Mitochondrial morphology in the spermatozoa of the Mexican axolotl, Ambystoma mexicanum.

Thin-section and freeze-fracture electron microscopy of immature and mature spermatozoa of the Mexican axolotl, Ambystoma mexicanum, revealed numerous small spherical mitochondria with diameters ranging from 0.15 to 0.22 micrometers. Both the spherical form and the small size of these mitochondria were confirmed by serial thin-section studies. In mature spermatozoa, the mitochondria are located in the midpiece region, in tight contact with each other, exhibiting an almost crystalline arrangement. They do not surround the midpiece, but form a semicircular sheet over the sustained filament. The portion of the midpiece on the side of the undulating membrane and the flagellum is devoid of mitochondria. The plasma membrane in the midpiece region is tightly apposed to the mitochondria, so that in freeze-fracture or scanning electron microscopy the mitochondria seem to protrude through the plasma membrane. We suggest that the unusual organization of mitochondria in axoloti sperm facilitates the oxidative processes and increases the efficiency of ATP production and/or distribution within the cell.