Electron cryo-microscopy of vitrified biological specimens: towards high spatial and temporal resolution.

A decade after the development of electron cryo-microscopy for vitrified specimens, its advantages and limitations are analysed. Indeed, recent work carried out by different laboratories strengthens the idea that electron cryo-microscopy might soon be an alternative method to X-ray crystallography and NMR techniques for determining the structure of biological assemblies with both high spatial and temporal resolutions. High pressure freezing allows vitrification of larger volumes of biological suspensions. Thick vitrified objects can be cryosectioned. Electron cryo-microscopy of the sections gives images having a resolution better than 2 nm. Although the high resolution imaging mode under low dose conditions is not yet fully understood, microscopes are being developed to provide better and better images. Image averaging is being facilitated by the development of both crystallization and computer methods. Thus, we can expect that electron microscopy will soon become a potential technique for structural determination at atomic resolution. Finally, much effort is being devoted to improving the temporal resolution of electron cryo-microscopy. Soon, we may be able to observe molecules during their biological activity.