Cryo-electron tomography: State of the art and future perspectives

The long prevailing view of a cell as a membrane-bound reaction compartment filled with freely diffusing and colliding macromolecules can no longer be maintained. There is growing awareness that fundamental cellular functions are carried out by ensembles of macromolecules, protein complexes or ‘molecular machines’. And, as in a factory, the operation of these machines must be coordinated to give rise to a stochastically variable supramolecular architecture. On this level of structure, the cell is, by and large, an uncharted territory. None of the existing imaging techniques enables the study of pleiomorphic structures, such as organelles or whole cells with a resolution of a few nanometers, as is required for identifying macromolecules in situ and for describing their interaction networks. Therefore, there is a strong incentive to develop methods, ideally non-invasive, to study the supramolecular architecture in a cellular context. However, cryo-electron tomograpy (ET) is an imaging technique which allows the three– dimensional (3D) visualisation of cells within 4 to 6 nm resolution. ET is by no means a new imaging technique, but with the advent of computer-controlled electron microscopes and the automation of elaborate image acquisition procedures, it became possible to obtai