Pros and cons: cryo-electron microscopic evaluation of block faces versus cryo-sections from frozen-hydrated skin specimens prepared by different techniques.

Over the last two decades, several different preparative techniques have been developed to investigate frozen-hydrated biological samples by electron microscopy. In this article, we describe an alternative approach that allows either ultrastructural investigations of frozen human skin at a resolution better than 15 nm or sample throughput that is sufficiently high enough for quantitative morphological analysis. The specimen preparation method we describe is fast, reproducible, does not require much user experience or elaborate equipment. We compare high-pressure freezing with plunge freezing, and block faces with frozen-hydrated slices (sections), to study variations in cell thickness upon hydration changes. Plunge freezing is optimal for morphological and stereological investigations of structures with low water content. By contrast, high-pressure freezing proved optimal for high-resolution studies and provided the best ultrastructural preservation. A combination of these fast-freezing techniques with cryo-ultramicrotomy yielded well-preserved block faces of the original biological material. Here we show that these block faces did not exhibit any of the artefacts normally associated with cryo-sections, and--after evaporating a heavy metal and carbon onto the surface--are stable enough in the electron beam to provide high-resolution images of large surface areas for statistical analysis in a cryo-SEM (scanning electron microscope). Because the individual preparation steps use only standard equipment and do not require much experience from the experimenter, they are generally more usable, making this approach an interesting alternative to other methods for the ultrastructural investigation of frozen-hydrated material.