Why does the sample size (tissue) have to be so small?
Samples (tissue slices) must be <3 mm3 for good morphology in electron microscopy. The main limitation is the speed and extent of penetration of fixatives (typically glutaraldehyde). If fixative doesn't penetrate sufficiently, then the specimen won't be sufficiently fixed for subsequent processing. In general, we will only section a 1 mm3 of that sample. Remember that TEM typically uses ~80 nm slices, so 1 mm provides ~12,500 slices — way too many to search for a rare feature. To "preview" or find interesting sections of the sample, we typically perform light microscopy of stained "semi-thin" sections (typically 0.5 µm). For rare features in the tissue, please be available to consult about areas to focus our efforts in thinner slices.
