Dimensional changes in cells and tissues during specimen preparation for the electron microscope

3D morphology of an outer-hair-cell hair bundle increases its displacement and dynamic range

In mammals, outer-hair-cell hair bundles (OHBs) transduce sound-induced forces into receptor currents and are required for the wide dynamic range and high sensitivity of hearing. OHBs differ conspicuously in morphology from other types of bundles. Here, we show that the 3D morphology of an OHB greatly impacts its mechanics and transduction. An OHB comprises rod-like stereocilia, which pivot on the surface of its sensory outer hair cell. Stereocilium pivot positions are arranged in columns and form a V shape. We measure the pivot positions and determine that OHB columns are far from parallel. To calculate the consequences of an OHB's V shape and far-from-parallel columns, we develop a mathematical model of an OHB that relates its pivot positions, 3D morphology, mechanics, and receptor current. We find that the 3D morphology of the OHB can halve its stiffness, can double its damping coefficient, and causes stereocilium displacements driven by stimulus forces to differ substantially across the OHB. Stereocilium displacements drive the opening and closing of ion channels through which the receptor current flows. Owing to the stereocilium-displacement differences, the currents passing through the ion channels can peak versus the stimulus frequency and vary considerably across the OHB. Consequently, the receptor current peaks versus the stimulus frequency. Ultimately, the OHB's 3D morphology can increase its receptor-current dynamic range more than twofold. Our findings imply that potential pivot-position changes owing to development, mutations, or location within the mammalian auditory organ might greatly alter OHB function.

Multiple origins of lipid-based structural colors contribute to a gradient of fruit colors in Viburnum (Adoxaceae)

Structural color is poorly known in plants relative to animals. In fruits, only a handful of cases have been described, including in Viburnum tinus where the blue color results from a disordered multilayered reflector made of lipid droplets. Here, we examine the broader evolutionary context of fruit structural color across the genus Viburnum. We obtained fresh and herbarium fruit material from 30 Viburnum species spanning the phylogeny and used transmission electron microscopy, optical simulations, and ancestral state reconstruction to identify the presence/absence of photonic structures in each species, understand the mechanism producing structural color in newly identified species, relate the development of cell wall structure to reflectance in Viburnum dentatum, and describe the evolution of cell wall architecture across Viburnum. We identify at least two (possibly three) origins of blue fruit color in Viburnum in species which produce large photonic structures made of lipid droplets embedded in the cell wall and which reflect blue light. Examining the full spectrum of mechanisms producing color in pl, including structural color as well as pigments, will yield further insights into the diversity, ecology, and evolution of fruit color.

Using structural colour to track length scale of cell-wall layers in developing Pollia japonica fruits

Helicoidally arranged layers of cellulose microfibrils in plant cell walls can produce strong and vivid coloration in a wide range of species. Despite its significance, the morphogenesis of cell walls, whether reflective or not, is not fully understood. Here we show that by optically monitoring the reflectance of Pollia japonica fruits during development we can directly map structural changes of the cell wall on a scale of tens of nanometres. Visible-light reflectance spectra from individual living cells were measured throughout the fruit maturation process and compared with numerical models. Our analysis reveals that periodic spacing of the helicoidal architecture remains unchanged throughout fruit development, suggesting that interactions in the cell-wall polysaccharides lead to a fixed twisting angle of cellulose helicoids in the cell wall. By contrast with conventional electron microscopy, which requires analysis of different fixed specimens at different stages of development, the noninvasive optical technique we present allowed us to directly monitor live structural changes in biological photonic systems as they develop. This method therefore is applicable to investigations of photonic tissues in other organisms.

