Multicolor three-photon fluorescence imaging with single-wavelength excitation deep in mouse brain

Rational Design of Semiconducting Oligomer for Third Harmonic Generation Bioimaging of Ultradeep Brain Imaging with NIR-IIb Excitation

Multiphoton microscopy (MPM) has unparalleled promise in high-spatiotemporal bioimaging within the tissue-transparent window of 1500 to 1700 nm, commonly referred to as the near-infrared-IIb (NIR-IIb) region. However, so far, surprisingly few cases of non-fluorescent MPM probes have been reported, and their imaging performances are relatively limited. Herein, this study introduces a highly efficient third harmonic generation (THG) probe based on semiconducting oligomer derivatives (BTICs), which exhibit strong THG responses under NIR-IIb (1700 nm) excitation. Leveraging halogen chemistry, semiconducting oligomers with varying halogen substitutions and nanoparticles (NPs) exhibit unexpectedly high THG performance across different aggregation states upon NIR-IIb excitation. The BTICs NPs exhibit a large THG conversion efficiency (1215 × 10-84 cm6 s2 photon-2) and exceptional resistance to photobleaching. Furthermore, the biocompatibility and in vivo THG angiography capabilities of BTICs NPs are validated, achieving the visualisation of deep-brain vasculature with unprecedented spatial resolution at a record-high imaging depth of 1745 µm. The pioneering exploitation of semiconducting oligomer-based THG probes establishes a new class of high-performance materials, enabling ultra-deep THG imaging of the brain and advancing the design of next-generation THG imaging platforms.

Three-photon in vivo imaging of neurons and glia in the medial prefrontal cortex with sub-cellular resolution

The medial prefrontal cortex (mPFC) is important for higher cognitive functions, including working memory, decision making, and emotional control. In vivo recordings of neuronal activity in the mPFC have been achieved via invasive electrical and optical approaches. Here we apply low invasive three-photon in vivo imaging in the mPFC of the mouse at unprecedented depth. Specifically, we measure neuronal and astrocytic Ca2+-transient parameters in awake head-fixed mice up to a depth of 1700 µm. Furthermore, we longitudinally record dendritic spine density (0.41 ± 0.07 µm-1) deeper than 1 mm for a week. Using 1650 nm wavelength to excite red fluorescent microglia, we quantify their processes' motility (58.9 ± 2% turnover rate) at previously unreachable depths (1100 µm). We establish three-photon imaging of the mPFC enabling neuronal and glial recordings with subcellular resolution that will pave the way for novel discoveries in this brain region.

Protocol to analyze marrow B lineage cell dynamics by in vivo three-photon microscopy in intact mouse tibia

Three-photon microscopy (3PM) allows deep-tissue imaging beyond the capabilities of two-photon microscopy (2PM) owing to infrared excitation. Here, we present a protocol for time-lapse 3D imaging of B lymphocytes in the tibia marrow of fluorescent reporter mice using 3PM at 1,650 nm. We describe steps for verifying microscope performance and tibia imaging. We then detail the cell dynamics analysis, including denoising, cell segmentation, and tracking. This protocol has potential application for immune cell tracking in other optically inaccessible organs in which 2PM fails. For complete details on the use and execution of this protocol, please refer to Rakhymzhan et al.1.

Deep-Penetrating and High-Resolution Continuous-Wave Nonlinear Microscopy Based on Homologous Dual-Emission Upconversion Adaptive Optics

Lanthanide-doped upconversion nanoparticles (UCNPs) are emerging as innovative nonlinear probes in biomedical studies, offering the unique capability to simultaneously emit both visible (VIS) and near-infrared (NIR) photons under continuous-wave (CW) NIR excitation. However, deep-tissue high-resolution imaging remains challenging due to the trade-off between VIS emission (higher resolution, limited penetration) and NIR emission (deeper penetration, lower resolution). Here we present a CW nonlinear microscopy based on homologous dual-emission upconversion adaptive optics, leveraging Tm3+/Yb3+ co-doped UCNPs' dual 455 nm/800 nm emission: the 800 nm emission for aberration measurement (guide-star) in deep tissues and the 455 nm emission for high-resolution imaging at matching depths. Using a home-built nonlinear laser scanning microscope with a 975 nm CW laser, we achieved near-diffraction-limited imaging (480 nm laterally) at a 500 μm depth in the mouse brain environment with significant optical aberrations. This strategy expands UCNPs' applications and innovates the exploration of deep-tissue optical features.

A highly effective curcumin-based fluorescent probe with single-wavelength excitation for simultaneous detection and bioimaging of Cys, Hcy and GSH

Cysteine (Cys), homocysteine (Hcy) and glutathione (GSH) act as significant roles in many physiological processes, and their abnormal proliferation will cause multiple diseases including Alzheimer's disease, Parkinson's disease, Cardiovascular disease, atherosclerosis, and soft tissue damage. However, It is challenging work to develop a efficient method for differentiating and detecting GSH, Cys and Hcy because of their significant similarity in structures and functions. In this work, a smart fluorescent probe FBCN based on curcumin was rationally devised and developed by etherifying the phenol hydroxyl group on FBC with NBD-Cl, which emitted strong green at 516 nm. FBCN distinguished Hcy from Cys/GSH with naked eyes based on the color variation of probe solution in sunlight. Meanwhile, GSH induced the powerful fluorescence quenching of probe solution, but the fluorescence color of FBCN solution transformed from green to luminous yellow accompanied with emission wavelength redshifted from 516 nm to 540 nm or 553 nm in the existence of Hcy and Cys, respectively. Probe FBCN had outstanding sensitivity and anti-interference, low detection limit (56.5 nM, 77.7 nM, and 288 nM corresponded to Cys, Hcy, and GSH, respectively), short response time (the response time of FBCN to Cys, Hcy and GSH was 1 min, 2 min and 5 min, respectively). The DFT calculation and HRMS had verified the sensing mechanism of FBCN to biothiols. In addition, the probe was successfully utilized to detect three biothiols levels in living cell and zebrafish.

In vivo three-photon fluorescence imaging of mouse brain vasculature labeled by Evans blue excited at the NIR-III window

Multiphoton fluorescence microscopy (MFM), renowned for its noninvasiveness and high spatiotemporal resolution, is extensively applied in brain structure imaging in vivo. Three-photon fluorescence (3PF) imaging, excited at the NIR-III window, can penetrate the deepest mouse cerebrovascular. Evans blue, a substance known for its low toxicity, high water solubility, and resistance to metabolism, is frequently employed to assess blood-brain barrier (BBB) permeability. However, its suitability for multiphoton fluorescence imaging of mouse cerebrovascular at the NIR-III window in vivo remains unexplored. In this paper, we conduct a comprehensive analysis of the multiphoton excitation and emission characterization of Evans blue when excited at the NIR-III window. Our findings indicate that 1) Evans blue can generate 3PF signals; 2) it exhibits a substantial three-photon action cross-section (ησ3 ) in plasma; 3) its three-photon emission spectrum measured in vivo agrees with that measured in plasma ex vivo. Drawing upon these findings, we successfully demonstrated the application of 3PF imaging of mouse brain vasculature labeled with Evans blue. Notably, the maximum depth of cerebrovascular is 1550 μm beneath the brain surface, spanning the entire gray matter layer and white matter layer and extending into the hippocampus. Evans blue is thus highly ideal for brain cerebrovascular 3PF imaging in vivo.

