title: Bimodal rheotactic behavior reflects flagellar beat asymmetry in human sperm cells.
abstract: Rheotaxis, the directed response to fluid velocity gradients, has been shown to facilitate stable upstream swimming of mammalian sperm cells along solid surfaces, suggesting a robust physical mechanism for long-distance navigation during fertilization. However, the dynamics by which a human sperm orients itself relative to an ambient flow is poorly understood. Here, we combine microfluidic experiments with mathematical modeling and 3D flagellar beat reconstruction to quantify the response of individual sperm cells in time-varying flow fields. Single-cell tracking reveals two kinematically distinct swimming states that entail opposite turning behaviors under flow reversal. We constrain an effective 2D model for the turning dynamics through systematic large-scale parameter scans, and find good quantitative agreement with experiments at different shear rates and viscosities. Using a 3D reconstruction algorithm to identify the flagellar beat patterns causing left or right turning, we present comprehensive 3D data demonstrating the rolling dynamics of freely swimming sperm cells around their longitudinal axis. Contrary to current beliefs, this 3D analysis uncovers ambidextrous flagellar waveforms and shows that the cell's turning direction is not defined by the rolling direction. Instead, the different rheotactic turning behaviors are linked to a broken mirror symmetry in the midpiece section, likely arising from a buckling instability. These results challenge current theoretical models of sperm locomotion.
authors.affiliation: St. Petersburg Academic University, St. Petersburg 194021, Russia; Institute for Analytical Instrumentation of the Russian Academy of Sciences, St. Petersburg 198095, Russia;
authors.forename: Anton
authors.initials: A
authors.lastname: Bukatin
authors.affiliation: Institute for Analytical Instrumentation of the Russian Academy of Sciences, St. Petersburg 198095, Russia; ITMO University, St. Petersburg 197101, Russia;
authors.forename: Igor
authors.initials: I
authors.lastname: Kukhtevich
authors.affiliation: Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA 02139-4307;
authors.forename: Norbert
authors.initials: N
authors.lastname: Stoop
authors.affiliation: Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA 02139-4307; dunkel@math.mit.edu.
authors.forename: Jörn
authors.initials: J
authors.lastname: Dunkel
authors.affiliation: Department of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom.
authors.forename: Vasily
authors.initials: V
authors.lastname: Kantsler
mesh_terms: D000465:Algorithms; D001696:Biomechanical Phenomena; D006801:Humans; D057446:Hydrodynamics; D008297:Male; D046210:Microfluidic Analytical Techniques / Q000379:methods; D008954:Models, Biological; D059010:Single-Cell Analysis / Q000379:methods; D013081:Sperm Motility / Q000502:physiology*; D013082:Sperm Tail / Q000502:physiology*; D013094:Spermatozoa / Q000166:cytology / Q000502:physiology*
keywords: fluid dynamics; microfluidics; rheotaxis; simulations; sperm swimming