Low proportions of sperm can bind to the zona pellucida of human oocytes

Background: Sperm binding to the zona pellucida (ZP) is required for human fertilization. Under experimental conditions not limited by ZP binding sites, the cumulative numbers of sperm binding tightly to the ZP will asymptote with time to the total number of sperm in the insemination medium capable of binding. Methods: Numbers of ZP-bound sperm were counted after groups of 10 oocytes were incubated with 2x10(4) motile sperm in 20 mul droplets. The time-course of sperm binding was measured in three consecutive 2 h incubation periods using fresh oocytes for each period (n=12). Using the kinetic theory of gases to model sperm-oocyte collision rates, the time-course results were extrapolated to give the total proportion of motile sperm capable of binding to the ZP. ZP binding of sperm after 4 h incubation was studied in 20 fertile and 20 normozoospermic subfertile men. Results: The percentage of motile sperm capable of binding was for fertile men: mean 14% (range 8-25) and for the subfertile: 4.3% (range 0.1-13, P<0.001). Sperm morphology correlated with the proportion of ZP-bound sperm. Conclusions: More than 75% of motile sperm from fertile men have no ability to bind to the ZP. This finding has important implications for improvement of semen analysis.