3D FIB-SEM structural insights into the architecture of sub-pellicular microtubules of Trypanosoma cruzi epimastigotes

BACKGROUND INFORMATION

Trypanosomatidae, which includes eukaryotic species agents of diseases like leishmaniasis, sleeping sickness, and Chagas disease, have special structures and organelles not found in mammalian cells. They present a layer of microtubules, known as subpellicular microtubules (SPMT), located underneath the plasma membrane and responsible for preserving cell morphology, cell polarity, the position of single copy organelles, and morphological changes that occur throughout the protozoan life cycle. Even though a lot of knowledge about the SPMT is available, we still do not know exactly how each microtubule in the system is organized in three dimensions. Here, we use focused ion beam scanning electron microscopy (FIB-SEM) to analyze the tridimensional organization of epimastigotes SPMT.

RESULTS

The high-resolution 3D analyses revealed that certain microtubules of the SPMT end more prematurely than the neighboring ones.

CONCLUSIONS

These microtubules could (1) be shorter or (2) have the same length as the neighboring ones, assuming that those end up earlier at their other end, might be treadmilling/catastrophe events that have not yet been described in trypanosomatids.

Single-Neuron Labeling in Fixed Tissue and Targeted Volume Electron Microscopy

The structural complexity of nervous tissue makes it very difficult to unravel the connectivity between neural elements at different scales. Numerous methods are available to trace long-range projections at the light microscopic level, and to identify the actual synaptic connections at the electron microscopic level. However, correlating mesoscopic and nanoscopic scales in the same cell, cell population or brain region is a problematic, laborious and technically demanding task. Here we present an effective method for the 3D reconstruction of labeled subcellular structures at the ultrastructural level, after single-neuron labeling in fixed tissue. The brain is fixed by intracardial perfusion of aldehydes and thick vibratome sections (250 μm) are obtained. Single cells in these vibratome sections are intracellularly injected with horseradish peroxidase (HRP), so that the cell body and its processes can be identified. The thick sections are later flat-embedded in epoxy resin and re-sectioned into a series of thinner (7 μm) sections. The sections containing the regions of interest of the labeled cells are then imaged with automated focused ion beam milling and scanning electron microscopy (FIB-SEM), acquiring long series of high-resolution images that can be reconstructed, visualized, and analyzed in 3D. With this methodology, we can accurately select any cellular segment at the light microscopic level (e.g., proximal, intermediate or distal dendrites, collateral branches, axonal segments, etc.) and analyze its synaptic connections at the electron microscopic level, along with other ultrastructural features. Thus, this method not only facilitates the mapping of the synaptic connectivity of single-labeled neurons, but also the analysis of the surrounding neuropil. Since the labeled processes can be located at different layers or subregions, this method can also be used to obtain data on the differences in local synaptic organization that may exist at different portions of the labeled neurons.

A cryogenic, coincident fluorescence, electron, and ion beam microscope

Cryogenic electron tomography (cryo-ET) combined with subtomogram averaging, allows in situ visualization and structure determination of macromolecular complexes at subnanometre resolution. Cryogenic focused ion beam (cryo-FIB) micromachining is used to prepare a thin lamella-shaped sample out of a frozen-hydrated cell for cryo-ET imaging, but standard cryo-FIB fabrication is blind to the precise location of the structure or proteins of interest. Fluorescence-guided focused ion beam (FIB) milling at target locations requires multiple sample transfers prone to contamination, and relocation and registration accuracy is often insufficient for 3D targeting. Here, we present in situ fluorescence microscopy-guided FIB fabrication of a frozen-hydrated lamella to address this problem: we built a coincident three-beam cryogenic correlative microscope by retrofitting a compact cryogenic microcooler, custom positioning stage, and an inverted widefield fluorescence microscope (FM) on an existing FIB scanning electron microscope. We show FM controlled targeting at every milling step in the lamella fabrication process, validated with transmission electron microscope tomogram reconstructions of the target regions. The ability to check the lamella during and after the milling process results in a higher success rate in the fabrication process and will increase the throughput of fabrication for lamellae suitable for high-resolution imaging.

