Microtrabecular lattice of the cytoplasmic ground substance. Artifact or reality

Electron-translucency and partial defects of synaptic basal lamina in the electrocyte synapse of an electric ray (Narke japonica) in 3D embedment-free section electron microscopy

The synaptic basal lamina of the electrocytes was disclosed to be electron-translucent to some extent when viewed in an en-face direction in embedment-free section transmission electron microscopy (EFS-TEM), and synaptic vesicles located close to the presynaptic membrane were seen through the synaptic basal lamina together with the presynaptic and postsynaptic membranes. This feature of translucency has the potential to analyze possible spatial interrelations in situ between bioactive molecules in the synaptic basal lamina and the synaptic vesicles in further studies. The synaptic basal lamina, appearing as an electron-dense line sandwiched by two parallel lines representing the presynaptic and postsynaptic membranes in ultrathin sections cut right to the synaptic junctional plane in conventional TEM, was not fully continuous but randomly intermittent along its trajectory. Compatible with the intermittent line appearance, the en-face 3D view in embedment-free section TEM revealed for the first time partial irregular defects of the synaptic basal lamina. Considering the known functional significance of several molecules contained in the synaptic basal lamina in the maintenance and exertion of the synapse, its partial defects may not represent its rigid structural features, but its immature structure under remodeling or its dynamic changes in consistency such as the sol/gel transition, whose validity needs further examination. RESEARCH HIGHLIGHTS: In embedment-free section TEM, a 3D en-face view of synaptic basal lamina in situ is reliably possible. The basal lamina en-face is electron-translucent, which makes it possible to analyze spatial interrelation between pre- and post-synaptic components. Partial irregular defects in the basal lamina are revealed in Torpedo electrocytes, suggesting its remodeling or dynamic changes in consistency.

Correlative microscopy of native surfaces of human lung: Color macrophotography, SEM, LM, TEM, HVEM, and low-temperature scanning electron microscopy

The power of microscopic analysis can be increased even though the resolution of the microscope remains constant if a native tissue surface is presented to the microscope. A surface that existsin vivobefore any cutting or handling of the tissue is a native surface. It preserves the most useful surface information present in the specimen and minimizes spurious information. in the case of human lung, the airway is dissected open as it branches, exposing its luminal surface. This preserves global properties such as connection and branching as well as local properties such as tissue and cellular structure. The surface of the airway is imaged at 100X with the scanning electron microscope (SEM) and the micrographs assembled into a montage, a 30"x40" map of the airway. These maps demonstrate the shape and branching pattern of the airway while preserving tissue detail with good resolution. The montage is a filing system for SEM micrographs that preserves positional information present in the sample. This information allows one to see patterns of ultrastructural features as a function of position along the airway, to see continuous variation without the need to break the airway down into a small number of regions. It allows precise localization of any structural feature along the airway for high magnification SEM and subsequent transmission electron microscopy (TEM) of the internal structure of the very same feature (figure 1).

Structural Organization of Detergent-Extracted Cells

Adequate visualization of the three-dimensional organization has always been a major problem in studies of cell architecture. Efforts of numerous investigators weredevoted to the question of how best information can be collected from specimens prepared with different procedures. In recent years, the potential of high voltage electron microscopy has been combined with a technique for sample preparation that circumvents embedding, namely critical point-drying from CO2, to study the three-dimensional fine structure of cells in culture. This approach has revealed new insights into the structural organization of the cytoplasm (1-4). A system of slender strands or microtrabeculae has been described to form an elaborate three-dimensional lattice in which other organelles are embedded. This system has been shown in some cells to undergo rapid conformational changes (3,5) and in general is believed to be an important component of the cytoskeleton, being responsible for the gelatious properties of the cytoplasm.

Sparse force-bearing bridges between neighboring synaptic vesicles

Most vesicles in the interior of synaptic terminals are clustered in clouds close to active zone regions of the plasma membrane where exocytosis occurs. Electron-dense structures, termed bridges, have been reported between a small minority of pairs of neighboring vesicles within the clouds. Synapsin proteins have been implicated previously, but the existence of the bridges as stable structures in vivo has been questioned. Here we use electron tomography to show that the bridges are present but less frequent in synapsin knockouts compared to wildtype. An analysis of distances between neighbors in wildtype tomograms indicated that the bridges are strong enough to resist centrifugal forces likely induced by fixation with aldehydes. The results confirm that the bridges are stable structures and that synapsin proteins are involved in formation or stabilization.

