Inside the Cryo-FIB Workflow That Makes 3D Imaging of Frozen Cells Possible

Cryo-electron tomography has become one of the most powerful tools in structural biology, allowing researchers to see the three-dimensional structures of biomolecular complexes and organelles inside cells and tissues in their near-native state. But there is a catch: most biological specimens are far too thick for electrons to pass through, and the technique only works when samples are thinned to an electron-transparent lamella that still contains the target of interest. A new review published in Applied Microscopy argues that the real bottleneck in cryo-electron tomography, or cryo-ET, is not the thinning technique itself but the design of an entire workflow that reliably produces high-quality, target-containing lamellae.

At the center of that workflow sits cryo-focused ion beam-scanning electron microscopy, or cryo-FIB-SEM. According to the review by Seonhye Son, Sujin Lee, Jihyun Kim and Chihong Song, this instrument is far more than peripheral equipment for shaving frozen samples down to size. It is simultaneously a lamella fabrication tool, a targeting platform and a quality-control station, integrating the machining of vitrified specimens with scanning electron microscopy assessment of surface geometry, cryogenic correlative light and electron microscopy for target localization, and repeated verification at intermediate milling stages. The authors describe cryo-FIB-SEM as the de facto standard platform for precisely machining frozen specimens, encompassing the management of major failure modes such as contamination, charging and curtaining, as well as downstream steps for data reconstruction and interpretation.

The review places cryo-ET within the broader multiscale spectrum of cryo-electron microscopy. Cryo-EM preserves specimens in a near-hydrated state through vitrification, either by plunge freezing or high-pressure freezing, and spans techniques from MicroED and single-particle analysis to cellular- and tissue-level imaging. Within that spectrum, cryo-ET stands out because it delivers direct three-dimensional structural information in situ, capturing the location of molecular machines and their surrounding environment rather than the averaged structures of purified protein complexes. That advantage, however, comes with steep technical demands. If a lamella is too thick, the signal-to-noise ratio drops and multiple scattering increases. If it is too thin or nonuniform, structural integrity and target retention become difficult to maintain. And even a perfectly prepared lamella is worthless if the target is not inside it.

The workflow begins earlier than many researchers might expect: at the freezing step. Plunge freezing, the most widely used standard method, works well for cultured cell monolayers and thin microbial samples. But once specimen thickness exceeds roughly 10 micrometers, plunge freezing often fails to transfer heat fast enough to the interior, raising the likelihood of vitrification failure. The authors also stress that how a specimen is positioned on the grid strongly influences whether it can later be milled and whether targets will be accessible. For in situ cryo-ET, they argue, ensuring that a target-containing region is stably presented for milling may matter more than achieving perfectly thin and uniform ice.

For thicker specimens, high-pressure freezing becomes the practical option. By applying high pressure during freezing, HPF alters the phase behavior of water in a way that suppresses ice crystal nucleation and growth, extending the achievable vitrified depth in thicker samples. This makes it a starting point for connecting tissues, high-density pellets and multicellular organisms to cryo-FIB-based workflows. One HPF-based variant highlighted in the review is the so-called waffle method, in which a vitrified sample is first formed as a relatively thick slab on the grid and subsequently processed into lamellae by cryo-FIB machining. According to the authors, this strategy can improve throughput and success rates by reconfiguring thick specimens into a form better suited for lamella fabrication, though it shifts the primary challenge from achieving vitrification to locating targets and designing appropriate milling strategies.

Locating those targets is where cryo-correlative light and electron microscopy, or cryo-CLEM, comes in. By combining the molecular specificity and wide-field target detection of fluorescence light microscopy with the high-resolution ultrastructural detail of electron microscopy, cryo-CLEM allows researchers to pinpoint where a specific protein or structure sits within the complex interior of a cell before milling begins. The review emphasizes that the practical value of cryo-CLEM lies not merely in overlaying images but in guiding where and how the specimen should be milled so the target survives in the final lamella. Because it is ultimately a coordinate-registration problem, errors can creep in from distortion during coordinate transformation, differences in magnification and specimen orientation between imaging modalities, fiducial distribution, and morphological changes during milling. When the final lamella is only a few hundred nanometers thick, even small cumulative deviations can be decisive, so the authors call for repeated verification that the predicted target position remains consistent before and during milling.

