Electron Microscope Tomography and Single-particle Reconstruction

Electron tomography progressively tilts a sample relative to the electron beam, capturing a series of 2D images from different angles. Computer software then stacks these 2D images to generate a 3D tomogram. This technique maintains the sample's in vivo structural integrity, making it ideal for visualizing cells, organelles, and macromolecules in their native environment.

Electron tomography examines intact samples at various angles to preserve native structure, while single-particle reconstruction averages thousands of 2D images of isolated molecules at random orientations. Tomography works in vivo for cells and organelles, whereas single-particle reconstruction requires purified, in vitro samples like isolated proteins or macromolecular complexes.

Cryo-electron tomography uses cryogenic conditions because fixation and dehydration can damage biological structures. This technique is limited to thin samples under 500 nanometers thick, as thicker specimens block the electron beam. It is primarily used for purified macromolecular complexes, viruses, and small cells such as bacterial cells.

Dual-axis tomography tilts the sample around two axes relative to the electron beam, generating two separate tomograms that are then aligned into a single 3D image. This approach provides better reconstruction of expanded features and increased sample depth resolution compared to single-axis tomography.

Single-particle analysis captures 2D images of thousands of isolated protein molecules distributed evenly on a grid at random orientations. These images are then averaged and aligned using computer software to reconstruct a 3D structure with near-atomic resolution. This technique is ideal for large or dynamic macromolecular complexes difficult to crystallize.

Single-particle reconstruction works best with large or dynamic macromolecular complexes that are difficult to crystallize, making it a substitute for X-ray crystallography. It has been successfully used to study membrane proteins, protein complexes, chromatin structure, and macromolecular machines such as ribosomes and proteasomes.

Scanning transmission electron microscopy (STEM) is a technique primarily used for obtaining tomograms of thick biological specimens. It combines sample surface scanning methodology with electron collection by detectors at each point where the electron beam hits the sample, enabling effective imaging of thicker samples than conventional transmission electron microscopy.