Cryo-electron microscopy (sometimes called cryo-EM or electron cryomicroscopy) is a form of electron microscopy (EM) where the sample is studied at cryogenic temperatures (generally liquid nitrogen temperatures). CryoEM is developing popularity in structural biology.

A version of electron cryomicroscopy is cryo-electron tomography (CET) where a 3D reconstruction of a sample is created from tilted 2D images, again at cryogenic temperatures (either liquid nitrogen or helium).

Biological specimens

The biological material is preserved in a frozen-hydrated state by rapid freezing, usually in liquid ethane near liquid nitrogen temperature. By maintaining specimens at liquid nitrogen temperature or colder, they can be introduced into the high-vacuum of the electron microscope column. Most biological specimens are extremely radiation sensitive, so they must be imaged with low-dose techniques (usefully, the low temperature of cryo-electron microscopy provides an additional protective factor against radiation damage).

Consequently, the images are extremely noisy. For some biological systems it is possible to average images to increase the signal to noise ratio and retrieve high-resolution information about the specimen. This approach requires that the things being averaged are identical (e.g. ribosome particles). Analysis of ordered arrays of protein, such as 2-D crystals of membrane proteins or helical arrays of proteins, also allows a kind of averaging which can provide high-resolution information about the specimen. This technique is called electron crystallography.

References

Microscopes | cell biology | Protein structure