Surface defects on thin cryosections.

Abstract

The electron microscopic observation of thin cryosections is frequently impaired by the occurrence of surface defects. To investigate the possible causes of these surface defects the structure of cryosections ( CTEM ) from frozen biological material was correlated with the structure of the block-face (SEM) from which the sections were obtained. Both sections and block-face showed steps with a periodicity larger than 0.5 micron. Upon dry sectioning of hard plastic comparable features were observed in the section ( CTEM /SEM) and block-face (SEM). Thin cryosections cut below 143 K were found to be smooth apart from defects introduced by imperfections of the knife. In addition to "long" wave length distortions, a periodical distortion smaller than 120 nm can be observed in cryosections from biological material. At a given temperature the frequency is related to the sectioning speed. At all sectioning temperatures studied, distortions of this high frequency/short wave length type have been observed although they were less conspicuous in thinner sections. The surface defects observed in cryosections from biological material resemble the defects found after metal cutting and chip-formation. Shear forces seem to be the main causes for the observed periodic deformations on both cryosections as well as on sections from metals and plastics. This may imply that material is collected and periodically shedded from a dead zone around the knife edge. In metal cutting such a dead zone can have the form of a built-up-edge on top of the knife or as a small overhang in front of the knife edge.