Scanning transmission electron microscopy and X-ray microanalysis of unstained biological thin sections

The scanning transmission electron microscope (STEM) has two distinct advantages over the conventional transmission electron microscope (CTEM) for the investigation of low contrast specimens such as unstained thin sections. These are the much higher collection efficiency for scattered electrons and the possibility of electronic enhancement of the image forming signal. The addition of an X-ray analysis system to STEM may also enable the identification of some of the elements contributing to the overall image contrast and to detect individual element concentrations in features of high contrast occurring in the specimens.Fresh tissue was fixed in 1-2.5% glutaraldehyde in PIPES buffer, quickly dehydrated in ethanol and embedded in epon. Thin (silver/grey) sections were collected on carbon coated copper or nylon grids and viewed in the STEM without further treatment. The microscope used was a Vacuum Generators HB5 STEM with field emission source operating at lOOkV with simultaneous bright field (BF) and dark field (DF) detection systems.