Acrosome reaction inducing activity in follicular fluid correlates with progesterone concentration but not with oocyte maturity or fertilizability.

Follicular fluid is a potent mediator of sperm acrosome reaction (AR) in vitro. The aim of this study was to investigate whether individual follicular fluids vary quantitatively in their ability to stimulate an AR, and whether such variability relates to fertilizability of the corresponding egg, its maturational level and/or progesterone content. Individual follicular fluids were obtained from 24 women undergoing in-vitro fertilization and assayed for their ability to induce an AR in normal human spermatozoa. After incubation in capacitation medium for 18 h, spermatozoa were challenged with the individual follicular fluids for 30 min. AR was detected by immunofluorescence, using fluorescein-labelled Pisum sativum lectin. We found that individual follicular fluids varied markedly in their ability to induce AR. Acrosome reaction correlated linearly with progesterone concentration (Spearman's r = 0.735, P = 0.01) at constant protein level, but no correlation was found between AR and protein concentration at constant progesterone level. Progesterone concentrations were not only higher (ANOVA, P = 0.002) in fluids from mature oocytes compared to those from less mature or post-mature eggs but also in fluids from fertilized compared to unfertilized eggs (ANOVA, P = 0.015, n = 13 patients with both fertilized and unfertilized eggs). In contrast, AR-inducing ability of individual follicular fluids did not differ for fertilized and unfertilized eggs. While AR-inducing ability appeared to increase with maturational stage of the egg, this trend was not statistically significant, probably due to small sample size. Our data suggest that progesterone rather than protein is the principal mediator of acrosome reaction induced by follicular fluid in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)