Ultrastructural analysis of dynamic cellular processes: a survey of current problems, pitfalls and perspectives

Dynamic phenomena in cells that can be analyzed on the ultrastructural level comprise so different aspects as ion shifts, conformational changes of macromolecules, membrane particle rearrangements, lipid phase transitions, protein-protein interactions (notably ligand-receptor interactions, including their sorting and sequestration), reversible membrane-to-membrane contacts, membrane fusions, transcellular transport phenomena, restructuring of cytoskeletal elements, ciliary and flagellar beat, cell shape changes, etc. Only some of these phenomena can be analyzed under stationary conditions, while others are unidirectional and sometimes very rapid. Therefore, the methodical approaches to be used (primary methods and follow-up procedures) might be widely different. Quite different methods are available, such as fast freezing, specific labeling, low temperature processing and/or analysis, x-ray-microanalysis, etc. Only occasionally are there alternative non-ultrastructural control methods available. This survey paper tries to analyze the degree of reliability (or uncertainty) of current methods and to pinpoint the goals and eventually also new methodical perspectives for an integrative approach to analyze dynamic cellular processes with the high temporal and spatial resolution provided by the electron microscope.