Rheotaxis facilitates upstream navigation of mammalian sperm cells

Abstract

A major puzzle in biology is how mammalian sperm maintain the correct swimming direction during various phases of the sexual reproduction process. Whilst chemotaxis may dominate near the ovum, it is unclear which cues guide spermatozoa on their long journey towards the egg. Hypothesized mechanisms range from peristaltic pumping to temperature sensing and response to fluid flow variations (rheotaxis), but little is known quantitatively about them. We report the first quantitative study of mammalian sperm rheotaxis, using microfluidic devices to investigate systematically swimming of human and bull sperm over a range of physiologically relevant shear rates and viscosities. Our measurements show that the interplay of fluid shear, steric surface-interactions, and chirality of the flagellar beat leads to stable upstream spiralling motion of sperm cells, thus providing a generic and robust rectification mechanism to support mammalian fertilisation. A minimal mathematical model is presented that accounts quantitatively for the experimental observations.DOI: http://dx.doi.org/10.7554/eLife.02403.001.

Keywords: fertilization; rheotaxis; sperm.

Figure 1.. Sperm swim on upstream spirals against shear flow.

(A) Background-subtracted micrograph showing the track of a bull sperm in a cylindrical channel (viscosity μ = 3 mPa·s shear rate $\dot{\gamma }=2.1s^{-1}$ ), channel boundary false-coloured with black, see Video 1 for raw data. (B) Schematic representation not drawn to scale. The conical envelope of the flagellar beat holds the sperm close to the surface (Kantsler et al., 2013). The vertical flow gradient exerts a torque that turns the sperm against the flow, but is counteracted by a torque from the chirality of the flagellar wave, resulting in a mean diagonal upstream motion. (C) Tracks of bull sperm near a flat channel surface. (D) Upstream and transverse mean velocities $\langle v_{y,x}\rangle$ vs shear flow speed u₂₀ at 20 μm from the surface for different viscosities. All velocities are normalised by the sample mean speed v_0μ at $\dot{\gamma }=0$. For human sperm, in order of increasing viscosity v_0μ = 53.5 ± 3.0, 46.8 ± 3.7, 36.8 ± 3.3, 29.7 ± 3.9 μm/s, and for bull sperm v_0μ = 70.4 ± 11.8, 45.6 ± 4.7, 32.4 ± 4.8, 29.6 ± 4.1 μm/s, where uncertainties are standard deviations of mean values from different experiments. Each data point is an average over >1000 sperms. (E) Histograms for selected points in (D). DOI: http://dx.doi.org/10.7554/eLife.02403.003

Figure 2.. Temporal response of sperm cells to a reversal of the flow direction depends sensitively on viscosity.

(A) At low viscosity, sperm perform sharp U-turns, see also Video 2. (B) At high viscosity, the typical radius of the U-turns increases substantially (Video 3). White/black arrows show orientations of several cells before/after turning. (C) Flow velocity at distance 5 μm from the channel surface (blue, ‘Flow’), mean upstream velocity $\langle v_y\rangle$ (red, ‘Up’) and mean transverse velocity $\langle v_x\rangle$ (green, ‘Trans’) as function of time. The typical response time of sperm cells after flow reversal increases with viscosity. Peaks reflect a short period when mean swimming direction and flow direction are aligned. The time series for human sperm also signal a suppression of the beat chirality at high viscosity, consistent with Figure 1D. DOI: http://dx.doi.org/10.7554/eLife.02403.008

Figure 3.. Model simulations reproduce main experimental observations.

(A) Upstream and transverse velocity for different values of the variability (effective noise) parameter D in units rad/s and dimensionless shape factors (α, β). (B) Time response of a chiral swimmer with χ = +1 (‘Human’) and a non-chiral swimmer with χ = 0 (‘Bull’) to a reversal of the flow direction at time t = 0. Blue dashed line shows fluid flow u_y at 5 μm from the boundary. Simulation parameters (N = 1000 trajectories, A = 10 μm, ℓ = 60 μm, V = 50 μm/s) were chosen to match approximately those for viscosity 1 mPa·s in Figure 2C. DOI: http://dx.doi.org/10.7554/eLife.02403.011

Figure 4.

(A) Schematic of the microfluidic channel and field of view (turquoise region) in the sperm motility measurements. (B) Velocity profile at the center of the channel. Red symbols are values of the vertical velocity profile v_y(z) measured by PTV for the flow rate 0.1 μl/s. The solid line shows the theoretically calculated flow profile for the same flow rate. In motility experiments, values for the velocity gradient near the boundary (pink region) were obtained by measuring the flow velocity at 20 μm from the boundary. (C) Theoretical 2D flow speed profile in (x,z)-plane at flow rate 0.1 μl/s. DOI: http://dx.doi.org/10.7554/eLife.02403.012