Higher harmonic generation microscopy for developmental biology

Third harmonic imaging contrast from tubular structures in the presence of index discontinuity

Accurate interpretation of third harmonic generation (THG) microscopy images in terms of sample optical properties and microstructure is generally hampered by the presence of excitation field distortions resulting from sample heterogeneity. Numerical methods that account for these artifacts need to be established. In this work, we experimentally and numerically analyze the THG contrast obtained from stretched hollow glass pipettes embedded in different liquids. We also characterize the nonlinear optical properties of 2,2[Formula: see text]-thiodiethanol (TDE), a water-soluble index-matching medium. We find that index discontinuity not only changes the level and modulation amplitude of polarization-resolved THG signals, but can even change the polarization direction producing maximum THG near interfaces. We then show that a finite-difference time-domain (FDTD) modeling strategy can accurately account for contrast observed in optically heterogeneous samples, whereas reference Fourier-based numerical approaches are accurate only in the absence of index mismatch. This work opens perspectives for interpreting THG microscopy images of tubular objects and other geometries.

Label-free imaging of red blood cells and oxygenation with color third-order sum-frequency generation microscopy

Mapping red blood cells (RBCs) flow and oxygenation is of key importance for analyzing brain and tissue physiology. Current microscopy methods are limited either in sensitivity or in spatio-temporal resolution. In this work, we introduce a novel approach based on label-free third-order sum-frequency generation (TSFG) and third-harmonic generation (THG) contrasts. First, we propose a novel experimental scheme for color TSFG microscopy, which provides simultaneous measurements at several wavelengths encompassing the Soret absorption band of hemoglobin. We show that there is a strong three-photon (3P) resonance related to the Soret band of hemoglobin in THG and TSFG signals from zebrafish and human RBCs, and that this resonance is sensitive to RBC oxygenation state. We demonstrate that our color TSFG implementation enables specific detection of flowing RBCs in zebrafish embryos and is sensitive to RBC oxygenation dynamics with single-cell resolution and microsecond pixel times. Moreover, it can be implemented on a 3P microscope and provides label-free RBC-specific contrast at depths exceeding 600 µm in live adult zebrafish brain. Our results establish a new multiphoton contrast extending the palette of deep-tissue microscopy.

Label-free third harmonic generation imaging and quantification of lipid droplets in live filamentous fungi

We report the utilization of Third-Harmonic Generation microscopy for label-free live cell imaging of lipid droplets in the hypha of filamentous fungus Phycomyces blakesleeanus. THG microscopy images showed bright spherical features dispersed throughout the hypha cytoplasm in control conditions and a transient increase in the number of bright features after complete nitrogen starvation. Colocalization analysis of THG and lipid-counterstained images disclosed that the cytoplasmic particles were lipid droplets. Particle Size Analysis and Image Correlation Spectroscopy were used to quantify the number density and size of lipid droplets. The two analysis methods both revealed an increase from 16 × 10^-3 to 23 × 10^-3 lipid droplets/µm² after nitrogen starvation and a decrease in the average size of the droplets (range: 0.5-0.8 µm diameter). In conclusion, THG imaging, followed by PSA and ICS, can be reliably used for filamentous fungi for the in vivo quantification of lipid droplets without the need for labeling and/or fixation. In addition, it has been demonstrated that ICS is suitable for THG microscopy.

Stimulated Raman scattering microscopy in chemistry and life science - Development, innovation, perspectives

In this review, we present a summary of the basics of the Stimulated Raman Scattering (SRS) phenomenon, methods of detecting the signal, and collection of the SRS images. We demonstrate the advantages of SRS imaging, and recent developments, but also the limitations, especially in image capture speeds and spatial resolution. We also compare the use of SRS microscopy in biological system studies with other techniques such as fluorescence microscopy, second-harmonic generation (SHG)-based microscopy, coherent anti-Stokes Raman scattering (CARS), and spontaneous Raman, and we show the compatibility of SRS-based systems with other discussed methods. The review is also focused on indicating innovations in SRS microscopy, on the background of which we present the layout and performance of our homemade setup built from commercially available elements enabling for imaging of the molecular structure of single cells over the spectral range of 800-3600 cm^-1. Methods of image analysis are discussed, including machine learning methods for obtaining images of the distribution of selected molecules and for the detection of pathological lesions in tissues or malignant cells in the context of clinical diagnosis of a wide range of diseases with the use of SRS microscopy. Finally, perspectives for the development of SRS microscopy are proposed.