IL1β and TNFα promote RANKL-dependent adseverin expression and osteoclastogenesis

Adseverin is an actin-binding protein involved in osteoclastogenesis, but its role in inflammation-induced bone loss is not well-defined. Here, we examined whether IL1β and TNFα regulate adseverin expression to control osteoclastogenesis in mouse primary monocytes and RAW264.7 cells. Adseverin was colocalized with subcortical actin filaments and was enriched in the fusopods of fusing cells. In precursor cells, adseverin overexpression boosted the formation of RANKL-induced multinucleated cells. Both IL1β and TNFα enhanced RANKL-dependent TRAcP activity by 1.6-fold and multinucleated cell formation (cells with ≥3 nuclei) by 2.6- and 3.3-fold, respectively. However, IL1β and TNFα did not enhance osteoclast formation in adseverin-knockdown cells. RANKL-dependent adseverin expression in bone marrow cells was increased by both IL1β (5.4-fold) and TNFα (3.3-fold). Luciferase assays demonstrated that this expression involved transcriptional regulation of the adseverin promoter. Activation of the promoter was restricted to a 1118 bp sequence containing an NF-κB binding site, upstream of the transcription start site. TNFα also promoted RANKL-induced osteoclast precursor cell migration. We conclude that IL1β and TNFα enhance RANKL-dependent expression of adseverin, which contributes to fusion processes in osteoclastogenesis.

Affine and non-affine deformations quantified in cytoskeletal networks through three-dimensional form-finding model

Actin filaments and cross-linkers are main components of cytoskeletal networks in eukaryotic cells, and they support bending moments and axial forces respectively. A three-dimensional form-finding model is proposed in this work to investigate affine and non-affine deformations in cytoskeletal networks. In recent studies, modeling of cytoskeletal networks turns out to be a key piece in the cell mechanics puzzle. We used form-finding analysis to compute and analyze cytoskeletal models. A three-dimensional model is much more flexible and contains more elements than a two-dimensional model, and non-linear finite element analysis is difficult to converge. Thus, vector form intrinsic finite element analysis is employed here for valid results. The three-dimensional model reveals new behaviors beyond earlier two-dimensional models and better aligns with available data. Relative density of actin filaments and height of the form-finding model both play important roles in determining cytoskeletal stiffness, positively and negatively, respectively. Real cytoskeletal networks are quite mixed in terms of affine and non-affine deformations, which are quantified by internal strain energy in actin filaments and cross-linkers. Results are also influenced by actin filament relative density and height of the model. The three-dimensional form-finding model does provide much more room for intensive studies on cytoskeletal networks. In our future study, microtubules, fluidics, viscoelastic-plastic cross-linkers and even the whole cell model may be taken into account gradually to improve the cytoskeletal form-finding model.

The Prediction of Key Cytoskeleton Components Involved in Glomerular Diseases Based on a Protein-Protein Interaction Network

Maintenance of the physiological morphologies of different types of cells and tissues is essential for the normal functioning of each system in the human body. Dynamic variations in cell and tissue morphologies depend on accurate adjustments of the cytoskeletal system. The cytoskeletal system in the glomerulus plays a key role in the normal process of kidney filtration. To enhance the understanding of the possible roles of the cytoskeleton in glomerular diseases, we constructed the Glomerular Cytoskeleton Network (GCNet), which shows the protein-protein interaction network in the glomerulus, and identified several possible key cytoskeletal components involved in glomerular diseases. In this study, genes/proteins annotated to the cytoskeleton were detected by Gene Ontology analysis, and glomerulus-enriched genes were selected from nine available glomerular expression datasets. Then, the GCNet was generated by combining these two sets of information. To predict the possible key cytoskeleton components in glomerular diseases, we then examined the common regulation of the genes in GCNet in the context of five glomerular diseases based on their transcriptomic data. As a result, twenty-one cytoskeleton components as potential candidate were highlighted for consistently down- or up-regulating in all five glomerular diseases. And then, these candidates were examined in relation to existing known glomerular diseases and genes to determine their possible functions and interactions. In addition, the mRNA levels of these candidates were also validated in a puromycin aminonucleoside(PAN) induced rat nephropathy model and were also matched with existing Diabetic Nephropathy (DN) transcriptomic data. As a result, there are 15 of 21 candidates in PAN induced nephropathy model were consistent with our predication and also 12 of 21 candidates were matched with differentially expressed genes in the DN transcriptomic data. By providing a novel interaction network and prediction, GCNet contributes to improving the understanding of normal glomerular function and will be useful for detecting target cytoskeleton molecules of interest that may be involved in glomerular diseases in future studies.

