Research ArticleBIOPHYSICS

Strictures of a microchannel impose fierce competition to select for highly motile sperm

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Science Advances  13 Feb 2019:
Vol. 5, no. 2, eaav2111
DOI: 10.1126/sciadv.aav2111
  • Fig. 1 Simulation of sperm motion below the stricture.

    (A) Two-dimensional velocity field of the sperm medium within the device at Z = 15 μm. (B) Velocity field of the medium demonstrated in Y-Z cut planes using contour levels. (C) Shear rate on the top surface of the channel. (D) Schematic of sperm butterfly-shaped motion, with depiction of all the variables. The terms ρ and ρ′ indicate the sperm perpendicular distance from both sidewalls, Embedded Image is the unit vector along the sperm orientation, and θ and θ′ are the angles between the sperm orientation and the unit vector normal to the sidewalls. These variables are used in Eqs. 2 to 4. (E) Microscopic image of the sperm and the direction of flow. (F) Sperm path below the stricture for sperm with different velocities (40 to 80 μm/s). (G) Influence of ΩIN on the sperm path. The value used as ΩIN was experimentally measured as 0.12 ± 0.06 s−1. (H) Top: Initial angle of the sperm with the sidewall at the contact point for ΩIN = −Ωmax, 0, and Ωmax, illustrated with red, green, and blue, respectively. Bottom: Time required for sperm to rotate upstream toward the stricture. (I) Total period (τ) required for sperm to depart from point A (C) and reach point C (A). The time elapsed in each mode is illustrated separately so that τf, τr, and τt correspond to the boundary, rotation, and transfer mode times.

  • Fig. 2 Sperm motion modes and the CI.

    (A) Schematic of sperm motion in different modes, including transfer, rotation, and boundary swimming modes, illustrated in red, green, and blue, respectively. The sperm projection in the X direction demonstrates a periodic motion, on which we based the CI. (B) The CI for sperm slower than a particular velocity drops depending on the value of a.

  • Fig. 3 Observation of the butterfly-shaped swimming path and withdrawal distance of sperm below a stricture.

    (A) Butterfly-shaped path extracted for a sperm with a velocity of 57 μm/s. (B) Schematic of the butterfly-shaped path based on the experimental results obtained in (A) for better visualization. (C) Trajectory of the sperm during two different periods to illustrate the consistency of the butterfly-shaped path over time. W is the withdrawal distance of the sperm. (D) Experimental values of the withdrawal distance extracted from 130 sperm with different velocities from three different samples in comparison with values expected by simulations.

  • Fig. 4 Experimentally measured times of the different swimming modes (i.e., transfer, rotation, and boundary swimming) and CI calculated for sperm.

    (A) Images of sperm (blue and yellow) moving in the transfer mode. (B) A single sperm (green) in the rotation mode, reorienting upstream. (C) A sperm (red) in the boundary swimming mode begins following the sidewall. (D) The times required for sperm to transfer (τt), rotate (τr), and follow the boundary (τf) were experimentally measured for 120 sperm. Since the rotation and the boundary swimming times overlapped, their sum (τr + τf) was reported and measured. (E) For a given a, as the velocity of the sperm decreases, its likelihood to maintain its X coordinate closer than a decays.

  • Fig. 5 Accumulation of sperm below the stricture and the twinkling effect.

    (A) Phase-contrast microscopy leads to twinkling of the bull sperm. (B) Low-magnification image of our device with a concentrated sample injected. Zones A and B are indicated in the image. (C) The number of live sperm in each zone for three different samples was counted manually to confirm the accumulation of the sperm.

  • Fig. 6 Gate-like role of the stricture.

    (A) Sperm number one is able to resist against the shear rate in the stricture, and therefore, it has approximately no movement in the −X direction. The other sperm (numbers 2 to 7) move in butterfly-shaped paths, but they cannot pass through the junction. Hierarchical swimming is discernible, and sperm with higher velocity are closer to the stricture and to each other. (B) A small decrease (7.98 to 7.16 s−1) in the injection flow rate led to sperm number 1 advancing and entering the adjacent compartment. Meanwhile, the slower sperm continue to swim on the butterfly-shaped path below the stricture.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/2/eaav2111/DC1

    Fig. S1. Sperm tilted orientation in the boundary swimming mode.

    Fig. S2. Schematic of the model used for lubrication theory.

    Fig. S3. Sperm intrinsic angular velocity and curvature.

    Fig. S4. Intrinsic angular velocities measured for sperm when the external flow was zero.

    Fig. S5. Impact of stricture mouth angle on the butterfly-shaped motion of sperm.

    Fig. S6. Butterfly-shaped motion of human sperm.

    Fig. S7. Transfer, rotation, and boundary swimming modes with corresponding times for human sperm.

    Fig. S8. Threshold sperm velocity versus shear rate of the stricture.

    Movie S1. Sperm intrinsic rotation.

    Movie S2. Bovine sperm swimming on butterfly-shaped paths.

    Movie S3. Human sperm swimming on butterfly-shaped paths.

    Movie S4. Accumulation of the sperm below the stricture.

    Movie S5. Gate-like role of the stricture.

    References (3540)

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. Sperm tilted orientation in the boundary swimming mode.
    • Fig. S2. Schematic of the model used for lubrication theory.
    • Fig. S3. Sperm intrinsic angular velocity and curvature.
    • Fig. S4. Intrinsic angular velocities measured for sperm when the external flow was zero.
    • Fig. S5. Impact of stricture mouth angle on the butterfly-shaped motion of sperm.
    • Fig. S6. Butterfly-shaped motion of human sperm.
    • Fig. S7. Transfer, rotation, and boundary swimming modes with corresponding times for human sperm.
    • Fig. S8. Threshold sperm velocity versus shear rate of the stricture.
    • References (3540)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Sperm intrinsic rotation.
    • Movie S2 (.mp4 format). Movie S2. Bovine sperm swimming on butterfly-shaped paths.
    • Movie S3 (.mp4 format). Movie S3. Human sperm swimming on butterfly-shaped paths.
    • Movie S4 (.mp4 format). Accumulation of the sperm below the stricture.
    • Movie S5 (.mp4 format). Gate-like role of the stricture.

    Files in this Data Supplement:

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