The role of follicular fluid in inducing the acrosome reaction of human spermatozoa incubated in vitro

Motile human sperm populations were prepared from liquefiedsemen (5 donors x 3 replicates) using Percoll gradients at 30–60 min post-ejaculation and preincubated in a complex ‘syntheitictubal fluid’ culture medium (STF) at 37° C under 5%CO₂ in air for 6h. Aliquots of these suspensions were then incubatedfor a further 2 h in STF containing 0, 5, 25, 50, 75 and 100%(v/v) pooled human follicular fluid (FF). Another aliquot wastreated with 10 µm A23187 in STF for 20min and then incubatedin fresh STF medium for a futher 2h to induce maximal acrosomeloss. Acrosome reactions were assessed using both the triple-staintechnique and fluorescent peanut agglutinin lectin-labelling.Sperm motility and movement characteristics were assessed fromvideorecordings using digital image analysis (CellSoft). Exposureto FF caused only relatively small proportions of the preincubatedspermatozoa to undergo acrosome reactions. The size of theseresponsive sub-populations was smaller than that capable ofresponding to a Ca²⁺ influx generated by A23187. Increased FFconcentrations induced a progressive loss of motility and trendsfor changes in movement characteristics that may have been relatedto reduced intracellular Ca²⁺. This interpretation of theseobervations is that while FF may act to stimulate or promotethe human sperm acrosome reaction it does not appear to be aspecific inducer of it. Consequently, a precise role for FFat the relatively low concentrations that would be expectedto be present in the tubal ampulla in the physiological regulationof human fertilization remains unproven