Advances in microscopy techniques

Deconvolution Microscopy: A Platform for Rapid On-Site Evaluation (ROSE) of Fine Needle Aspiration (FNA) Specimens that Enables Recovery of the Sample

CONTEXT

Rapid on-site evaluation (ROSE) optimizes the performance of cytology, but requires skilled handling, and smearing can make the material unavailable for some ancillary tests. There is a need to facilitate ROSE without sacrificing part of the sample.

OBJECTIVE

We evaluated the image quality of inexpensive deconvolution fluorescence microscopy for optically sectioning non-smeared FNA tissue fragments.

DESIGN

A portion of residual material from 14 FNA samples was stained for 3 minutes in Hoechst 33342 and Sypro^TM Red to label DNA and protein respectively, transferred to an imaging chamber, and imaged at 200X or 400X magnification at 1 micron intervals using a GE DeltaVision inverted fluorescence microscope. A deconvolution algorithm was applied to remove out of plane signal, and resulting images were inverted and pseudocolored to resemble an H&E section. Five cytopathologists blindly diagnosed 2 to 4 representative image stacks per case (total 70 evaluations), and later compared them to conventional epifluorescent images.

RESULTS

Accurate definitive diagnoses were rendered in 45 of 70 (64%) total evaluations; equivocal diagnoses (atypical or suspicious) were made in 21 of 70 (30%). There were two false positive and two false negative "definite" diagnoses in three cases (4/70; 6%). Cytopathologists preferred deconvolved images compared to raw images (p< 0.01). The imaged fragments were recovered and prepared into a ThinPrep or cell block without discernable alteration.

CONCLUSIONS

Deconvolution improves image quality of FNA fragments compared to epifluorescence, often allowing definitive diagnosis while enabling the ROSE material to be subsequently triaged.

Skin Complications of Diabetes Mellitus Revealed by Polarized Hyperspectral Imaging and Machine Learning

Aging and diabetes lead to protein glycation and cause dysfunction of collagen-containing tissues. The accompanying structural and functional changes of collagen significantly contribute to the development of various pathological malformations affecting the skin, blood vessels, and nerves, causing a number of complications, increasing disability risks and threat to life. In fact, no methods of non-invasive assessment of glycation and associated metabolic processes in biotissues or prediction of possible skin complications, e.g., ulcers, currently exist for endocrinologists and clinical diagnosis. In this publication, utilizing emerging photonics-based technology, innovative solutions in machine learning, and definitive physiological characteristics, we introduce a diagnostic approach capable of evaluating the skin complications of diabetes mellitus at the very earlier stage. The results of the feasibility studies, as well as the actual tests on patients with diabetes and healthy volunteers, clearly show the ability of the approach to differentiate diabetic and control groups. Furthermore, the developed in-house polarization-based hyperspectral imaging technique accomplished with the implementation of the artificial neural network provides new horizons in the study and diagnosis of age-related diseases.

Molecularly targeted nanoparticles: an emerging tool for evaluation of expression of the receptor for advanced glycation end products in a murine model of peripheral artery disease

Background

Molecular imaging with molecularly targeted probes is a powerful tool for studying the spatio-temporal interactions between complex biological processes. The pivotal role of the receptor for advanced glycation end products (RAGE), and its involvement in numerous pathological processes, aroused the demand for RAGE-targeted imaging in various diseases. In the present study, we evaluated the use of a diagnostic imaging agent for RAGE quantification in an animal model of peripheral artery disease, a multimodal dual-labeled probe targeted at RAGE (MMIA-CML).

Methods

PAMAM dendrimer was conjugated with Nε-carboxymethyl-lysine (CML) modified albumin to synthesize the RAGE-targeted probe. A control untargeted agent carried native non-modified human albumin (HSA). Bifunctional p-SCN-Bn-NOTA was used to conjugate the ⁶⁴Cu radioisotope. Surgical right femoral artery ligation was performed on C57BL/6 male mice. One week after femoral artery ligation, mice were injected with MMIA-CML or MMIA-HSA labeled with ⁶⁴Cu radioisotope and 60 min later in vivo microPET-CT imaging was performed. Immediately after PET imaging studies, the murine hindlimb muscle tissues were excised and prepared for gene and protein expression analysis. RAGE gene and protein expression was assessed using real-time qPCR and Western blot technique respectively. To visualize RAGE expression in excised tissues, microscopic fluorescence imaging was performed using RAGE-specific antibodies and RAGE-targeted and -control MMIA.

