Abstract
The distribution of membrane sterols in the acrosome reaction of Japanese abalone sperm was studied by freeze-fracture techniques using a polyene antibiotic, filipin, as a probe for membrane sterols. In the unreacted sperm, the plasma membrane covering the acrosome showed a random distribu-tion of filipin-sterol complexes, while in the outer acrosomal membrane, filipin-sterol complexes were rarely observed. In the initial stage of the acrosome reac-tion, filipin-sterol complexes were found densely distributed in the plasma mem-brane throughout the acrosomal region. On the subjacent outer acrosomal mem-brane, filipin-sterol complexes were densely packed in the apical area, cor-responding to the truncated cone region. This apical area consistently showed such regionalization of filipin-sterol complexes during the acrosome reaction. As the acrosomal process elongated, the intramembrane particle (IMP)-free areas which appeared in the acrosomal process membrane were found to be displaced by densely distributed filipin-sterol complexes. These results indicate a possible regionalization of membrane sterols related to the acrosome reaction.
