Flow Orientation as a Possible Explanation of ‘Wave-Motion’ and ‘Rheotaxis’ of Spermatozoa

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Abstract
Flow orientation of spermatozoa in dense suspensions has been demonstrated by optical and electrical methods. Relaxation time, i.e. the time required for a suspension of elongated cells, which have been orientated by flow, to regain a certain degree of random distribution when flow ceases, has been shown to be closely and inversely related to motility, as measured by impedance change frequency in the suspension. Flow orientation may provide a physical explanation of ‘wave-motion’ and ‘rheotaxis’ of spermatozoa. Wave-motion occurs in a suspension of elongate cells which possess progressive motility in the direction of the long axis of the cell. Orientation of the cells may be brought about by ‘short-range order’ or flow. Disorientation may be brought about by streams of orientated cells moving in random directions. In the suspension as a whole an equilibrium will be reached such that the forces tending towards orientation (short-range order or flow) are balanced by forces tending towards disorientation (streaming). In small regions, however, statistical variation in the direction and velocity of the streams set up by sperm movements will produce local disturbance of the equilibrium and turbulence or wave-motion will result.