Читать книгу Cell Biology - Stephen R. Bolsover - Страница 25

In principle the smaller the wavelength of the radiation used to image a structure, the better the resolution. This fact led to the invention in 1931 of the transmission electron microscope by Max Knoll and Ernst Ruska. An electron gun generates a beam of electrons by heating a thin, V‐shaped piece of tungsten wire to 3000 °C. A large voltage accelerates the beam down the microscope column, which is under vacuum because the electrons are slowed and scattered if they collide with air molecules. The beam passes through the specimen and is bent by powerful magnets to form a highly magnified image (Figure 1.11b). This image can be viewed on a fluorescent screen that emits light when struck by electrons. While the electron microscope offers enormous resolution, electron beams are potentially highly destructive, and biological material must be subjected to a complex processing schedule before it can be examined. The preparation of cells for electron microscopy is summarized in Figure 1.12. The transmission electron microscope produces a detailed image but one that is static, two‐dimensional, and highly processed (Figure 1.4). Often, only a small region of what was once a dynamic, living, three‐dimensional cell is revealed. Moreover, the picture revealed is essentially a snapshot taken at the particular instant that the cell was killed (“fixed”). Clearly, such images must be interpreted with great care. Also, electron microscopes are large, expensive, and require a skilled operator. Nevertheless, since they can resolve objects down to about 0.2 nm in size, they are the main source of information on the organization of the cell at the nanometer scale, the ultrastructure of the cell.

Figure 1.10. Cultured human cells on a hemocytometer grid under bright‐field and phase contrast.

Figure 1.11. Cell structure as seen through light and transmission electron microscopes. For the identity of the structures revealed by the electron microscope, see Figure 1.2.

Figure 1.12. Preparation of tissue for electron microscopy.