Ultrastructure of Cytoplasmic Matrix

Abstract

Because of the limitations inherent in conventional transmission electron microscopy (TEM) using epoxy ultrathin sections for a clear recognition of biological entities having electron densities similar to or lower than that of epoxy resin, embedment-free sectioning for TEM has been developed. Embedment-free section TEM is reliably performed using water-soluble polyethylene glycol (PEG) as a transient embedding media, with subsequent de-embedment of PEG by immersion into water, followed by critical point-drying (CPD) of the embedment-free section. The present author has stressed that this approach clearly discloses structures whose contours and/or appearance are accordingly vague and/or fuzzy in conventional TEM, but that it does not reveal any new artefactual structures. Based on embedment-free section transmission electron microscopy (PEG-TEM), this paper presents five major findings regarding strand- or microtrabecular lattices which have been clearly revealed to occur in the cytoplasmic matrix?an impossibility with conventional TEM. These are 1) the appearance of lattices of different compactness in various cells and in intracellular domains of a given cell; 2) the faithful reproduction from an albumin solution in vitro of strand-lattices with correspondingly increasing compactness following increasing concentrations; 3) the appearance of more compact lattices from gelated gelatin than from solated gelatin at a given concentration in vitro; 4) the appearance of either greater or less lattice-compactness by hyper- or hypo-tonic pre-treatments of cells; and 5) the appearance of certain intracellular proteins confined to the centripetal demilune-domain of centrifuged ganglion cells which is occupied with strand-lattices of a substantial compactness, whereas certain other proteins remained evenly throughout the centripetal and centrifugal domains of the cells. From these findings, questions now arise as to the biological significance of the individual strands themselves in the microtrabecular lattices in PEG-TEM. In contrast, it may be plausible that the appearance of strand-lattices as a whole in a given biological domain represents the presence of soluble proteins; the lattice-compactness indicates the concentration of soluble proteins in the domain, which are in favor of the solution-idea for the aqueous cytoplasm. Furthermore, the appearance of two contiguous domains exhibiting differing degrees of lattice-compactness in a given cell indicates that cytoplasmic proteins are solated in a domain with more compact lattices, whereas they are gelated in the other domain, which is in favor of the non-solution/structure-association or structure-forming idea of the aqueous cytoplasm. Therefore, it is appropriate to understand that the aqueous cytoplasm behaves as a dynamic combination of the solution- and the structured situations. These proposed interpretations need to be confirmed by further studies. If confirmed, the control mechanisms of the localization and movement of intracellular organelles could then be understood on the basis not only of information about the cytoskeletons but also of cell ultrastructure-related information on the concentration and sol/gel states of intracellular proteins, representing a resurrection of the cytoplasmic sol/gel doctrine.