Exocytosis during Acrosomal Reaction



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An early event in the acrosomal reaction is the exocytotic fusion of the acrosomal vacuole with the plasma membrane.  This results in the exposure of the vacuole membrane, hitherto occluded, and the vavuole membrane eventually fuses with the membrane of an egg.  In the movie (fluorescence image at left, phase-contrast image at right), the medium surrounding the sperm contains a lipophilic fluorescent dye.  The dye initially stains only the plasma membrane.  At time 0, exoctosis takes place.  At this moment, the vacuole membrane is yet dark in the fluorescence image, indicating that it has really been occuluded and disconnected from the plasma membrane.  The next moment the vacuole membrane starts to fluoresce brightly, showing that it is now exposed to the medium.  The newly exposed membrane is pushed toward an egg as the acrosomal process elongates (by actin polymerization).
                                                               Sase, I. et al., J. Cell Biol. 131, 963-973 (1995).