Characterization of the Reorientation in Sperm Motility*

To fertilize an egg, sperm must orient their motion according to the complex environment in the female reproductive tract. Here, using bovine sperm as a model, we look to identify the reorientation mechanisms that make such migration possible by analyzing the movement of individual sperm heads in a neutral environment. ​In a microfluidic device, we image sperm moving at the interface of a viscoelastic medium and a solid substrate. By training a machine learning model to automatically track the head of a sperm cell from each frame, we obtain the trajectories over time, and the autocorrelation of the head velocities reveals that their dominant reorientation mechanism is a circular motion, albeit with sinusoidal-like deviations from the perfect circular trajectory. Fourier analysis indicates that these deviations stem from the beating of the tail. Our preliminary analysis of the tail beating confirms the beating frequency. More work is needed to fully determine the relationship between tail beating and head movement direction. As all the known molecular motors are located along the sperm tail, this understanding will be the foundation for understanding how sperm navigate the complex environment.