Deep and dynamic metabolic and structural imaging in living tissues

Label-free imaging through two-photon autofluorescence of NAD(P)H allows for nondestructive, high-resolution visualization of cellular activities in living systems. However, its application to thick tissues has been restricted by its limited penetration depth within 300 μm, largely due to light scattering. Here, we demonstrate that the imaging depth for NAD(P)H can be extended to more than 700 μm in living engineered human multicellular microtissues by adopting multimode fiber-based, low repetition rate, high peak power, three-photon excitation of NAD(P)H at 1100 nm. This is achieved by having more than 0.5 megawatts peak power at the band of 1100 ± 25 nm through adaptively modulating multimodal nonlinear pulse propagation with a compact fiber shaper. Moreover, the eightfold increase in pulse energy enables faster imaging of monocyte behaviors in the living multicellular models. These results represent a substantial advance for deep and dynamic imaging of intact living biosystems. The modular design is anticipated to allow wide adoption for demanding imaging applications, including cancer research, immune responses, and tissue engineering.

Modulating Aggregation and Deaggregation Based on Assembling Strategy to Switch on NIR-II Light-Excited Fluorescence for Self-Reporting Viability of Eliminating Cancer Cell

The fabrication of self-reporting photosensitizers (PSs), enabling real-time evaluation of the extent of elimination of cancer cells, holds significant scientific importance in the photodynamic therapy (PDT) process. To address the intrinsic challenge of the short-wavelength light source, this work proposed an innovative approach of rational design second near-infrared (NIR-II, 1000-1700 nm) light-excited fluorescent PS systems (named HOEt-PI, Me-PI, and Et-PI, respectively) through modulating aggregation and deaggregation based on assembling strategy. Therein, the suitable interplanar distance of adjacent Et-PI linked with C-H···π interactions was an idea for relieving compact π···π packing for fluorescent imaging as well as elevating the spin-orbit coupling for reactive oxygen species (ROS) generation. With ROS continuously increasing, Et-PI underwent cell membrane-to-mitochondria migration, ultimately accumulated in nucleoli, symbolizing programmed cell death, thus distinguishing dead/live cells via three-photon fluorescence imaging (excited on 1250 nm) under photogeneration ROS. Meaningfully, the three-photon fluorescence of Et-PI was triggered by RNA of nucleoli, for which the higher signal-to-noise ratio and in-depth fluorescence imaging observed cancer cellular viability. Collectively, the proposed findings presented a constructing strategy for NIR-II light-mediated self-reporting PS for guiding the PDT of deep cancerous tissue in the future.

Optical Transparency Windows in Near-Infrared and Short-Wave Infrared for the Skin, Skull, and Brain: Fluorescence Bioimaging Using PbS Quantum Dots

Fluorescence imaging (FI) employing near-infrared (NIR) light within the range of ~750-1350 nm enables biomedical imaging several millimeters beneath the tissue surface. More recent investigations into the short-wave IR (SWIR) transparency windows between ~1550-1870 and 2100-2300 nm highlight their superior capabilities. This research presents a comparison of IR-FI of PbS quantum dots, emitting at 990, 1310, and 1580 nm, through the mouse scalp skin, skull, and brain. The SWIR fluorescence is the most effectively transmitted signal, showing particularly significant enhancement when passing through the skull, which causes high light scattering. For the analysis of the imaging results and light propagation through the organs, their spectra of attenuation, absorption, and scattering coefficients are measured. In view of biomedical imaging, attenuation due to light scattering is a more destructive factor. Hence, the spatial resolution and imaging contrast can be improved by operating in SWIR due to decreased light scattering.

Dual Infrared 2-Photon Microscopy Achieves Minimal Background Deep Tissue Imaging in Brain and Plant Tissues

Traditional deep fluorescence imaging has primarily focused on red-shifting imaging wavelengths into the near-infrared (NIR) windows or implementation of multi-photon excitation approaches. Here, we combine the advantages of NIR and multiphoton imaging by developing a dual-infrared two-photon microscope to enable high-resolution deep imaging in biological tissues. We first computationally identify that photon absorption, as opposed to scattering, is the primary contributor to signal attenuation. We next construct a NIR two-photon microscope with a 1640 nm femtosecond pulsed laser and a NIR PMT detector to image biological tissues labeled with fluorescent single-walled carbon nanotubes (SWNTs). We achieve spatial imaging resolutions close to the Abbe resolution limit and eliminate blur and background autofluorescence of biomolecules, 300 μm deep into brain slices and through the full 120 μm thickness of a Nicotiana benthamiana leaf. We also demonstrate that NIR-II two-photon microscopy can measure tissue heterogeneity by quantifying how much the fluorescence power law function varies across tissues, a feature we exploit to distinguish Huntington's Disease afflicted mouse brain tissues from wildtype. Our results suggest dual-infrared two-photon microscopy could accomplish in-tissue structural imaging and biochemical sensing with a minimal background, and with high spatial resolution, in optically opaque or highly autofluorescent biological tissues.

Optimized intravital three-photon imaging of intact mouse tibia links plasma cell motility to functional states

Intravital deep bone marrow imaging is crucial to studying cellular dynamics and functions but remains challenging, and minimally invasive methods are needed. We employed a high pulse-energy 1650 nm laser to perform three-photon microscopy in vivo, reaching ≈400 μm depth in intact mouse tibia. Repetition rates of 3 and 4 MHz allowed us to analyze motility patterns of fast and rare cells within unperturbed marrow and to identify a bi-modal migratory behavior for plasma cells. Third harmonic generation (THG) was identified as a label-free marker for cellular organelles, particularly endoplasmic reticulum, indicating protein synthesis capacity. We found a strong THG signal, suggesting high antibody secretion, in one-third of plasma cells while the rest showed low signals. We discovered an inverse relationship between migratory behavior and THG signal, linking motility to functional plasma cell states. This method may enhance our understanding of marrow microenvironment effects on cellular functions.

