Cryo-ET

Core Idea

Cryo-electron tomography (cryo-ET) images biological structures in their native cellular context by collecting a tilt series — a sequence of cryo-EM images of the same specimen tilted from approximately -60 to +60 degrees — and computationally reconstructing a three-dimensional volume (tomogram). Unlike single-particle cryo-EM (which images purified molecules in isolation), cryo-ET can visualize macromolecular complexes directly inside cells, revealing their spatial organization, interactions with other cellular components, and functional states in situ. Focused ion beam (FIB) milling thins frozen cells to electron-transparent lamellae (~100-200 nm), and subtomogram averaging of repeated structures within tomograms can achieve sub-nanometer resolution.

Explainer

Single-particle cryo-EM produces beautiful atomic-resolution structures, but of purified molecules in isolation. The molecule has been removed from the cell, stripped of its interaction partners, and frozen in a thin layer of ice. Cryo-electron tomography takes the opposite approach: it images molecules where they actually function — inside cells, attached to membranes, assembled into higher-order structures — revealing not just what a molecule looks like but where it is and what it does in its native environment.

The principle is analogous to medical CT scanning. A tilt series is collected: the specimen is imaged at many different tilt angles (typically -60 to +60 degrees in 1-3 degree increments), producing a set of 2D projections from different viewing angles. These projections are computationally combined (back-projected) to reconstruct a 3D volume — the tomogram. Each tomogram is a complete 3D snapshot of a biological scene at the moment of vitrification: ribosomes decorating the ER surface, vesicles budding from the Golgi, cytoskeletal filaments spanning the cytoplasm, all captured in their native spatial relationships.

The resolution of a single tomogram (~20-40 Angstroms) is limited by the low electron dose (to prevent radiation damage), the missing wedge (the specimen cannot be tilted to 90 degrees, creating a gap in angular coverage), and the specimen thickness (thicker samples scatter electrons more, reducing image quality). FIB-milling addresses the thickness problem: a focused ion beam is used to thin a frozen cell to a ~100-200 nm lamella, creating an electron-transparent window into the cell interior. This technology has opened essentially any cell type to tomographic imaging.

Subtomogram averaging bridges the resolution gap between cellular tomography and atomic structural biology. When a macromolecular complex appears many times in tomograms (ribosomes on the ER, nuclear pore complexes in the nuclear envelope, coat proteins on vesicles), each instance can be extracted as a small 3D volume, and these volumes can be aligned and averaged — identical in principle to the averaging that drives single-particle cryo-EM. With sufficient copies (thousands to tens of thousands), subtomogram averaging achieves sub-nanometer resolution while preserving the cellular context. Recent studies have determined near-atomic resolution structures of ribosomes, proteasomes, and viral capsid proteins directly inside cells — a goal that seemed impossibly ambitious just a decade ago. Cryo-ET is the frontier of structural biology, connecting molecular structure to cellular function in a way that no other technique can match.