Structure of membrane domains and matrix components of the bovine acrosome

The acrosomal membrane system of bovine spermatozoa was examined by thin-section, freeze-fracture, surface-replica, and negative staining techniques in order to identify structural differentiations of specific acrosomal membrane domains. The outer acrosomal membrane of the apical and principal segments is characterized by a prominent electron-dense complex associated with its luminal face and a random intramembranous particle distribution. In the equatorial segment, the two-dimensional organization of bridging elements extending between the outer and inner acrosomal membrane was determined and correlated to freeze-fracture images. The inner acrosomal membrane lacked the electron-dense assembly noted on the outer acrosomal membrane and in freeze-fracture it appears crystalline. Further studies identified the distribution of the electron-dense subacrosomal material in the space between the inner acrosomal membrane and outer nuclear membrane. Finally, new observations on the structural organization of the acrosomal matrix are presented.