The
electron microscope (EM) is among the most valuable tools used in the study of
biology and has revolutionized the discovery and characterization of the
subcellular structure of the cell. Viewing these structures was not possible
with the standard light microscope (LM) that had been developed at the end of
the sixteenth century.
IT’S A VERY SMALL WORLD
The LM magnifies objects up to 2,000 times, while the EM
visualizes objects up to 2,000,000 times and at much higher resolution. The two
basic EMs are the transmission electron microscope (TEM) and the scanning
electron microscope (SEM). The TEM transmits electrons through thin tissue
slices, and its two-dimensional images are used to examine the internal
structure of cells. In the SEM, the electron beam sweeps across the sample and
is used to study the surface detail of solid living specimens. It produces
excellent three-dimensional images but is only one-tenth as powerful as the TEM
and provides lower resolution.
The
extraordinary magnification of EMs comes at a number of costs: EMs are very
expensive to purchase and maintain; researchers need considerable training in
their use and the preparation of biological specimens; TEM specimens must be
stained and visualized in a vacuum that precludes the study of living samples;
and EMs are large, and must be housed in vibration-free rooms.
The
EM was developed at the University of Berlin by physicist Ernst Ruska and his
professor, Max Knoll. Knoll knew that optical resolution (the ability to
distinguish between two points, a measure of detail) was dependent upon the
wavelength of the source of illumination, and that the wavelength of electrons
is 1/100,000 that of light particles. Based on this relationship, using a beam
of electrons that was focused on the specimen with electromagnets, they
developed the first EM in 1931. It was improved and commercialized in 1939, and
Ruska was the recipient of the 1986 Nobel Prize in Physics. In the 1950s,
George Palade at the Rockefeller Institute (now Rockefeller University) used
the EM to make discoveries on the fundamental organization of cells, for which
he was a co-recipient of the 1974 Nobel Prize in Medicine.
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