Results

Animals subjected to PET imaging exhibited greater MMIA-CML uptake in ischemic hindlimbs than non-ischemic hindlimbs. We observed a high correlation between fluorescent signal detection and radioactivity measurement. Significant RAGE gene and protein overexpression were observed in ischemic hindlimbs compared to non-ischemic hindlimbs at one week after surgical ligation. Fluorescence microscopic staining revealed significantly increased uptake of RAGE-targeted nanoparticles in both ischemic and non-ischemic muscle tissues compared to the control probe but at a higher level in ischemic hindlimbs. Ischemic tissue exhibited explicit RAGE dyeing following anti-RAGE antibody and high colocalization with the MMIA-CML targeted at RAGE.

Conclusions

The present results indicate increased expression of RAGE in the ischemic hindlimb and enable the use of multimodal nanoparticles in both in vitro and in vivo experimental models, creating the possibility for imaging structural and functional changes with a RAGE-targeted tracer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s11658-021-00253-0.

Rethinking resolution estimation in fluorescence microscopy: from theoretical resolution criteria to super-resolution microscopy

Resolution is undoubtedly the most important parameter in optical microscopy by providing an estimation on the maximum resolving power of a certain optical microscope. For centuries, the resolution of an optical microscope is generally considered to be limited only by the numerical aperture of the optical system and the wavelength of light. However, since the invention and popularity of various advanced fluorescence microscopy techniques, especially super-resolution fluorescence microscopy, many new methods have been proposed for estimating the resolution, leading to confusions for researchers who need to quantify the resolution of their fluorescence microscopes. In this paper, we firstly summarize the early concepts and criteria for predicting the resolution limit of an ideal optical system. Then, we discuss some important influence factors that deteriorate the resolution of a certain fluorescence microscope. Finally, we provide methods and examples on how to measure the resolution of a fluorescence microscope from captured fluorescence images. This paper aims to answer as best as possible the theoretical and practical issues regarding the resolution estimation in fluorescence microscopy.

On detection thresholds-a review on diagnostic approaches in the infectious disease laboratory and the interpretation of their results

Diagnostic testing in the infectious disease laboratory facilitates decision-making by physicians at the bedside as well as epidemiological assessments and surveillance at study level. Problems may arise if test results are uncritically considered as being the same as the unknown true value. To allow a better understanding, the influence of external factors on the interpretation of test results is introduced with the example of prevalence, followed by the presentation of strengths and weaknesses of important techniques in the infectious disease laboratory like microscopy, cultural diagnostics, serology, mass spectrometry, nucleic acid amplification and hypothesis-free metagenomic sequencing with focus on basic, high-technology and potential future approaches. Special problems like multiplex testing as well as uncertainty of test evaluations, if no gold standard is available, are also stressed with a final glimpse on emerging future technologies for the infectious disease laboratory. In the conclusions, suitability for point-of-care-testing and field laboratory applications is summarized. The aim is to illustrate the limitations of diagnostic accuracy to both clinicians and study planners and to stress the importance of close cooperation with experts in laboratory disciplines so as to avoid potentially critical misunderstandings due to inappropriate interpretation of diagnostic test results.

The coming paradigm shift: A transition from manual to automated microscopy

The field of pathology has used light microscopy (LM) extensively since the mid-19^th century for examination of histological tissue preparations. This technology has remained the foremost tool in use by pathologists even as other fields have undergone a great change in recent years through new technologies. However, as new microscopy techniques are perfected and made available, this reliance on the standard LM will likely begin to change. Advanced imaging involving both diffraction-limited and subdiffraction techniques are bringing nondestructive, high-resolution, molecular-level imaging to pathology. Some of these technologies can produce three-dimensional (3D) datasets from sampled tissues. In addition, block-face/tissue-sectioning techniques are already providing automated, large-scale 3D datasets of whole specimens. These datasets allow pathologists to see an entire sample with all of its spatial information intact, and furthermore allow image analysis such as detection, segmentation, and classification, which are impossible in standard LM. It is likely that these technologies herald a major paradigm shift in the field of pathology.

