Follicular fluid-induced acrosome reaction distinguishes a subgroup of men with unexplained infertility not identified by semen analysis.

We compared the ability of sperm to undergo follicular fluid-induced acrosome reaction in vitro in fertile men and patients with unexplained infertility. After capacitation under optimum conditions, 28% of sperm from fertile men undergo acrosome reaction after follicular fluid exposure, whereas only 7% of the cells react spontaneously. In 15 men with unexplained infertility, 6 patients showed lack of acrosome reaction, whereas 9 men had sperm acrosome reactions similar to that of fertile men. However, in this cohort under study, semen characteristics of AR(+) and AR(-) patients were similar. In addition to inducing sperm acrosome reaction, follicular fluid also promoted significant changes in motion characteristics of capacitated sperm. Sperm curvilinear velocity (Vc) increased significantly after exposure to follicular fluid though linearity remained unchanged. The largest difference in cumulative Vc occurred at 90 microns/s. Assessing the ability of capacitated sperm to acrosome react may have clinical significance in predicting whether such sperm are capable of fertilizing an ovum.