Form-finding model shows how cytoskeleton network stiffness is realized

In eukaryotic cells the actin-cytoskeletal network provides stiffness and the driving force that contributes to changes in cell shape and cell motility, but the elastic behavior of this network is not well understood. In this paper a two dimensional form-finding model is proposed to investigate the elasticity of the actin filament network. Utilizing an initially random array of actin filaments and actin-cross-linking proteins the form-finding model iterates until the random array is brought into a stable equilibrium configuration. With some care given to actin filament density and length, distance between host sites for cross-linkers, and overall domain size the resulting configurations from the form-finding model are found to be topologically similar to cytoskeletal networks in real cells. The resulting network may then be mechanically exercised to explore how the actin filaments deform and align under load and the sensitivity of the network’s stiffness to actin filament density, length, etc. Results of the model are consistent with the experimental literature, e.g. actin filaments tend to re-orient in the direction of stretching; and the filament relative density, filament length, and actin-cross-linking protein’s relative density, control the actin-network stiffness. The model provides a ready means of extension to more complicated domains and a three-dimensional form-finding model is under development as well as models studying the formation of actin bundles.

Advantages of embedment-free section transmission electron microscopy

The usefulness of embedment-free section transmission electron microscopy (TEM) is stressed for present and future morphological analyses, and several examples are demonstrated which are revealed in sections for the first time by this method: en-face views of slit diaphragm of renal glomerulus and fenestrated diaphragm of capillary endothelium, transparency of neural myelin, attenuated endothelium and some basement laminae, labyrinth architecture of vacuoles within lipid droplets, and enhanced 3D effect of ultrastructures, the latter of which is the case in electron tomography. In addition, the biological significance of structured appearance (microtrabecular lattices) of the cytoplasmic matrix, which is disclosed by this method, are briefly reviewed in relation to the sol-gel transition of cytoplasmic heterogenous proteins. Since the ultrastructures of various cells and tissues in this method are confirmed to be well correspondent to those in conventional epoxy section TEM except for isotropic dimensional changes, and because there is no necessity for any special expensive equipments other than those for the conventional TEM, the embedment-free section TEM method with these advantages, deserves much more wide application to the morphological research including electron tomography.

Hydrogel formation by multivalent IDPs: A reincarnation of the microtrabecular lattice?

Based on high-voltage electron microscopic (HVEM) data of fixed cultured cells, an elaborate three-dimensional network of filaments, including and interconnecting other elements of the cytoskeleton, was observed in cells some half a century ago. Despite many attempts and comparative studies, this “microtrabecular lattice” (MTL) of the cytoplasmic ground substance could not be established as a genuine component of the eukaryotic cell, and is mostly considered today as a sample-preparation artifact of protein adherence and cross-linking to the cytoskeleton. Here we elaborate on the provocative idea that recent observations of hydrogel-forming phase transitions of repetitive regions of intrinsically disordered proteins (IDPs) bear resemblance in creation, organization and physical appearance to the MTL. We review this phenomenon in detail, and suggest that phase transitions of actin regulatory proteins, neurofilament side-arms and other proteins could generate non-uniform spatial distribution of cytoplasmic material in the vicinity of the cytoskeleton that might even give rise to fixation phenomena resembling the MTL. Whether such hydrogel formation by IDPs is a general physical phenomenon, will remain to be seen, nevertheless, the underlying organizational principle provokes novel experimental studies to uncover the ensuing higher-level regulation of cell physiology, in which the despised and long-forgotten concept of MTL might give some interesting leads.

Septin-containing barriers control the differential inheritance of cytoplasmic elements

Fusion of haploid cells of Saccharomyces cerevisiae generates zygotes. We observe that the zygote midzone includes a septin annulus and differentially affects redistribution of supramolecular complexes and organelles. Redistribution across the midzone of supramolecular complexes (polysomes and Sup35p-GFP [PSI+]) is unexpectedly delayed relative to soluble proteins; however, in [psi-] × [PSI+] crosses, all buds eventually receive Sup35p-GFP [PSI+]. Encounter between parental mitochondria is further delayed until septins relocate to the bud site, where they are required for repolarization of the actin cytoskeleton. This delay allows rationalization of the longstanding observation that terminal zygotic buds preferentially inherit a single mitochondrial genotype. The rate of redistribution of complexes and organelles determines whether their inheritance will be uniform.

Morphological effects of transcardially perfused SDS on the rat brain

BACKGROUND INFORMATION

For explanation of the formation of 'dark' neurons, an enigmatic phenomenon in neuropathology, we hypothesized recently that all spaces between the ultrastructural elements visible in the traditional transmission electron microscope are filled with a gel structure that stores free energy in the form of non-covalent interactions, is continuous in the whole soma-dendrite domains of neurons, and is capable of whole-cell phase transition. This hypothesis was deduced from the fact that 'dark' neurons can be formed, even under conditions extremely unfavourable for enzyme-mediated biochemical processes, if initiated by a physical damage. In order to gain further information on this gel structure, we perfused transcardially rats for 5 min with physiological saline containing 1 mM SDS before the perfusion of a fixative for electron microscopy.

RESULTS

Dramatic compaction of visibly intact ultrastructural elements was caused in the whole soma-dendrite domains of thinly scattered neurons ('dark' neurons), whereas substantial cytoplasmic swelling and patchy ultrastructural disintegration occurred in numerous other neurons ('light' neurons). Similar morphological changes were observed in scattered astrocytes, oligodendrocytes, pericytes and endothelial cells.

