Signal improvement in multiphoton microscopy by reflection with simple mirrors near the sample

Characterizing Vocal Fold Injury Recovery in a Rabbit Model With Three-Dimensional Virtual Histology

OBJECTIVES/HYPOTHESIS

In animal studies of vocal fold scarring and treatment, imaging-based evaluation is most often conducted by tissue slicing and histological staining. Given variation in anatomy, injury type, severity, and sacrifice timepoints, planar histological sections provide limited spatiotemporal details of tissue repair. Three-dimensional (3D) virtual histology may provide additional contextual spatial information, enhancing objective interpretation. The study's aim was to evaluate the suitability of magnetic resonance imaging (MRI), microscale computed tomography (CT), and nonlinear laser-scanning microscopy (NM) as virtual histology approaches for rabbit studies of vocal fold scarring.

METHODS

A unilateral injury was created using microcup forceps in the left vocal fold of three New Zealand White rabbits. Animals were sacrificed at 3, 10, and 39 days postinjury. ex vivo imaging of excised larynges was performed with MRI, CT, and NM modalities.

RESULTS

The MRI modality allowed visualization of injury location and morphological internal features with 100-μm spatial resolution. The CT modality provided a view of the injury defect surface with 12-μm spatial resolution. The NM modality with optical clearing resolved second-harmonic generation signal of collagen fibers and two-photon autofluorescence in vocal fold lamina propria, muscle, and surrounding cartilage structures at submicrometer spatial scales.

CONCLUSIONS

Features of vocal fold injury and wound healing were observed with MRI, CT, and NM. The MRI and CT modalities provided contextual spatial information and dissection guidance, whereas NM resolved extracellular matrix structure. The results serve as a proof of concept to motivate incorporation of 3D virtual histology techniques in future vocal fold injury animal studies.

LEVEL OF EVIDENCE

NA Laryngoscope, 131:1578-1587, 2021.

High-speed, multi-modal, label-free imaging of pathological slices with a Bessel beam

Optical imaging of stained pathological slices has become the gold standard for disease diagnosis. However, the procedure of sample preparation is complex and time-consuming. Multiphoton microscopy (MPM) is promising for label-free imaging, but the imaging speed is limited, especially for whole slice imaging. Here we propose a high-speed, multi-modal, label-free MPM by Bessel scan-based strip mosaicking. With a Bessel beam for excitation, the extended depth-of-focus not only enables full axial information acquisition at once, but also alleviates the demanding requirement of sample alignment. With the strip mosaicking protocol, we can save the time of frequent sample transferring. Besides, we add a closely-attached reflection mirror under the sample for enhancing epi-detection signals, and employ circularly polarized beams for recording comprehensive information. We demonstrate its application in multi-modal, label-free imaging of human gastric cancer slices and liver cancer slices, and show its potential in rapid disease diagnosis.

Imaging of Murine Whole Lung Fibrosis by Large Scale 3D Microscopy aided by Tissue Optical Clearing

Pulmonary fibrosis, characterized by excessive collagen deposition in the lungs, comprises a key and debilitating component of chronic lung diseases. Methods are lacking for the direct visualization of fibrillar collagen throughout the whole murine lung, a capability that would aid the understanding of lung fibrosis. We combined an optimized organ-level optical clearing (OC) approach with large-scale, label-free multiphoton microscopy (MPM) and second harmonic generation microscopy (SHGM) to reveal the complete network of fibrillar collagen in whole murine lungs. An innate inflammation-driven model based on repetitive poly(I:C) challenge was evaluated. Following OC, mosaic MPM/SHGM imaging with 3D reconstruction and whole organ quantitative analysis revealed significant differences in collagen deposition between PBS and poly(I:C) treated lungs. Airway specific analysis in whole lung acquisitions revealed significant sub-epithelial fibrosis evident throughout the proximal conductive and distal airways with higher collagen deposition in the poly(I:C) group vs PBS group. This study establishes a new, powerful approach based on OC and MPM/SHGM imaging for 3D analysis of lung fibrosis with macroscopic views of lung pathology based on microscopy and providing a new way to analyze the whole lung while avoiding regional sampling bias.