Protocols for Generating Surfaces and Measuring 3D Organelle Morphology Using Amira

High-resolution 3D images of organelles are of paramount importance in cellular biology. Although light microscopy and transmission electron microscopy (TEM) have provided the standard for imaging cellular structures, they cannot provide 3D images. However, recent technological advances such as serial block-face scanning electron microscopy (SBF-SEM) and focused ion beam scanning electron microscopy (FIB-SEM) provide the tools to create 3D images for the ultrastructural analysis of organelles. Here, we describe a standardized protocol using the visualization software, Amira, to quantify organelle morphologies in 3D, thereby providing accurate and reproducible measurements of these cellular substructures. We demonstrate applications of SBF-SEM and Amira to quantify mitochondria and endoplasmic reticulum (ER) structures.

Lunapark-dependent formation of a virus-induced ER exit site contains multi-tubular ER junctions that promote viral ER-to-cytosol escape

Viruses rearrange host membranes to support different entry steps. Polyomavirus simian virus 40 (SV40) reorganizes the endoplasmic reticulum (ER) membrane to generate focus structures that enable virus ER-to-cytosol escape, a decisive infection step. The molecular architecture of the ER exit site that might illuminate why it is ideally suited for membrane penetration is unknown. Here 3D focused ion beam scanning electron microscopy (FIB-SEM) reconstruction reveals that the ER focus structure consists of multi-tubular ER junctions where SV40 preferentially localizes, suggesting that tubular branch points are virus ER-to-cytosol penetration sites. Functional analysis demonstrates that lunapark-an ER membrane protein that typically stabilizes three-way ER junctions-relocates to the ER foci, where it supports focus formation, leading to SV40 ER escape and infection. Our results reveal how a virus repurposes the activity of an ER membrane protein to form a virus-induced ER substructure required for membrane escape and suggest that ER tubular junctions are vulnerable sites exploited by viruses for membrane penetration.

3D Mesoscopic Architecture of a Heterogeneous Cellular Network in the Cementum-Periodontal Ligament-Alveolar Bone Complex

Cell-to-cell communication orchestrates various cell and tissue functions. This communication enables cells to form cellular networks with each other through direct contact via intercellular junctions. Because these cellular networks are closely related to tissue and organ functions, elucidating the morphological characteristics of cellular networks could lead to the development of novel therapeutic approaches. The tooth, periodontal ligament (PDL), and alveolar bone form a complex via collagen fibres. Teeth depend on the co-ordinated activity of this complex to maintain their function, with cellular networks in each of its three components. Imaging methods for three-dimensional (3D) mesoscopic architectural analysis include focused ion beam/scanning electron microscopy (FIB/SEM), which is characterised by its ability to select observation points and acquire data from complex tissue after extensive block-face imaging, without the need to prepare numerous ultrathin sections. Previously, we employed FIB/SEM to analyse the 3D mesoscopic architecture of hard tissue including the PDL, which exists between the bone and tooth root. The imaging results showed that the cementum, PDL, and alveolar bone networks are in contact and form a heterogeneous cellular network. This cellular network may orchestrate mechanical loading-induced remodelling of the cementum-PDL-alveolar bone complex as the remodelling of each complex component is coordinated, as exemplified by tooth movement due to orthodontic treatment and tooth dislocation due to occlusal loss. In this review, we summarise and discuss the 3D mesoscopic architecture of cellular networks in the cementum, PDL, and alveolar bone as observed in our recent mesoscopic and morphological studies.

Microscopes, tools, probes, and protocols: A guide in the route of correlative microscopy for biomedical investigation

In the last decades, the advancements of microscopes technology, together with the development of new imaging approaches, are trying to address some biological questions that have been unresolved in the past: the need to combine in the same analysis temporal, functional and morphological information on the biological sample has become pressing. For this reason, the use of correlative microscopy, in which two or more imaging techniques are combined in the same analysis, is getting increasingly widespread. In fact, correlative microscopy can overcome limitations of a single imaging method, giving access to a larger amount of information from the same specimen. However, correlative microscopy can be challenging, and appropriate protocols for sample preparation and imaging methods must be selected. Here we review the state of the art of correlating electron microscopy with different imaging methods, focusing on sample preparation, tools, and labeling methods, with the aim to provide a comprehensive guide for those scientists who are approaching the field of correlative methods.