Deep Mouse Brain Two-Photon Near-Infrared Fluorescence Imaging Using a Superconducting Nanowire Single-Photon Detector Array

Two-photon microscopy (2PM) has become an important tool in biology to study the structure and function of intact tissues in vivo. However, adult mammalian tissues such as the mouse brain are highly scattering, thereby putting fundamental limits on the achievable imaging depth, which typically reside at around 600-800 μm. In principle, shifting both the excitation as well as (fluorescence) emission light to the shortwave near-infrared (SWIR, 1000-1700 nm) region promises substantially deeper imaging in 2PM, yet this shift has proven challenging in the past due to the limited availability of detectors and probes in this wavelength region. To overcome these limitations and fully capitalize on the SWIR region, in this work, we introduce a novel array of superconducting nanowire single-photon detectors (SNSPDs) and associated custom detection electronics for use in near-infrared 2PM. The SNSPD array exhibits high efficiency and dynamic range as well as low dark-count rates over a wide wavelength range. Additionally, the electronics and software permit a seamless integration into typical 2PM systems. Together with an organic fluorescent dye emitting at 1105 nm, we report imaging depth of >1.1 mm in the in vivo mouse brain, limited mostly by available labeling density and laser properties. Our work establishes a promising, and ultimately scalable, new detector technology for SWIR 2PM that facilitates deep tissue biological imaging.

Multiplexed, image-based pooled screens in primary cells and tissues with PerturbView

Optical pooled screening (OPS) is a scalable method for linking image-based phenotypes with cellular perturbations. However, it has thus far been restricted to relatively low-plex phenotypic readouts in cancer cell lines in culture due to limitations associated with in situ sequencing of perturbation barcodes. Here, we develop PerturbView, an OPS technology that leverages in vitro transcription to amplify barcodes before in situ sequencing, enabling screens with highly multiplexed phenotypic readouts across diverse systems, including primary cells and tissues. We demonstrate PerturbView in induced pluripotent stem cell-derived neurons, primary immune cells and tumor tissue sections from animal models. In a screen of immune signaling pathways in primary bone marrow-derived macrophages, PerturbView uncovered both known and novel regulators of NF-κB signaling. Furthermore, we combine PerturbView with spatial transcriptomics in tissue sections from a mouse xenograft model, paving the way to in situ screens with rich optical and transcriptomic phenotypes. PerturbView broadens the scope of OPS to a wide range of models and applications.

Near-Infrared Emissive π-Conjugated Oligomer Nanoparticles for Three- and Four-Photon Deep-Brain Microscopic Imaging Beyond 1700 nm Excitation

High-resolution visualization of the deep brain is still a challenging and very significant issue. Multiphoton microscopy (MPM) holds great promise for high-spatiotemporal deep-tissue imaging under NIR-III and NIR-IV excitation. However, thus far, their applications have been seriously restricted by the scarcity of efficient organic probes. Herein, we designed and synthesized two donor-acceptor-donor-type conjugated small molecules (TNT and TNS) for in vivo mouse deep-brain imaging with three- and four-photon microscopy under 1700 and 2200 nm excitation. With a selenium (Se) substitution, we synthesized two conjugated small molecules to promote their emission into the deep near-infrared region with high quantum yields of 55% and 20% in THF solvent, respectively, and their water-dispersive nanoparticles have relatively large absorption cross-sections in the 1700 and 2200 nm windows, respectively, with good biosafety. With these superiorities, these organic NPs achieve high-resolution deep-brain imaging via three-photon and four-photon microscopy with excitation at 1700 and 2200 nm windows, and 1620 μm deep in the brain vasculature can be visualized in vivo. This study demonstrates the efficiency of NIR-emissive conjugated small molecules for high-performance MPM imaging in the NIR-III and NIR-IV window and provides a route for the future design of organic MPM probes.

Deep intravital brain tumor imaging enabled by tailored three-photon microscopy and analysis

Intravital 2P-microscopy enables the longitudinal study of brain tumor biology in superficial mouse cortex layers. Intravital microscopy of the white matter, an important route of glioblastoma invasion and recurrence, has not been feasible, due to low signal-to-noise ratios and insufficient spatiotemporal resolution. Here, we present an intravital microscopy and artificial intelligence-based analysis workflow (Deep3P) that enables longitudinal deep imaging of glioblastoma up to a depth of 1.2 mm. We find that perivascular invasion is the preferred invasion route into the corpus callosum and uncover two vascular mechanisms of glioblastoma migration in the white matter. Furthermore, we observe morphological changes after white matter infiltration, a potential basis of an imaging biomarker during early glioblastoma colonization. Taken together, Deep3P allows for a non-invasive intravital investigation of brain tumor biology and its tumor microenvironment at subcortical depths explored, opening up opportunities for studying the neuroscience of brain tumors and other model systems.

Noninvasive in vivo microscopy of single neutrophils in the mouse brain via NIR-II fluorescent nanomaterials

In vivo microscopy of single cells enables following pathological changes in tissues, revealing signaling networks and cell interactions critical to disease progression. However, conventional intravital microscopy at visible and near-infrared wavelengths <900 nm (NIR-I) suffers from attenuation and is typically performed following the surgical creation of an imaging window. Such surgical procedures cause the alteration of the local vasculature and induce inflammation in skin, muscle and skull, inevitably altering the microenvironment in the imaging area. Here, we detail the use of near-infrared fluorescence (NIR-II, 1,000-1,700 nm) for in vivo microscopy to circumvent attenuation in living tissues. This approach enables the noninvasive visualization of cell migration in deep tissues by labeling specific cells with NIR-II lanthanide downshifting nanoparticles exhibiting high physicochemical stability and photostability. We further developed a NIR-II fluorescence microscopy setup for in vivo imaging through the intact skull with high spatiotemporal resolution, which we use for the real-time dynamic visualization of single-neutrophil behavior in the deep brain of a mouse model of ischemic stroke. The labeled downshifting nanoparticle synthesis takes 5-6 d, the imaging system setup takes 1-2 h, the in vivo cell labeling takes 1-3 h, the in vivo NIR-II microscopic imaging takes 3-5 h and the data analysis takes 3-8 h. The procedures can be performed by users with standard laboratory training in nanomaterials research and appropriate animal handling.

Demixing fluorescence time traces transmitted by multimode fibers

Optical methods based on thin multimode fibers (MMFs) are promising tools for measuring neuronal activity in deep brain regions of freely moving mice thanks to their small diameter. However, current methods are limited: while fiber photometry provides only ensemble activity, imaging techniques using of long multimode fibers are very sensitive to bending and have not been applied to unrestrained rodents yet. Here, we demonstrate the fundamentals of a new approach using a short MMF coupled to a miniscope. In proof-of-principle in vitro experiments, we disentangled spatio-temporal fluorescence signals from multiple fluorescent sources transmitted by a thin (200 µm) and short (8 mm) MMF, using a general unconstrained non-negative matrix factorization algorithm directly on the raw video data. Furthermore, we show that low-cost open-source miniscopes have sufficient sensitivity to image the same fluorescence patterns seen in our proof-of-principle experiment, suggesting a new avenue for novel minimally invasive deep brain studies using multimode fibers in freely behaving mice.

