Resolving exit strategies of mycobacteria in Dictyostelium discoideum by combining high-pressure freezing with 3D-correlative light and electron microscopy.

The infection course of Mycobacterium tuberculosis is highly dynamic and comprises sequential stages that require damaging and crossing of several membranes to enable the translocation of the bacteria into the cytosol or their escape from the host. Many important breakthroughs such as the restriction of mycobacteria by the autophagy pathway and the recruitment of sophisticated host repair machineries to the Mycobacterium-containing vacuole have been gained in the Dictyostelium discoideum/M. marinum system. Despite the availability of well-established light and advanced electron microscopy techniques in this system, a correlative approach integrating both methods with near-native ultrastructural preservation is currently lacking. This is most likely due to the low ability of D. discoideum to adhere to surfaces, which results in cell loss even after fixation. To address this problem, we improved the adhesion of cells and developed a straightforward and convenient workflow for 3D-correlative light and electron microscopy. This approach includes high-pressure freezing, which is an excellent technique for preserving membranes. Thus, our method allows to monitor the ultrastructural aspects of vacuole escape which is of central importance for the survival and dissemination of bacterial pathogens.

Cyto-architecture of Byblis glands and leaf cells based on freeze-substitution and conventional TEM.

BACKGROUND AND AIMS: Byblis liniflora (Byblidaceae) is a carnivorous plant that has developed sticky fly paper traps with two types of glandular trichomes producing digestive enzymes and sticky mucilage. This study aimed to analyze the ultrastructure of these glandular leaf trichomes based on rapid freeze fixation and conventional chemical fixation in the attempt to understand their functional contribution to the carnivorous performance of the plants. METHODS: The Byblis cells were studied in TEM, SEM and STEM using cryo techniques for fixation and substitution in addition to conventional chemical fixation. KEY RESULTS: We show in detail the architecture of both the digestive glands and the mucilage glands with their relevant sets of organelles. Both mitochondria and plastids have a conspicuous plasticity, with branches and constrictions, and they associate to form clusters. The glandular cells appear to be transfer cells with cell wall ingrowths. Digestive glands occur in different states of development. Their cuticle forms discontinuities which are unique among glands of carnivorous plants. They look like cuticular holes -- the cuticle separates from the cell wall in only one spot and then ruptures. Cuticular discontinuities thus differ from cuticular gaps and cuticular pores so far described in carnivorous plants. We therefore propose for them the term cuticular holes. CONCLUSIONS: Application of cryo-techniques made it possible to show the true structure of the cell wall and the relationship between cell wall ingrowths and organelles, as well as the morphology and structure of organelles and their associations.

Rehydration of Freeze Substituted Brain Tissue for Pre-embedding Immunoelectron Microscopy.

Electron microscopy (EM) volume reconstruction is a powerful tool for investigating the fundamental structure of brain circuits, but the full potential of this technique is limited by the difficulty of integrating molecular information. High quality ultrastructural preservation is necessary for EM reconstruction, and intact, highly contrasted cell membranes are essential for following small neuronal processes through serial sections. Unfortunately, the antibody labeling methods used to identify most endogenous molecules result in compromised morphology, especially of membranes. Cryofixation can produce superior morphological preservation and has the additional advantage of allowing indefinite storage of valuable samples. We have developed a method based on cryofixation that allows sensitive immunolabeling of endogenous molecules, preserves excellent ultrastructure, and is compatible with high-contrast staining for serial EM reconstruction.

Single-Molecule Localization Microscopy of Presynaptic Active Zones in Drosophila melanogaster after Rapid Cryofixation.