Nuquantus: Machine learning software for the characterization and quantification of cell nuclei in complex immunofluorescent tissue images

Determination of fundamental mechanisms of disease often hinges on histopathology visualization and quantitative image analysis. Currently, the analysis of multi-channel fluorescence tissue images is primarily achieved by manual measurements of tissue cellular content and sub-cellular compartments. Since the current manual methodology for image analysis is a tedious and subjective approach, there is clearly a need for an automated analytical technique to process large-scale image datasets. Here, we introduce Nuquantus (Nuclei quantification utility software) - a novel machine learning-based analytical method, which identifies, quantifies and classifies nuclei based on cells of interest in composite fluorescent tissue images, in which cell borders are not visible. Nuquantus is an adaptive framework that learns the morphological attributes of intact tissue in the presence of anatomical variability and pathological processes. Nuquantus allowed us to robustly perform quantitative image analysis on remodeling cardiac tissue after myocardial infarction. Nuquantus reliably classifies cardiomyocyte versus non-cardiomyocyte nuclei and detects cell proliferation, as well as cell death in different cell classes. Broadly, Nuquantus provides innovative computerized methodology to analyze complex tissue images that significantly facilitates image analysis and minimizes human bias.

Material properties of biofilms-a review of methods for understanding permeability and mechanics

Microorganisms can form biofilms, which are multicellular communities surrounded by a hydrated extracellular matrix of polymers. Central properties of the biofilm are governed by this extracellular matrix, which provides mechanical stability to the 3D biofilm structure, regulates the ability of the biofilm to adhere to surfaces, and determines the ability of the biofilm to adsorb gases, solutes, and foreign cells. Despite their critical relevance for understanding and eliminating of biofilms, the materials properties of the extracellular matrix are understudied. Here, we offer the reader a guide to current technologies that can be utilized to specifically assess the permeability and mechanical properties of the biofilm matrix and its interacting components. In particular, we highlight technological advances in instrumentation and interactions between multiple disciplines that have broadened the spectrum of methods available to conduct these studies. We review pioneering work that furthers our understanding of the material properties of biofilms.

The diagnostic pathology of the nuclear envelope in human cancers

Cancer is still diagnosed on the basis of altered tissue and cellular morphology. The criteria that pathologists use for diagnosis include many morphologically distinctive alterations in the nuclear envelope (NE). With the expectation that diagnostic NE changes will have biological relevance to cancer, a classification of the various types of NE structural changes into three groups is proposed. The first group predicts chromosomal instability. The changes in this group include pleomorphism of lamina size and shape, as if constraints to maintain a spherical shape were lost. Also characteristic of chromosomal instability are the presence of micronuclei, a specific structural feature likely related to the newly described physiology of chromothripsis. The second group is predicted to be functionally important during clonal evolution, because the NE changes in this group are conserved during the clonal evolution of genetically unstable tumors. Two examples of this group include increased ratio of nuclear volume to cytoplasmic volume and the relatively fragile nuclei of small-cell carcinomas. The third and most interesting group develops in a near-diploid, genetically stable background. Many of these (perhaps ultimately all) are directly related to the activation of particular oncogenes. The changes in this group so far include long inward folds of the NE and spherical invaginations of cytoplasm projecting partially into the nucleus ("intranuclear cytoplasmic inclusions"). This group is exemplified by papillary thyroid carcinoma in which RET and TRK tyrosine kinases, and probably B-Raf mutations, directly lead to diagnostic longitudinal folds of the lamina ("nuclear grooves") and intranuclear cytoplasmic inclusions. B-Raf activation may also be linked to intranuclear cytoplasmic inclusions in melanoma and to nuclear grooves in Langerhans cell histiocytosis. Nuclear grooves in granulosa cell tumor may be related to mutations in the FOXL2 oncogene. Uncovering the precise mechanistic basis for any of these lamina alterations would provide a valuable objective means for improving diagnosis, and will likely reflect new types of functional changes, relevant to particular forms of cancer.

Five Top Stories in Cytopathology

Context.—Cytology relies heavily on morphology to make diagnoses, and morphologic criteria have not changed much in recent years. The field is being shaped predominantly by new techniques for imaging and for acquiring and processing samples, advances in molecular diagnosis and therapeutics, and regulatory issues.

Objective.—To review the importance of classical morphology in the future of cytopathology, to identify areas in which cytology is expanding or contracting in its scope, and to identify factors that are shaping the field.

Data Sources.—Literature review.