High-Resolution 3D Reconstruction of Human Oocytes Using Focused Ion Beam Scanning Electron Microscopy

The egg plays a pivotal role in the reproduction of our species. Nevertheless, its fundamental biology remains elusive. Transmission electron microscopy is traditionally used to inspect the ultrastructure of female gametes. However, two-dimensional micrographs contain only fragmentary information about the spatial organization of the complex oocyte cytoplasm. Here, we employed the Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) to explore human oocyte intracellular morphology in three dimensions (3D). Volume reconstruction of generated image stacks provided an unprecedented view of ooplasmic architecture. Organelle distribution patterns observed in nine donor oocytes, representing three maturational stages, documented structural changes underlying the process by which the egg acquires developmental competence. 3D image segmentation was performed to extract information about distinct organelle populations, and the following quantitative analysis revealed that the mitochondrion occupies ∼ 4.26% of the maturing oocyte cytoplasm. In summary, this proof-of-concept study demonstrates the potential of large volume electron microscopy to study rare samples of delicate female gametes and paves the way for applying the FIB-SEM technique in human oocyte research.

Electron Microscopy Techniques for Investigating Structure and Composition of Hair-Cell Stereociliary Bundles

Hair cells—the sensory cells of the vertebrate inner ear—bear at their apical surfaces a bundle of actin-filled protrusions called stereocilia, which mediate the cells’ mechanosensitivity. Hereditary deafness is often associated with morphological disorganization of stereocilia bundles, with the absence or mislocalization within stereocilia of specific proteins. Thus, stereocilia bundles are closely examined to understand most animal models of hereditary hearing loss. Because stereocilia have a diameter less than a wavelength of light, light microscopy is not adequate to reveal subtle changes in morphology or protein localization. Instead, electron microscopy (EM) has proven essential for understanding stereocilia bundle development, maintenance, normal function, and dysfunction in disease. Here we review a set of EM imaging techniques commonly used to study stereocilia, including optimal sample preparation and best imaging practices. These include conventional and immunogold transmission electron microscopy (TEM) and scanning electron microscopy (SEM), as well as focused-ion-beam scanning electron microscopy (FIB-SEM), which enables 3-D serial reconstruction of resin-embedded biological structures at a resolution of a few nanometers. Parameters for optimal sample preparation, fixation, immunogold labeling, metal coating and imaging are discussed. Special attention is given to protein localization in stereocilia using immunogold labeling. Finally, we describe the advantages and limitations of these EM techniques and their suitability for different types of studies.

Structural Analysis of the Caenorhabditis elegans Dauer Larval Anterior Sensilla by Focused Ion Beam-Scanning Electron Microscopy

At the end of the first larval stage, the nematode Caenorhabditis elegans developing in harsh environmental conditions is able to choose an alternative developmental path called the dauer diapause. Dauer larvae exhibit different physiology and behaviors from non-dauer larvae. Using focused ion beam-scanning electron microscopy (FIB-SEM), we volumetrically reconstructed the anterior sensory apparatus of C. elegans dauer larvae with unprecedented precision. We provide a detailed description of some neurons, focusing on structural details that were unknown or unresolved by previously published studies. They include the following: (1) dauer-specific branches of the IL2 sensory neurons project into the periphery of anterior sensilla and motor or putative sensory neurons at the sub-lateral cords; (2) ciliated endings of URX sensory neurons are supported by both ILso and AMso socket cells near the amphid openings; (3) variability in amphid sensory dendrites among dauers; and (4) somatic RIP interneurons maintain their projection into the pharyngeal nervous system. Our results support the notion that dauer larvae structurally expand their sensory system to facilitate searching for more favorable environments.

Hierarchical ultrastructure: An overview of what is known about tendons and future perspective for tendon engineering

Abnormal tendons are rarely ever repaired to the natural structure and morphology of normal tendons. To better guide the repair and regeneration of injured tendons through a tissue engineering method, it is necessary to have insights into the internal morphology, organization, and composition of natural tendons. This review summarized recent researches on the structure and function of the extracellular matrix (ECM) components of tendons and highlight the application of multiple detection methodologies concerning the structure of ECMs. In addition, we look forward to the future of multi-dimensional biomaterial design methods and the potential of structural repair for tendon ECM components. In addition, focus is placed on the macro to micro detection methods for tendons, and current techniques for evaluating the extracellular matrix of tendons at the micro level are introduced in detail. Finally, emphasis is given to future extracellular matrix detection methods, as well as to how future efforts could concentrate on fabricating the biomimetic tendons.

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Summarize recent research on the structure and function of the extracellular matrix (ECM) components of tendons.

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Comments on current research methods concerning the structure of ECMs.

