Preparation of Biological Samples for Electron Microscopy

Abstract

Biological structures of living organisms are highly dynamic, they are always in an aqueous environment and the components are mainly made of elements of low atomic number: H, C, O, N, P, S. Last but not least most of the organism are rather large. The physical properties of electrons define the preparation methods of biological samples. Electrons have a short mean free pathway of about 1 cm in air and even shorter in denser structures, therefore the electron microscope needs to be evacuated. Thus an ideal sample for electron microscopy is tiny, does not move, is rigid, contains no liquids and has a high contrast. In addition for transmission electron microscopy the sample must be thin. The opposing properties and demands of electrons versus biology led to cumbersome path on biological sample preparation. In the beginning viruses and bacteria were studied [1, 2]. They were adsorbed on support film and initially directly imaged with the electron beam. The poor contrast was improved by heavy metal shadowing. For the visualization of molecules and virus particles in the 50ies the spreading technique introduced by Kleinschmidt [3] as well as the negative staining technique pioneered by Brenner & Horne [4] were cornerstones.