Conclusions.—Five stories paint a picture in which classical cytomorphology will continue to have essential importance, both for diagnosis and for improving our understanding of cancer biology. New endoscopy and imaging techniques are replacing surgical biopsies with cytology samples. New molecularly targeted therapies offer a chance for cytology to play a major role, but they pose new challenges. New molecular tests have the potential to synergize with, but not replace, morphologic interpretation of thyroid fine-needle aspirations. Ultrasound-guided fine-needle aspiration performed by cytopathologists is opening a new field of “interventional cytopathology” with unique value. For the productive evolution of the field, it will be important for cytopathologists to play an active role in clinical trials that document the ability of cytology to achieve cost-effective health care outcomes.

Intraoperative fluorescent imaging of intracranial tumors: a review

A review of fluorescent imaging for intracranial neoplasms is presented. Complete resection of brain cancer is seldom possible because of the goal to preserve brain tissue and the inability to visualize individual infiltrative tumor cells. Verification of histology and identification of tumor invasion in macroscopically normal-appearing brain tissue determine prognosis after resection of malignant gliomas. Therefore, imaging modalities aim to facilitate intraoperative decision-making. Intraoperative fluorescent imaging techniques have the potential to enable precise histopathologic diagnosis and to detect tumor remnants in the operative field. Macroscopic fluorescence imaging is effective for gross tumor detection. Microscopic imaging techniques enhance the sensitivity of the macroscopic observations and provide real-time histological information. Further development of clinical grade fluorescent agents specifically targeting tumor cells could improve the diagnostic and prognostic yield of intraoperative imaging.

Ca2+-imaging techniques to analyze Ca2+ signaling in cells and to monitor neuronal activity in the retina

Ca(2+) is an important regulator of many cell functions including proliferation, apoptosis, movements, secretion, contraction, excitation, and differentiation. The regulation of these different cell functions is encoded by the specific temporal and spatial distribution of Ca(2+) signals. In degenerative diseases mutations can lead to changes in cell functions in the worst case to apoptosis. Thus analysis of signals arising as changes in intracellular free Ca(2+) represent an important step towards the understanding of mutation-dependent or environmental impact into cell function. The classic approach to study changes in intracellular free Ca(2+) is the measurement of intracellular Ca(2+) by using Ca(2+)-sensitive fluorescence dyes in conjunction with fluorescence microscopy as a method called Ca(2+) imaging. In this chapter the basic method and a short theoretical background will be provided to perform Ca(2+)-imaging experiments. As a model cultured retinal pigment epithelial cells will be used. The basic steps of the method are the loading of the cells with the fluorescence dye by incubation with a membrane permeable ester of the dye. The next step would be the application of an agonist which can be further analyzed by blockers of enzymes or by manipulating the different Ca(2+)-storing compartments which contribute to changes in intracellular free Ca(2+). At the end of an experiment an on-cell type of calibration will be performed to calculate the underlying concentration of intracellular free Ca(2+). Furthermore, the successful calibration of an experiment can be used as a measure of a reliable experiment. In addition to that, three examples for basic experiments will be given which can lead to a first insight into the mechanism underlying changes in cytosolic free Ca(2+)as a second messenger.

A molecular imaging primer: modalities, imaging agents, and applications

Molecular imaging is revolutionizing the way we study the inner workings of the human body, diagnose diseases, approach drug design, and assess therapies. The field as a whole is making possible the visualization of complex biochemical processes involved in normal physiology and disease states, in real time, in living cells, tissues, and intact subjects. In this review, we focus specifically on molecular imaging of intact living subjects. We provide a basic primer for those who are new to molecular imaging, and a resource for those involved in the field. We begin by describing classical molecular imaging techniques together with their key strengths and limitations, after which we introduce some of the latest emerging imaging modalities. We provide an overview of the main classes of molecular imaging agents (i.e., small molecules, peptides, aptamers, engineered proteins, and nanoparticles) and cite examples of how molecular imaging is being applied in oncology, neuroscience, cardiology, gene therapy, cell tracking, and theranostics (therapy combined with diagnostics). A step-by-step guide to answering biological and/or clinical questions using the tools of molecular imaging is also provided. We conclude by discussing the grand challenges of the field, its future directions, and enormous potential for further impacting how we approach research and medicine.