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Perspective on the future of multi-dimensional detection techniques and structural repair of tendon ECM components.

Three-Dimensional Synaptic Organization of Layer III of the Human Temporal Neocortex

In the present study, we have used focused ion beam/scanning electron microscopy (FIB/SEM) to perform a study of the synaptic organization of layer III of Brodmann's area 21 in human tissue samples obtained from autopsies and biopsies. We analyzed the synaptic density, 3D spatial distribution, and type (asymmetric/symmetric), as well as the size and shape of each synaptic junction of 4945 synapses that were fully reconstructed in 3D. Significant differences in the mean synaptic density between autopsy and biopsy samples were found (0.49 and 0.66 synapses/μm3, respectively). However, in both types of samples (autopsy and biopsy), the asymmetric:symmetric ratio was similar (93:7) and most asymmetric synapses were established on dendritic spines (75%), while most symmetric synapses were established on dendritic shafts (85%). We also compared several electron microscopy methods and analysis tools to estimate the synaptic density in the same brain tissue. We have shown that FIB/SEM is much more reliable and robust than the majority of the other commonly used EM techniques. The present work constitutes a detailed description of the synaptic organization of cortical layer III. Further studies on the rest of the cortical layers are necessary to better understand the functional organization of this temporal cortical region.

Invadopodia Structure in 3D Environment Resolved by Near-Infrared Branding Protocol Combining Correlative Confocal and FIB-SEM Microscopy

Cancer cell invasion through tissue barriers is the intrinsic feature of metastasis, the most life-threatening aspect of cancer. Detailed observation and analysis of cancer cell behaviour in a 3D environment is essential for a full understanding of the mechanisms of cancer cell invasion. The inherent limits of optical microscopy resolution do not allow to for in-depth observation of intracellular structures, such as invadopodia of invading cancer cells. The required resolution can be achieved using electron microscopy techniques such as FIB-SEM. However, visualising cells in a 3D matrix using FIB-SEM is challenging due to difficulties with localisation of a specific cell deep within the resin block. We have developed a new protocol based on the near-infrared branding (NIRB) procedure that extends the pattern from the surface grid deep inside the resin. This 3D burned pattern allows for precise trimming followed by targeted 3D FIB-SEM. Here we present detailed 3D CLEM results combining confocal and FIB-SEM imaging of cancer cell invadopodia that extend deep into the collagen meshwork.

Maturation of Heterogeneity in Afferent Synapse Ultrastructure in the Mouse Cochlea

Auditory nerve fibers (ANFs) innervating the same inner hair cell (IHC) may have identical frequency tuning but different sound response properties. In cat and guinea pig, ANF response properties correlate with afferent synapse morphology and position on the IHC, suggesting a causal structure-function relationship. In mice, this relationship has not been fully characterized. Here we measured the emergence of synaptic morphological heterogeneities during maturation of the C57BL/6J mouse cochlea by comparing postnatal day 17 (p17, ∼3 days after hearing onset) with p34, when the mouse cochlea is mature. Using serial block face scanning electron microscopy and three-dimensional reconstruction we measured the size, shape, vesicle content, and position of 70 ribbon synapses from the mid-cochlea. Several features matured over late postnatal development. From p17 to p34, presynaptic densities (PDs) and post-synaptic densities (PSDs) became smaller on average (PDs: 0.75 to 0.33; PSDs: 0.58 to 0.31 μm²) and less round as their short axes shortened predominantly on the modiolar side, from 770 to 360 nm. Membrane-associated synaptic vesicles decreased in number from 53 to 30 per synapse from p17 to p34. Anatomical coupling, measured as PSD to ribbon distance, tightened predominantly on the pillar side. Ribbons became less spherical as long-axes lengthened only on the modiolar side of the IHC, from 372 to 541 nm. A decreasing gradient of synaptic ribbon size along the modiolar-pillar axis was detected only at p34 after aligning synapses of adjacent IHCs to a common reference frame (median volumes in nm³ × 10⁶: modiolar 4.87; pillar 2.38). The number of ribbon-associated synaptic vesicles scaled with ribbon size (range 67 to 346 per synapse at p34), thus acquiring a modiolar-pillar gradient at p34, but overall medians were similar at p17 (120) and p34 (127), like ribbon surface area (0.36 vs. 0.34 μm²). PD and PSD morphologies were tightly correlated to each other at individual synapses, more so at p34 than p17, but not to ribbon morphology. These observations suggest that PDs and PSDs mature according to different cues than ribbons, and that ribbon size may be more influenced by cues from the IHC than the surrounding tissue.

