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

Motile human sperm populations were prepared from liquefied semen (5 donors x 3 replicates) using Percoll gradients at 30-60 min post-ejaculation and preincubated in a complex 'synthetic tubal fluid' culture medium (STF) at 37 degrees C under 5% CO2 in air for 6 h. Aliquots of these suspensions were then incubated for a further 2 h in STF containing 0, 5, 10, 25, 50, 75 and 100% (v/v) pooled human follicular fluid (FF). Another aliquot was treated with 10 micron A23187 in STF for 20 min and then incubated in fresh STF medium for a further 2 h to induce maximal acrosome loss. Acrosome reactions were assessed using both the triple-stain technique and fluorescent peanut agglutinin lectin-labelling. Sperm motility and movement characteristics were assessed from videorecordings using digital image analysis (CellSoft). Exposure to FF caused only relatively small proportions of the preincubated spermatozoa to undergo acrosome reactions. The size of these responsive sub-populations was smaller than that capable of responding to a Ca2+ influx generated by A23187. Increased FF concentrations induced a progressive loss of motility and trends for changes in movement characteristics that may have been related to reduced intracellular Ca2+. This interpretation of these observations is that while FF may act to stimulate or promote the human sperm acrosome reaction it does not appear to be a specific inducer of it. Consequently, a precise role for FF at the relatively low concentrations that would be expected to be present in the tubal ampulla in the physiological regulation of human fertilization remains unproven.