Single-molecule localization microscopy (SMLM) greatly advances structural studies of diverse biological tissues. For example, presynaptic active zone (AZ) nanotopology is resolved in increasing detail. Immunofluorescence imaging of AZ proteins usually relies on epitope preservation using aldehyde-based immunocompetent fixation. Cryofixation techniques, such as high-pressure freezing (HPF) and freeze substitution (FS), are widely used for ultrastructural studies of presynaptic architecture in electron microscopy (EM). HPF/FS demonstrated nearer-to-native preservation of AZ ultrastructure, e.g., by facilitating single filamentous structures. Here, we present a protocol combining the advantages of HPF/FS and direct stochastic optical reconstruction microscopy (dSTORM) to quantify nanotopology of the AZ scaffold protein Bruchpilot (Brp) at neuromuscular junctions (NMJs) of Drosophila melanogaster. Using this standardized model, we tested for preservation of Brp clusters in different FS protocols compared to classical aldehyde fixation. In HPF/FS samples, presynaptic boutons were structurally well preserved with ~22% smaller Brp clusters that allowed quantification of subcluster topology. In summary, we established a standardized near-to-native preparation and immunohistochemistry protocol for SMLM analyses of AZ protein clusters in a defined model synapse. Our protocol could be adapted to study protein arrangements at single-molecule resolution in other intact tissue preparations.

Potassium permanganate is an excellent alternative to osmium tetroxide in freeze-substitution.

High-pressure freezing followed by freeze-substitution is a valuable method for ultrastructural analyses of resin-embedded biological samples. The visualization of lipid membranes is one of the most critical aspects of any ultrastructural study and can be especially challenging in high-pressure frozen specimens. Historically, osmium tetroxide has been the preferred fixative and staining agent for lipid-containing structures in freeze-substitution solutions. However, osmium tetroxide is not only a rare and expensive material, but also volatile and toxic. Here, we introduce the use of a combination of potassium permanganate, uranyl acetate, and water in acetone as complementing reagents during the freeze-substitution process. This mix imparts an intense en bloc stain to cellular ultrastructure and membranes, which makes poststaining superfluous and is well suited for block-face imaging. Thus, potassium permanganate can effectively replace osmium tetroxide in the freeze-substitution solution without sacrificing the quality of ultrastructural preservation.

Characterisation of early ultrastructural changes in the cerebral white matter of CADASIL small vessel disease using high-pressure freezing/freeze-substitution.

AIMS: The objective of this study was to elucidate the early white matter changes in CADASIL small vessel disease. METHODS: We used high-pressure freezing and freeze substitution (HPF/FS) in combination with high-resolution electron microscopy (EM), immunohistochemistry and confocal microscopy of brain specimens from control and CADASIL (TgNotch3R169C ) mice aged 4-15 months to study white matter lesions in the corpus callosum. RESULTS: We first optimised the HPF/FS protocol in which samples were chemically prefixed, frozen in a sample carrier filled with 20% polyvinylpyrrolidone and freeze-substituted in a cocktail of tannic acid, osmium tetroxide and uranyl acetate dissolved in acetone. EM analysis showed that CADASIL mice exhibit significant splitting of myelin layers and enlargement of the inner tongue of small calibre axons from the age of 6 months, then vesiculation of the inner tongue and myelin sheath thinning at 15 months of age. Immunohistochemistry revealed an increased number of oligodendrocyte precursor cells, although only in older mice, but no reduction in the number of mature oligodendrocytes at any age. The number of Iba1 positive microglial cells was increased in older but not in younger CADASIL mice, but the number of activated microglial cells (Iba1 and CD68 positive) was unchanged at any age. CONCLUSION: We conclude that early WM lesions in CADASIL affect first and foremost the myelin sheath and the inner tongue, suggestive of a primary myelin injury. We propose that those defects are consistent with a hypoxic/ischaemic mechanism.

Combining high-pressure freezing with pre-embedding immunogold electron microscopy and tomography.