Lamella fabrication itself proceeds through protective layer deposition, trenching and rough milling, fine milling, and transfer to a cryo-transmission electron microscope. The protective layer reduces curtaining and charging and mitigates surface damage, while rough milling establishes the lamella geometry and secures a volume likely to contain the target. Fine milling reaches the final thickness, but the review argues that uniformity, flatness, target retention and minimization of contamination matter more than thinness alone. The authors catalog the failure modes that plague the process: contamination accumulates over time and demands careful management of transfers and chamber conditions; charging harms contrast, focus and drift and must be mitigated through coating and grounding; curtaining arises from material heterogeneity or inappropriate milling conditions; and devitrification can cause irreversible damage if temperature, vacuum and transfer time are not controlled. In their framing, lamella fabrication is not a single technique but an operational problem of systematically reducing the probability of failure.

Once lamellae are secured, cryo-ET moves to tilt-series acquisition, alignment, reconstruction, contrast transfer function correction and, when needed, subtomogram averaging, in which repeated macromolecular particles extracted from tomograms are aligned, classified and averaged to improve resolution. Acquisition decisions matter enormously here. Dose-symmetric tilt schemes are often preferred because they distribute the electron dose more evenly and help preserve high-resolution information, while careful management of the cumulative dose is essential for thin lamellae where radiation damage can rapidly degrade structural integrity. The accessible tilt range is frequently constrained by lamella geometry, influencing angular coverage and reconstruction fidelity. The standard software toolchain is relatively mature, with the IMOD/Etomo family widely used for alignment and reconstruction, and subtomogram averaging available in ecosystems such as RELION tomography, Dynamo and emClarity. Deep learning-based correction, restoration and particle detection are expanding rapidly, but the authors caution that such methods can generate plausible yet erroneous results, making embedded validation mechanisms essential.

The review also surveys extended strategies for thicker specimens. Serial slice-and-view imaging repeatedly removes thin layers and images the exposed surface by SEM to build three-dimensional volumes over relatively wide fields of view, providing broader cellular and organelle context for the local high-resolution information cryo-ET delivers. Because datasets are large and the interpretive bottleneck shifts to segmentation, the authors regard slice-and-view as an optional complement rather than an essential element, best adopted with clearly defined quality-control criteria from the acquisition stage. For the largest specimens, they note that high-pressure freezing followed by freeze-substitution, or resin embedding after conventional chemical fixation, can provide more stable and reliable large-area reconstruction.

Serial Lift-Out extends the toolkit further by addressing a fundamental constraint of conventional on-grid preparation, where each lamella exposes only a thin, spatially restricted region of the specimen. Lift-out strategies extract a larger volume and relocate it onto a receiver grid for thinning, and Serial Lift-Out goes further by serially sectioning a single extracted volume into multiple lamellae, expanding sampling coverage and enabling more continuous three-dimensional reconstruction across a larger specimen axis. The approach is exemplified in studies of the nematode Caenorhabditis elegans, where serial lamellae arranged along the organism’s body axis served as high-quality substrates for cryo-ET and downstream analyses such as subtomogram averaging. Successful implementation, the authors note, still depends on managing redeposition during milling, maintaining the mechanical stability of serial lamellae during transfer and imaging, and balancing increased sampling coverage against added preparation complexity. Their overall conclusion is that future progress in cryo-ET will depend less on better instruments and more on standardized workflows, stepwise decision-making and rigorous validation.

Subject of Research: Cryo-focused ion beam-scanning electron microscopy workflows for preparing and imaging frozen biological specimens in three-dimensional cryo-electron tomography

Article Title: Cryo-FIB–enabled 3D imaging of biological specimens

Article References: Son, S., Lee, S., Kim, J., & Song, C. (2026). Cryo-FIB–enabled 3D imaging of biological specimens. Applied Microscopy, 56(1), Article 10. https://doi.org/10.1186/s42649-026-00131-z

Image Credits: AI Generated

DOI: 10.1186/s42649-026-00131-z

Keywords: cryo-electron tomography, cryo-FIB-SEM, lamella preparation, cryo-CLEM, high-pressure freezing, waffle method, Serial Lift-Out, subtomogram averaging, vitrification, slice-and-view imaging, structural biology, electron microscopy