Introduction:
Transmission electron microscope (TEM) is the type of a microscope that uses a beam of electron rather than visible light to illuminate the specimen. Besides, while light microscope uses glass lens to focus light to the specimen, TEM uses electromagnetic lenses for focusing of the electron beam. TEM has a higher resolving power than the light microscope because the electron beam it uses for illumination has a much lower wavelength than visible light. During the operation of TEM, some electrons passing through the specimen get scattered and disappear from the beam. However, the unscattered electrons reach a fluorescence screen giving rise to a shadow image of the specimen (“The Transmission Electron Microscope”,n.d., Magnetic Lenses Guide the Electrons, para. 1). The image displays various parts of the specimen with different darkness depending on the density of the parts of the specimen. This image can either be studied directly, or photographed (The Transmission Electron Microscope.”, n.d., Magnetic Lenses Guide the Electrons, para. 1).
History of electron microscope
The emergence of TEM came as a result of the quest to increase the resolving power of the light microscope. However, these attempts failed and by the middle of the 19th century, it was unanimously concluded that the resolving power of the light microscope could not go beyond one micrometre (John Innes Centre, n.d., The History of EM, para. 1). Cathode rays were first regarded as a wave motion by Hertz and it marked a significant milestone in the journey towards the development of the first TEM. In 1899, Weichert discovered that cathode rays could be concentrated into a small spot by using an axial magnetic field (John Innes Centre, n.d., The History of EM, para. 1). In 1926, Busch showed theoretically that a short solenoid can converge a beam of electrons in a similar manner in which a glass lens converges light from the sun (John Innes Centre, n.d., The History of EM, para. 1).
An electron image was first magnified in 1931 by two German engineers: Ernst Ruska and Maximillion Knoll (John Innes Centre, n.d., The History of EM, para. 2). In 1933, Ruska built the first prototype of TEM with a resolving power of 50nm (John Innes Centre, n.d., The History of EM, para. 2). However, the microscope was not fit for practical use. The Metropolitan Vickers for Imperial College, London, built the first commercially available electron microscope and called it EM1. Its resolution was not satisfying enough. The first modern electron microscope was built in 1938at the University of Toronto by Eli Franklin Burton and students James Hillier, Albert Prebus, and Cecil Hall (John Innes Centre, n.d., The History of EM, para. 2). Since then, the resolving power of TEM has improved over the years.
Negative staining
Negative staining is mainly used to determine the cell size of specimens and their arrangements (“Negative Stain.” (n.d, para. 1). Furthermore, it can be used to stain cells that would otherwise be destroyed if heat-fixed. In this type of staining, an optically opaque stain is used to stain the background rather than the actual specimen. Therefore, what is visible is the specimen in the form of a clear spot against a dark background.
References
“The Transmission Electron Microscope.” (n.d.). Nobel Prize. Retrieved from http://www.nobelprize.org/educational/physics/microscopes/tem/
John Innes Centre (n.d.). What is Electron Microscopy? Retrieved from https://www.jic.ac.uk/microscopy/intro_EM.html
“Negative Stain.” (n.d.). Welcome to Microbugz. Retrieved from http://www.austincc.edu/microbugz/negative_stain.php
