Characterization of rheotaxis of bull sperm using microfluidics.

We study rheotaxis of bull sperm inside microchannels to characterize the effects of flow and wall shape on sperm swimming behavior. We found that a large percentage of sperm cells, 80 to 84%, exhibited positive rheotaxis (sperm cells swimming against the flow) within flow velocities of 33 to 134 μm s(-1). Sperm cells were also found to reverse their swimming direction when the liquid flow direction was reversed. Time taken by sperm cells to reverse their swimming direction was inversely proportional to the flow velocity. Sperm behavior was significantly affected by the sperm position with respect to the channel wall. Sperm cells close to the channel wall moved upstream faster than sperm cells moving along the channel centerline. Shear stress, which is an indicator of velocity distribution, was found to play an important role in regulating rheotactic behavior of sperm cells. Side pockets were added to some microchannels to mimic storage sites in mucosal folds and pockets in the fallopian tube of the female reproductive system and sperm interaction with these pockets was monitored. We found that sperm cells tend to follow channel walls and enter these pockets without any chemical binding, which further confirms the wall tracking behavior of mammalian sperm cells. Our results confirm that sperm rheotaxis is a strong mechanism for guiding sperm cells towards the oocyte along the female genital tract.