Cryoultramicrotomy of ultra-rapidly frozen specimens.

The physical events of the cryoultramicrotomy at the level of organelles and macromolecules are not completely understood. The extent to which tissue is either cut by the edge of the knife or fractured ahead of the knife is one such event. This issue of cryofracturing versus cryosectioning during cryoultramicrotomy has been examined in quick frozen, uncryoprotected rat liver. Cryosectioned specimens were freeze-substituted and edge-on views of the sectioned surface were examined in TEM. In tissue regions showing no obvious ice crystals, fracturing was rare. Regions with less adequate freezing however had numerous fractured structures. These results indicate that high quality freezing promotes sectioning over fracturing and thus works to eliminate this serious artifact.