Optical Electrophysiology: Toward the Goal of Label-Free Voltage Imaging

Measuring and monitoring the electrical signals transmitted between neurons is key to understanding the communication between neurons that underlies human perception, information processing, and decision-making. While electrode-based electrophysiology has been the gold standard, optical electrophysiology has opened up a new area in the past decade. Voltage-dependent fluorescent reporters enable voltage imaging with high spatial resolution and flexibility to choose recording locations. However, they exhibit photobleaching as well as phototoxicity and may perturb the physiology of the cell. Label-free optical electrophysiology seeks to overcome these hurdles by detecting electrical activities optically, without the incorporation of exogenous fluorophores in cells. For example, electrochromic optical recording detects neuroelectrical signals via a voltage-dependent color change of extracellular materials, and interferometric optical recording monitors membrane deformations that accompany electrical activities. Label-free optical electrophysiology, however, is in an early stage, and often has limited sensitivity and temporal resolution. In this Perspective, we review the recent progress to overcome these hurdles. We hope this Perspective will inspire developments of label-free optical electrophysiology techniques with high recording sensitivity and temporal resolution in the near future.

Presence of intralesional melanocytes as a histopathological feature of actinic keratosis based on in vivo harmonic generation microscopy in Asians

BACKGROUND

Most patients with actinic keratosis (AK) present with more than one lesion. Although histopathological examination is the gold standard for diagnosing this condition, performing an invasive skin biopsy for each AK is impractical. Thus, this study aimed to identify AK's morphological characteristics based on harmonic generation microscopy (HGM). Moreover, the correlation between features observed using HGM and histopathological grading of AK was examined.

METHODS

Lesions of seven patients were examined using HGM (n = 1, ex vivo and n = 6, in vivo), and histopathological examinations of the biopsy specimens were also performed. The features of each AK, based on HGM, were assessed and compared with corresponding standard histopathological findings.

RESULTS

Using the histopathological findings as a standard reference, HGM's accuracy in detecting features of AK lesions, such as hyperkeratosis, epidermal thinning, abnormal architecture, and atypical honeycomb pattern, was 100%. Approximately five (72%) patients had similar histopathological grades. Moreover, based on HGM, except for one patient with grade 1 AK, six (85.71%) patients had lesions with intraepidermal dendritic cell-like cells, representing melanocytes.

CONCLUSION

Harmonic generation microscopy can be used in vivo to provide critical diagnostic information with a resolution comparable to histopathological examination. In addition, intralesional melanocytes in AK, which may be correlated with disease severity, can be specifically enhanced using HGM.

Review of Stimulated Raman Scattering Microscopy Techniques and Applications in the Biosciences

Stimulated Raman scattering (SRS) microscopy is a nonlinear optical imaging method for visualizing chemical content based on molecular vibrational bonds. Featuring high speed, high resolution, high sensitivity, high accuracy, and 3D sectioning, SRS microscopy has made tremendous progress toward biochemical information acquisition, cellular function investigation, and label-free medical diagnosis in the biosciences. In this review, the principle of SRS, system design, and data analysis are introduced, and the current innovations of the SRS system are reviewed. In particular, combined with various bio-orthogonal Raman tags, the applications of SRS microscopy in cell metabolism, tumor diagnosis, neuroscience, drug tracking, and microbial detection are briefly examined. The future prospects for SRS microscopy are also shared.