Brain slice on a chip: opportunities and challenges of applying microfluidic technology to intact tissues

Isolated brain tissue, especially brain slices, are valuable experimental tools for studying neuronal function at the network, cellular, synaptic, and single channel levels. Neuroscientists have refined the methods for preserving brain slice viability and function and converged on principles that strongly resemble the approach taken by engineers in developing microfluidic devices. With respect to brain slices, microfluidic technology may 1) overcome the traditional limitations of conventional interface and submerged slice chambers and improve oxygen/nutrient penetration into slices, 2) provide better spatiotemporal control over solution flow/drug delivery to specific slice regions, and 3) permit successful integration with modern optical and electrophysiological techniques. In this review, we highlight the unique advantages of microfluidic devices for in vitro brain slice research, describe recent advances in the integration of microfluidic devices with optical and electrophysiological instrumentation, and discuss clinical applications of microfluidic technology as applied to brain slices and other non-neuronal tissues. We hope that this review will serve as an interdisciplinary guide for both neuroscientists studying brain tissue in vitro and engineers as they further develop microfluidic chamber technology for neuroscience research.

Review of advanced imaging techniques

Pathology informatics encompasses digital imaging and related applications. Several specialized microscopy techniques have emerged which permit the acquisition of digital images (“optical biopsies”) at high resolution. Coupled with fiber-optic and micro-optic components, some of these imaging techniques (e.g., optical coherence tomography) are now integrated with a wide range of imaging devices such as endoscopes, laparoscopes, catheters, and needles that enable imaging inside the body. These advanced imaging modalities have exciting diagnostic potential and introduce new opportunities in pathology. Therefore, it is important that pathology informaticists understand these advanced imaging techniques and the impact they have on pathology. This paper reviews several recently developed microscopic techniques, including diffraction-limited methods (e.g., confocal microscopy, 2-photon microscopy, 4Pi microscopy, and spatially modulated illumination microscopy) and subdiffraction techniques (e.g., photoactivated localization microscopy, stochastic optical reconstruction microscopy, and stimulated emission depletion microscopy). This article serves as a primer for pathology informaticists, highlighting the fundamentals and applications of advanced optical imaging techniques.

Correlative light and electron microscopy (CLEM) as a tool to visualize microinjected molecules and their eukaryotic sub-cellular targets

The eukaryotic cell relies on complex, highly regulated, and functionally distinct membrane bound compartments that preserve a biochemical polarity necessary for proper cellular function. Understanding how the enzymes, proteins, and cytoskeletal components govern and maintain this biochemical segregation is therefore of paramount importance. The use of fluorescently tagged molecules to localize to and/or perturb subcellular compartments has yielded a wealth of knowledge and advanced our understanding of cellular regulation. Imaging techniques such as fluorescent and confocal microscopy make ascertaining the position of a fluorescently tagged small molecule relatively straightforward, however the resolution of very small structures is limited. On the other hand, electron microscopy has revealed details of subcellular morphology at very high resolution, but its static nature makes it difficult to measure highly dynamic processes with precision. Thus, the combination of light microscopy with electron microscopy of the same sample, termed Correlative Light and Electron Microscopy (CLEM), affords the dual advantages of ultrafast fluorescent imaging with the high-resolution of electron microscopy. This powerful technique has been implemented to study many aspects of cell biology. Since its inception, this procedure has increased our ability to distinguish subcellular architectures and morphologies at high resolution. Here, we present a streamlined method for performing rapid microinjection followed by CLEM (Fig. 1). The microinjection CLEM procedure can be used to introduce specific quantities of small molecules and/or proteins directly into the eukaryotic cell cytoplasm and study the effects from millimeter to multi-nanometer resolution (Fig. 2). The technique is based on microinjecting cells grown on laser etched glass gridded coverslips affixed to the bottom of live cell dishes and imaging with both confocal fluorescent and electron microscopy. Localization of the cell(s) of interest is facilitated by the grid pattern, which is easily transferred, along with the cells of interest, to the Epon resin used for immobilization of samples and sectioning prior to electron microscopy analysis (Fig. 3). Overlay of fluorescent and EM images allows the user to determine the subcellular localization as well as any morphological and/or ultrastructural changes induced by the microinjected molecule of interest (Fig. 4). This technique is amenable to time points ranging from ≤5 s up to several hours, depending on the nature of the microinjected sample.