Size, Shape, and Distribution of Multivesicular Bodies in the Juvenile Rat Somatosensory Cortex: A 3D Electron Microscopy Study

Multivesicular bodies (MVBs) are membrane-bound organelles that belong to the endosomal pathway. They participate in the transport, sorting, storage, recycling, degradation, and release of multiple substances. They interchange cargo with other organelles and participate in their renovation and degradation. We have used focused ion beam milling and scanning electron microscopy (FIB-SEM) to obtain stacks of serial sections from the neuropil of the somatosensory cortex of the juvenile rat. Using dedicated software, we have 3D-reconstructed 1618 MVBs. The mean density of MVBs was 0.21 per cubic micron. They were unequally distributed between dendrites (39.14%), axons (18.16%), and nonsynaptic cell processes (42.70%). About one out of five MVBs (18.16%) were docked on mitochondria, representing the process by which the endosomal pathway participates in mitochondrial maintenance. Other features of MVBs, such as the presence of tubular protrusions (6.66%) or clathrin coats (19.74%) can also be interpreted in functional terms, since both are typical of early endosomes. The sizes of MVBs follow a lognormal distribution, with differences across cortical layers and cellular compartments. The mean volume of dendritic MVBs is more than twice as large as the volume of axonic MVBs. In layer I, they are smaller, on average, than in the other layers.

Inherited nuclear pore substructures template post-mitotic pore assembly

Nuclear envelope assembly during late mitosis includes rapid formation of several thousand complete nuclear pore complexes (NPCs). This efficient use of NPC components (nucleoporins or "NUPs") is essential for ensuring immediate nucleocytoplasmic communication in each daughter cell. We show that octameric subassemblies of outer and inner nuclear pore rings remain intact in the mitotic endoplasmic reticulum (ER) after NPC disassembly during prophase. These "inherited" subassemblies then incorporate into NPCs during post-mitotic pore formation. We further show that the stable subassemblies persist through multiple rounds of cell division and the accompanying rounds of NPC mitotic disassembly and post-mitotic assembly. De novo formation of NPCs from newly synthesized NUPs during interphase will then have a distinct initiation mechanism. We postulate that a yet-to-be-identified modification marks and "immortalizes" one or more components of the specific octameric outer and inner ring subcomplexes that then template post-mitotic NPC assembly during subsequent cell cycles.

The cytostome-cytopharynx complex of intracellular and extracellular amastigotes of Trypanosoma cruzi exhibit structural and functional differences

Endocytosis in Trypanosoma cruzi is mainly performed through a specialised membrane domain called cytostome-cytopharynx complex. Its ultrastructure and dynamics in endocytosis are well characterized in epimastigotes, being absent in trypomastigotes, that lack endocytic activity. Intracellular amastigotes also possess a cytostome-cytopharynx but participation in endocytosis of these forms is not clear. Extracellular amastigotes can be obtained from the supernatant of infected cells or in vitro amastigogenesis. These amastigotes share biochemical and morphological features with intracellular amastigotes but retain trypomastigote's ability to establish infection. We analysed and compared the ultrastructure of the cytostome-cytopharynx complex of intracellular amastigotes and extracellular amastigotes using high-resolution tridimensional electron microscopy techniques. We compared the endocytic ability of intracellular amastigotes, obtained through host cell lysis, with that of extracellular amastigotes. Intracellular amastigotes showed a cytostome-cytopharynx complex similar to epimastigotes'. However, after isolation, the complex undergoes ultrastructural modifications that progressively took to an impairment of endocytosis. Extracellular amastigotes do not possess a cytostome-cytopharynx complex nor the ability to endocytose. Those observations highlight morpho functional differences between intra and extracellular amastigotes regarding an important structure related to cell metabolism. TAKE AWAYS: T. cruzi intracellular amastigotes endocytose through the cytostome-cytopharynx complex. The cytostome-cytopharynx complex of intracellular amastigotes is ultrastructurally similar to the epimastigote. Intracellular amastigotes, once outside the host cell, disassembles the cytostome-cytopharynx membrane domain. Extracellular amastigotes do not possess a cytostome-cytopharynx either the ability to endocytose.