A TICT-AIE activated dual-channel fluorescence-on probe to reveal the dynamics mechanosensing of lipid droplets during ferroptosis

Mechanical forces play a crucial role in cellular processes, including ferroptosis, a form of regulated cell death associated with various diseases. However, the mechanical aspects of organelle lipid droplets (LDs) during ferroptosis are poorly understood. In this study, we designed and synthesized a fluorescent probe, TPE-V1, to enable real-time monitoring of LDs' viscosity using a dual-channel fluorescence-on model (red channel at 617 nm and NIR channel at 710 nm). The fluorescent imaging of using TPE-V1 was achieved due to the integrated mechanisms of the twisted intramolecular charge transfer (TICT) and aggregation-induced emission (AIE). Through dual-emission channel fluorescence imaging, we observed the enhanced mechanical energy of LDs triggering cellular mechanosensing, including ferroptosis and cell deformation. Theoretical calculations confirmed the probe's behavior, showing that high-viscosity media prevented the rotation processes and restored fluorescence quenching in low viscosity. These findings suggest that our TICT-TPE design strategy provides a practical approach to study LDs' mechanical properties during ferroptosis. This development enhances our understanding of the interplay between mechanical forces and LDs, contributing to the knowledge of ferroptotic cell death and potential therapeutic interventions targeting dysregulated cell death processes.

Inspiring a convergent engineering approach to measure and model the tissue microenvironment

Understanding the molecular and physical complexity of the tissue microenvironment (TiME) in the context of its spatiotemporal organization has remained an enduring challenge. Recent advances in engineering and data science are now promising the ability to study the structure, functions, and dynamics of the TiME in unprecedented detail; however, many advances still occur in silos that rarely integrate information to study the TiME in its full detail. This review provides an integrative overview of the engineering principles underlying chemical, optical, electrical, mechanical, and computational science to probe, sense, model, and fabricate the TiME. In individual sections, we first summarize the underlying principles, capabilities, and scope of emerging technologies, the breakthrough discoveries enabled by each technology and recent, promising innovations. We provide perspectives on the potential of these advances in answering critical questions about the TiME and its role in various disease and developmental processes. Finally, we present an integrative view that appreciates the major scientific and educational aspects in the study of the TiME.

Improving Three-Photon Fluorescence of Near-Infrared Quantum Dots for Deep Brain Imaging by Suppressing Biexciton Decay

Three-photon fluorescence microscopy (3PFM) is a promising brain research tool with submicrometer spatial resolution and high imaging depth. However, only limited materials have been developed for 3PFM owing to the rigorous requirement of the three-photon fluorescence (3PF) process. Herein, under the guidance of a band gap engineering strategy, CdTe/CdSe/ZnS quantum dots (QDs) emitting in the near-infrared window are designed for constructing 3PF probes. The formation of type II structure significantly increased the three-photon absorption cross section of QDs and caused the delocalization of electron-hole wave functions. The time-resolved transient absorption spectroscopy confirmed that the decay of biexcitons was significantly suppressed due to the appropriate band gap alignment, which further enhanced the 3PF efficiency of QDs. By utilizing QD-based 3PF probes, high-resolution 3PFM imaging of cerebral vasculature was realized excited by a 1600 nm femtosecond laser, indicating the possibility of deep brain imaging with these 3PF probes.

Measurement of third order coherence by in situ autocorrelation for determining three-photon cross-sections

We show theoretically that the third order coherence at zero delay can be obtained by measuring the second and third order autocorrelation traces of a pulsed laser. Our theory enables the measurement of a fluorophore's three-photon cross-section without prior knowledge of the temporal profile of the excitation pulse by using the same fluorescent medium for both the measurement of the third order coherence at zero delay as well as the cross-section. Such an in situ measurement needs no assumptions about the pulse shape nor group delay dispersion of the optical system. To verify the theory experimentally, we measure the three-photon action cross-section of Alexa Fluor 350 and show that the measured value of the three-photon cross-section remains approximately constant despite varied amounts of chirp on the excitation pulses.

Long-term in vivo three-photon imaging reveals region-specific differences in healthy and regenerative oligodendrogenesis

The generation of new myelin-forming oligodendrocytes in the adult central nervous system is critical for cognitive function and regeneration following injury. Oligodendrogenesis varies between gray and white matter regions, suggesting that local cues drive regional differences in myelination and the capacity for regeneration. However, the layer- and region-specific regulation of oligodendrocyte populations is unclear due to the inability to monitor deep brain structures in vivo. Here we harnessed the superior imaging depth of three-photon microscopy to permit long-term, longitudinal in vivo three-photon imaging of the entire cortical column and subcortical white matter in adult mice. We find that cortical oligodendrocyte populations expand at a higher rate in the adult brain than those of the white matter. Following demyelination, oligodendrocyte replacement is enhanced in the white matter, while the deep cortical layers show deficits in regenerative oligodendrogenesis and the restoration of transcriptional heterogeneity. Together, our findings demonstrate that regional microenvironments regulate oligodendrocyte population dynamics and heterogeneity in the healthy and diseased brain.

Emission of five OAM dispersive waves in dispersion-engineered double-ring core fiber

Beams carrying orbital angular momentum (OAM) have exhibited significant potential across various fields, such as metrology, image coding, and optical communications. High-performance broadband coherent OAM sources are critical to the operation of optical systems. The emission of dispersive waves facilitates the efficient transfer of energy to distant spectral domains while preserving the coherence among the generated frequency components. Light sources that maintain consistency over a wide range can increase the efficiency of optical communication systems and improve the measurement accuracy in imaging and metrology. In this work, we propose a germanium-doped double ring-core fiber for five OAM dispersive waves (DWs) generation. The OAM1,1 mode supported in the fiber exhibits three zero-dispersion wavelengths (ZDWs) located at 1275, 1720 and 2325 nm. When pumped under normal dispersion, the output spectrum undergoes broadening and exhibits five DWs, situated around 955, 1120, 1450, 2795 and 2965 nm, respectively. Concomitant with blue-shifted and red-shifted dispersive waves, the spectrum spans from 895 to 3050 nm with high coherence. The effect of the fiber and input pulse parameters on DWs generation, as well as the underlying dynamics of the dispersive wave generation process, are discussed. As expected, the number and location of DWs generated in the output spectrum have agreement with the prediction of the phase-matching condition. Overall, this multiple DWs generation method in the proposed fiber paves the way for developing efficient and coherent OAM light sources in fiber-based optical systems.

Toward a brighter constellation: multiorgan neuroimaging of neural and vascular dynamics in the spinal cord and brain

Significance

Pain comprises a complex interaction between motor action and somatosensation that is dependent on dynamic interactions between the brain and spinal cord. This makes understanding pain particularly challenging as it involves rich interactions between many circuits (e.g., neural and vascular) and signaling cascades throughout the body. As such, experimentation on a single region may lead to an incomplete and potentially incorrect understanding of crucial underlying mechanisms.

Aim

We aimed to develop and validate tools to enable detailed and extended observation of neural and vascular activity in the brain and spinal cord. The first key set of innovations was targeted to developing novel imaging hardware that addresses the many challenges of multisite imaging. The second key set of innovations was targeted to enabling bioluminescent (BL) imaging, as this approach can address limitations of fluorescent microscopy including photobleaching, phototoxicity, and decreased resolution due to scattering of excitation signals.

