Swimming by spinning: spinning-top type rotations regularize sperm swimming into persistently symmetric paths in 3D

TL;DR

This study reveals that sperm spinning stabilizes their forward swimming paths in 3D, despite asymmetric flagellar beating, by analyzing fluid mechanics and orientation, offering new insights into sperm navigation mechanisms.

Contribution

It demonstrates that sperm spinning regularizes swimming into symmetric paths in 3D, and introduces a method to infer beat symmetry from orientation without tracking the flagellum.

Findings

Spinning-top rotations drive sperm to swim symmetrically in 3D.

Orientation analysis reveals beat symmetry without flagellum tracking.

Spinning stabilizes forward propulsion despite beat asymmetries.

Abstract

Sperm modulate their flagellar symmetry to navigate through complex physico-chemical environments and achieve reproductive function. Yet it remains elusive how sperm swim forwards despite the inherent asymmetry of several components that constitutes the flagellar engine. Despite the critical importance of symmetry, or the lack of it, on sperm navigation and its physiological state, there is no methodology to date that can robustly detect the symmetry state of the beat in free-swimming sperm in 3D.How does symmetric progressive swimming emerge even for asymmetric beating, and how can beating (a)symmetry be inferred experimentally? Here, we numerically resolve the fluid mechanics of swimming around asymmetrically beating spermatozoa. This reveals that sperm spinning critically regularizes swimming into persistently symmetric paths in 3D, allowing sperm to swim forwards despite any imperfections on the beat. The sperm orientation in three-dimensions, and not the swimming path, can inform the symmetry state of the beat, eliminating the need of tracking the flagellum in 3D. We report a surprising correspondence between the movement of sperm and spinning-top experiments, indicating that the flagellum drives ''spinning-top'' type rotations during sperm swimming, and that this parallel is not a mere analogy. These results may prove essential in future studies on the role of (a)symmetry in spinning and swimming microorganisms and micro-robots, as body orientation detection has been vastly overlooked in favour of swimming path detection. Altogether, sperm rotation may provide a foolproof mechanism for forward propulsion and navigation in nature that would otherwise not be possible for flagella with broken symmetry.