The transmission electron microscope is a form of magnifying device which uses electrons to pass through the object being studied. The resolution is a thousand times greater than light microscopy. Atoms or molecules of high mass do not allow electrons to pass and this is detected as decreased transmission from this area relative to a surrounding area on a photographic plate or electron-sensitive fluorescent imaging screen.
Electrons are produced by a heated metal filament - the cathode. A large electrical potential is applied from cathode to anode further along the microscope. This attraction to a region of positive potential accelerates the electrons into a high speed beam that is focussed by a series of electromagnets onto the specimen held within a vacuum.
The specimen must be fixed and stained in a way that does not disrupt microstructure and that also accentuates the contrast between regions. For example, fixatives include glutaraldehyde which preserves proteins, and stains include osmium compounds, the heavy metal being of adequate mass to prevent electron transmission. The specimen is fixed in plastic and cut by diamond-tipped microtome in order to ensure a suitably thin section that will not stop all electrons from passing.
Freeze fracture techniques may be used to show great surface detail of a given specimen. The tissue is rapidly cooled to around -200 degrees Farenheit and than cleaved with a sharp blow. The energy literally fractures molecular bonds along tissue planes, for example, through the surface of the cell membrane. One of the fractured surfaces is then coated with a heavy metal film and it is this cast that is visualised under the electron microscope.
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