Time-dependent measure of a nanoscale force-pulse driven by the axonemal dynein motors in individual live sperm cells

Vertical deflection of the force sensor-vs.-time records an individual sperm cell contacting and then swimming against the sensor with mechanical coupling. Fresh semen was diluted 200-fold in Tris-saline buffer pH 7.4 and injected into a 50-μL fluid chamber containing a calibrated¹⁹ force sensor 460 μm in length. The propulsive forces of the sperm cell are represented by a regular oscillating pattern, clearly visible in the right-hand portion of the plot. The frequency of these discrete force pulses ranged between 3.5 and 19.5 Hz, varying from one cell to the next. The force-pulse frequency seems to hold a 1:1 correspondence with the known beat frequency of the bovine flagellum. Alternatively, because flagellar beat frequency will decrease to some degree under the experimental conditions used (23°C), the possibility that two force-pulses are produced during a single beat cycle (a 2:1 correspondence) cannot be excluded. In all cases the reported longitudinal force is calculated from oscillations in the flat region of the scan (2.31–2.97 seconds) after any initial transient has vanished.

Force measurement and determination of the force vector alignment. Sensor movements in the (A, C) horizontal and (B, D) vertical planes were recorded simultaneously for two different sperm cells (A, B and C, D). The horizontal signal records twisting motions of the cantilevered sensor with a spring constant of 195 pN/nm. The vertical signal records the up-down bending of the sensor. When (D) the vertical motions are maximal and (C) the twisting motions of the rectangular sensor are minimal, the force of the forward propulsion is measured as the amplitude of the vertical signal. These data are used to calculate the propulsion force and energy (see text). If the sperm cell is misaligned and not swimming in the vertical plane of the sensor, then (A) the horizontal signal is relatively strong and (B) the vertical signal weak.

The longitudinal propulsive forces measured using six different force sensors (k = 50–261 pN/nm) cluster near 9.88 nN (7.35–14.36 nN) regardless of beat frequency (3.5–19.5 Hz). The force constant (k, in pN/nm) and vertical displacement (y_tip, in nm), respectively, are provided adjacent to the data markers (squares) for the sensor used for the measurement. The datum with spring constant 195 pN/nm corresponds to curve D in Figure 2.