ATG4 family proteins drive phagophore growth independently of the LC3/GABARAP lipidation system

The sequestration of damaged mitochondria within double-membrane structures termed autophagosomes is a key step of PINK1/Parkin mitophagy. The ATG4 family of proteases are thought to regulate autophagosome formation exclusively by processing the ubiquitin-like ATG8 family (LC3/GABARAPs). We discover that human ATG4s promote autophagosome formation independently of their protease activity and of ATG8 family processing. ATG4 proximity networks reveal a role for ATG4s and their proximity partners, including the immune-disease protein LRBA, in ATG9A vesicle trafficking to mitochondria. Artificial intelligence-directed 3D electron microscopy of phagophores shows that ATG4s promote phagophore-ER contacts during the lipid-transfer phase of autophagosome formation. We also show that ATG8 removal during autophagosome maturation does not depend on ATG4 activity. Instead, ATG4s can disassemble ATG8-protein conjugates, revealing a role for ATG4s as deubiquitinating-like enzymes. These findings establish non-canonical roles of the ATG4 family beyond the ATG8 lipidation axis and provide an AI-driven framework for rapid 3D electron microscopy.

Collagen Pentablock Copolymers Form Smectic Liquid Crystals as Precursors for Mussel Byssus Fabrication

Protein-based biological materials are important role models for the design and fabrication of next generation advanced polymers. Marine mussels (Mytilus spp.) fabricate hierarchically structured collagenous fibers known as byssal threads via bottom-up supramolecular assembly of fluid protein precursors. The high degree of structural organization in byssal threads is intimately linked to their exceptional toughness and self-healing capacity. Here, we investigated the hypothesis that multidomain collagen precursor proteins, known as preCols, are stored in secretory vesicles as a colloidal liquid crystal (LC) phase prior to thread self-assembly. Using advanced electron microscopy methods, including scanning TEM and FIB-SEM, we visualized the detailed smectic preCol LC nanostructure in 3D, including various LC defects, confirming this hypothesis and providing quantitative insights into the mesophase structure. In light of these findings, we performed an in-depth comparative analysis of preCol protein sequences from multiple Mytilid species revealing that the smectic organization arises from an evolutionarily conserved ABCBA pentablock copolymer-like primary structure based on demarcations in hydropathy and charge distribution as well as terminal pH-responsive domains that trigger fiber formation. These distilled supramolecular assembly principles provide inspiration and strategies for sustainable assembly of nanostructured polymeric materials for potential applications in engineering and biomedical applications.

A Workflow for High-pressure Freezing and Freeze Substitution of the Caenorhabditis elegans Embryo for Ultrastructural Analysis by Conventional and Volume Electron Microscopy

The free-living nematode Caenorhabditis elegans is a popular model system for studying developmental biology. Here we describe a detailed protocol to high-pressure freeze the C. elegans embryo (either ex vivo after dissection, or within the intact worm) followed by quick freeze substitution. Processed samples are suitable for ultrastructural analysis by conventional electron microscopy (EM) or newer volume EM (vEM) approaches such as Focused Ion Beam Scanning Electron Microscopy (FIB-SEM). The ultrastructure of cellular features such as the nuclear envelope, chromosomes, endoplasmic reticulum and mitochondria are well preserved after these experimental procedures and yield accurate 3D models for visualization and analysis ( Chang et al., 2020 ). This protocol was used in the 3D reconstruction of membranes and chromosomes after pronuclear meeting in the C. elegans zygote ( Rahman et al., 2020 ).

FIB-SEM as a Volume Electron Microscopy Approach to Study Cellular Architectures in SARS-CoV-2 and Other Viral Infections: A Practical Primer for a Virologist

The visualization of cellular ultrastructure over a wide range of volumes is becoming possible by increasingly powerful techniques grouped under the rubric “volume electron microscopy” or volume EM (vEM). Focused ion beam scanning electron microscopy (FIB-SEM) occupies a “Goldilocks zone” in vEM: iterative and automated cycles of milling and imaging allow the interrogation of microns-thick specimens in 3-D at resolutions of tens of nanometers or less. This bestows on FIB-SEM the unique ability to aid the accurate and precise study of architectures of virus-cell interactions. Here we give the virologist or cell biologist a primer on FIB-SEM imaging in the context of vEM and discuss practical aspects of a room temperature FIB-SEM experiment. In an in vitro study of SARS-CoV-2 infection, we show that accurate quantitation of viral densities and surface curvatures enabled by FIB-SEM imaging reveals SARS-CoV-2 viruses preferentially located at areas of plasma membrane that have positive mean curvatures.

