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 co rresponding egg, its maturational level and/or progesterone content. I ndividual follicular fluids were obtained from 24 women undergoing in- vitro fertilization and assayed for their ability to induce an AR in n ormal human spermatozoa. After incubation in capacitation medium for 1 8 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 flu ids varied markedly in their ability to induce AR. Acrosome reaction c orrelated linearly with progesterone concentration (Spearman's r = 0.7 35, P = 0.01) at constant protein level, but no correlation was found between AR and protein concentration at constant progesterone level. P rogesterone 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 unfertilize d eggs (ANOVA, P = 0.015, n = 13 patients with both fertilized and unf ertilized eggs). In contrast, AR-inducing ability of individual follic ular fluids did not differ for fertilized and unfertilized eggs. While AR-inducing ability appeared to increase with maturational stage of t he egg, this trend was not statistically significant, probably due to small sample size. Our data suggest that progesterone rather than prot ein is the principal mediator of acrosome reaction induced by follicul ar fluid in vitro. Though progesterone concentration correlates with b oth the ability of the fluid to induce an AR in normal spermatozoa and with fertilization of the corresponding oocyte by husband's spermatoz oa, the lack of direct correlation between AR-inducing ability and fer tilization implies that other aspects of gamete function are also impo rtant in determining fertilization success.