Molecular mobility and activity in an intravital imaging setting - implications for cancer progression and targeting

Molecular mobility, localisation and spatiotemporal activity are at the core of cell biological processes and deregulation of these dynamic events can underpin disease development and progression. Recent advances in intravital imaging techniques in mice are providing new avenues to study real-time molecular behaviour in intact tissues within a live organism and to gain exciting insights into the intricate regulation of live cell biology at the microscale level. The monitoring of fluorescently labelled proteins and agents can be combined with autofluorescent properties of the microenvironment to provide a comprehensive snapshot of in vivo cell biology. In this Review, we summarise recent intravital microscopy approaches in mice, in processes ranging from normal development and homeostasis to disease progression and treatment in cancer, where we emphasise the utility of intravital imaging to observe dynamic and transient events in vivo We also highlight the recent integration of advanced subcellular imaging techniques into the intravital imaging pipeline, which can provide in-depth biological information beyond the single-cell level. We conclude with an outlook of ongoing developments in intravital microscopy towards imaging in humans, as well as provide an overview of the challenges the intravital imaging community currently faces and outline potential ways for overcoming these hurdles.

Two-photon microscopy allows imaging and characterization of cochlear microvasculature in vivo

Impairment of cochlear blood flow has been discussed as factor in the pathophysiology of various inner ear disorders. However, the microscopic study of cochlear microcirculation is limited due to small scale and anatomical constraints. Here, two-photon fluorescence microscopy is applied to visualize cochlear microvessels. Guinea pigs were injected with Fluorescein isothiocyanate- or Texas red-dextrane as plasma marker. Intravital microscopy was performed in four animals and explanted cochleae from four animals were studied. The vascular architecture of the cochlea was visualized up to a depth of 90.0±22.7 μm. Imaging yielded a mean contrast-to-noise ratio (CNR) of 3.3±1.7. Mean diameter in vivo was 16.5±6.0 μm for arterioles and 8.0±2.4 μm for capillaries. In explanted cochleae, the diameter of radiating arterioles and capillaries was measured with 12.2±1.6 μm and 6.6±1.0 μm, respectively. The difference between capillaries and arterioles was statistically significant in both experimental setups (P<0.001 and P=0.022, two-way ANOVA). Measured vessel diameters in vivo and ex vivo were in agreement with published data. We conclude that two-photon fluorescence microscopy allows the investigation of cochlear microvessels and is potentially a valuable tool for inner ear research.

Label-free determination of hemodynamic parameters in the microcirculaton with third harmonic generation microscopy

Determination of blood flow velocity and related hemodynamic parameters is an important aspect of physiological studies which in many settings requires fluorescent labeling. Here we show that Third Harmonic Generation (THG) microscopy is a suitable tool for label-free intravital investigations of the microcirculation in widely-used physiological model systems. THG microscopy is a non-fluorescent multi-photon scanning technique combining the advantages of label-free imaging with restriction of signal generation to a focal spot. Blood flow was visualized and its velocity was measured in adult mouse cremaster muscle vessels, non-invasively in mouse ear vessels and in Xenopus tadpoles. In arterioles, THG line scanning allowed determination of the flow pulse velocity curve and hence the heart rate. By relocating the scan line we obtained velocity profiles through vessel diameters, allowing shear rate calculations. The cell free layer containing the glycocalyx was also visualized. Comparison of the current microscopic resolution with theoretical, diffraction limited resolution let us conclude that an about sixty-fold THG signal intensity increase may be possible with future improved optics, optimized for 1200-1300 nm excitation. THG microscopy is compatible with simultaneous two-photon excited fluorescence detection. It thus also provides the opportunity to determine important hemodynamic parameters in parallel to common fluorescent observations without additional label.