Photobiomodulation of avian embryos by red laser

The current research focuses on the effect of variable doses of red laser on the chick embryonic development. He-Ne laser of 632-nm wavelength was used as an irradiation source in the first 48 h post-laying of chicken eggs. We have used five different doses: 2, 1, 0.3, 0.2, and 0.1 mJ/cm² that needed a time range for about 400-20 s. Those irradiated embryos were left for additional 11 days for incubation in normal conditions, where they are blindly studied after the 11^th day. Light microscopy was used in this study to investigate the histological and pathological features of the different experimental groups compared to the control one. However, electron microcopy was utilized to trace the apoptotic distribution in the developmental embryos. Minor abnormalities that are dependent on the laser dose have been shown in the irradiated embryos when compared to the sham group, where the highest laser dose showed about 12% embryonic development anomalies when related to the other irradiated groups. Irradiated embryos were found to express more INF-γ and IL-2 as circulating cytokines relative to the unexposed group, where the levels of IL-2 were highly significantly increased by all laser doses (0.3 mJ/cm² light dose recipient group showed significant increase only when compared to the control group). IFN-γ levels were significantly increased as well by light doses above 0.2 mJ/cm². This IFN-γ increase trend seemed to be laser dose-dependent. Simultaneously, these combined results propose the ability of high laser doses in inducing incurable changes in the embryonic development and consequently such alterations can have potential therapeutic applications through what is known as photobiomodulation.

Toxicity of polyelectrolyte-functionalized titania nanoparticles in zebrafish (Danio rerio) embryos

We investigated the effects of short-term exposure of bare TiO2NPs and polyelectrolyte-coated TiO2NPs in the 5–25 nm size range, at relatively high concentrations (of 500 and 1000 mg/L) under light or dark conditions, in D. rerio embryos. The biological endpoints investigated included embryo viability and mRNA transcript levels of antioxidant and membrane transport genes relative to control embryos. The presence of nanoparticles on the surface of embryos was assessed using TEM. The results confirm an accumulation of TiO2NPs on the outer surface (chorion) of the embryo, but not within the embryo. No significant difference in embryo viability was detected following each exposure regime. The expression of antioxidant biomarker, SOD2, was significantly impacted by the type of TiO2NP, with TiO2NPs/PSS/PAH coating exposure showing down regulation; the concentration of the nanoparticles, with down regulation at 500 mg/L; and dark/light condition with down regulation in the light. The expression levels of the hypoxia and membrane markers, HIF1 and Pxmp2, were not significantly impacted by any factor. The study indicates that SOD2 mRNA expression levels may be useful in the detection of apparent oxidative stress induced by the titania nanoparticle build up on the embryo chorion surface.

Slide-free clinical imaging of melanin with absolute quantities using label-free third-harmonic-generation enhancement-ratio microscopy

The capability to image the 3D distribution of melanin in human skin in vivo with absolute quantities and microscopic details will not only enable noninvasive histopathological diagnosis of melanin-related cutaneous disorders, but also make long term treatment assessment possible. In this paper, we demonstrate clinical in vivo imaging of the melanin distribution in human skin with absolute quantities on mass density and with microscopic details by using label-free third-harmonic-generation (THG) enhancement-ratio microscopy. As the dominant absorber in skin, melanin provides the strongest THG nonlinearity in human skin due to resonance enhancement. We show that the THG-enhancement-ratio (erTHG) parameter can be calibrated in vivo and can indicate the melanin mass density. With an unprecedented clinical imaging resolution, our study revealed erTHG-microscopy's unique capability for long-term treatment assessment and direct clinical observation of melanin's micro-distribution to shed light into the unknown pathway and regulation mechanism of melanosome transfer and translocation.