Chemical effects of diceCT staining protocols on fluid-preserved avian specimens

Diffusible iodine-based contrast-enhanced computed tomography (diceCT) techniques allow visualization of soft tissues of fluid-preserved specimens in three dimensions without dissection or histology. Two popular diceCT stains, iodine-potassium iodide (I2KI) dissolved in water and elemental iodine (I2) dissolved in 100% ethanol (EtOH), yield striking results. Despite the widespread use of these stains in clinical and biological fields, the molecular mechanisms that result in color change and radiopacity attributed to iodine staining are poorly understood. Requests to apply these stains to anatomical specimens preserved in natural history museums are increasing, yet curators have little information about the potential for degradation of treated specimens. To assess the molecular effects of iodine staining on typical museum specimens, we compared the two popular stains and two relatively unexplored stains (I2KI in 70% EtOH, I2 in 70% EtOH). House sparrows (Passer domesticus) were collected and preserved under uniform conditions following standard museum protocols, and each was then subjected to one of the stains. Results show that the three ethanol-based stains worked equally well (producing fully stained, life-like, publication quality scans) but in different timeframes (five, six, or eight weeks). The specimen in I2KI in water became degraded in physical condition, including developing flexible, demineralized bones. The ethanol-based methods also resulted in some demineralization but less than the water-based stain. The pH of the water-based stain was notably acidic compared to the water used as solvent in the stain. Our molecular analyses indicate that whereas none of the stains resulted in unacceptable levels of protein degradation, the bones of a specimen stained with I2KI in water demineralized throughout the staining process. We conclude that staining with I2KI or elemental I2 in 70% EtOH can yield high-quality soft-tissue visualization in a timeframe that is similar to that of better-known iodine-based stains, with lower risk of negative impacts on specimen condition.

Optimization of ultrastructural preservation of human brain for transmission electron microscopy after long post-mortem intervals

Electron microscopy (EM) provides the necessary resolution to visualize the finer structures of nervous tissue morphology, which is important to understand healthy and pathological conditions in the brain. However, for the interpretation of the micrographs the tissue preservation is crucial. The quality of the tissue structure is mostly influenced by the post mortem interval (PMI), the time of death until the preservation of the tissue. Therefore, the aim of this study was to optimize the preparation-procedure for the human frontal lobe to preserve the ultrastructure as well as possible despite the long PMIs. Combining chemical pre- and post-fixation with cryo-fixation and cryo-substitution ("hybrid freezing"), it was possible to improve the preservation of the neuronal profiles of human brain samples compared to the "standard" epoxy resin embedding method. In conclusion short PMIs are generally desirable but up to a PMI of 16 h the ultrastructure can be preserved on an acceptable level with a high contrast using the "hybrid freezing" protocol described here.

From two to three dimensions: The importance of the third dimension for evaluating the limits to neuronal miniaturization in insects

Most studies dealing with the limits to miniaturization in insect brains have until now relied on information based on data collected in two dimensions: either histological sections imaged by light microscopy, or electron micrographs of single ultrathin sections imaged by transmission electron microscopy (TEM). To test the validity of transferring information gained from two-dimensional images to the third dimension, we examined a 3D image stack from serial-section TEM (ssTEM) of the optic neuropiles of the miniature parasitic wasp Trichogramma brassicae (Bezdenko, 1968). We reinvestigated the proposed lower limit of 2 µm for the diameters of neuronal somata and found average volumes of 6.5 μm³ for lamina cells and 3.8 μm³ for medulla cells. We likewise found a limiting factor for the volume of nuclei, which averages 41.9% and 49.2% of the cell body volume, respectively, but that in turn the compactness of heterochromatin was not a limiting factor in the minimal volume of the nuclei. Finally, we also found a minimum axon diameter of 98 nm that could nevertheless accommodate axoplasmic mitochondria. Incorporating the third dimension thus proves critically important in avoiding volumetric misinterpretations of these values. We discuss the limitations of analyzing the effects of miniaturization from profile data of neurons and demonstrate that miniaturization within the nervous system can lie beyond previously described limits and in some cases is already present in the optic lobe neurons of T. brassicae.

Evaluating the diffusion coefficient of dopamine at the cell surface during amperometric detection: disk vs ring microelectrodes

During exocytosis, small quantities of neurotransmitters are released by the cell. These neurotransmitters can be detected quantitatively using electrochemical methods, principally with disk carbon fiber microelectrode amperometry. An exocytotic event then results in the recording of a current peak whose characteristic features are directly related to the mechanisms of exocytosis. We have compared two exocytotic peak populations obtained from PC12 cells with a disk carbon fiber microelectrode and with a pyrolyzed carbon ring microelectrode array, with a 500 nm ring thickness. The specific shape of the ring electrode allows for precise analysis of diffusion processes at the vicinity of the cell membrane. Peaks obtained with a ring microelectrode array show a distorted average shape, owing to increased diffusion pathways. This result has been used to evaluate the diffusion coefficient of dopamine at the surface of a cell, which is up to an order of magnitude smaller than that measured in free buffer. The lower rate of diffusion is discussed as resulting from interactions with the glycocalyx.

