Transmission electron microscopy allows the direct visualization of the individual components of the cell nucleus. Unlike fluorescence microscopy, which relies upon the use of fluorescent probes to tag structures, TEM is capable of visualizing the structures themselves. The theoretical resolution of the transmission electron microscope is sufficient to resolve the molecular constituents of the individual nuclear compartments and structures. To prepare specimens for transmission electron microscopy, cells are fixed (typically with glutaraldehyde), infiltrated with a polymer that is later polymerized to generate a hardened specimen, and sliced into very thin sections. The accelerating voltage of the electron microscope ultimately determines what thickness can be directly visualized. Traditionally, specimens have been cut in the range of 70-90 nm in thickness. The resulting TEM image is most commonly a projection of this thickness onto a single plane (much like an optical section or fluorescence microscopy image). 3-D data sets can be collected by tilting the specimen to give 3-D information by generating projections from different angles or through the collection of serial sections. Tilted specimens have higher resolution of depth.