Highlights on selected microscopy techniques to study zebrafish developmental biology

Bio-imaging is a tedious task when it concerns exploring cell functions, developmental mechanisms, and other vital processes in vivo. Single-cell resolution is challenging due to different issues such as sample size, the scattering of intact and opaque tissue, pigmentation in untreated animals, the movement of living organs, and maintaining the sample under physiological conditions. These factors might lead researchers to implement microscopy techniques with a suitable animal model to mimic the nature of the living cells. Zebrafish acquired its prestigious reputation in the biomedical research field due to its transparency under advanced microscopes. Therefore, various microscopy techniques, including Multi-Photon, Light-Sheet Microscopy, and Second Harmonic Generation, simplify the discovery of different types of internal functions in zebrafish. In this review, we briefly discuss three recent microscopy techniques that are being utilized because they are non-invasive in investigating developmental events in zebrafish embryo and larvae.

Comparative analysis of intrinsic skin aging between Caucasian and Asian subjects by slide-free in vivo harmonic generation microscopy

Phenotypical and functional differences in the intrinsic skin aging process of individuals between Caucasians and Asians have generated considerable interest in dermatology and cosmetic industry. Most of the studies focused on the stratum corneum, and in some other studies inter-individual differences overwhelms the racial difference. None of the studies comparatively analyzes the difference from the histopathological point of view. Here we report our harmonic generation microscopy study to analyze the difference of intrinsic aging between Caucasian and Asian skin from a histopathological point of view. As a result, the cellular and nuclear areas of basal cells in Caucasian subjects were found to increase at the same rate as the Asian subjects, ideal for scoring age. The maximum thickness of the viable epidermis, the dermal papilla (DP) volume per unit area and the depth of the DP zone in Caucasians were found to decrease at faster rates than those in Asians.

Additive-color multi-harmonic generation microscopy for simultaneous label-free differentiation of plaques, tangles, and neuronal axons

Multicolor fluorescence imaging has been widely used by neuroscientists to simultaneously observe different neuropathological features of the brain. However, these optical modalities rely on exogenous labeling. Here, we demonstrate, for the first time, a label-free additive-color multi-harmonic generation microscopy to elucidate, concurrently with different hues, Alzheimer's disease (AD) neuropathological hallmarks: amyloid β (Aβ) plaques and neurofibrillary tangles (NFT). By treating third harmonic generation (THG) and second harmonic generation (SHG) as two primary colors, our study can simultaneously label-free differentiate AD hallmarks by providing different additive colors between Aβ plaques, NFT, and neuronal axons, with weaker THG presentation from NFT in most places of the brain. Interestingly our pixel-based quantification and Pearson's correlation results further corroborated these findings. Our proposed label-free technique fulfills the unmet challenge in the clinical histopathology for stain-free slide-free differential visualization of neurodegenerative disease pathologies, with a sub-femtoliter resolution in a single image field-of-view.

In vivo third-harmonic generation microscopy study on vitiligo patients

Melanin is known to provide strong third-harmonic generation (THG) contrast in human skin. With a high concentration in basal cell cytoplasm, THG contrast provided by melanin overshadows other THG sources in human skin studies. For better understanding of the THG signals in keratinocytes without the influence of melanin, an in vivo THG microscopy (THGM) study was first conducted on vitiliginous skin. As a result, the THG-brightness ratio between the melanin-lacking cytoplasm of basal cells and collagen fibers is about 1.106 at the dermal-epidermal junctions of vitiliginous skin, indicating high sensitivity of THGM for the presence of melanin. We further applied the in vivo THGM to assist evaluating the therapeutic outcome from the histopathological point of view for those showed no improvement under narrowband ultraviolet B therapy based on the seven-point Physician Global Assessment score. Our clinical study indicates the high potential of THGM to assist the histopathological assessment of the therapeutic efficacy of vitiligo treatments.