CONCLUSIONS

These observations: (i) support the existence of the above intracellular gel structure in neurons; (ii) allow the conclusion that this gel structure is present in the form of an ubiquitous trabecular network surrounded by a confluent system of fluid cytoplasm; (iii) draw attention to the possibility that the previous two statements also apply to other cell types of the brain tissue; and (iv) suggest that pressure-induced direct channels exist between neurons and astrocytes.

Revisiting the microtrabecular lattice

The 'microtrabecular lattice' (MTL) that Keith Porter described in the 1970s and 1980s is reconsidered as a proposed fundamental cytoplasmic structure of eukaryotic cells. Although considered to be an artefact by most cell biologists of his time (and probably ours), the case is made that something like the MTL may well exist, but in a much more dynamic form than images from electron microscopy imply and convey.

Disclosure in 3D of slit-membrane as well as -strands, and en-face basal lamina in situ of renal glomerulus of normal rats in embedment-free section transmission electron microscopy

With higher contrast and transparency due to the absence of epon and stereo-viewing effect due to thicker sections than conventional electron microscopy as methodological advantages, the renal glomerular slits were re-examined in embedment-free section electron microscopy. In addition to clear demonstration of strands bridging the slits in forms of ladders with highly irregular intervals and various extension-directions and length, this study disclosed clearly for the first time in the "section" TEM thin sheets which partially spanned the slit together with the strand-ladders. No strands were found to align in forms of typical zippers in normal kidney. Furthermore, en-face ultrastructure of the basal lamina in situ was clearly demonstrated in superimposed sites of the endothelial fenestrae with the slits.

The origins and evolution of freeze-etch electron microscopy

The introduction of the Balzers freeze-fracture machine by Moor in 1961 had a much greater impact on the advancement of electron microscopy than he could have imagined. Devised originally to circumvent the dangers of classical thin-section techniques, as well as to provide unique en face views of cell membranes, freeze-fracturing proved to be crucial for developing modern concepts of how biological membranes are organized and proved that membranes are bilayers of lipids within which proteins float and self-assemble. Later, when freeze-fracturing was combined with methods for freezing cells that avoided the fixation and cryoprotection steps that Moor still had to use to prepare the samples for his original invention, it became a means for capturing membrane dynamics on the millisecond time-scale, thus allowing a deeper understanding of the functions of biological membranes in living cells as well as their static ultrastructure. Finally, the realization that unfixed, non-cryoprotected samples could be deeply vacuum-etched or even freeze-dried after freeze-fracturing opened up a whole new way to image all the other molecular components of cells besides their membranes and also provided a powerful means to image the interactions of all the cytoplasmic components with the various membranes of the cell. The purpose of this review is to outline the history of these technical developments, to describe how they are being used in electron microscopy today and to suggest how they can be improved in order to further their utility for biological electron microscopy in the future.

A cytoplasmic gel network capable of mediating the conversion of chemical energy to mechanical work in diverse cell processes: a speculation

Enigmatic morphological features of the formation and fate of "dark" (hyper-basophilic, hyper-argyrophilic and hyper-electrondense) neurons suggest that the mechanical work causing their dramatic shrinkage (whole-cell ultrastructural compaction) is done by a previously "unknown" ultrastructural component residing in the spaces between their "known" (i.e. visible in the conventional transmission electron microscopy) ultrastructural constituents. Embedment-free section electron microscopy revealed in these spaces the existence of a continuous network of gel microdomains, which is embedded in a continuous network of fluid-filled lacunae. We gathered experimental facts suggesting that this gel network is capable of a volume-reducing phase-transition (an established physico-chemical phenomenon), which could be the motor of the whole-cell ultrastructural compaction. The present paper revisits our relevant observations and speculates how such a continuous whole-cell gel network can do both whole-cell and compartmentalized mechanical work.

Ultrastructural consideration on the nature, sol and gel, of the aqueous cytoplasm in embedment-free section electron microscopy

TEM of any in situ cells in embedment-free sections--regardless of specimen-fixation methods--clearly shows strand-lattices occupying the cytoplasmic matrix. The cytoplasmic matrix is assumed to be a site of soluble proteins; however, it appears indistinct as conventional TEM cannot target it. Strand-lattices similar to the cytoplasmic ones are duplicated in bovine serum albumin as well as solated gelatin fixed at warm temperatures and at appropriate concentrations, while lattices from gelatin gelated by cooling before fixation are much more compact than those from solated gelatin at a given concentration. Based on the finding of the in vitro proteins, a new interpretation of cell ultrastructures in embedment-free section TEM is proposed: first, differences in the compactness of cytoplasmic lattices represent those in the protein concentration in the cytoplasmic matrix; second, when loose and compact lattices are contiguous within a cell, the cytoplasmic matrix domain occupied by the compact lattice is in a gel state while the remaining domain of the same cell is in a sol state. The explanation for the states of the gel and sol based on the lattice-compactness is applicable to changes in the lattice-compactness of the cytoplasmic matrix of neurohypophyseal axons under intense secretion.

