Electron microscopy, electron diffraction, and element analysis of wet biological specimens.

Temperature controlled differentially pumped environmental chambers now allow more routine examination of wet specimens in the electron microscope. A sensitive test of their efficiency is the ability to provide high resolution electron diffraction patterns from wet, unfixed protein microcrystals. Fortunately, wet specimens can be prepared with only a few tens of nanometers thickness of remaining water, so extraneous electron scattering by liquid water can be kept to a minimum. It still remains to be determined whether microprobe analysis (X-ray or electron energy-loss spectroscopy) using wet specimens gives better element localization in cells than the current freezing methods. More extensive comparisons are also required of the ultrastructural preservation and visibility of macromolecules immersed in a thin layer of water vs immersion in a thin layer of amorphous ice. However, the recent introduction of commercial forms of the necessary equipment now make these comparisons more feasible.