Arachidonic acid-induced acrosomal loss in the spermatozoa of a marsupial, the tammar wallaby (Macropus eugenii).

Tammar wallaby spermatozoa were induced to undergo acrosomal loss when incubated with arachidonic acid (AA). Ultrastructural examination indicated that the AA-induced acrosomal loss occurred via multiple point fusions between the outer acrosomal membrane and the overlying plasma membrane. This form of acrosomal loss mimicked the physiological acrosome reaction (AR) seen in the sperm of eutherian mammals. The fusion event was limited to the acrosomal region of the plasma membrane and did not proceed past the peri-acrosomal ring. The entire acrosome was lost after AA treatment leaving no evidence of a persistent equatorial segment-like region. Ultrastructural evidence of AR-like membrane fusion was seen immediately on addition of 50 microg mL(-1) AA and a large proportion of sperm examined after five min incubation were in the late stages of membrane fusion. Longer-term incubation with AA had deleterious effects on wallaby sperm motility. It remains to be determined whether the AA-induced membrane fusion observed here indicates that AA is involved in the marsupial AR. However, pretreatment of sperm with the protein kinase C (PKC) inhibitor HMG significantly reduced AA-induced acrosomal loss suggesting that AA may have acted via PKC. If this is so, AA is probably physiologically significant and a novel pathway may be operating during AR induction in marsupials.