Indirect immunofluorescence microscopy and freeze-fracture have been used to identify overlapping subdomains at the peripheral rim of the sperm-head plasma membrane (PM) and the margin of the outer acrosomal membrane (OAM) comprising the principal segment of the acrosome of the boar spermatozoon. An array of ridge-like structures (spaced 12–16 nm centre-to-centre), originally observed on the OAM by Aguas & Pinto da Silva, lies just beneath an area of the PM that is sparsely populated with large intramembranous particles compared to that of other regions of the head PM. This region has a high specificity for the lectin arachis hypogaea (peanut agglutinin). We suggest that the OAM at the rim of the sperm head may be rich in acidic phospholipids and that the close apposition of this membrane with a region of the PM relatively poor in integral membrane proteins may provide sites for initiating the acrosome reaction.

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