Cytoplasmic matrix in embedment-free electron microscopy: non-molecular biological histology

Embedment-free electron microscopy using polyethylene glycol as a transient embedment has revealed that slender strands, originally termed microtrabeculae and microtrabecular lattices, interconnect every organelle and conventional cytoskeletons as well as plasma membranes, resulting in the formation of 3-D meshworks in all portions of the cytoplasmic matrix of every cell. The microtrabeculae correspond well to the wispy components in the cytoplasmic matrix of conventionally epoxy-sectioned cell specimens that have been looked at but often neglected because of their poorly defined images due to the presence of embedding media having a substantial electron-scattering property. Because of the occurrence of similar meshworks in specimens that are supposed to be unstructured, such as the intramitochondrial matrix and blood plasma, together with the failure to detect any predictable changes of the microtrabecular lattices by experimental manipulation of cellular environments, it is inaccurate to conclude that all microtrabecular lattice represent structures equivalent to those in a living state of cells simply because of their clear appearance. Instead, three possible interpretations are newly proposed for the biological significance of the microtrabecular lattices. The first is that the appearance of lattices represents the presence of proteins, and that their approximate concentrations are speculated based on the compactness of the lattice. The second is that when an intracellular microdomain composed of more compact lattices is contiguous with another domain composed of looser lattices in a given cell, the former might represent the gelated state and the latter the solated state. Possible examples for these two interpretations are also proposed, possibly leading us to further elaborate the significance of microtrabecular lattices.