From protoplasmic theory to cellular systems biology: a 150-year reflection

Present-day cellular systems biology is producing data on an unprecedented scale. This field has generated a renewed interest in the holistic, "system" character of cell structure-and-function. Underlying the data deluge, however, there is a clear and present need for a historical foundation. The origin of the "system" view of the cell dates to the birth of the protoplasm concept. The 150-year history of the role of "protoplasm" in cell biology is traced. It is found that the "protoplasmic theory," not the "cell theory," was the key 19th-century construct that drove the study of the structure-and-function of living cells and set the course for the development of modern cell biology. The evolution of the "protoplasm" picture into the 20th century is examined by looking at controversial issues along the way and culminating in the current views on the role of cytological organization in cellular activities. The relevance of the "protoplasmic theory" to 21st-century cellular systems biology is considered.

The actin cytoskeleton in whole mount preparations and sections

In non-muscle cells, the actin cytoskeleton plays a key role by providing a scaffold contributing to the definition of cell shape, force for driving cell motility, cytokinesis, endocytosis, and propulsion of pathogens, as well as tracks for intracellular transport. A thorough understanding of these processes requires insight into the spatial and temporal organisation of actin filaments into diverse higher-order structures, such as networks, parallel bundles, and contractile arrays. Transmission and scanning electron microscopy can be used to visualise the actin cytoskeleton, but due to the delicate nature of actin filaments, they are easily affected by standard preparation protocols, yielding variable degrees of ultrastructural preservation. In this chapter, we describe different conventional and cryo-approaches to visualise the actin cytoskeleton using transmission electron microscopy and discuss their specific advantages and drawbacks. In the first part, we present three different whole mount techniques, which allow visualisation of actin in the peripheral, thinly spread parts of cells grown in monolayers. In the second part, we describe specific issues concerning the visualisation of actin in thin sections. Techniques for three-dimensional visualisation of actin, protein localisation, and correlative light and electron microscopy are also included.

Unravelling the structure of the lamellipodium

Summary Pushing at the cell front is the business of lamellipodia and understanding how lamellipodia function requires knowledge of their structural organization. Analysis of extracted, critical-point-dried cells by electron microscopy has led to a current dogma that the lamellipodium pushes as a branched array of actin filaments, with a branching angle of 70 degrees , defined by the Arp2/3 complex. Comparison of different preparative methods indicates that the critical-point-drying-replica technique introduces distortions into actin networks, such that crossing filaments may appear branched. After negative staining and from preliminary studies by cryo-electron tomography, no clear evidence could be found for actin filament branching in lamellipodia. From recent observations of a sub-class of actin speckles in lamellipodia that exhibit a dynamic behaviour similar to speckles in the lamella region behind, it has been proposed that the lamellipodium surfs on top of the lamella. Negative stain electron microscopy and cryo-electron microscopy of fixed cells, which reveal the entire complement of filaments in lamellipodia show, however, that there is no separate, second array of filaments beneath the lamellipodium network. From present data, we conclude that the lamellipodium is a distinct protrusive entity composed of a network of primarily unbranched actin filaments. Cryo-electron tomography of snap-frozen intact cells will be required to finally clarify the three-dimensional arrangement of actin filaments in lamellipodia in vivo.

First disclosure of lipid droplet substructure and myelin translucency in embedment-free section electron microscopy

In conventional transmission electron microscopy (EM), thinly sectioned specimens embedded in epoxy resin are observed. However, because of a substantial level of electron density of epoxy resin, the possibility cannot be ruled out that bio-structures having electron density similar to that of epoxy resin are not clearly recognized and thus are neglected or misinterpreted in conventional EM. This was the reason to require for embedment-free EM. Embedment-free sections have already been made available reliably by transient embedding in polyethylene glycol (PEG) and subsequent de-embedding through immersion in water, and further by critical-point drying, and this embedment-free EM is thus termed PEG-EM. However, this PEG-EM has not been successful to attract reasonable attention from electron microscopists and instead been misunderstood as a non-reliable method. In this paper, the remarkably enhanced contrast and electron translucency of any observation targets in PEG-EM are clearly demonstrated by comparing with images in conventional EM of adipocytes and neural myelin as examples. These features of PEG-EM, together with faithful correspondence in EM images of any individual substructures between the two methods, confirm the reliability of PEG-EM. Furthermore, the much higher thickness of embedment-free sections together with these features makes the PEG-EM more advantageous than the conventional EM for three-dimensional appreciation of structural elements, which is made by stereo-viewing of sections or by EM tomography. Therefore, the PEG-EM is regarded as an important adjunct to the conventional EM for histological studies and wide application of this method may unravel a new level of histology.