Mucoadhesive nanoparticles may disrupt the protective human mucus barrier by altering its microstructure

Mucus secretions typically protect exposed surfaces of the eyes and respiratory, gastrointestinal and female reproductive tracts from foreign entities, including pathogens and environmental ultrafine particles. We hypothesized that excess exposure to some foreign particles, however, may cause disruption of the mucus barrier. Many synthetic nanoparticles are likely to be mucoadhesive due to hydrophobic, electrostatic or hydrogen bonding interactions. We therefore sought to determine whether mucoadhesive particles (MAP) could alter the mucus microstructure, thereby allowing other foreign particles to more easily penetrate mucus. We engineered muco-inert probe particles 1 µm in diameter, whose diffusion in mucus is limited only by steric obstruction from the mucus mesh, and used them to measure possible MAP-induced changes to the microstructure of fresh human cervicovaginal mucus. We found that a 0.24% w/v concentration of 200 nm MAP in mucus induced a ∼10-fold increase in the average effective diffusivity of the probe particles, and a 2- to 3-fold increase in the fraction capable of penetrating physiologically thick mucus layers. The same concentration of muco-inert particles, and a low concentration (0.0006% w/v) of MAP, had no detectable effect on probe particle penetration rates. Using an obstruction-scaling model, we determined that the higher MAP dose increased the average mesh spacing (“pore” size) of mucus from 380 nm to 470 nm. The bulk viscoelasticity of mucus was unaffected by MAP exposure, suggesting MAP may not directly impair mucus clearance or its function as a lubricant, both of which depend critically on the bulk rheological properties of mucus. Our findings suggest mucoadhesive nanoparticles can substantially alter the microstructure of mucus, highlighting the potential of mucoadhesive environmental or engineered nanoparticles to disrupt mucus barriers and cause greater exposure to foreign particles, including pathogens and other potentially toxic nanomaterials.

Measurement of single-cell adhesion strength using a microfluidic assay

Despite the importance of cell adhesion in numerous physiological, pathological, and biomaterial-related responses, our understanding of adhesion strength at the cell-substrate interface and its relationship to cell function remains incomplete. One reason for this deficit is a lack of accessible experimental approaches that quantify adhesion strength at the single-cell level and facilitate large numbers of tests. The current work describes the design, fabrication, and use of a microfluidic-based method for single-cell adhesion strength measurements. By applying a monotonically increasing flow rate in a microfluidic channel in combination with video microscopy, the adhesion strength of individual NIH3T3 fibroblasts cultured for 24 h on various surfaces was measured. The small height of the channel allows high shear stresses to be generated under laminar conditions, allowing strength measurements on well-spread, strongly adhered cells that cannot be characterized in most conventional assays. This assay was used to quantify the relationship between morphological characteristics and adhesion strength for individual well-spread cells. Cell adhesion strength was found to be positively correlated with both cell area and circularity. Computational fluid dynamics (CFD) analysis was performed to examine the role of cell geometry in determining the actual stress applied to the cell. Use of this method to examine adhesion at the single-cell level allows the detachment of strongly-adhered cells under a highly-controllable, uniform loading to be directly observed and will enable the characterization of biological events and relationships that cannot currently be achieved using existing methods.

Contribution of high-resolution correlative imaging techniques in the study of the liver sieve in three-dimensions

Correlative microscopy has become increasingly important for the analysis of the structure, function, and dynamics of cells. This is largely due to the result of recent advances in light-, probe-, laser- and various electron microscopy techniques that facilitate three-dimensional studies. Furthermore, the improved understanding in the past decade of imaging cell compartments in the third dimension has resulted largely from the availability of powerful computers, fast high-resolution CCD cameras, specifically developed imaging analysis software, and various probes designed for labeling living and or fixed cells. In this paper, we review different correlative high-resolution imaging methodologies and how these microscopy techniques facilitated the accumulation of new insights in the morpho-functional and structural organization of the hepatic sieve. Various aspects of hepatic endothelial fenestrae regarding their structure, origin, dynamics, and formation will be explored throughout this paper by comparing the results of confocal laser scanning-, correlative fluorescence and scanning electron-, atomic force-, and whole-mount electron microscopy. Furthermore, the recent advances of vitrifying cells with the vitrobot in combination with the glove box for the preparation of cells for cryo-electron microscopic investigation will be discussed. Finally, the first transmission electron tomography data of the liver sieve in three-dimensions are presented. The obtained data unambiguously show the involvement of special domains in the de novo formation and disappearance of hepatic fenestrae, and focuses future research into the (supra)molecular structure of the fenestrae-forming center, defenestration center and fenestrae-, and sieve plate cytoskeleton ring by using advanced cryo-electron tomography.