Approach

We designed 3D-printed brain and spinal cord implants to enable effective surgical implantations and optical access with wearable miniscopes or an open window (e.g., for one- or two-photon microscopy or optogenetic stimulation). We also tested the viability for BL imaging and developed a novel modified miniscope optimized for these signals (BLmini).

Results

We describe "universal" implants for acute and chronic simultaneous brain-spinal cord imaging and optical stimulation. We further describe successful imaging of BL signals in both foci and a new miniscope, the "BLmini," which has reduced weight, cost, and form-factor relative to standard wearable miniscopes.

Conclusions

The combination of 3D-printed implants, advanced imaging tools, and bioluminescence imaging techniques offers a coalition of methods for understanding spinal cord-brain interactions. Our work has the potential for use in future research into neuropathic pain and other sensory disorders and motor behavior.

Field-programmable gate array and deep neural network-accelerated spatial-spectral interferometry for rapid optical dispersion analysis

Spatial-spectral interferometry (SSI) is a technique used to reconstruct the electrical field of an ultrafast laser. By analyzing the spectral phase distribution, SSI provides valuable information about the optical dispersion affecting the spectral phase, which is related to the energy distribution of the laser pulses. SSI is a single-shot measurement process and has a low laser power requirement. However, the reconstruction algorithm involves numerous Fourier transform and filtering operations, which limits the applicability of SSI for real-time dispersion analysis. To address this issue, this Letter proposes a field-programmable gate array (FPGA)-based deep neural network to accelerate the spectral phase reconstruction and dispersion estimation process. The results show that the analysis time is improved from 124 to 9.27 ms, which represents a 13.4-fold improvement on the standard Fourier transform-based reconstruction algorithm.

Three-dimensional, dual-color nanoscopy enabled by migrating photon avalanches with one single low-power CW beam

The development of super-resolution fluorescence microscopy is very essential for understanding the physical and biological fundamentals at nanometer scale. However, to date most super-resolution modalities require either complicated/costly purpose-built systems such as multiple-beam architectures or complex post-processing procedures with intrinsic artifacts. Achieving three-dimensional (3D) or multi-channel sub-diffraction microscopic imaging using a simple method remains a challenging and struggling task. Herein, we proposed 3D highly-nonlinear super-resolution microscopy using a single-beam excitation strategy, and the microscopy principle was modelled and studied based on the ultrahigh nonlinearity enabled by photon avalanches. According to the simulation, the point spread function of highly nonlinear microscopy is switchable among different modes and can shrink three-dimensionally to sub-diffraction scale at the photon avalanche mode. Experimentally, we demonstrated 3D optical nanoscopy assisted with huge optical nonlinearities in a simple laser scanning configuration, achieving a lateral resolution down to 58 nm (λ/14) and an axial resolution down to 185 nm (λ/5) with one single beam of low-power, continuous-wave, near-infrared laser. We further extended the photon avalanche effect to many other emitters to develop multi-color photon avalanching nanoprobes based on migrating photon avalanche mechanism, which enables us to implement single-beam dual-color sub-diffraction super-resolution microscopic imaging.

Supercontinuum intrinsic fluorescence imaging heralds free view of living systems

Optimal imaging strategies remain underdeveloped to maximize information for fluorescence microscopy while minimizing the harm to fragile living systems. Taking hint from the supercontinuum generation in ultrafast laser physics, we generated supercontinuum fluorescence from untreated unlabeled live samples before nonlinear photodamage onset. Our imaging achieved high-content cell phenotyping and tissue histology, identified bovine embryo polarization, quantified aging-related stress across cell types and species, demystified embryogenesis before and after implantation, sensed drug cytotoxicity in real-time, scanned brain area for targeted patching, optimized machine learning to track small moving organisms, induced two-photon phototropism of leaf chloroplasts under two-photon photosynthesis, unraveled microscopic origin of autumn colors, and interrogated intestinal microbiome. The results enable a facility-type microscope to freely explore vital molecular biology across life sciences.

Simultaneous Two- and Three-Photon Deep Imaging of Autofluorescence in Bacterial Communities

The intrinsic fluorescence of bacterial samples has a proven potential for label-free bacterial characterization, monitoring bacterial metabolic functions, and as a mechanism for tracking the transport of relevant components through vesicles. The reduced scattering and axial confinement of the excitation offered by multiphoton imaging can be used to overcome some of the limitations of single-photon excitation (e.g., scattering and out-of-plane photobleaching) to the imaging of bacterial communities. In this work, we demonstrate in vivo multi-photon microscopy imaging of Streptomyces bacterial communities, based on the excitation of blue endogenous fluorophores, using an ultrafast Yb-fiber laser amplifier. Its parameters, such as the pulse energy, duration, wavelength, and repetition rate, enable in vivo multicolor imaging with a single source through the simultaneous two- and three-photon excitation of different fluorophores. Three-photon excitation at 1040 nm allows fluorophores with blue and green emission spectra to be addressed (and their corresponding ultraviolet and blue single-photon excitation wavelengths, respectively), and two-photon excitation at the same wavelength allows fluorophores with yellow, orange, or red emission spectra to be addressed (and their corresponding green, yellow, and orange single-photon excitation wavelengths). We demonstrate that three-photon excitation allows imaging over a depth range of more than 6 effective attenuation lengths to take place, corresponding to an 800 micrometer depth of imaging, in samples with a high density of fluorescent structures.

The Cousa objective: a long-working distance air objective for multiphoton imaging in vivo

Multiphoton microscopy can resolve fluorescent structures and dynamics deep in scattering tissue and has transformed neural imaging, but applying this technique in vivo can be limited by the mechanical and optical constraints of conventional objectives. Short working distance objectives can collide with compact surgical windows or other instrumentation and preclude imaging. Here we present an ultra-long working distance (20 mm) air objective called the Cousa objective. It is optimized for performance across multiphoton imaging wavelengths, offers a more than 4 mm2 field of view with submicrometer lateral resolution and is compatible with commonly used multiphoton imaging systems. A novel mechanical design, wider than typical microscope objectives, enabled this combination of specifications. We share the full optical prescription, and report performance including in vivo two-photon and three-photon imaging in an array of species and preparations, including nonhuman primates. The Cousa objective can enable a range of experiments in neuroscience and beyond.

Towards a Brighter Constellation: Multi-Organ Neuroimaging of Neural and Vascular Dynamics in the Spinal Cord and Brain

Significance

Pain is comprised of a complex interaction between motor action and somatosensation that is dependent on dynamic interactions between the brain and spinal cord. This makes understanding pain particularly challenging as it involves rich interactions between many circuits (e.g., neural and vascular) and signaling cascades throughout the body. As such, experimentation on a single region may lead to an incomplete and potentially incorrect understanding of crucial underlying mechanisms.

Aim

Here, we aimed to develop and validate new tools to enable detailed and extended observation of neural and vascular activity in the brain and spinal cord. The first key set of innovations were targeted to developing novel imaging hardware that addresses the many challenges of multi-site imaging. The second key set of innovations were targeted to enabling bioluminescent imaging, as this approach can address limitations of fluorescent microscopy including photobleaching, phototoxicity and decreased resolution due to scattering of excitation signals.

