350-μm side-view optical probe for imaging the murine brain in vivo from the cortex to the hypothalamus

Recent advances in intravital microscopy for investigation of dynamic cellular behavior in vivo

Currently, most biological research relies on conventional experimental techniques that allow only static analyses at certain time points in vitro or ex vivo. However, if one could visualize cellular dynamics in living organisms, that would provide a unique opportunity to study key biological phenomena in vivo. Intravital microscopy (IVM) encompasses diverse optical systems for direct viewing of objects, including biological structures and individual cells in live animals. With the current development of devices and techniques, IVM addresses important questions in various fields of biological and biomedical sciences. In this mini-review, we provide a general introduction to IVM and examples of recent applications in the field of immunology, oncology, and vascular biology. We also introduce an advanced type of IVM, dubbed real-time IVM, equipped with video-rate resonant scanning. Since the real-time IVM can render cellular dynamics with high temporal resolution in vivo, it allows visualization and analysis of rapid biological processes.

3D Whole-Brain Imaging Approaches to Study Brain Tumors

Simple Summary

Brain tumors integrate into the brain and consist of tumor cells with different molecular alterations. During brain tumor pathogenesis, a variety of cell types surround the tumors to either inhibit or promote tumor growth. These cells are collectively referred to as the tumor microenvironment. Three-dimensional and/or longitudinal visualization approaches are needed to understand the growth of these tumors in time and space. In this review, we present three imaging modalities that are suitable or that can be adapted to study the volumetric distribution of malignant or tumor-associated cells in the brain. In addition, we highlight the potential clinical utility of some of the microscopy approaches for brain tumors using exemplars from solid tumors.

Abstract

Although our understanding of the two-dimensional state of brain tumors has greatly expanded, relatively little is known about their spatial structures. The interactions between tumor cells and the tumor microenvironment (TME) occur in a three-dimensional (3D) space. This volumetric distribution is important for elucidating tumor biology and predicting and monitoring response to therapy. While static 2D imaging modalities have been critical to our understanding of these tumors, studies using 3D imaging modalities are needed to understand how malignant cells co-opt the host brain. Here we summarize the preclinical utility of in vivo imaging using two-photon microscopy in brain tumors and present ex vivo approaches (light-sheet fluorescence microscopy and serial two-photon tomography) and highlight their current and potential utility in neuro-oncology using data from solid tumors or pathological brain as examples.

Scanning and Actuation Techniques for Cantilever-Based Fiber Optic Endoscopic Scanners-A Review

Endoscopes are used routinely in modern medicine for in-vivo imaging of luminal organs. Technical advances in the micro-electro-mechanical system (MEMS) and optical fields have enabled the further miniaturization of endoscopes, resulting in the ability to image previously inaccessible small-caliber luminal organs, enabling the early detection of lesions and other abnormalities in these tissues. The development of scanning fiber endoscopes supports the fabrication of small cantilever-based imaging devices without compromising the image resolution. The size of an endoscope is highly dependent on the actuation and scanning method used to illuminate the target image area. Different actuation methods used in the design of small-sized cantilever-based endoscopes are reviewed in this paper along with their working principles, advantages and disadvantages, generated scanning patterns, and applications.

Micro-endoscopy for Live Small Animal Fluorescent Imaging

Intravital microscopy has emerged as a powerful technique for the fluorescent visualization of cellular- and subcellular-level biological processes in vivo. However, the size of objective lenses used in standard microscopes currently makes it difficult to access internal organs with minimal invasiveness in small animal models, such as mice. Here we describe front- and side-view designs for small-diameter endoscopes based on gradient-index lenses, their construction, their integration into laser scanning confocal microscopy platforms, and their applications for in vivo imaging of fluorescent cells and microvasculature in various organs, including the kidney, bladder, heart, brain, and gastrointestinal tracts, with a focus on the new techniques developed for each imaging application. The combination of novel fluorescence techniques with these powerful imaging methods promises to continue providing novel insights into a variety of diseases.

Endoscopic Optical Imaging Technologies and Devices for Medical Purposes: State of the Art

The growth and development of optical components and, in particular, the miniaturization of micro-electro-mechanical systems (MEMSs), has motivated and enabled researchers to design smaller and smaller endoscopes. The overarching goal of this work has been to image smaller previously inaccessible luminal organs in real time, at high resolution, in a minimally invasive manner that does not compromise the comfort of the subject, nor introduce additional risk. Thus, an initial diagnosis can be made, or a small precancerous lesion may be detected, in a small-diameter luminal organ that would not have otherwise been possible. Continuous advancement in the field has enabled a wide range of optical scanners. Different scanning techniques, working principles, and the applications of endoscopic scanners are summarized in this review.

Label-free multiphoton imaging of β-amyloid plaques in Alzheimer's disease mouse models

β -Amyloid ( A β ) plaque, representing the progressive accumulation of the protein that mainly consists of A β , is one of the prominent pathological hallmarks of Alzheimer's disease (AD). Label-free imaging of A β plaques holds the potential to be a histological examination tool for diagnosing AD. We applied label-free multiphoton microscopy to identify extracellular A β plaque as well as intracellular A β accumulation for the first time from AD mouse models. We showed that a two-photon-excited fluorescence signal is a sensitive optical marker for revealing the spatial-temporal progression and the surrounding morphological changes of A β deposition, which demonstrated that both extracellular and intracellular A β accumulations play an important role in the progression of AD. Moreover, combined with a custom-developed image-processing program, we established a rapid method to visualize different degrees of A β deposition by color coding. These results provide an approach for investigating pathophysiology of AD that can complement traditional biomedical procedures.

