Effects of surrounding fluid on motility of hyperactivated bovine sperm.

Mammalian spermatozoa in organisms with internal fertilization are required to swim in the cervical and oviductal mucus, whose rheological properties differ substantially from those of water. Moreover, on the way to the oviduct, a change in sperm motility called hyperactivation may occur. In the present study, we focused on the motion characteristics of hyperactivated bovine sperm and investigated the effect of the surrounding fluid on motility. We prepared two kinds of polyacrylamide with high-viscosity non-Newtonian fluid properties, similar to the actual cervical and oviductal mucus. Using semen from Japanese cattle, we evaluated curvilinear velocity (VCL), straight-line velocity (VSL), and average path velocity (VAP). Additionally, we estimated linearity (LIN), straightness (STR), and wobble (WOB) as sperm motility parameters for several surrounding fluids. We successfully induced hyperactivation of bovine sperm in high-viscosity non-Newtonian fluid. Hyperactivation resulted in an increase in VCL and a decrease in VSL. In the high-viscosity non-Newtonian fluid, the hyperactivated sperm moved in a zig-zag pattern with regularity, different from the movement observed in a diluted solution. The increase in WOB in the non-Newtonian fluid suggests that hyperactivated sperm efficiently progress along the groove that exists on the oviductal mucus wall. These results improve our understanding of the motility of bovine sperm when they undergo hyperactivation in the actual cervical and oviductal mucus.