Approach

We designed 3D-printed brain and spinal cord implants to enable effective surgical implantations and optical access with wearable miniscopes or an open window (e.g., for one- or two-photon microscopy or optogenetic stimulation). We also tested the viability for bioluminescent imaging, and developed a novel modified miniscope optimized for these signals (BLmini).

Results

Here, we describe novel 'universal' implants for acute and chronic simultaneous brain-spinal cord imaging and optical stimulation. We further describe successful imaging of bioluminescent signals in both foci, and a new miniscope, the 'BLmini,' which has reduced weight, cost and form-factor relative to standard wearable miniscopes.

Conclusions

The combination of 3D printed implants, advanced imaging tools, and bioluminescence imaging techniques offers a new coalition of methods for understanding spinal cord-brain interactions. This work has the potential for use in future research into neuropathic pain and other sensory disorders and motor behavior.

Fluorescent probe for sensitive discrimination of GSH and Hcy/Cys with single-wavelength excitation in biological systems via different emission

Biothiols (GSH, Hcy, Cys) are important active sulfur substances in biological systems and widely participate in various physiological processes. The three kinds of biothiols have similar chemical structures, including the sulfhydryl group (-SH) and an amino group (-NH2), so distinguishing two or more of them simultaneously is an important challenge. Herein, a nopinone-based fluorescent probe 3-(3-((4-nitrobenzoxadiazole vinyl) nopinyl difluoride (NF-NBD) was designed to distinguish GSH and Hcy/Cys by generating different fluorescence channels with a single excitation wavelength. The nitrobenzodioxazole (NBD) was introduced in the fluorescent probe by ether bounds that can quench fluorescence and selectively discriminate GSH and Hcy/Cys. After reacting with GSH and Hcy/Cys, NF-NBD exhibited strong fluorescence (green for GSH and yellow for Hcy/Cys). NF-NBD displayed a wide linear range, low detection limit, a rapid response time, and superior selectivity for biothiols. Furthermore, NF-NBD was applied to image and distinguish different biothiols in living cells and zebrafish via different fluorescence signals at a single excitation wavelength.

Recent strategies for neural dynamics observation at a larger scale and wider scope

The tremendous technical progress in neuroscience offers opportunities to observe a more minor or/and broader dynamic picture of the brain. Moreover, the large-scale neural activity of individual neurons enables the dissection of detailed mechanistic links between neural populations and behaviors. To measure neural activity in-vivo, multi-neuron recording, and neuroimaging techniques are employed and developed to acquire more neurons. The tools introduced concurrently recorded dozens to hundreds of neurons in the coordinated brain regions and elucidated the neuronal ensembles from a massive population perspective of diverse neurons at cellular resolution. In particular, the increasing spatiotemporal resolution of neuronal monitoring across the whole brain dramatically facilitates our understanding of additional nervous system functions in health and disease. Here, we will introduce state-of-the-art neuroscience tools involving large-scale neural population recording and the long-range connections spanning multiple brain regions. Their synergic effects provide to clarify the controversial circuitry underlying neuroscience. These challenging neural tools present a promising outlook for the fundamental dynamic interplay across levels of synaptic cellular, circuit organization, and brain-wide. Hence, more observations of neural dynamics will provide more clues to elucidate brain functions and push forward innovative technology at the intersection of neural engineering disciplines. We hope this review will provide insight into the use or development of recent neural techniques considering spatiotemporal scales of brain observation.

Long-term in vivo three-photon imaging reveals region-specific differences in healthy and regenerative oligodendrogenesis

The generation of new myelin-forming oligodendrocytes in the adult CNS is critical for cognitive function and regeneration following injury. Oligodendrogenesis varies between gray and white matter regions suggesting that local cues drive regional differences in myelination and the capacity for regeneration. Yet, the determination of regional variability in oligodendrocyte cell behavior is limited by the inability to monitor the dynamics of oligodendrocytes and their transcriptional subpopulations in white matter of the living brain. Here, we harnessed the superior imaging depth of three-photon microscopy to permit long-term, longitudinal in vivo three-photon imaging of an entire cortical column and underlying subcortical white matter without cellular damage or reactivity. Using this approach, we found that the white matter generated substantially more new oligodendrocytes per volume compared to the gray matter, yet the rate of population growth was proportionally higher in the gray matter. Following demyelination, the white matter had an enhanced population growth that resulted in higher oligodendrocyte replacement compared to the gray matter. Finally, deep cortical layers had pronounced deficits in regenerative oligodendrogenesis and restoration of the MOL5/6-positive oligodendrocyte subpopulation following demyelinating injury. Together, our findings demonstrate that regional microenvironments regulate oligodendrocyte population dynamics and heterogeneity in the healthy and diseased brain.

Fluorescence-amplified nanocrystals in the second near-infrared window for in vivo real-time dynamic multiplexed imaging

Optical imaging in the second near-infrared window (NIR-II, 1,000-1,700 nm) holds great promise for non-invasive in vivo detection. However, real-time dynamic multiplexed imaging remains challenging due to the lack of available fluorescence probes and multiplexing techniques in the ideal NIR-IIb (1,500-1,700 nm) 'deep-tissue-transparent' sub-window. Here we report on thulium-based cubic-phase downshifting nanoparticles (α-TmNPs) with 1,632 nm fluorescence amplification. This strategy was also validated for the fluorescence enhancement of nanoparticles doped with NIR-II Er3+ (α-ErNPs) or Ho3+ (α-HoNPs). In parallel, we developed a simultaneous dual-channel imaging system with high spatiotemporal synchronization and accuracy. The NIR-IIb α-TmNPs and α-ErNPs facilitated the non-invasive real-time dynamic multiplexed imaging of cerebrovascular vasomotion activity and the single-cell-level neutrophil behaviour in mouse subcutaneous tissue and ischaemic stroke model.

Measurement of three-photon excitation cross-sections of fluorescein from 1154 nm to 1500 nm

Measurements of three-photon action cross-sections for fluorescein (dissolved in water, pH ∼11.5) are presented in the excitation wavelength range from 1154 to 1500 nm in ∼50 nm steps. The excitation source is a femtosecond wavelength tunable non-collinear optical parametric amplifier, which has been spectrally filtered with 50 nm full width at half maximum band pass filters. Cube-law power dependance is confirmed at the measurement wavelengths. The three-photon excitation spectrum is found to differ from both the one- and two-photon excitation spectra. The three-photon action cross-section at 1154 nm is more than an order of magnitude larger than those at 1450 and 1500 nm (approximately three times the wavelength of the one-photon excitation peak), which possibly indicates the presence of resonance enhancement.

