Question

In certain electron microscopy methods, the specimen is not directly imaged. How do these methods provide information about cellular structures, and what type of structures do they visualize?

Answer 1

The electron microscopes are of two types called transmission electron microscope (TEM) and scanning electron microscope (SEM). The TEM is used to view detailed structure of cells and viruses. The image produced by these microscopes is by transmission of electrons through specimen. The specimens are stained or coated with the metals and this will enhance the contrast of the image. The specimen is cut into thin slices of 20 to 100nm thickness.

The electrons passing through the specimen travel into fluorescent screen and display the image. As a result, the darker and lighter parts of image represent more and less dense parts on specimen. The scanning electron microscope provides detailed three dimensional views of biological specimens such as dental plaque or tape worm heads.

In SEM, the electrons bombard the surface of metal coated specimen and this would allow scanning of the specimen back and forth. The electrons strike back from the surface are detected by a detector and the pattern of electrons is displayed as image on screen or monitor to reveal fine details of the structure. The specimens, that reveals smooth and flat surfaces with light microscope appear and reveal distinct features with SEM. The advanced electron microscopes currently used arerscanning probe microscope and atomic force microscope