How to "read" a vitreous section

Cryo-electron microscopy of vitreous sections (CEMOVIS) makes it possible to observe biological specimen in a close-to-native state. There is no chemical fixation, no stain, and no aggregation of the biological material; all the water is present, immobilized in a vitreous state. An image obtained with CEMOVIS looks very different from a conventionally prepared dry section. The information it contains must be interpreted differently. First of all, the section looks smooth and homogeneous because, globally, the molecules are uniformly distributed in the cytosol. Contrast appears at the level of macromolecules and their complexes, such as membranes, ribosomes and nucleosomes, fibers, and even single molecules, when they are large enough and appropriately arranged. The limitation on the amount of information that can be retrieved from an image is set by the plethora of overlapping structures in the two-dimensional image of the three-dimensional section. Computer electron tomography may overcome this limitation. It is, however, in the nature of vitreous sections to be deformed during the cutting process. This effect takes place without loss of information, as long as the deformation is homogeneous. Beam damage may be spectacular while one is imaging vitreous biological material, but it is probably less severe than in conventional preparations, as long as meaningful resolution is the limiting factor. A few observations, which require more elaborate interpretation, are presented.