The Neurovasculome: Key Roles in Brain Health and Cognitive Impairment: A Scientific Statement From the American Heart Association/American Stroke Association

BACKGROUND

Preservation of brain health has emerged as a leading public health priority for the aging world population. Advances in neurovascular biology have revealed an intricate relationship among brain cells, meninges, and the hematic and lymphatic vasculature (the neurovasculome) that is highly relevant to the maintenance of cognitive function. In this scientific statement, a multidisciplinary team of experts examines these advances, assesses their relevance to brain health and disease, identifies knowledge gaps, and provides future directions.

METHODS

Authors with relevant expertise were selected in accordance with the American Heart Association conflict-of-interest management policy. They were assigned topics pertaining to their areas of expertise, reviewed the literature, and summarized the available data.

RESULTS

The neurovasculome, composed of extracranial, intracranial, and meningeal vessels, as well as lymphatics and associated cells, subserves critical homeostatic functions vital for brain health. These include delivering O2 and nutrients through blood flow and regulating immune trafficking, as well as clearing pathogenic proteins through perivascular spaces and dural lymphatics. Single-cell omics technologies have unveiled an unprecedented molecular heterogeneity in the cellular components of the neurovasculome and have identified novel reciprocal interactions with brain cells. The evidence suggests a previously unappreciated diversity of the pathogenic mechanisms by which disruption of the neurovasculome contributes to cognitive dysfunction in neurovascular and neurodegenerative diseases, providing new opportunities for the prevention, recognition, and treatment of these conditions.

CONCLUSIONS

These advances shed new light on the symbiotic relationship between the brain and its vessels and promise to provide new diagnostic and therapeutic approaches for brain disorders associated with cognitive dysfunction.

Chirped pulse amplification based on praseodymium-doped fluoride fibers

We have developed a chirped pulse amplification system for ultrashort 1300 nm pulses based on praseodymium-doped fluoride fibers (Pr:ZBLAN). The 1300 nm seed pulse is generated through soliton-dispersive wave coupling in a highly nonlinear fiber pumped by a pulse from an erbium-doped fiber laser. The seed pulse is stretched with a grating stretcher to ∼150 ps and amplified with a two-stage Pr:ZBLAN amplifier. The average power reaches ∼112 mW at the repetition rate of 40 MHz. The pulse is compressed to 225 fs by using a pair of gratings without serious phase distortion.

Deep Imaging to Dissect Microvascular Contributions to White Matter Degeneration in Rodent Models of Dementia

The increasing socio-economic burden of Alzheimer disease (AD) and AD-related dementias has created a pressing need to define targets for therapeutic intervention. Deficits in cerebral blood flow and neurovascular function have emerged as early contributors to disease progression. However, the cause, progression, and consequence of small vessel disease in AD/AD-related dementias remains poorly understood, making therapeutic targets difficult to pinpoint. Animal models that recapitulate features of AD/AD-related dementias may provide mechanistic insight because microvascular pathology can be studied as it develops in vivo. Recent advances in in vivo optical and ultrasound-based imaging of the rodent brain facilitate this goal by providing access to deeper brain structures, including white matter and hippocampus, which are more vulnerable to injury during cerebrovascular disease. Here, we highlight these novel imaging approaches and discuss their potential for improving our understanding of vascular contributions to AD/AD-related dementias.

Fluorescence microscopic platforms imaging mitochondrial abnormalities in neurodegenerative diseases

Neurodegenerative diseases (NDs) are progressive disorders that cause the degeneration of neurons. Mitochondrial dysfunction is a common symptom in NDs and plays a crucial role in neuronal loss. Mitochondrial abnormalities can be observed in the early stages of NDs and evolve throughout disease progression. Visualizing mitochondrial abnormalities can help understand ND progression and develop new therapeutic strategies. Fluorescence microscopy is a powerful tool for dynamically imaging mitochondria due to its high sensitivity and spatiotemporal resolution. This review discusses the relationship between mitochondrial dysfunction and ND progression, potential biomarkers for imaging dysfunctional mitochondria, advances in fluorescence microscopy for detecting organelles, the performance of fluorescence probes in visualizing ND-associated mitochondria, and the challenges and opportunities for developing new generations of fluorescence imaging platforms for monitoring mitochondria in NDs.

A single-wavelength excited NIR fluorescence probe for distinguishing GSH/H2S and Cys/Hcy in living cells and zebrafish through separated dual-channels

Biothiols and hydrogen sulfide, as critical sulfur-containing reactive substances, serve essential functions in various human pathological processes, making it challenging to simultaneously distinguish them due to their similar reactivity and structures (-SH). Here, we rationalized the development of a single-wavelength excitation near-infrared (NIR) fluorescence probe, FC-NBD, for distinguishing GSH/H₂S and Cys/Hcy by separated fluorescence dual channels. In this probe, FC-NBD, composed of coumarin-benzopyrylium derivatives linked with nitro benzoxadiazole (NBD) via ether bonds, could quantitatively and selectively distinguish GSH/H₂S and Cys/Hcy with a low limit of detection (LOD) of 0.199/0.177 μM and 0.106/0.076 μM, respectively. As expected, under single-wavelength excitation (470 nm), FC-NBD demonstrated distinctly separable green and NIR fluorescence emissions towards Cys/Hcy at 550 and 660 nm, but only exhibited a noticeable NIR fluorescence emission towards GSH/H₂S at 660 nm. Moreover, FC-NBD could simultaneously visualize and discriminate GSH/H₂S and Cys/Hcy in living cells as well as zebrafish through green and NIR channels at a single excitation wavelength.

In Vivo Deep-Brain 3- and 4-Photon Fluorescence Imaging of Subcortical Structures Labeled by Quantum Dots Excited at the 2200 nm Window

Multiphoton microscopy (MPM) is an enabling technology for visualizing deep-brain structures at high spatial resolution in vivo. Within the low tissue absorption window, shifting to longer excitation wavelengths reduces tissue scattering and boosts penetration depth. Recently, the 2200 nm excitation window has emerged as the last and longest window suitable for deep-brain MPM. However, multiphoton fluorescence imaging at this window has not been demonstrated, due to the lack of characterization of multiphoton properties of fluorescent labels. Here we demonstrate technologies for measuring both the multiphoton excitation and emission properties of fluorescent labels at the 2200 nm window, using (1) 3-photon (ησ₃) and 4-photon action cross sections (ησ₄) and (2) 3-photon and 4-photon emission spectra both ex vivo and in vivo of quantum dots. Our results show that quantum dots have exceptionally large ησ₃ and ησ₄ for efficient generation of multiphoton fluorescence. Besides, the 3-photon and 4-photon emission spectra of quantum dots are essentially identical to those of one-photon emission, which change negligibly subject to the local environment of circulating blood. Based on these characterization results, we further demonstrate deep-brain vasculature imaging in vivo. Due to the superb multiphoton properties of quantum dots, 3-photon and 4-photon fluorescence imaging reaches a maximum brain imaging depth of 1060 and 940 μm below the surface of a mouse brain, respectively, which enables the imaging of subcortical structures. We thus fill the last gap in multiphoton fluorescence imaging in terms of wavelength selection.

