Biological ultrastructure as revealed by high resolution cryo-SEM of blockfaces after cryo-sectioning

Ultrastructural information was obtained by imaging the block face of high- pressure-frozen cryo-sectioned biological samples in a high-resolution cryo -SEM, Cryo-sectioning leads to a well-defined flat artificial surface in co ntrast to cryo-fracturing, Typical artefacts of cryo-sections such as compr ession and crevasses were not visible on the block face, The ultrastructura l features known from resin sections and from freeze-fractures could also b e found on the block faces. The cytoplasms show particles of different size which most likely represent proteins. The effects of radiation damage coul d be reduced considerably by applying the double layer coating technique an d backscattered electron imaging, High quality cryo-sections are only obtained from vitrified material. Reaso nably flat block faces were, however, also obtained from adequately frozen microcrystalline samples, thereby facilitating ultrastructural studies in t he frozen hydrated state.