We are using electron tomography to investigate the internal organization of mitochondria and the interactions of these organelles with other cellular components. Two-axis tilt-series are recorded from 0.2-1.0-μm sections using either an AEI EM7 high-voltage or JEOL 4000 intermediate-voltage electron microscope. Reconstructions are computed using programs in SPIDER and visualized with the Sterecon system.

Three-dimensional reconstructions of conventionally fixed and plastic-embedded rat-liver mitochondria (isolated and in situ) indicate that the cristae, the infoldings of the inner membrane, are pleiomorphic, defining compartments that are connected to each other and to the surface of the inner membrane by narrow (30-40-nm diameter) tubular regions (Fig. 1). This is contrary to the “baffle” model commonly shown in textbooks and has important implications for the diffusion of ions, metabolites, and macromolecules inside the organelle. The same basic organization is also observed in reconstructions of mitochondria in rat-liver sections prepared by the cryo-fixation method of Tokuyasu(Fig. 2).