Cryo-focused-ion-beam applications in structural biology.

The ability to precisely control the preparation of biological samples for investigations by electron cryo-microscopy is becoming increasingly important for ultrastructural imaging in biology. Precision machining instruments such as the focused ion beam microscope (FIB) were originally developed for applications in materials science. However, today we witness a growing use of these tools in the life sciences mainly due to their versatility, since they can be used both as manipulation and as imaging devices, when complemented with a scanning electron microscope (SEM). The advent of cryo-preparation equipment and accessories made it possible to pursue work on frozen-hydrated biological specimens with these two beam (FIB/SEM) instruments. In structural biology, the cryo-FIB can be used to site-specifically thin vitrified specimens for transmission electron microscopy (TEM) and tomography. Having control over the specimen thickness is a decisive factor for TEM imaging, as the thickness of the object under scrutiny determines the attainable resolution. Besides its use for TEM preparation, the FIB/SEM microscope can be additionally used to obtain three-dimensional volumetric data from biological specimens. The unique combination of an imaging and precision manipulation tool allows sequentially removing material with the ion beam and imaging the milled block faces by scanning with the electron beam, an approach known as FIB/SEM tomography. This review covers both fields of cryo-FIB applications: specimen preparation for TEM cryo-tomography and volume imaging by cryo-FIB/SEM tomography.