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The Electron Microscope
The most commonly used type of electron microscope in biology is the transmission electron microscope (TEM, so named because electrons are transmitted through the specimen towards the observer). The components of a transmission electron microscope do essentially the same job as the components of a light microscope, but the whole instrument is the other way up. An electron source generates a beam of electrons by heating a thin V-shaped piece of tungsten wire to 3,000'C. A large voltage accelerates the beam down the microscope column, which is under vacuum because otherwise the electrons would be slowed by collision with air molecules. An image is obtained by a similar arrangement of lenses to that used in the light microscope, although electromagnets are used for focussing rather than glass. While the electron microscope offers great improvements in resolution, electron beams are potentially highly destructive, and biological material must be extensively processed before it can be examined. The preparation of cells for electron microscopy is summarized in EXP 1.
The image produced by the transmission electron microscope is rich in detail. However, the image is static, two-dimensional, and artificial. Often only a small region of what was once a dynamic, living, three-dimensional cell is revealed. The image is essentially a snapshot taken at the particular instant that the cell was killed. Such images must be interpreted with great care; nevertheless, the transmission electron microscope is the main source of our information on cell ultrastructure.
The Scanning Electron Microscope
Whereas the image in a transmission electron microscope is formed by electrons transmitted through the specimen, in the scanning electron microscope (SEM) it is formed from electrons that are reflected back from the surface of a specimen as the electron beam scans rapidly back and forth over it. These reflected electrons are processed to generate a picture on a television monitor. The scanning electron microscope operates over a wide magnification range, from lOx to 100,000x. Its greatest advantage is a large depth of focus, which gives a threedimensional image. The scanning electron microscope therefore provides useful topographical information about the surfaces of cells or tissues. Modern instruments have a resolution of about I nm.
EXP 1.
 A small piece of tissue (-1 mm3) is immersed in glutaralclehyde and osmium tetroxide. These chemicals bind all the component parts of the cells together; the tissue is said to be fixed. It is then washed thoroughly.
The tissue is dehydrated by soaking in acetone or ethanol.
The tissue is embedded in resin which is then baked hard.
Sections (thin slices less than 100 nm thick) are machine called an ultramicrotome.
The sections are placed on a small copper grid and stained with uranyl acetate and lead citrate. When viewed in the electron microscope, regions that have bound lots of uranium and lead will appear dark because they are a barrier to the electron beam.
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