Sperm navigate through the female reproductive tract, encountering varying viscosities, to reach the oocyte for fertilisation [1]. Understanding sperm motion and flagellar patterns in a physiologically relevant context is crucial for reproductive insights and identifying infertility causes. Viscosity notably affects sperm motility, and it is suggested to influence long-range in vivo guidance [2]. However, the biomechanics underlying sperm flagellar activity in relation to fluid viscosity remain understood. Lack of high-speed high-resolution imaging techniques with automated image processing capabilities has been the main barrier to fully describing the flagellar beating behaviour. Here, we used a custom built high-speed, high-resolution dark-field microscopy system to study bull and mouse sperm flagellar dynamics in a physiologically relevant range of media viscosity, from 1 mPa·s to 200 mPa·s. Automated image analysis was used to extract sperm flagellar waveform and characterise bull and mouse sperm flagellar dynamics.
Differences emerged in the beating pattern for sperm in low versus high-viscosity conditions. Bull sperm exhibited a lower flagellar beating amplitude along the distal end of the tail when swimming in a high-viscosity compared to a low-viscosity buffer. However, mouse sperm in a high-viscosity buffer had a lower flagellar beating amplitude across the principal piece and a higher beating amplitude across the distal end of the tail compared to the low-viscosity buffer. Notably, bull sperm demonstrated a transition mode at 5 mPa·s and a regular circular pattern at 1 mPa·s and above 5 mPa·s. Conversely, mouse sperm maintained periodic flagellar beating in high-viscosity media but displayed distorted loops in low-viscosity media.
In conclusion, we resolved the dynamics of free-swimming bull and mouse sperm in viscoelastic media (1-200 mPa·s). Our findings indicated a highly reproducible flagellar waveform for both species in high-viscosity media.