Advances in microscopy and complementary imaging techniques to assess the fate of drugs ex vivo in respiratory drug delivery: an invited paper

The technical advances in microscopy imaging techniques have been applied to assess the fate of drugs for researching respiratory drug delivery in ex vivo and in vivo experiments. Recent developments in optical imaging (confocal microscopy, multi-photon microscopy, fluorescence imaging (FLI) and bioluminescence imaging (BLI)), and in non-optical imaging (magnetic resonance imaging (MRI), computing tomography (CT), positron-emission tomography (PET) and single-photon-emission computed tomography (SPECT)) are presented with their derivative medical devices. Novel microscopy have been utilized to address many biological questions in basic research and are becoming powerful clinical tools for non-invasive objective diagnosis, guided treatment, and monitoring therapies. The goal of this paper is to present recent advances in microscopy imaging techniques and to discuss their novel applications in respiratory drug delivery imaging.

Dissecting T-cell activation with high-resolution live-cell microscopy

Results from live-cell microscopy suggest that the behaviour of isolated components of the T-cell activation machinery in vitro does not represent the reality inside cells. Understanding the cellular-scale dynamics of microcluster migration can only be accomplished by in situ observation. Developments in 'super-resolution' microscopy have permitted investigators to move beyond tracking the movements of individual molecules, allowing the recognition of protein islands and nanodomains present in quiescent and active T cells. Many high-resolution techniques have their own susceptibilities to artefacts, so it is important to take a multifaceted approach to confirm results. A major challenge for the future will be to integrate all the new information into a coherent model of antigen recognition and T-cell activation.

Recent advances in single-cell studies of gene regulation

A mechanistic understanding of gene regulatory network dynamics requires quantitative single-cell data of multiple network components in response to well-defined perturbations. Recent advances in the development of fluorescent biomarkers for proteins, detection of RNA and interactions, microfluidic technology, and high-resolution imaging have set the stage for a host of new studies that elucidate the important roles of stochasticity and cell-cell variability in response to external perturbations. In this review, we briefly describe methods for high-resolution visualization and the control of gene expression, along with application of these novel methods to recent studies involving gene networks.

Confocal Laser Endomicroscopy: A Primer for Pathologists

Context.—The advent of new endoscopic optical techniques is likely to change pathologists' role in diagnosis.

Objective.—To describe how confocal laser endomicroscopy (CLE) works, show its advantages and limitations compared to cytohistologic biopsy, and explore how it may affect the practice of pathology.

Data Sources.—Literature review.

Conclusions.—Confocal laser endomicroscopy is proving its ability to provide histology-like images of tissues in vivo to help avoid risks and costs of conventional biopsies. Confocal imaging restricts light to 1 plane, emulating a paraffin section, and topical or systemic optical contrast agents allow subcellular resolution. New contrast agents could theoretically permit molecular characterization. In vivo imaging has begun to demonstrate novel, dynamic types of diagnostic features. Decreased histologic biopsies can be anticipated for a few scenarios. Significant limitations of CLE include the inability to create a tissue archive for broad molecular classification, suboptimal contrast agents, small fields of view and shallow penetration, paucity of clinical validation studies, and problems with reimbursement. Confocal laser endomicroscopy exposes new opportunities for pathologists: CLE technologies can be exploited in pathology, and diagnostic criteria expanded based on endoscopists' discoveries. Potential synergy exists between CLE and cytology, allowing the low-magnification diagnostic architectural changes by CLE and cytomorphology to emulate the full diagnostic information in a histologic biopsy while providing an archive of material for molecular or immunohistochemical studies. Confocal laser endomicroscopy will decrease some types of biopsies, but offers an opportunity for pathologists to find new ways to provide value and improve patient care.

New insights into HTLV-1 particle structure, assembly, and Gag-Gag interactions in living cells

Human T-cell leukemia virus type 1 (HTLV-1) has a reputation for being extremely difficult to study in cell culture. The challenges in propagating HTLV-1 has prevented a rigorous analysis of how these viruses replicate in cells, including the detailed steps involved in virus assembly. The details for how retrovirus particle assembly occurs are poorly understood, even for other more tractable retroviral systems. Recent studies on HTLV-1 using state-of-the-art cryo-electron microscopy and fluorescence-based biophysical approaches explored questions related to HTLV-1 particle size, Gag stoichiometry in virions, and Gag-Gag interactions in living cells. These results provided new and exciting insights into fundamental aspects of HTLV-1 particle assembly-which are distinct from those of other retroviruses, including HIV-1. The application of these and other novel biophysical approaches promise to provide exciting new insights into HTLV-1 replication.