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A Transmission Electron Microscope (TEM, also refers to the process transmission electron microscopy) utilizes a focused stream of electrons in a vacuum environment to visualize biological samples, with a maximum resolution of 0.1-0.2 nanometers (nm). Though no microscope is ‘ideal’ for every laboratory task, TEMs have many singular advantages which cannot be claimed by the standard brightfield/widefield microscopes. Unlike a scanning electron microscope (SEM), TEMs produce two-dimensional (2-D) images of the visualized sample, in black and white. They are exceptionally useful for visualizing thin sections of sample and showing the intricacies of the ultra-structure therein.
Advantages
Transmission Electron Microscopes can be used for a variety of different techniques, which set them apart from other forms of microscope.
Metal Shadowing or Negative Staining
The first is metal shadowing or negative staining. TEM microscopes require heavy metal staining to prevent damage to the tissue or structure of the sample by the focused electron beam. The heavy metal staining also helps to create different densities of electron absorption. Metal shadowing as a technique involves staining from an angle oblique to the sample, allowing one surface of it to accumulate greater deposits of metal and therefore focus them in the generated image.
Ideal Microscope for Visualization
Transmission Electron Microscope technology can also be used to visualize both molecules and viruses. Virions, or whole viruses, measure between 20-300 nm usually, although some giant virus strains may measure up to 1,400 nm in length. The minuscule size of these molecules makes TEMs the ideal form of microscopy for their visualization.
Because of the high-resolution capability, TEMs are vital for the visualization of known and novel cell organelles and the associations between them. In medicine, they play an important role and in understanding the dynamics which occur between human cells, bacteria, parasites, and viruses. For example, it is transmission electron microscopy which has allowed us to better understand the development of the blood-stage forms of the malaria parasite and how exactly it interacts with our cells. How exactly viruses look and how they interact with the surfaces of our cells.
Cryotransmission electron microscopy is a branch of transmission electron microscopy which holds great appeal in structural biology as it does not require fixing or staining to properly image the sample. This is advantageous as it allows visualization of the sample in its native environment, which is otherwise impossible in standard Transmission Electron Microscope preparations.
Freeze Fracture and Freeze Etch Techniques
A key advantage which exists with Transmission Electron Microscope technology is that it can be used in combination with freeze fracture and freeze etch techniques. These techniques allow visualization of internal structures by rapidly freezing then fracturing weak points in the structure of the sample. For example, this technique is noted for allowing the separation of the cell membrane lipid bilayer, which is composed of a double layer of lipid molecules, which have hydrophobic tails which orientate towards each other, creating a membrane isolating the cytosol from the external environment and creating what we understand as the self -contained “cell”. Thus, this technique combined with electron microscopy is of great importance in understanding the lipid, protein and carbohydrate content of the cell membrane. It, therefore, represents an important tool in the repertoire of structural biologists.
Application
Transmission electron microscopes have become a necessity in recent years for the study of biological and molecular ultra-structures, some universities even going as far as to work together with others to gain access to this equipment. The sheer detail and accuracy of imaging which is made possible by these microscopes make them a relative staple in the arsenal of structural and molecular biologists.
Sources and Further Reading
- Collier, L. and Oxford, J. (2006). Human virology. Oxford: Oxford University Press.
- Radiology Key. (2019). Metal Shadowing for Electron Microscopy. [online] Available at: https://radiologykey.com/metal-shadowing-for-electron-microscopy/ [Accessed 15 Aug. 2019].
- Severs, N. (2007). Freeze-fracture electron microscopy. Nature Protocols, 2(3), pp.547-576.
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