What we have learned and will learn from cell ultrastructure in embedment-free section electron microscopy

The limitations inherent in conventional electron microscopy (EM) using epoxy ultrathin sections for a clear recognition of biological entities having electron densities similar to or lower than that of epoxy resin have led to the development of embedment-free sectioning for EM. Embedment-free section EM is reliably performed using water-soluble polyethylene glycol (PEG) as a transient embedding medium, 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 EM, but does not reveal any new structures. Based on embedment-free electron microscopy (PEG-EM), this article presents five major findings regarding strand- or microtrabecular lattices which have been clearly revealed to occur in the cytoplasmic matrix-an impossibility with conventional EM. 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 hypotonic pretreatments 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. From these findings, questions now arise as to the biological significance of the individual strand itself in the microtrabecular lattices in PEG-EM. In addition, it may be that the appearance of strand-lattices in a given biological domain represents the presence of soluble proteins; the lattice-compactness indicates the concentration of soluble proteins in the domain, and the aqueous cytoplasm is equivalent to the aqueous solution. Further, the appearance of two contiguous lattice domains exhibiting differing degrees of compactness in a given cell indicates that cytoplasmic proteins are solated in a domain with less compact lattices, whereas they are gelated in the other domain. 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. In addition, possible interpretations of the significance of strand-lattices in PEG-EM are also applicable to the nucleoplasm, especially extra-heterochromatin (euchromatin) areas. Finally, several potential uses/advantages of PEG-EM in the cell-ultrastructure have also been demonstrated, especially in three-dimensional reconstructions of nonmembranous structures including stereo-viewing using a pair of EM images with appropriate tilting as well as electron microscopic tomography.

Chaperones as parts of cellular networks

The most important interactions between cellular molecules have a high affinity, are unique and specific, and require a network approach for a detailed description. Molecular chaperones usually have many first and second neighbors in protein-protein interaction networks and they play a prominent role in signaling and transcriptional regulatory networks of the cell. Chaperones may uncouple protein, signaling, membranous, organellar and transcriptional networks during stress, which gives an additional protection for the cell at the network-level. Recent advances uncovered that chaperones act as genetic buffers stabilizing the phenotype of various cells and organisms. This chaperone effect on the emergent properties of cellular networks may be generalized to proteins having a specific, central position and low affinity, weak links in protein networks. Cellular networks are preferentially remodeled in various diseases and aging, which may help us to design novel therapeutic and anti-aging strategies.

Cell mechanics and stress: from molecular details to the ‘universal cell reaction’ and hormesis

The 'universal cell reaction' (UCR), a coordinated biphasic response to external (noxious and other) stimuli observed in all living cells, was described by Nasonov and his colleagues in the mid-20th century. This work has received no attention from cell biologists in the West, but the UCR merits serious consideration. Although it is non-specific, it is likely to be underpinned by precise mechanisms and, if these mechanisms were characterized and their relationship to the UCR elucidated, then our understanding of the integration of cellular function could be improved. As a step towards identifying such mechanisms, I review some recent advances in understanding cell mechanics and the stress response and I suggest potentially testable hypotheses. There is a particular need for time-course studies of cellular responses to different stimulus doses or intensities. I also suggest a correspondence with hormesis; re-investigation of the UCR using modern biophysical and molecular-biological techniques might throw light on this much-discussed phenomenon.

