Table 4 Direct effects of surfaces on sperm performance

Vogel [12] gives a particularly disquieting illustration of the problem wall effects may present in studies of microscopic movement: at a Reynolds number of 10^-3 (slightly less than that of a sea urchin spermatozoon), a wall 50 diameters away can significantly influence drag of a cylinder moving parallel to the wall. Changes in swimming speed due to wall effects are predicted to be modulated by drag effects, but as often with fluid dynamics, this effect is not always intuitive: sperm are predicted to swim faster within 10 body lengths of a wall, than in an unbounded fluid [78, 79]. Using sea urchin (Arbacia punctulata) sperm Gee & Zimmer-Faust [80] found significant differences in speed between sperm swimming at two different distances from a wall and concluded that wall effects can "substantially exaggerate swimming speed" (p 3185). This finding is supported by the theoretical predictions of [81] (Figure 3). Vogel [12] suggests a rule of thumb derived from White [82], that for Re < 1, we can be reasonably sure that wall effects can be ignored if

$\frac{y}{L}>\frac{20}{\mathrm{Re}}$(5)

   where y is the distance to the nearest wall, and L is the characteristic length of the object (in this case total sperm length).

   The attraction of sperm to walls, such as glass coverslips, and cell surfaces (such as that of the egg) seems to have been first noted by Dewitz (1886) and quantified initially by Rothschild (1963). Since then several empirical and theoretical studies have been conducted on this phenomenon. Winet et al [83] used human sperm to study accumulation at boundaries, while Woolley [84] used a selection of sperm from mice, chinchillas, Xenopus and eels, and Cosson et al. [85] worked with sea urchin sperm.

   Fauci & MacDonald [81] used a numerical approach to further explore the effects of boundaries on sperm motion, concluding that hydrodynamic effects lead to the attraction of sperm to boundaries. However, the exact mechanisms involved may depend on the type of swimming motion [84] or asymmetries in the head-flagellum connection [85].