Aggregation-Induced Nonlinear Optical Effects of AIEgen Nanocrystals for Ultradeep In Vivo Bioimaging

Nonlinear optical microscopy has become a powerful tool in bioimaging research due to its unique capabilities of deep optical sectioning, high-spatial-resolution imaging, and 3D reconstruction of biological specimens. Developing organic fluorescent probes with strong nonlinear optical effects, in particular third-harmonic generation (THG), is promising for exploiting nonlinear microscopic imaging for biomedical applications. Herein, a simple method for preparing organic nanocrystals based on an aggregation-induced emission (AIE) luminogen (DCCN) with bright near-infrared emission is successfully demonstrated. Aggregation-induced nonlinear optical effects, including two-photon fluorescence (2PF), three-photon fluorescence (3PF), and THG, of DCCN are observed in nanoparticles, especially for crystalline nanoparticles. The nanocrystals of DCCN are successfully applied for 2PF microscopy at 1040 nm NIR-II excitation and THG microscopy at 1560 nm NIR-II excitation, respectively, to reconstruct the 3D vasculature of the mouse cerebral vasculature. Impressively, the THG microscopy provides much higher spatial resolution and brightness than the 2PF microscopy and can visualize small vessels with diameters of ≈2.7 µm at the deepest depth of 800 µm in a mouse brain. Thus, this is expected to inspire new insights into the development of advanced AIE materials with multiple nonlinearity, in particular THG, for multimodal nonlinear optical microscopy.

Harmonic Generation Microscopy 2.0: New Tricks Empowering Intravital Imaging for Neuroscience

Optical harmonic generation, e.g., second- (SHG) and third-harmonic generation (THG), provides intrinsic contrasts for three-dimensional intravital microscopy. Contrary to two-photon excited fluorescence (TPEF), however, they have found relatively specialized applications, such as imaging collagenous and non-specific tissues, respectively. Here we review recent advances that broaden the capacity of SHG and THG for imaging the central nervous system in particular. The fundamental contrast mechanisms are reviewed as they encode novel information including molecular origin, spectroscopy, functional probes, and image analysis, which lay foundations for promising future applications in neuroscience.

Three-dimensional intravital imaging in bone research

Intravital imaging has emerged as a novel and efficient tool for visualization of in situ dynamics of cellular behaviors and cell-microenvironment interactions in live animals, based on desirable microscopy techniques featuring high resolutions, deep imaging and low phototoxicity. Intravital imaging, especially based on multi-photon microscopy, has been used in bone research for dynamics visualization of a variety of physiological and pathological events at the cellular level, such as bone remodeling, hematopoiesis, immune responses and cancer development, thus, providing guidance for elucidating novel cellular mechanisms in bone biology as well as guidance for new therapies. This review is aimed at interpreting development and advantages of intravital imaging in bone research, and related representative discoveries concerning bone matrices, vessels, and various cells types involved in bone physiologies and pathologies. Finally, current limitations, further refinement, and extended application of intravital imaging in bone research are concluded.

Molecular understanding of label-free second harmonic imaging of microtubules

Microtubules are a vital component of the cell’s cytoskeleton and their organization is crucial for healthy cell functioning. The use of label-free SH imaging of microtubules remains limited, as sensitive detection is required and the true molecular origin and main determinants required to generate SH from microtubules are not fully understood. Using advanced correlative imaging techniques, we identified the determinants of the microtubule-dependent SH signal. Microtubule polarity, number and organization determine SH signal intensity in biological samples. At the molecular level, we show that the GTP-bound tubulin dimer conformation is fundamental for microtubules to generate detectable SH signals. We show that SH imaging can be used to study the effects of microtubule-targeting drugs and proteins and to detect changes in tubulin conformations during neuronal maturation. Our data provide a means to interpret and use SH imaging to monitor changes in the microtubule network in a label-free manner.

Microtubules (MTs) are well-studied cytoskeleton components, but have primarily been investigated using fixation or invasive techniques. Here, the authors use label-free second harmonic (SH) fluorescence and correlative light electron microscopy to pinpoint determinants required for SH from MTs.

WITHDRAWN: Effect of He-Ne laser irradiation on embryonic development in chicken eggs

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). These articles are retracted at the request of the authors. The joint Editors-in-Chief agree with this decision.

