Vitreous freezing offers a way to study cells and tissue in a near-native state by cryo-transmission electron microscopy (cryo-TEM), which is important when structural information at the macromolecular level is required. Many cells -- especially those in tissue -- are too thick to study intact in the cryo-TEM. Cryo focused-ion-beam (cryo-FIB) milling is being used in a few laboratories to thin vitreously frozen specimens, thus avoiding the artifacts and difficulties of cryo-ultramicrotomy. However, the technique is challenging because of the need to avoid devitrification and frost accumulation during the entire process, from the initial step of freezing to the final step of loading the specimen into the cryo-TEM. We present a robust workflow that makes use of custom fixtures and devices that can be used for high-pressure-frozen bulk tissue samples as well as for samples frozen on TEM grids.

Cryo-electron tomography (cryo-ET) is a well-established technique for studying 3D structural details of subcellular macromolecular complexes and organelles in their nearly native context in the cell. A primary limitation of the application of cryo-ET is the accessible specimen thickness, which is less than the diameters of almost all eukaryotic cells. It has been shown that focused ion beam (FIB) milling can be used to prepare thin, distortion-free lamellae of frozen biological material for high-resolution cryo-ET. Commercial cryosystems are available for cryo-FIB specimen preparation, however re-engineering and additional fixtures are often essential for reliable results with a particular cryo-FIB and cryo-transmission electron microscope (cryo-TEM). Here, we describe our optimized protocol and modified instrumentation for cryo-FIB milling to produce thin lamellae and subsequent damage-free cryotransfer of the lamellae into our cartridge-type cryo-TEM.

Cryo-electron microscopy (cryo-EM) enables the study of protein complexes, cytoskeletal elements, and organelles in three dimensions without the use of chemical fixation. Most cryo-EM studies focus on vitreously frozen individual cells separated from their native tissue contexts. This reliance on imaging of single cells is primarily due to technical challenges associated with preparing fresh tissue sections at a thinness sufficient for visualization via cryo-EM. Highly heterogenous and specialized tissues, such as brain, are especially affected by this limitation as the cellular, subcellular, and synaptic milieus can significantly vary across neuroanatomical locations. To address this limitation, we established new instrumentation and a workflow that consists of: 1) high-pressure freezing of fresh brain tissue; 2) tissue trimming followed by cryo-focused ion beam milling via the H-bar approach to generate ultrathin lamellae; and 3) cryo-EM imaging. Here, we apply this workflow to visualize the fine ultrastructural details of organelles, as well as cytoskeletal and synaptic elements that comprise the cortical neuropil within fresh, unfixed mouse brain tissue. Moreover, we present initial studies that apply principles of the above workflow to the analysis of postmortem human brain tissue. Overall, our work integrates the strengths of cryo-electron microscopy and tissue-based approaches to produce a generalizable workflow capable of visualizing subcellular structures within complex tissue environments.