging of beating patterns in tethered mouse sperm

“…To quantitatively analyze beat patterns in a statistically meaningful way, we need to image swimming sperm over several beat cycles. While rapid progress is being made on full three-dimensional tracking [25][26][27], it is unlikely that su cient beat cycles can be reliably recorded with freely swimming sperm that can quickly move out of focal plane or the field of view [28]. Instead, we image flagella beating freely in the focal plane in cells tethered chemically at their heads to a cover-slip.…”