ABOUT CRYO-EM

How does cryoEM work?

CryoEM will result in high-resolution structures of homogeneous samples such as a protein, RNA, virus, and complexes. A few microliters of the solution containing a purified homogeneous sample are placed onto a special grid, blotted to a thin layer, and plunged into liquid ethane to vitrify the sample. This vitrification process is typically performed by a robot so that the parameters can be repeated. Vitrification captures the sample like a snapshot in time, preserving the objects in a hydrated and biologically relevant state. Importantly, because the sample was in solution and homogeneous, when it is captured on the grid, many different orientations are present. Vitrification also protects the sample from damage during data collection using the electron microscope.

The information gathered is analyzed using computer software that groups together similar orientations of the sample. With enough different views of the object, the software can reconstruct a 3D map of the structure. Such maps are valuable tools for researchers to use in their research to understand the mechanisms of vital biological processes, track drug resistance, and develop biologicals and other therapeutics, in search of cures.

What is CryoET?

Cryogenic-sample electron tomography (cryoET) is used to resolve unique or pleomorphic samples, such as slices of tissue, bacteria, cells, and non-homogeneous virus like measles, coronavirus, and influenza.

With cryoET, the sample is vitrified and placed into the microscope. However, the sample is tilted as data are collected, resulting in many different views from the tilt series. For logistical reasons, the grid stage can only be tilted up to 60 degrees, which results in some loss of information. Consequently, the reconstructed map is not as high-resolution as what results from cryoEM. Even so, the information that can be gathered from tomography remains extremely valuable.

“It’s [tomography is] still extremely powerful, because we can see pleomorphic things [that is, things that can alter their size or shape under different environmental conditions] instead of all the same thing that we can average altogether. Now we can see things like HIV, or coronavirus—because each one is different. Otherwise, we wouldn’t get a look at it.” — Dr. Susan Hafenstein

CryoEM and cryoET: How should we write them?

CryoEM and cryoET: to hyphenate, or not to hyphenate? What exactly is cryoEM or cryoET short for? These questions are frequently debated in scientific circles, with calls to move toward more standardized usages and definitions to limit confusion as the technology expands.

One recent call for standardization came from Richard Henderson himself, one of the three 2017 Nobel Prize in Chemistry awardees for advancements made in cryoEM. In an IUCrJ paper published in 2023, Henderson and co-author Samar Hasnain suggest the “succinct” wording of “cryogenic-sample Electron Microscopy” and its abbreviation, “cryoEM.”

This is a distinction from the commonly used “cryo-electron microscopy” and hyphenated abbreviation “cryo-EM,” which some have pointed out imply that it is the electrons that are subjected to cryogenic temperatures, when that is not the case.

Thus, as suggested in the paper, these are the usages we currently subscribe to at The Hormel Institute cryoEM Core Facilities:

cryogenic-sample Electron Microscopy (cryoEM)
cryogenic-sample Electron Tomography (cryoET)