In vivo longitudinal depth-wise visualization of tumorigenesis by needle-shaped side-view confocal endomicroscopy

In vivo, longitudinal observation of tumorigenesis in a live mouse model over an extended time period has been actively pursued to obtain a better understanding of the cellular and molecular mechanism in a highly complex tumor microenvironment. However, common intravital imaging approaches based on a conventional laser scanning confocal or a two-photon microscope have been mostly limited to the observation of superficial parts of the solid tumor tissue. In this work, we implemented a small diameter needle-shaped side-view confocal endomicroscope that can be directly inserted into a solid tumor in a minimally-invasive manner in vivo. By inserting the side-view endomicroscope into the breast tumor from the surface, we achieved in vivo depth-wise cellular-level visualization of microvasculature and fluorescently labeled tumor cells located deeply inside the tumor. In addition, we successfully performed longitudinal depth-wise visualization of a growing breast tumor over three weeks in a live mouse model, which revealed dynamic changes in microvasculature such as a decreasing amount of intratumoral blood vessels over time.

Rapid, label-free identification of cerebellar structures using multiphoton microscopy

The cerebellum is the prominent laminar structure of the mammalian brain that has been implicated in various psychiatric and neurological diseases. Although clinical brain imaging techniques have provided precise anatomic images of cerebellar structures, a definitive diagnosis still requires adequate resolution to identify individual layers in cerebellar cortex, the extent of tumor, even requires the histological tissue examination during surgical procedures. In this study, multiphoton microscopy (MPM), based on second harmonic generation (SHG) and two-photon excited fluorescence (TPEF), was perform on the rat cerebellar structures and pathology with the combination of image analysis methods. Results show that MPM can reveal the cerebellar vermis, hemispheres, medulla, and ventricle, as well as axon bundles, Purkinje cells, capillaries, and the pia mater of the cerebellum. Together with custom-developed image processing algorithms, MPM could further differentiate between the gray and white matter, as well as evaluate the Purkinje cell layer, identify the cerebellar tumor boundary, and distinguish between the tumor core and peritumor regions. Our results establish a direct visualization and rapid assessment approach for the cerebellar structures, as well as suggest the feasibility of in vivo multiphoton microendoscopes and fiberscopes as clinical tools for neuropathological diagnoses.

In Vivo Voltage-Sensitive Dye Imaging of Subcortical Brain Function

The whisker system of rodents is an excellent model to study peripherally evoked neural activity in the brain. Discrete neural modules represent each whisker in the somatosensory cortex (“barrels”), thalamus (“barreloids”), and brain stem (“barrelettes”). Stimulation of a single whisker evokes neural activity sequentially in its corresponding barrelette, barreloid, and barrel. Conventional optical imaging of functional activation in the brain is limited to surface structures such as the cerebral cortex. To access subcortical structures and image sensory-evoked neural activity, we designed a needle-based optical system using gradient-index (GRIN) rod lens. We performed voltage-sensitive dye imaging (VSDi) with GRIN rod lens to visualize neural activity evoked in the thalamic barreloids by deflection of whiskers in vivo. We stimulated several whiskers together to determine the sensitivity of our approach in differentiating between different barreloid responses. We also carried out stimulation of different whiskers at different times. Finally, we used muscimol in the barrel cortex to silence the corticothalamic inputs while imaging in the thalamus. Our results show that it is possible to obtain functional maps of the sensory periphery in deep brain structures such as the thalamic barreloids. Our approach can be broadly applicable to functional imaging of other core brain structures.

In vivo fluorescence microscopy: lessons from observing cell behavior in their native environment

Microscopic imaging techniques to visualize cellular behaviors in their natural environment play a pivotal role in biomedical research. Here, we review how recent technical advances in intravital microscopy have enabled unprecedented access to cellular physiology in various organs of mice in normal and diseased states.

Controlling feeding behavior by chemical or gene-directed targeting in the brain: what's so spatial about our methods?

Intracranial chemical injection (ICI) methods have been used to identify the locations in the brain where feeding behavior can be controlled acutely. Scientists conducting ICI studies often document their injection site locations, thereby leaving kernels of valuable location data for others to use to further characterize feeding control circuits. Unfortunately, this rich dataset has not yet been formally contextualized with other published neuroanatomical data. In particular, axonal tracing studies have delineated several neural circuits originating in the same areas where ICI injection feeding-control sites have been documented, but it remains unclear whether these circuits participate in feeding control. Comparing injection sites with other types of location data would require careful anatomical registration between the datasets. Here, a conceptual framework is presented for how such anatomical registration efforts can be performed. For example, by using a simple atlas alignment tool, a hypothalamic locus sensitive to the orexigenic effects of neuropeptide Y (NPY) can be aligned accurately with the locations of neurons labeled by anterograde tracers or those known to express NPY receptors or feeding-related peptides. This approach can also be applied to those intracranial "gene-directed" injection (IGI) methods (e.g., site-specific recombinase methods, RNA expression or interference, optogenetics, and pharmacosynthetics) that involve viral injections to targeted neuronal populations. Spatial alignment efforts can be accelerated if location data from ICI/IGI methods are mapped to stereotaxic brain atlases to allow powerful neuroinformatics tools to overlay different types of data in the same reference space. Atlas-based mapping will be critical for community-based sharing of location data for feeding control circuits, and will accelerate our understanding of structure-function relationships in the brain for mammalian models of obesity and metabolic disorders.

Optical fine-needle imaging biopsy of the brain

We demonstrate optical fine-needle imaging biopsy (FNIB), combining a fine needle (22 gauge) and a high-resolution side-view probe (350-μm diameter) for minimally invasive interrogation of brain tissue in situ. We apply this technique to examine pathogenesis in murine models of neurodegeneration, brain metastasis of melanoma, and arterial occlusion, respectively. The demonstrated ability to obtain cellular images in the deep brain without craniotomy may be useful in the longitudinal studies of brain diseases.