Hyperactivation of capacitated human sperm correlates with the zona pellucida-induced acrosome reaction of zona pellucida-bound sperm

Background: The aim of this study was to determine the relationship between human sperm hyperactivation (HA), sperm-zona pellucida (ZP) binding and the ZP-induced acrosome reaction (AR) of ZP-bound sperm in vitro. Methods: Sperm samples from 129 infertile men were studied. Motile sperm (2 x 10(6)) selected by Pure sperm were incubated with four oocytes in 1 ml human tubal fluid supplemented with 10% human serum. After 2-h incubation, the number of sperm bound to the ZP and the AR of ZP-bound sperm were examined. Velocities and HA of sperm in insemination medium were assessed by Hamilton-Thorn Sperm Analyzer. Results: The HA was highly correlated with the ZP- induced AR in all the subjects (rho 5 0.626, P < 0.001). In the 69 men with <= 100 sperm bound/ ZP, allowing accurate counts, HA was not significantly correlated with sperm-ZP binding. Men with < 7% HA sperm were more likely to have very low ZP- induced AR. Only normal sperm morphology was significantly correlated with sperm-ZP binding ( rho 5 0.346 and 0.446 in semen and insemination medium, respectively, both P < 0.001). Sperm motility and velocities were significantly correlated with sperm morphology but not with either sperm-ZP binding or the ZP-induced AR. Conclusions: The correlation of HA with the ZP-induced AR of ZP-bound sperm suggests a mechanistic link between HA and the physiological AR in humans. Assessment of HA of capacitated sperm in vitro may be a useful clinical test for male infertility associated with defective ZP-induced AR that does not require the use of human oocytes.