Immunogold labeling of permeabilized whole-mount cells or thin-sectioned material is widely used for the subcellular localization of biomolecules at the high spatial resolution of electron microscopy (EM). Those approaches are well compatible with either 3-dimensional (3D) reconstruction of organelle morphology and antigen distribution or with rapid cryofixation-but not easily with both at once. We describe here a specimen preparation and labeling protocol for animal cell cultures, which represents a novel blend of specifically adapted versions of established techniques. It combines the virtues of reliably preserved organelle ultrastructure, as trapped by rapid freezing within milliseconds followed by freeze-substitution and specimen rehydration, with the advantages of robust labeling of intracellular constituents in 3D through means of pre-embedding NANOGOLD-silver immunocytochemistry. So obtained thin and semi-thick epoxy resin sections are suitable for transmission EM imaging, as well as tomographic reconstruction and modeling of labeling patterns in the 3D cellular context.

In situ electron microscopy characterization of intracellular ion pools in mineral forming microalgae.

The formation of coccoliths, intricate calcium carbonate scales that cover the cells of unicellular marine microalgae, is a highly regulated biological process. For decades, scientists have tried to elucidate the cellular, chemical, and structural mechanisms that control the precise mineralogy and shape of the inorganic crystals. Transmission electron microscopy was pivotal in characterizing some of the organelles that orchestrate this process. However, due to the difficulties in preserving soluble inorganic phases during sample preparation, only recently, new intracellular ion-pools were detected using state-of-the-art cryo X-ray and electron microscopy techniques. Here, we combine a completely non-aqueous sample preparation procedure and room temperature electron microscopy, to investigate the presence, cellular location, and composition, of mineral phases inside mineral forming microalga species. This methodology, which fully preserves the forming coccoliths and the recently identified Ca-P-rich bodies, allowed us to identify a new class of ion-rich compartments that have complex internal structure. In addition, we show that when carefully choosing heavy metal stains, elemental analysis of the mineral phases can give accurate chemical signatures of the inorganic phases. Applying this approach to mineral forming microalgae will bridge the gap between the low-preservation power for inorganic phases of conventional chemical-fixation based electron microscopy, and the low-yield of advanced cryo techniques.

On the structural organization of the bacillary band of Trichuris muris under cryopreparation protocols and three-dimensional electron microscopy.

Whipworms of the genus Trichuris are nematode parasites that infect mammals and can lead to various intestinal diseases of human and veterinary interest. The most intimate interaction between the parasite and the host intestine occurs through the anterior region of the nematode body, inserted into the intestinal mucosa during infection. One of the most prominent structures of the nematode surface found at the infection site is the bacillary band, a surface domain formed by a number of cells, mostly stichocytes and bacillary glands, whose structure and function are still under debate. Here, we used confocal microscopy, field emission scanning electron microscopy, helium ion microscopy, transmission electron microscopy and FIB-SEM tomography to unveil the functional role of the bacillary gland cell. We analyzed the surface organization as well as the intracellular milieu of the bacillary glands of Trichuris muris in high pressure frozen/freeze-substituted samples. Results showed that the secretory content is preserved in all gland openings, presenting a projected pattern. FIB-SEM analysis showed that the lamellar zone within the bacillary gland chamber is formed by a set of lacunar structures that may exhibit secretory or absorptive functions. In addition, incubation of parasites with the fluid phase endocytosis marker sulforhodamine B showed a time-dependent uptake by the parasite mouth, followed by perfusion through different tissues with ultimate secretion through the bacillary gland. Taken together, the results show that the bacillary gland possess structural characteristics of secretory and absorptive cells and unequivocally demonstrate that the bacillary gland cell functions as a secretory structure.

High-pressure freezing followed by freeze substitution of a complex and variable density miniorgan: the wool follicle.

Cryofixation by high-pressure freezing (HPF) followed by freeze substitution (FS) is a preferred method to prepare biological specimens for ultrastructural studies. It has been shown to achieve uniform vitrification and ultrastructure preservation of complex structures in different cell types. One limitation of HPF is the small sample volume of <200 µm thickness and about 2000 µm across. A wool follicle is a rare intact organ in a single sample about 200 µm thick. Within each follicle, specialized cells derived from multiple cell lineages assemble, mature and cornify to make a wool fibre, which contains 95% keratin and associated proteins. In addition to their complex structure, large density changes occur during wool fibre development. Limited water movement and accessibility of fixatives are some issues that negatively affect the preservation of the follicle ultrastructure via conventional chemical processing. Here, we show that HPF-FS of wool follicles can yield high-quality tissue preservation for ultrastructural studies using transmission electron microscopy.

