Characterization of the Reorientation and Fluctuation in Orientation in Sperm Motility

Living cells often need to find and move to a better environment, and the mechanism enabling them to do so requires a step of reorientation. For example, E.coli chemotaxis is achieved by modulating tumbling rates. Using bovine sperm as a model, we work to identify and quantify the effects of different reorientation mechanisms of mammalian sperm since there is no tumbling. Within a microfluidic device at the interface of polymer solution and solid substrate, sperm move close to the surface, making the motion mostly two-dimensional. After tracking sperm trajectories, we computed the orientation autocorrelation function. From both the autocorrelation and time evolution of sperm orientation, we observed strong evidence that the dominant motion is circular, with a period that provides a timescale. Analysis of orientation fluctuations around this circular motion revealed a Gaussian fluctuation with no distinct features in its time autocorrelation. These analyses help us understand the reorientation mechanism of mammalian sperm, which will help us devise potential chemo and thermotaxis. Further analysis of the Gaussian fluctuation suggests that it originates from the deterministic motion driven by sperm flagellum. Revealing the root cause of this noise helps us understand the motility strategy overall. The timescales of these mechanisms also allow us to estimate the swimming pressure of sperm and experimentally verify theoretical calculations and simulations of swim pressure in active matter systems.