Glycocalyx modulation is a physiological means of regulating cell adhesion

Here we present experimental evidence that phagocytic cells use modulation of specific components of their glycocalyx to regulate their binding capacity. Particles coated with antibodies specific for the CD32 medium affinity IgG receptor were driven along human monocytic THP-1 cells (expressing CD32) in a flow chamber operated at low shear rate. Surprisingly, only minimal adhesion was observed. However, when cells were activated by exposure to fibronectin-coated surfaces and/or soluble γ interferon, adhesion efficiency was dramatically increased, whereas the apparent glycocalyx thickness displayed 20% decrease, and the surface density of CD43/leukosialin carbohydrate epitopes displayed 30–40% decrease on activated cells. The existence of a causal link between adhesion increase and glycocalyx alteration was strongly supported by the finding that (i) both phenomena displayed similar kinetics, (ii) an inverse relationship between THP-1 cell binding capacity and glycocalyx density was demonstrated at the individual cell level, and (iii) adhesion enhancement could not be ascribed to an increased binding site density or improved functional capacity of activated cells. Additional experiments revealed that cell-to-particle adhesion resulted in delayed (i.e. more than a few minutes) egress of CD43/leukosialin from contact areas. Since the time scale of particle attachment was less than a second, surface mobility should not affect the potential of CD43 to impair the initial step of adhesion. Finally, studies performed with fluorescent lectins suggested that THP-1 cell activation and increased adhesive potential were related to a decrease of O-glysosylation rather than N-glycosylation of surface glycoproteins.

Comparison of different drying procedures for scanning electron microscopy using human leukocytes

Using human leukocytes as test specimens, three different drying procedures for scanning electron microscopy: critical-point drying (CPD), Peldri II, and tetramethylsilane (TMS), were compared. All three procedures produced identical surface morphology preservation. An equal amount of volume shrinkage was observed regardless of the dehydrants and drying techniques employed. Considering the simplicity, convenience, and time saved, air-drying with TMS is by far the best choice for preparing animal cells for scanning electron microscopy.

Volumes of chick and rat osteoclasts cultured on glass

We have examined the relationship between the number of nuclei of an osteoclast and its volume. Chick and rat cells were released from long bones by chopping the shafts and flushing the fragments in Eagle's Minimum Essential Medium with added 10% fetal calf serum. The bone cell suspension was seeded onto glass coverslips. In Experiment 1, rat and chick cells were allowed to settle for 15 minutes, more medium was then added, and the cells were cultured in 5% CO2 at 37 degrees C for 4 hours. In Experiment 2, only rat cells were used, and the cells were cultured in the presence or absence of 10(-6) M 3-amino-1-hydroxypropylidene-1,1-bisphosphonate (APD) in the medium for 4 or 6 hours. The coverslips were washed in 37 degrees C phosphate-buffered saline and fixed for 24 hours in 2.5% glutaraldehyde in isotonic cacodylate buffer (initially 37 degrees C). The chick cells were critical point dried (CPD) or freeze dried (FD); all rat cells were FD. After drying, cells were coated with gold by vacuum evaporation. The volumes and areas of osteoclasts were measured using a video-rate, line-confocal reflection laser scanning microscope and the number of nuclei in each cell was counted. The volumes and volumes per nucleus of the FD cells were larger than those of the CPD cells but there was no significant difference in plan-areas. Rat osteoclasts were larger than chick cells in all the measured parameters except the mean number of nuclei/cell. The correlation coefficients for the areas, volumes, and the numbers of nuclei for rat and chick cells were all high (r > 0.725). The volumes and volumes per nucleus, but not the areas or areas per nucleus, of the osteoclasts cultured with APD were significantly smaller than control cells. We conclude that FD causes less shrinkage than CPD; chick osteoclasts are about two-thirds the size of rat osteoclasts; and 10(-6) M APD caused a reduction of rat osteoclast volume and volume per nucleus of 21%.