Cryopreservation of articular cartilage. Part 3: The liquidus-tracking method.

Although it is relatively straightforward to cryopreserve living isolated chondrocytes, at the present time there is no satisfactory method to preserve surgical grafts between the time of procurement or manufacture and actual use. In earlier papers we have established that the cryoprotectants dimethyl sulphoxide or propylene glycol do penetrate into this tissue very rapidly. Chondrocytes are not unusually susceptible to osmotic stress; in fact they appear to be particularly resistant. It appears that damage is associated with the formation of ice per se, even at cooling rates that are optimal for the cryopreservation of isolated chondrocytes. We then showed that current methods of cartilage cryopreservation involve the nucleation and growth of ice crystals within the chondrons rather than ice being restricted to the surrounding acellular matrix. This finding established the need to avoid the crystallization of ice-in other words, vitrification. Song and his colleagues have published a vitrification method that is based on the use of one of Fahy's vitrification formulations. We confirmed the effectiveness of this method but found it to be very dependent on ultra rapid warming. However, we were able to develop a 'liquidus-tracking' method that completely avoids the crystallization of ice and does not require rapid warming. The ability of cartilage preserved in this way to incorporate sulphate into newly synthesized glycosaminoglycans (GAGs) approached 70% of that of fresh control cartilage. In this method the rates of cooling and warming can be very low, which is essential for any method that is to be used in Tissue Banks to process the bulky grafts that are required by orthopaedic surgeons. Work is continuing to refine this method for Tissue Bank use.

High subzero cryofixation: A technique for observing ice within tissues.

While various fixation techniques for observing ice within tissues stored at high sub-zero temperatures currently exist, these techniques require either different fixative solution compositions when assessing different storage temperatures or alteration of the sample temperature to enable alcohol-water substitution. Therefore, high-subzero cryofixation (HSC), was developed to facilitate fixation at any temperature above -80 °C without sample temperature alteration. Rat liver sections (1 cm2) were frozen at a rate of -1 °C/min to -20 °C, stored for 1 h at -20 °C, and processed using classical freeze-substitution (FS) or HSC. FS samples were plunged in liquid nitrogen and held for 1 h before transfer to -80 °C methanol. After 1, 3, or 5 days of -80 °C storage, samples were placed in 3% glutaraldehyde on dry ice and allowed to sublimate. HSC samples were stored in HSC fixative at -20 °C for 1, 3, or 5 days prior to transfer to 4 °C. Tissue sections were paraffin embedded, sliced, and stained prior to quantification of ice size. HSC fixative permeation was linear with time and could be mathematically modelled to determine duration of fixation required for a given tissue depth. Ice grain size within the inner regions of 5 d samples was consistent between HSC and FS processing (p = 0.76); however, FS processing resulted in greater ice grains in the outer region of tissue. This differed significantly from HSC outer regions (p = 0.016) and FS inner regions (p = 0.038). No difference in ice size was observed between HSC inner and outer regions (p = 0.42). This work demonstrates that HSC can be utilized to observe ice formed within liver tissue stored at -20 °C. Unlike isothermal freeze fixation and freeze substitution alternatives, the low melting point of the HSC fixative enables its use at a variety of temperatures without alteration of sample temperature or fixative composition.

FIB-SEM imaging properties of Drosophila melanogaster tissues embedded in Lowicryl HM20.

