The spontaneous acrosome reaction of human spermatozoa incubated in vitro.

Motile human sperm populations were prepared from liquefied semen (10 donors x 3 replicates) using Percoll density gradients at 30-60 min post-ejaculation. Sperm suspensions were incubated in a complex 'synthetic tubal fluid' culture medium (STF) at 37 degrees C under 5% CO2 in air for up to 36 h. Parallel aliquots were incubated with 50 microM A23187 to induce maximum acrosome loss (ARMAX). Acrosome reactions were assessed using both the triple-stain (TS) technique and fluorescent peanut agglutinin (PNA) lectin-labelling. During incubation, the proportion of TS acrosome reacted spermatozoa increased from 9.1 to 54.3% with ARMAX being 68.3%. Spermatozoa showing intact acrosomes by PNA labelling decreased from 68.4 to 26.1% over 36 h of incubation (ARMAX = 13.8%). Simultaneously, spermatozoa showing complete acrosomal loss (no PNA labelling) increased from 8.1 to 27.0% (ARMAX = 46.3%). Therefore, while only 23.5% of cells were actually undergoing acrosomal changes at the start of incubation, this had increased to 46.9% after 36 h (ARMAX = 40.7%). These experiments clearly show that even in selected populations, not all human spermatozoa are capable of undergoing an acrosome reaction. However, the incidence of acrosomal changes after 36 h of incubation did approach the ARMAX. These levels of spontaneous occurrence of the human sperm acrosome reaction were higher than those reported in many other in-vitro incubation studies: an improvement that may be attributable to the more physiological nature of the STF culture medium.