Three-dimensional structure analysis of melanocytes and keratinocytes in senile lentigo

Senile lentigo or age spots are hyperpigmented macules of skin that commonly develop following long-term exposure to ultraviolet radiation. This condition is caused by accumulation of large numbers of melanosomes (melanin granules) produced by melanocytes within neighboring keratinocytes. However, there is still no consensus regarding the melanosome transfer mechanism in senile lentigo. To date, most pathohistological studies of skin have been two-dimensional and do not provide detailed data on the complex interactions of the melanocyte-keratinocyte network involved in melanosome transfer. We performed a three-dimensional reconstruction of the epidermal microstructure in senile lentigo using three different microscopic modalities to visualize the topological melanocyte-keratinocyte relationship and melanosome distribution. Confocal laser microscopy images showed that melanocyte dendritic processes are more frequently branched and elongated in senile lentigo skin than in normal skin. Serial transmission electron micrographs showed that dendritic processes extend into intercellular spaces between keratinocytes. Focused ion beam-scanning electron micrographs showed that dendritic processes in senile lentigo encircle adjacent keratinocytes and accumulate large numbers of melanosomes. Moreover, melanosomes transferred to keratinocytes are present not only in the supranuclear area but throughout the perinuclear area except on the basal side. The use of these different microscopic methods helped to elucidate the three-dimensional morphology and topology of melanocytes and keratinocytes in senile lentigo. We show that the localization of melanosomes in dendritic processes to the region encircling recipient keratinocytes contributes to efficient melanosome transfer in senile lentigo.

Enhanced Cells Anchoring to Electrospun Hybrid Scaffolds With PHBV and HA Particles for Bone Tissue Regeneration

Hybrid materials combining organic and inorganic compounds used as scaffolds are highly beneficial in bone regeneration. In this study, we successfully produced by blend electrospinning poly(3-hydroxybutyric acid-co-3-hydrovaleric acid) (PHBV) scaffolds enriched with hydroxyapatite (HA) particles to biomimic bone tissue for improved and faster regeneration processes. The morphology, fiber diameters, and composition of the scaffolds were investigated by scanning electron microscopy (SEM) techniques followed by focused ion beam (FIB) sectioning to verify HA particles integration with PHBV fibers. In vitro cell culture was performed for 7 days and followed with the cell proliferation test (CellTiter-Blue^® Assay). Additionally, cell integration with the scaffold was visualized by confocal and SEM imaging. We developed a simple way of obtaining hybrid scaffolds by electrospinning PHBV solution with HA particles without any post-processing. The PHBV + HA scaffold enhanced cell proliferation and filopodia formation responsible for cell anchoring within the created 3D environment. The obtained results show the great potential in the development of hybrid scaffolds stimulating bone tissue regeneration.

Focused ion beam-SEM 3D analysis of mineralized osteonal bone: lamellae and cement sheath structures

The mineralized collagen fibril is the basic building block of bone, and hence is the key to understanding bone structure and function. Here we report imaging of mineralized pig bone samples in 3D using the focused ion beam-scanning electron microscope (FIB-SEM) under conditions that reveal the 67 nm D-banding of mineralized collagen fibrils. We show that in adult pig osteons, the lamellar bone comprises alternating layers with either collagen fibrils predominantly aligned in one direction, and layers in which fibrils are predominantly aligned in two directions. The cement sheath contains thin layers of both these motifs, but its dominant structural component comprises a very complex layer of fibrils predominantly aligned in three or more directions. The degree of mineralization of the cement sheath is comparable to that of the osteon interior. The extent of alignment (dispersion) of the collagen fibrils in the osteonal lamellar bone is significantly higher than in the cement sheath. Canaliculi within the cement sheath are mainly aligned parallel to the cement sheath boundary, whereas in the lamellar bone they are mainly aligned perpendicular to the lamellar boundaries. This study further characterizes the presence of two types of collagen fibril arrangements previously identified in demineralized lamellar bone from other species. The simple sample preparation procedure for mineralized bone and the lower risk of introducing artifacts opens the possibility of using FIB-SEM to study more samples, to obtain automatic quantitative information on collagen fibril organization and to evaluate the degrees of mineralization all in relatively large volumes of bone.