Embedment-free section electron microscopy.

Because of potential hindrance of clear viewing in epoxy sections of biological entities having an electron density similar to and lower than that of epoxy resin, the author has stressed that the embedment-free section electron microscopy is necessary for re-examination and/or clarification of biological specimen structures, and that the embedment-free electron microscopy is reliably done by using water-soluble polyethylene glycol (PEG) as a transient embedding media and by critical point-drying of embedment-free sections after de-embedment of PEG by immersion of semithin sections into water. With the embedment-free electron microscopy, the author has presented five major findings: the appearance of microtrabecular lattices with different compactnesses in various cells and in intracellular domains of a given cell, the faithful reproduction of microtrabecula-like strand lattices in vitro with increasing compactnesses from artificial protein solutions at correspondingly increasing concentrations, the appearance of more compact lattices from gelated gelatin than from solated gelatin at a given concentration in vitro, the changeability in compactnesses of the microtrabecular lattices by hyper- or hypotonic shock treatments of cells, and the confined appearance of an intracellular protein in the centripetal demilune of centrifuged ganglion cells which is occupied with the microtrabecular lattices of a substantial compactness. From these findings, several conclusions are drawn: individual strands themselves of the microtrabeculae are meaningless, the appearance of microtrabeculae represents the presence of proteins at a certain concentration, and it is therefore likely that the aqueous cytoplasm is equivalent to the aqueous solution. In addition, it is possible that the appearance of two contiguous lattice domains exhibiting different compactnesses in a given cell may represent the occurrence of a contiguity of sol to gel states of cytoplasmic domains. It is thus proposed that the localization and movement of intracellular organelles are controlled not only by the cytoskeletons but also by the concentration and sol/gel states of intracellular proteins. In addition, several potential usefulness of the embedment-free electron microscopy has also been demonstrated.