Multimodal imaging platform for optical virtual skin biopsy enabled by a fiber-based two-color ultrafast laser source

We demonstrate multimodal label-free nonlinear optical microscopy in human skin enabled by a fiber-based two-color ultrafast source. Energetic femtosecond pulses at 775 nm and 1250 nm are simultaneously generated by an Er-fiber laser source employing frequency doubling and self-phase modulation enabled spectral selection. The integrated nonlinear optical microscope driven by such a two-color femtosecond source enables the excitation of endogenous two-photon excitation fluorescence, second-harmonic generation, and third-harmonic generation in human skin. Such a 3-channel imaging platform constitutes a powerful tool for clinical application and optical virtual skin biopsy.

Third harmonic generation imaging and analysis of the effect of low gravity on the lacuno-canalicular network of mouse bone

The lacuno-canalicular network (LCN) hosting the osteocytes in bone tissue represents a biological signature of the mechanotransduction activity in response to external biomechanical loading. Using third-harmonic generation (THG) microscopy with sub-micrometer resolution, we investigate the impact of microgravity on the 3D LCN structure in mice following space flight. A specific analytical procedure to extract the LCN characteristics from THG images is described for ex vivo studies of bone sections. The analysis conducted in different anatomical quadrants of femoral cortical bone didn’t reveal any statistical differences between the control, habitat control and flight groups, suggesting that the LCN connectivity is not affected by one month space flight. However, significant variations are systematically observed within each sample. We show that our current lack of understanding of the extent of the LCN heterogeneity at the organ level hinders the interpretation of such investigations based on a limited number of samples and we discuss the implications for future biomedical studies.

Discovering Macrophage Functions Using In Vivo Optical Imaging Techniques

Macrophages are an important component of host defense and inflammation and play a pivotal role in immune regulation, tissue remodeling, and metabolic regulation. Since macrophages are ubiquitous in human bodies and have versatile physiological functions, they are involved in virtually every disease, including cancer, diabetes, multiple sclerosis, and atherosclerosis. Molecular biological and histological methods have provided critical information on macrophage biology. However, many in vivo dynamic behaviors of macrophages are poorly understood and yet to be discovered. A better understanding of macrophage functions and dynamics in pathogenesis will open new opportunities for better diagnosis, prognostic assessment, and therapeutic intervention. In this article, we will review the advances in macrophage tracking and analysis with in vivo optical imaging in the context of different diseases. Moreover, this review will cover the challenges and solutions for optical imaging techniques during macrophage intravital imaging.

Label-free imaging of bone multiscale porosity and interfaces using third-harmonic generation microscopy

Interfaces provide the structural basis of essential bone functions. In the hierarchical structure of bone tissue, heterogeneities such as porosity or boundaries are found at scales ranging from nanometers to millimeters, all of which contributing to macroscopic properties. To date, however, the complexity or limitations of currently used imaging methods restrict our understanding of this functional integration. Here we address this issue using label-free third-harmonic generation (THG) microscopy. We find that the porous lacuno-canalicular network (LCN), revealing the geometry of osteocytes in the bone matrix, can be directly visualized in 3D with submicron precision over millimetric fields of view compatible with histology. THG also reveals interfaces delineating volumes formed at successive remodeling stages. Finally, we show that the structure of the LCN can be analyzed in relation with that of the extracellular matrix and larger-scale structures by simultaneously recording THG and second-harmonic generation (SHG) signals relating to the collagen organization.

Third-harmonic generation Stokes-Mueller polarimetric microscopy

An experimental implementation of the nonlinear Stokes-Mueller polarimetric (NSMP) microscopy in third-harmonic generation modality is presented. The technique is able to extract all eight 2D-accessible χ⁽³⁾ components for any sample from 64 polarization measurements, and can be applied to noninvasive ultrastructural characterization. The polarization signature of an isotropic glass coverslip is presented, and carotenoid crystallites in the root of orange carrot (Daucus carota) are investigated, showing complex χ⁽³⁾ components with a significant chiral contribution.