Tutorial: multiphoton microscopy to advance neuroscience research

Multiphoton microscopy (MPM) employs ultrafast infrared lasers for high-resolution deep three-dimensional imaging of live biological samples. The goal of this tutorial is to provide a practical guide to MPM imaging for novice microscopy developers and life-science users. Principles of MPM, microscope setup, and labeling strategies are discussed. Use of MPM to achieve unprecedented imaging depth of whole mounted explants and intravital imaging via implantable glass windows of the mammalian nervous system is demonstrated.

Three-photon excited fluorescence imaging in neuroscience: From principles to applications

The development of three-photon microscopy (3PM) has greatly expanded the capability of imaging deep within biological tissues, enabling neuroscientists to visualize the structure and activity of neuronal populations with greater depth than two-photon imaging. In this review, we outline the history and physical principles of 3PM technology. We cover the current techniques for improving the performance of 3PM. Furthermore, we summarize the imaging applications of 3PM for various brain regions and species. Finally, we discuss the future of 3PM applications for neuroscience.

Contribution of Intravital Neuroimaging to Study Animal Models of Multiple Sclerosis

Multiple sclerosis (MS) is a complex and long-lasting neurodegenerative disease of the central nervous system (CNS), characterized by the loss of myelin within the white matter and cortical fibers, axonopathy, and inflammatory responses leading to consequent sensory-motor and cognitive deficits of patients. While complete resolution of the disease is not yet a reality, partial tissue repair has been observed in patients which offers hope for therapeutic strategies. To address the molecular and cellular events of the pathomechanisms, a variety of animal models have been developed to investigate distinct aspects of MS disease. Recent advances of multiscale intravital imaging facilitated the direct in vivo analysis of MS in the animal models with perspective of clinical transfer to patients. This review gives an overview of MS animal models, focusing on the current imaging modalities at the microscopic and macroscopic levels and emphasizing the importance of multimodal approaches to improve our understanding of the disease and minimize the use of animals.

In vivo three- and four-photon fluorescence microscopy using a 1.8 µm femtosecond fiber laser system

Multiphoton microscopy has enabled us to image cellular dynamics in vivo. However, the excitation wavelength for imaging with commercially available lasers is mostly limited between 0.65-1.04 µm. Here we develop a femtosecond fiber laser system that produces ∼150 fs pulses at 1.8 µm. Our system starts from an erbium-doped silica fiber laser, and its wavelength is converted to 1.8 µm using a Raman shift fiber. The 1.8 µm pulses are amplified with a two-stage Tm:ZBLAN fiber amplifier. The final pulse energy is ∼1 µJ, sufficient for in vivo imaging. We successfully observe TurboFP635-expressing cortical neurons at a depth of 0.7 mm from the brain surface by three-photon excitation and Clover-expressing astrocytes at a depth of 0.15 mm by four-photon excitation.

Multi-Photon Microscopy

In this series of papers on light microscopy imaging, we have covered the fundamentals of microscopy, super-resolution microscopy, and lightsheet microscopy. This last review covers multi-photon microscopy with a brief reference to intravital imaging and Brainbow labeling. Multi-photon microscopy is often referred to as two-photon microscopy. Indeed, using two-photon microscopy is by far the most common way of imaging thick tissues; however, it is theoretically possible to use a higher number of photons, and three-photon microscopy is possible. Therefore, this review is titled "multi-photon microscopy." Another term for describing multi-photon microscopy is "non-linear" microscopy because fluorescence intensity at the focal spot depends upon the average squared intensity rather than the squared average intensity; hence, non-linear optics (NLO) is an alternative name for multi-photon microscopy. It is this non-linear relationship (or third exponential power in the case of three-photon excitation) that determines the axial optical sectioning capability of multi-photon imaging. In this paper, the necessity for two-photon or multi-photon imaging is explained, and the method of optical sectioning by multi-photon microscopy is described. Advice is also given on what fluorescent markers to use and other practical aspects of imaging thick tissues. The technique of Brainbow imaging is discussed. The review concludes with a description of intravital imaging of the mouse. © 2023 Wiley Periodicals LLC.

Multiscale imaging informs translational mouse modeling of neurological disease

Multiscale neurophysiology reveals that simple motor actions are associated with changes in neuronal firing in virtually every brain region studied. Accordingly, the assessment of focal pathology such as stroke or progressive neurodegenerative diseases must also extend widely across brain areas. To derive mechanistic information through imaging, multiple resolution scales and multimodal factors must be included, such as the structure and function of specific neurons and glial cells and the dynamics of specific neurotransmitters. Emerging multiscale methods in preclinical animal studies that span micro- to macroscale examinations fill this gap, allowing a circuit-based understanding of pathophysiological mechanisms. Combined with high-performance computation and open-source data repositories, these emerging multiscale and large field-of-view techniques include live functional ultrasound, multi- and single-photon wide-scale light microscopy, video-based miniscopes, and tissue-penetrating fiber photometry, as well as variants of post-mortem expansion microscopy. We present these technologies and outline use cases and data pipelines to uncover new knowledge within animal models of stroke, Alzheimer's disease, and movement disorders.

Deep tissue multi-photon imaging using adaptive optics with direct focus sensing and shaping

High-resolution optical imaging deep in tissues is challenging because of optical aberrations and scattering of light caused by the complex structure of living matter. Here we present an adaptive optics three-photon microscope based on analog lock-in phase detection for focus sensing and shaping (ALPHA-FSS). ALPHA-FSS accurately measures and effectively compensates for both aberrations and scattering induced by specimens and recovers subcellular resolution at depth. A conjugate adaptive optics configuration with remote focusing enables in vivo imaging of fine neuronal structures in the mouse cortex through the intact skull up to a depth of 750 µm below the pia, enabling near-non-invasive high-resolution microscopy in cortex. Functional calcium imaging with high sensitivity and high-precision laser-mediated microsurgery through the intact skull were also demonstrated. Moreover, we achieved in vivo high-resolution imaging of the deep cortex and subcortical hippocampus up to 1.1 mm below the pia within the intact brain.

The Development and Application of Opto-Chemical Tools in the Zebrafish

The zebrafish is one of the most widely adopted animal models in both basic and translational research. This popularity of the zebrafish results from several advantages such as a high degree of similarity to the human genome, the ease of genetic and chemical perturbations, external fertilization with high fecundity, transparent and fast-developing embryos, and relatively low cost-effective maintenance. In particular, body translucency is a unique feature of zebrafish that is not adequately obtained with other vertebrate organisms. The animal's distinctive optical clarity and small size therefore make it a successful model for optical modulation and observation. Furthermore, the convenience of microinjection and high embryonic permeability readily allow for efficient delivery of large and small molecules into live animals. Finally, the numerous number of siblings obtained from a single pair of animals offers large replicates and improved statistical analysis of the results. In this review, we describe the development of opto-chemical tools based on various strategies that control biological activities with unprecedented spatiotemporal resolution. We also discuss the reported applications of these tools in zebrafish and highlight the current challenges and future possibilities of opto-chemical approaches, particularly at the single cell level.