Lowicryl resins enable processing of biological material for electron microscopy at the lowest temperatures compatible with resin embedding. When combined with high-pressure freezing and freeze-substitution, Lowicryl embedding supports preservation of fine structural details and fluorescent markers. Here, we analysed the applicability of Lowicryl HM20 embedding for focused ion beam (FIB) scanning electron microscopy (SEM) tomography of Drosophila melanogaster embryonic and larval model systems. We show that the freeze-substitution with per-mill concentrations of uranyl acetate provided sufficient contrast and an image quality of SEM imaging in the range of similar samples analysed by transmission electron microscopy (TEM). Preservation of genetically encoded fluorescent proteins allowed correlative localization of regions of interest (ROI) within the embedded tissue block. TEM on sections cut from the block face enabled evaluation of structural preservation to allow ROI ranking and thus targeted, time-efficient FIB-SEM tomography data collection. The versatility of Lowicryl embedding opens new perspectives for designing hybrid SEM-TEM workflows to comprehensively analyse biological structures. LAY DESCRIPTION: Focused ion beam scanning electron microscopy is becoming a widely used technique for the three-dimensional analysis of biological samples at fine structural details beyond levels feasible for light microscopy. To withstand the abrasion of material by the ion beam and the imaging by the scanning electron beam, biological samples have to be embedded into resins, most commonly these are very dense epoxy-based plastics. However, dense resins generate electron scattering which interferes with the signal from the biological specimen. Furthermore, to improve the imaging contrast, epoxy embedding requires chemical treatments with e.g. heavy metals, which deteriorate the ultrastructure of the biological specimen. In this study we explored the applicability of an electron lucent resin, Lowicryl HM 20, for focused ion beam scanning electron microscopy. The Lowicryl embedding workflow operates at milder chemical treatments and lower temperatures, thus preserving the sub-cellular and sub-organellar organization, as well as fluorescent markers visible by light microscopy. Here we show that focus ion beam scanning electron microscopy of Lowicryl-embedded fruit flies tissues provides reliable imaging revealing fine structural details. Our workflow benefited from use of transmission electron microscopy for the quality control of the ultrastructural preservation and fluorescent light microscopy for localization of regions of interest. The versatility of Lowicryl embedding opens up new perspectives for designing hybrid workflows combining fluorescent light, scanning, and transmission electron microscopy techniques to comprehensively analyze biological structures.

Thin polyester filters as versatile sample substrates for high-pressure freezing of bacterial biofilms, suspended microorganisms and adherent eukaryotic cells.

High-pressure freezing limits the size of biological samples, because only small samples can be frozen without ice damage. Additionally, these samples must fit into the dimensions of the sample holder provided by the high-pressure freezer. We explored the potential of a 10 µm thin polyester filter membrane (PE-filter) as a versatile sample substrate for high-pressure freezing. Planktonic bacteria, bacterial spores and suspended eukaryotic cells could be concentrated on the PE-filter, whereas biofilm, bacterial microcolonies and HeLa cells were able to grow directly on the PE-filter. These microorganism-loaded PE-filters were used for high-pressure freezing, freeze-substitution and plastic embedding in Epon or Lowicryl. Embedded filters were cross-sectioned so that the interface between microorganism and substrate as well as the overlying medium was revealed. Although the structural preservation was good for thin samples and samples with lower water content, such as biofilms, adherent HeLa-cell cultures were likewise sufficiently preserved for transmission electron microscopy imaging. The fact that microorganism-loaded PE-filters could be also examined with confocal laser scanning fluorescence microscopy under fully hydrated conditions, and freeze-substituted PE-filters samples with scanning electron microscopy, demonstrates the versatility of the PE-filter as a sample substrate for a wide array of microorganisms. LAY DESCRIPTION: In order to investigate biological samples in the transmission electron microscope it is imperative to remove all their water content, or the specimens will be destroyed by boiling in the high vacuum of the microscope. In order to avoid dramatic morphology-changes due to drying artefacts or the impact of chemical stabilisers, high-pressure freezing (HPF) was developed. This protocol allows freezing biological samples in an instant (within a few milliseconds) down to -196°C while applying high pressure at the same time so that the specimen retains all its water in a solidified noncrystalline form. However, the formation of morphology-destroying ice crystals is only avoided, if the cooling of the sample is faster than the ice crystal formation, which is only possible with very thin samples (up to a maximum of 200 µm in optimal cases). High-pressure freezing is regarded as the gold-standard for sample preparation of cells, tissues and small organisms. However, all of these samples must fit into the dimensions of the specific sample holder of the high-pressure freezer and their transfer into the high-pressure freezing machine must be achieved without significant impact on sample physiology. Additionally, it may also necessary to concentrate and immobilise a biological specimen before they can be placed in the HPF sample holder. Although a few number of strategies and sample substrates have been used for different types of biological samples, we explored the potential of a 10 µm thin polyester filter membrane (PE-filter) as a versatile sample substrate for HPF. In culture medium suspended bacteria, suspended bacterial spores and in medium suspended higher cells could be concentrated on the PE-filter, whereas bacterial biofilm or bacterial microcolonies from an agar plate, and surface-adhering higher cells were able to grow directly on the PE-filter. These microorganism-loaded PE-filters could be directly used for high-pressure freezing, and were finally embedded in a plastic resin like Epon or Lowicryl. Embedded filters were cross-sectioned so that the interface between microorganism and substrate or overlying medium was revealed. Although the structural preservation was good for thin samples and samples with lower water content, such as biofilms, adherent HeLa-cell cultures were likewise sufficiently preserved for transmission electron microscopy imaging. The fact that microorganism-loaded PE-filters could be also examined with confocal laser scanning fluorescence microscopy under fully hydrated conditions, and freeze-substituted PE-filters samples with scanning-electron microscopy, demonstrates the versatility of the PE-filter as a sample substrate for a wide array of microorganisms.

