part of inner acrosomal membrane The acrosomal membrane region that underlies the acrosomal vesicle and is located toward the sperm nucleus. This region is responsible for molecular interactions allowing the sperm to penetrate the zona pellucida and fuses with the egg plasma membrane. GO:0002079 cellular_component Note that this term is not a descendant of 'organelle inner membrane ; GO:0019866' because the outer acrosomal membrane is a portion of the acrosomal membrane; the latter is a single lipid bilayer. acrosomal membrane cytoplasmic vesicle membrane