Compact non-contact total emission detection for in vivo multiphoton excitation microscopy

We describe a compact, non-contact design for a total emission detection (c-TED) system for intra-vital multiphoton imaging. To conform to a standard upright two-photon microscope design, this system uses a parabolic mirror surrounding a standard microscope objective in concert with an optical path that does not interfere with normal microscope operation. The non-contact design of this device allows for maximal light collection without disrupting the physiology of the specimen being examined. Tests were conducted on exposed tissues in live animals to examine the emission collection enhancement of the c-TED device compared to heavily optimized objective-based emission collection. The best light collection enhancement was seen from murine fat (5×-2× gains as a function of depth), whereas murine skeletal muscle and rat kidney showed gains of over two and just under twofold near the surface, respectively. Gains decreased with imaging depth (particularly in the kidney). Zebrafish imaging on a reflective substrate showed close to a twofold gain throughout the entire volume of an intact embryo (approximately 150 μm deep). Direct measurement of bleaching rates confirmed that the lower laser powers, enabled by greater light collection efficiency, yielded reduced photobleaching in vivo. The potential benefits of increased light collection in terms of speed of imaging and reduced photo-damage, as well as the applicability of this device to other multiphoton imaging methods is discussed.

Label-free 3D visualization of cellular and tissue structures in intact muscle with second and third harmonic generation microscopy

Second and Third Harmonic Generation (SHG and THG) microscopy is based on optical effects which are induced by specific inherent physical properties of a specimen. As a multi-photon laser scanning approach which is not based on fluorescence it combines the advantages of a label-free technique with restriction of signal generation to the focal plane, thus allowing high resolution 3D reconstruction of image volumes without out-of-focus background several hundred micrometers deep into the tissue. While in mammalian soft tissues SHG is mostly restricted to collagen fibers and striated muscle myosin, THG is induced at a large variety of structures, since it is generated at interfaces such as refraction index changes within the focal volume of the excitation laser. Besides, colorants such as hemoglobin can cause resonance enhancement, leading to intense THG signals. We applied SHG and THG microscopy to murine (Mus musculus) muscles, an established model system for physiological research, to investigate their potential for label-free tissue imaging. In addition to collagen fibers and muscle fiber substructure, THG allowed us to visualize blood vessel walls and erythrocytes as well as white blood cells adhering to vessel walls, residing in or moving through the extravascular tissue. Moreover peripheral nerve fibers could be clearly identified. Structure down to the nuclear chromatin distribution was visualized in 3D and with more detail than obtainable by bright field microscopy. To our knowledge, most of these objects have not been visualized previously by THG or any label-free 3D approach. THG allows label-free microscopy with inherent optical sectioning and therefore may offer similar improvements compared to bright field microscopy as does confocal laser scanning microscopy compared to conventional fluorescence microscopy.

Efficient large core fiber-based detection for multi-channel two-photon fluorescence microscopy and spectral unmixing

Low-magnification high-numerical aperture objectives maximize the collection efficiency for scattered two-photon excited fluorescence (2PEF), but non-descanned detection schemes for such objectives demand optical components much bigger than standard microscope optics. Fiber coupling offers the possibility of removing bulky multi-channel detectors from the collection site, but coupling and transmission losses are generally believed to outweigh the benefits of optical fibers. We present here two new developments based on large-core fiber-optic fluorescence detection that illustrate clear advantages over conventional air-coupled 2PEF detection schemes. First, with minimal modifications of a commercial microscope, we efficiently couple the output of a 20×/NA0.95 objective to a large-core liquid light guide and we obtain a 7-fold collection gain when imaging astrocytes at 100 μm depth in acute brain slices of adult ALDH1L1-GFP mice. Second, combining 2PEF microscopy and 4-color detection on a custom microscope, mode scrambling inside a 2-mm plastic optical fiber is shown to cancel out the spatially non-uniform spectral sensitivity observed with air-coupled detectors. Spectral unmixing of images of brainbow mice taken with a fiber-coupled detector revealed a uniform color distribution of hippocampal neurons across a large field of view. Thus, fiber coupling improves both the efficiency and the homogeneity of 2PEF collection.