X-ray microanalysis of biological specimens by high voltage electron microscopy

For the purpose of analyzing and imaging chemical components of cells and tissues at the electron microscopic level, 3 fundamental methods are available, chemical, physical and biological. Among the physical methods, two methods qualifying and quantifying the elements in the structural components are very often employed. The first method is radioautography which can demonstrate the localization of radiolabeled compounds which were incorporated into cells and tissues after the administration of radiolabeled compounds. The second method is X-ray microanalysis which can qualitatively analyze and quantify the total amounts of elements present in cells and tissues. We have developed the two methodologies in combination with intermediate high or high voltage transmission electron microscopy (200-400 kV) and applied them to various kinds of organic and inorganic compounds present in biological materials. As for the first method, radioautography, I had already contributed a chapter to PHC (37/2). To the contrary, this review deals with another method, X-ray microanalysis, using semi-thin sections and intermediate high voltage electron microscopy developed in our laboratory. X-ray microanalysis is a useful method to qualify and quantify basic elements in biological specimens. We first quantified the end-products of histochemical reactions such as Ag in radioautographs, Ce in phosphatase reaction and Au in colloidal gold immunostaining using semithin sections and quantified the reaction products observing by intermediate high voltage transmission electron microscopy at accelerating voltages from 100 to 400 kV. The P/B ratios of all the end products Ag, Ce and Au increased with the increase of the accelerating voltages from 100 to 400 kV. Then we analyzed various trace elements such as Zn, Ca, S and Cl which originally existed in cytoplasmic matrix or cell organelles of various cells, or such elements as Al which was absorbed into cells and tissues after oral administration, using both conventional chemical fixation and cryo-fixation followed by cryo-sectioning and freeze-drying, or freeze-substitution and dry-sectioning, or freeze-drying and dry-sectioning producing semithin sections similarly to radioautography. As the results, some trace elements which originally existed in cytoplasmic matrix or cell organelles of various cells in different organs such as Zn, Ca, S and Cl, were effectively detected. Zn was demonstrated in Paneth cell granules of mouse intestines and its P/B ratios showed a peak at 300 kV. Ca was found in human ligaments and rat mast cells with a maximum of P/B ratios at 350 kV. S and Cl were detected in mouse colonic goblet cells with maxima of P/B ratios at 300 kV. On the other hand, some elements which were absorbed by experimental administration into various cells and tissues in various organs, such as Al in lysosomes of hepatocytes and uriniferous tubule cells in mice was detected with a maximum of P/B ratios at 300 kV. From the results, it was shown that X-ray microanalysis using semi-thin sections observed by intermediate high voltage transmission electron microscopy at 300-400 kV was very useful resulting in high P/B ratios for quantifying some trace elements in biological specimens. These methodologies should be utilized in microanalysis of various compounds and elements in various cells and tissues in various organs.

Cellular Reactions of Osteoblasts to Micron- and Submicron-Scale Porous Structures of Titanium Surfaces

Osteoblast reactions to topographic structures of titanium play a key role in host tissue responses and the final osseointegration. Since it is difficult to fabricate micro- and nano-scale structures on titanium surfaces, little is known about the mechanism whereby the topography of titanium surfaces exerts its effects on cell behavior at the cellular level. In the present study, the titanium surface was structured in micron- and submicron-scale ranges by anodic oxidation in either 0.2 M H3PO4 or 0.03 M calcium glycerophosphate with 0.15 calcium acetate. The average dimensions of pores in the structured surface were about 0.5 and 2 microm in diameter, with roughness averaging at 0.2 and 0.4 microm, respectively. Enhanced attachment of cells (SaOS-2) was shown on micron- and submicron-scale structures. Initial cell reactions to different titanium surfaces, e.g. the development of the actin-containing structures, are determined by the different morphology of the surfaces. It is demonstrated that on either micron- or submicron-structured surfaces, many well-developed filopodia were observed to be primary adhesion structures in cell-substrate interactions, and some of them entered pores using their distinct tips or points along their length for initial attachment. Therefore, porous structures at either micro- or submicrometre scale supply positive guidance cues for anchorage-dependent cells to attach, leading to enhanced cell attachment. In contrast, the cells attached to a smooth titanium surface by focal contacts around their periphery as predominant adhesion structures, since repulsive signals from the environment led to retraction of the filopodia back to the cell bodies. These cells showed well-organized stress fibres, which exert tension across the cell body, resulting in flattened cells.

Molecular mechanisms for organizing the neuronal cytoskeleton

Neurofilaments and microtubules are important components of the neuronal cytoskeleton. In axons or dendrites, these filaments are aligned in parallel arrays, and separated from one another by nonrandom distances. This distinctive organization has been attributed to cross bridges formed by NF side arms or microtubule-associated proteins. We recently proposed a polymer-brush-based mechanism for regulating interactions between neurofilaments and between microtubules. In this model, the side arms of neurofilaments and the projection domains of microtubule-associated proteins are highly unstructured and exert long-range repulsive forces that are largely entropic in origin; these forces then act to organize the cytoskeleton in axons and dendrites. Here, we review the biochemical, biophysical, genetic and cell biological data for the polymer-brush and cross-bridging models. We explore how the data traditionally used to support cross bridging may be reconciled with a polymer-brush mechanism and compare the implications of recent experimental insights into axonal transport and physiology for each model.

A new look at the cellular scaffold by embedment-free electron microscopy method

The basic scaffold of most cells is afforded by the cytoskeleton (comprising microfilaments, intermediate filaments and the microtubules). The conventional methods of electron microscopy fail to visualize filamentous cell structure. They can show only these filaments lying at the section surface. Heavy metal staining (I), and the optical properties of the resins used for embedding are similar to those of proteins hence most proteinaceous structures remain unresolved and the cytoplasm seems to be quite homogenous (II). Aldehyde fixation could cross-link proteins and lead to the emergence of artificial structures (III). These limitations may be overcome by the use of the embedment-free electron microscopy (EF-EM). This technique present cellular scaffold as a purified, isolated, three-dimensional network with various thickness of filaments. Our study on the dynamic aspect of cellular scaffold indicate that the thickness and arrangement of filaments depend on cell type and both physiological or pathological environments. Thank also to the adaptation of immunocytochemistry to EF-EM it was possible to understand the nuclear matrix and cytomatrix structure in relation to function. Thus, combination these methods revealed findings suggesting the nuclear homing of proapoptotic proteins and their association with intermediate filaments.