Intracellular lumen formation in Drosophila proceeds via a novel subcellular compartment.

Cellular tubes have diverse morphologies, including multicellular, unicellular and subcellular architectures. Subcellular tubes are found prominently within the vertebrate vasculature, the insect breathing system and the nematode excretory apparatus, but how such tubes form is poorly understood. To characterize the cellular mechanisms of subcellular tube formation, we have refined methods of high pressure freezing/freeze substitution to prepare Drosophila larvae for transmission electron microscopic (TEM) analysis. Using our methods, we have found that subcellular tube formation may proceed through a previously undescribed multimembrane intermediate composed of vesicles bound within a novel subcellular compartment. We have also developed correlative light/TEM procedures to identify labeled cells in TEM-fixed larval samples. Using this technique, we have found that Vacuolar ATPase (V-ATPase) and the V-ATPase regulator Rabconnectin-3 are required for subcellular tube formation, probably in a step resolving the intermediate compartment into a mature lumen. In general, our ultrastructural analysis methods could be useful for a wide range of cellular investigations in Drosophila larvae.

STEM tomography of high-pressure frozen and freeze-substituted cells: a comparison of image stacks obtained at 200 kV or 300 kV.

Scanning transmission electron microscopic (STEM) tomography of high-pressure frozen, freeze-substituted semi-thin sections is one of multiple approaches for three-dimensional recording and visualization of electron microscopic samples. Compared to regular TEM tomography thicker sample sections can be investigated since chromatic aberration due to inelastic scattering is not a limit. The method is ideal to investigate subcellular compartments or organelles such as synapses, mitochondria, or microtubule arrangements. STEM tomography fills the gap between single-particle electron cryo-tomography, and methods that allow investigations of large volumes, such as serial block-face SEM and FIB-SEM. In this article, we discuss technical challenges of the approach and show some applications in cell biology. It is ideal to use a 300-kV electron microscope with a very small convergence angle of the primary beam ("parallel" beam). These instruments are expensive and tomography is rather time consuming, and therefore, access to such a high-end microscope might be difficult. In this article, we demonstrate examples of successful STEM tomography in biology using a more standard 200-kV microscope equipped with a field emission tip.

Preservation of Photoreceptor Nanostructure for Electron Tomography Using Transcardiac Perfusion Followed by High-Pressure Freezing and Freeze-Substitution.