Ultrastructure of the epithelial cells of the endolymphatic duct in the rat

The ultrastructure of the epithelial cells of the endolymphatic duct in the rat is described, following vascular perfusion-fixation of live, anaesthetised and artificially respirated animals. The animals were fixed by means of a pressure feed-back controlled peristaltic pump and an isotonic perfusate-fixative containing glutaraldehyde and Dextran. The endolymphatic duct was isolated by microdissection after the perfusion-fixation, to omit the step of a demineralization procedure. The proximal, intermediate and juxta-saccular parts of the endolymphatic duct were embedded, sectioned and studied separately in the electron microscope. Postfixation in a solution containing OsO4 and potassium ferricyanide revealed a well-developed tubulo-cisternal endoplasmic reticulum (TER), not previously described. Serial sectioning and computerized three-dimensional reconstruction demonstrated a continuity of the TER through the cell from subsurface cisterns abutting on the apical cell membrane to subsurface cisterns abutting on the basolateral cell membrane. The TER resembles that found in solute transporting epithelia, e.g., renal proximal tubule, gall bladder, small intestine and choroid plexus. A fluid resorptive capacity of the epithelial cells of the endolymphatic duct is compatible with the fine structure revealed in the present study. Epithelial cells in the juxta-saccular part of the duct display morphological indications of a secretory activity; furthermore, multivesicular bodies were observed in the epithelial cells throughout the endolymphatic duct.

Improvements in the technique of vascular perfusion-fixation employing a fluorocarbon-containing perfusate and a peristaltic pump controlled by pressure feedback

A new improved technique for whole-body perfusion-fixation of rats and other small animals is described. The driving force is a peristaltic pump which is feedback regulated by a pressure transducer that monitors the blood-perfusion pressure in the left ventricle of the heart. The primary perfusate-fixative is composed of a blood substitute--13.3% oxygenated fluorocarbon FC-75--in 0.05 M cacodylate buffer (pH 7.4) with a 2% glutaraldehyde. The secondary perfusate-fixative is composed of 2% glutaraldehyde in 0.05 M cacodylate buffer (pH 7.4) with 20 mM CaCl2. A double-barrelled, self-holding cannula is used to cannulate the heart; the outer and inner barrels of the cannula are connected to the peristaltic pump and to the pressure transducer, respectively. The tissue oxygen tension in the rat is monitored by a subcutaneous oxygen electrode. Measurements showed that tissue hypoxia/anoxia did not develop before or during the perfusion-fixation. Thus, the technique permits study of specimens which do not exhibit fixation gradients and do not contain cells fixed in a state of asphyxia. This is substantiated by electron micrographs of cells from different organs, revealing new fine structural elements. By adding oxygenated fluorocarbon to glutaraldehyde perfusate-fixatives, enough oxygen is made accessible for cellular respiration as well as for the oxygen-consuming chemical reactions of glutaraldehyde with the tissue. Data on anaesthesia, operative manoeuvres, mechanical components of the system, preparation of fixatives and flow of the perfusate-fixatives are furnished and discussed.

Ultrastructure of the endolymphatic duct in the rat. Fixation and preservation

Ten rats were vascular-perfused at subphysiologic as well as physiologic pressures, 80 mmHg and 120 mmHg, respectively, employing a pressure feed-back controlled peristaltic pump and an isotonic perfusate/fixative with colloids (2% Dextran) and a hypertonic perfusate/fixative without colloids, 300 and 530 mOsm, respectively. In both experiments the endolymphatic duct and sac were isolated by microdissection after primary fixation. When comparing micrographs from the two experiments we observed that rats perfused at low pressure in isotonic fixative with colloids added had non-dilated lateral intercellular spaces and a subepithelial ground substance loaded with uniformly arranged microfibrils, not previously discovered. In rats perfused at high pressure in hypertonic fixative with no colloids added, we observed an edematous state in the subepithelial space between the solid bony aqueduct and the endolymphatic duct, with a concomitant derangement of previously well organized microfibrils. Furthermore, the epithelium was quite clearly displaced from the capillaries, the intercellular spaces were widely dilated and the endolymphatic duct was compressed into its lumen. Methods of fixation and current theories of endolymph resorption by the endolymphatic duct are discussed.