| Abstract: | Freeze-fracturing may be accomplished either under vacuum, or at atmospheric pressure. The devices available for freeze-cleaving are discussed in relation to the vacuum and temperature conditions prevailing during cleaving, etching and replication. It is concluded that most specimens can be satisfactorily cleaved (even at 4 K), and processed using simple cleavage devices and systems that provide ample cold-trapping protection for the specimen. Only for special purposes are microtome assemblies or ultra-high vacuum units essential. The fracturing process may produce artefacts by plastic deformation. Such artefacts have been noted in both non-biological and biological polymers cleaved at temperatures as low as 4 K. Contamination of the frozen surface need not be a problem provided that the specimen is transferred under ‘safe’ conditions, and is protected by well-designed cold traps. Freeze-cleavage and freeze-sectioning are compared. It is considered that there will be a temperature range for most heterogeneous specimens within which both cleavage and fracturing may occur, depending upon the nature of the molecules in the cleavage/sectioning plane. Local heating during freeze-sectioning will produce deformation artifacts as in freeze-cleavage, and may also lead to more general surface ‘flow’. The relationships between sectioning and fracturing biological specimens at low temperature require further clarification. |