The phototransductive membrane disks of a vertebrate photoreceptor outer segment (OS) are highly susceptible to perturbations during preservation for electron microscopy. To optimize their preservation for nanostructural studies, such as with electron tomography (ET), we developed a protocol, using a combination of chemical and physical fixation approaches, including transcardiac perfusion, high-pressure freezing, and freeze-substitution.

Candida albicans biofilms: comparative analysis of room-temperature and cryofixation for scanning electron microscopy.

Biofilms are frequently related to invasive fungal infections and are reported to be more resistant to antifungal drugs than planktonic cells. The structural complexity of the biofilm as well as the presence of a polymeric extracellular matrix (ECM) is thought to be associated with this resistant behavior. Scanning electron microscopy (SEM) after room temperature glutaraldehyde-based fixation, have been used to study fungal biofilm structure and drug susceptibility but they usually fail to preserve the ECM and, therefore, are not an optimised methodology to understand the complexity of the fungal biofilm. Thus, in this work, we propose a comparative analysis of room-temperature and cryofixation/freeze substitution of Candida albicans biofilms for SEM observation. Our experiments showed that room-temperature fixative protocols using glutaraldehyde and osmium tetroxide prior to alcohol dehydration led to a complete extraction of the polymeric ECM of biofilms. ECM from fixative and alcohol solutions were recovered after all processing steps and these structures were characterised by biochemistry assays, transmission electron microscopy and mass spectrometry. Cryofixation techniques followed by freeze-substitution lead to a great preservation of both ECM structure and C. albicans biofilm cells, allowing the visualisation of a more reliable biofilm structure. These findings reinforce that cryofixation should be the indicated method for SEM sample preparation to study fungal biofilms as it allows the visualisation of the EMC and the exploration of the biofilm structure to its fullest, as its structural/functional role in interaction with host cells, other pathogens and for drug resistance assays.

The CryoCapsule: simplifying correlative light to electron microscopy.

Correlating complementary multiple scale images of the same object is a straightforward means to decipher biological processes. Light microscopy and electron microscopy are the most commonly used imaging techniques, yet despite their complementarity, the experimental procedures available to correlate them are technically complex. We designed and manufactured a new device adapted to many biological specimens, the CryoCapsule, that simplifies the multiple sample preparation steps, which at present separate live cell fluorescence imaging from contextual high-resolution electron microscopy, thus opening new strategies for full correlative light to electron microscopy. We tested the biological application of this highly optimized tool on three different specimens: the in vitro Xenopus laevis mitotic spindle, melanoma cells over-expressing YFP-langerin sequestered in organized membranous subcellular organelles and a pigmented melanocytic cell in which the endosomal system was labeled with internalized fluorescent transferrin.

An improved high pressure freezing and freeze substitution method to preserve the labile vaccinia virus nucleocapsid.

In recent years, high pressure freezing and freeze substitution have been widely used for electron microscopy to reveal viral and cellular structures that are difficult to preserve. Vaccinia virus, a member of the Poxviridae family, presents one of the most complex viral structures. The classical view of vaccinia virus structure consists of an envelope surrounding a biconcave core, with a lateral body in each concavity of the core. This classical view was challenged by Peters and Muller (1963), who demonstrated the presence of a folded tubular structure inside the virus core and stated the difficulty in visualizing this structure, possibly because it is labile and cannot be preserved by conventional sample preparation. Therefore, this tubular structure, now called the nucleocapsid, has been mostly neglected over the years. Earlier studies were able to preserve the nucleocapsid, but with low efficiency. In this study, we report the protocol (and troubleshooting) that resulted in preservation of the highest numbers of nucleocapsids in several independent preparations. Using this protocol, we were able to demonstrate an interdependence between the formation of the virus core wall and the nucleocapsid, leading to the hypothesis that an interaction exists between the major protein constituents of these compartments, A3 (core wall) and L4 (nucleocapsid). Our results show that high pressure freezing and freeze substitution can be used in more in-depth studies concerning the nucleocapsid structure and function.