A model for cryosectioning based on the morphology of vitrified ultrathin sections

Electron microscopy of vitrified ultrathin sections allows cell ultrastructure to be studied in the hydrated state. Sectioning of the frozen material is, however, a limiting step, since the cutting forces cause severe mechanical deformation. In order to address this problem, we have investigated the surface of cryosections. It is shown that cryosections have two fundamentally different surfaces. One surface is rough, deformed by cutting-induced deformation lines which are orientated perpendicular to the cutting direction. The other surface, in comparison, is not affected by those deformation lines. Except for knife marks it is smooth. In order to explain the observations, the following model is proposed. The rough relief corresponds to the former block face. Its roughness originates from material that is squeezed out of the section plane when the section is compressed in the cutting direction and bent away from the specimen block. The smooth section surface is the surface in contact with the knife during the sectioning. This contact keeps the surface smooth while imprinting the knife marks.