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.