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.

Chaperones come of age

Chaperone function plays a key role in repairing proteotoxic damage, in the maintenance of cell architecture, and in cell survival. Here, we summarize our current knowledge about changes in chaperone expression and function in the aging process, as well as their involvement in longevity and cellular senescence.

The evolving complexity of cytoplasmic structure

Our knowledge of the structure of the cytoplasm has grown with the advent of advanced techniques and equipment that have allowed us to study cellular components. Over the past 150 years, such advances have steadily improved our realization of the complexity of cytoplasmic organization.

A nonconventional role of molecular chaperones: involvement in the cytoarchitecture

A hallmark of chaperone action is assistance in protein folding. Indeed, folding of nascent prokaryotic proteins proceeds mostly as a chaperone-assisted, posttranslational event. On the contrary, in nonstressed eukaryotic cells folding-related tasks of eukaryotic chaperones are restricted to a subset of proteins, and "jobless" chaperones may form an extension of the cytoarchitecture, facilitating intracellular traffic of proteins and other macromolecules.

Three-dimensional high voltage electron microscopy of thick biological specimens

The procedures recently developed in our laboratory to observe three-dimensional structures of cell organelles in thick biological specimens by means of high voltage electron microscopy are reviewed. Thick biological specimens such as whole mount cultured cells seeded and grown on grid meshes in culture vessels or thick sections cut from embedded tissues and stained by histochemical reactions can be readily observed three-dimensionally by high voltage transmission electron microscopy at 400-1000kV. Cultured cells used were both primary cultures from animal tissues and established cell lines maintained in our laboratory. The livers of adult Wistar rats were isolated by collagenase perfusion, and hepatocytes were suspended in a Leibovitz medium and seeded on formval coated gold grid meshes in Petri dishes, incubated in a CO(2) incubator in a humidified atmosphere containing 5% CO(2) in air at 37 degrees C for a few days. Established cell lines, CHO-K1 cells, were cultured in Ham's F12 medium, while HeLa cells were cultured in Eagle's MEM under the same condition. Some of the cells were cultured under experimental conditions such as hepatocyte culture in the medium containing peroxisome proliferating agents such as clofibrate or bezafibrate and some of them were labeled with (3)H-thymidine, (3)H-uridine, (3)H-labeled precursors and (14)C-bezafibrate. Also some cells were incubated in medium containing HRP to induce pinocytosis. All the whole mount cultured cells on grid meshes were prefixed in buffered 2.5% glutaraldehyde, stained with various histochemical reactions and postfixed in 1% osmium tetroxide. The histochemical reactions used were glucose-6-phosphatase (G-6-Pase), thiamine pyrophosphatase (TPPase), cytochrome oxidase, acid phosphatase (AcPase), DAB, ZIO, PA-TCH-SP reactions and radioautography was performed after labeling with radiolabeled compounds. The whole mount cultured cells were dried in a critical point dryer and were observed with JEOL JEM-4000EX or Hitachi H-1250M high voltage electron microscopes at 400-1000kV. By tilting the specimens' stereo-pair micrographs were recorded and they were observed with stereoscopes. Rat liver, mouse intestine and pancreas tissues, fixed and stained as above, were embedded in Epoxy resin, thick sectioned at 1-2 microm and were observed as for the whole mount cultured cells at 1000kV. Stereo-pairs were further analyzed with an image analyzer JEOL JIM-5000 (JEOL, Tokyo, Japan), producing two contour lines plotted from the micrographs at a thickness of 0.2 microm and were observed with anaglyph type glasses, demonstrating the depth or heights of respective cell organelles. The results show that whole mount cultured cells and thick sections stained with histochemical reactions reveal cell organelles corresponding to marker enzymes, such as G-6-Pase in endoplasmic reticulum, TPPase and ZIO in Golgi apparatus, cytochrome oxidase in mitochondria, AcPase in lysosomes, DAB in peroxisomes and pinocytotic vesicles, PA-TCH-SP in secretory granules, (3)H-thymidine and (3)H-uridine in nuclei, (3)H-animo acids in endoplasmic reticulum and secretory granules, (14)C-bezafibrate around ER and peroxisomes. The ultrastructure of these cell organelles as well as the structural relationship between them can be demonstrated three-dimensionally with stereo-pair images. Overall, these procedures are useful for analyzing stereologically the ultrastructure of cell organelles in cells and tissues.