Structural organization of surface domains of sperm mitochondria.

Sperm mitochondria are assembled into an organized sheath surrounding the outer dense fibers and axoneme of the flagellar midpiece. Each mitochondrion is arranged so that its surface faces three different cellular organelles including the plasma membrane, neighboring mitochondria and the outer dense fiber-axoneme complex. In this manuscript we present data on structural differentiations of these three different surface domains of the outer mitochondrial membrane. We demonstrate that the apposed surfaces of adjacent mitochondria are joined by a two dimensional network of studs unique to this domain. By contrast, the surface domain facing the outer dense fiber-axoneme complex exhibits a different, but highly ordered structural organization, evident as a parcrystalline network of parallel stripes; this domain is further distinguished by its exclusive association with a midpiece-specific cytoskeletal complex. These differentiations are not seen on the surface domain of mitochondria which faces the plasma membrane. The implications of the mosaic composition of the outer mitochondrial membrane in the assembly and function of the mitochondrial sheath are discussed.