To provide contrast in the
Most biological materials show little contrast with their surroundings unless they are stained. In the case of light microscopy, contrast can be enhanced by using coloured stains which selectively absorb certain wavelengths. The electrons in the electron microscope are absorbed very little by biological material and contrast is obtained mainly by electron scattering.
To heighten the contrast between viruses and the background, use is made of electron-dense "stains". These are usually compounds of heavy metals of high atomic number, that serve to scatter the electrons from regions covered with the stain. If virus particles are coated with stain (positive staining), fine detail may be obscured. Negative staining overcomes this problem by staining the background and leaving the virus relatively untouched. The negative stain is moulded round the virus particle, outlining its structure, and is also able to penetrate between small surface projections and to delineate them. If there are cavities within the virus particle that are accessible to the stain, these will be revealed and some of the internal structure of the virus may be disclosed.
One-sided staining provides more accurate information about the organization of capsomers, etc., although these particles are less well supported and tend to "collapse", resulting in apparent increase in size.
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© Copyright Linda M Stannard, 1995.