CUTTING ARTIFACTS ON ULTRATHIN CRYOSECTIONS OF BIOLOGICAL BULK SPECIMENS

Cryoelectron microscopy of ultrathin cyrosections permits study of the ultrastructure of cells in the hydrated state. However, cryosectioning deforms the cut Volume as the sections are compressed in the cutting direction while their thickness is increased with respect to the cutting feed. The compression has been measured on the elliptical deformation of lipid droplets in liver cells. It is about 40% at 48 degrees cutting angle while it is reduced to 30% at 38 degrees. The section deformation involves the underlying ultrastructure as far as the specimen is deformable. Three deformation modes are discussed. Crystalline ice is broken into chips which are tilted and pressed out of the section plane to account for the section deformation. Within one chip, the ultrastructure of the ice is preserved. In contrast, hydrated catalase crystals deform plastically. Cutting shortens the crystal distances the same amount as cryosections are compressed. Distances in hexagonal DNA crystals are also influenced by cutting since the spots of diffraction patterns are blurred in the cutting direction. However, the crystal spacing is not shortened in the cutting direction. The crystal apparently reorganizes during cutting deformation.