Two-color multiphoton in vivo imaging with a femtosecond diamond Raman laser

Two-color multiphoton microscopy through wavelength mixing of synchronized lasers has been shown to increase the spectral window of excitable fluorophores without the need for wavelength tuning. However, most currently available dual output laser sources rely on the costly and complicated optical parametric generation approach. In this report, we detail a relatively simple and low cost diamond Raman laser pumped by a ytterbium fiber amplifier emitting at 1055 nm, which generates a first Stokes emission centered at 1240 nm with a pulse width of 100 fs. The two excitation wavelengths of 1055 and 1240 nm, along with the effective two-color excitation wavelength of 1140 nm, provide an almost complete coverage of fluorophores excitable within the range of 1000-1300 nm. When compared with 1055 nm excitation, two-color excitation at 1140 nm offers a 90% increase in signal for many far-red emitting fluorescent proteins (for example, tdKatushka2). We demonstrate multicolor imaging of tdKa-tushka2 and Hoechst 33342 via simultaneous two-color two-photon, and two-color three-photon microscopy in engineered 3D multicellular spheroids. We further discuss potential benefits and applications for two-color three-photon excitation. In addition, we show that this laser system is capable of in vivo imaging in mouse cortex to nearly 1 mm in depth with two-color excitation.

Small teleost fish provide new insights into human skeletal diseases

Small teleost fish such as zebrafish and medaka are increasingly studied as models for human skeletal diseases. Efficient new genome editing tools combined with advances in the analysis of skeletal phenotypes provide new insights into fundamental processes of skeletal development. The skeleton among vertebrates is a highly conserved organ system, but teleost fish and mammals have evolved unique traits or have lost particular skeletal elements in each lineage. Several unique features of the skeleton relate to the extremely small size of early fish embryos and the small size of adult fish used as models. A detailed analysis of the plethora of interesting skeletal phenotypes in zebrafish and medaka pushes available skeletal imaging techniques to their respective limits and promotes the development of new imaging techniques. Impressive numbers of zebrafish and medaka mutants with interesting skeletal phenotypes have been characterized, complemented by transgenic zebrafish and medaka lines. The advent of efficient genome editing tools, such as TALEN and CRISPR/Cas9, allows to introduce targeted deficiencies in genes of model teleosts to generate skeletal phenotypes that resemble human skeletal diseases. This review will also discuss other attractive aspects of the teleost skeleton. This includes the capacity for lifelong tooth replacement and for the regeneration of dermal skeletal elements, such as scales and fin rays, which further increases the value of zebrafish and medaka models for skeletal research.

Two-Color, Two-Photon Imaging at Long Excitation Wavelengths Using a Diamond Raman Laser

We demonstrate that the second-Stokes output from a diamond Raman laser, pumped by a femtosecond Ti:Sapphire laser, can be used to efficiently excite red-emitting dyes by two-photon excitation at 1,080 nm and beyond. We image HeLa cells expressing red fluorescent protein, as well as dyes such as Texas Red and Mitotracker Red. We demonstrate the potential for simultaneous two-color, two-photon imaging with this laser by using the residual pump beam for excitation of a green-emitting dye. We demonstrate this for the combination of Alexa Fluor 488 and Alexa Fluor 568. Because the Raman laser extends the wavelength range of the Ti:Sapphire laser, resulting in a laser system tunable to 680-1,200 nm, it can be used for two-photon excitation of a large variety and combination of dyes.

Art on the Nanoscale and Beyond

Methods of forming and patterning materials at the nano- and microscales are finding increased use as a medium of artistic expression, and as a vehicle for communicating scientific advances to a broader audience. While sharing many attributes of other art forms, miniaturized art enables the direct engagement of sensory aspects such as sight and touch for materials and structures that are otherwise invisible to the eye. The historical uses of nano-/microscale materials and imaging techniques in arts and sciences are presented. The motivations to create artwork at small scales are discussed, and representations in scientific literature and exhibitions are explored. Examples are presented using semiconductors, microfluidics, and nanomaterials as the artistic media; these utilized techniques including micromachining, focused ion beam milling, two-photon polymerization, and bottom-up nanostructure growth. Finally, the technological factors that limit the implementation of artwork at miniature scales are identified, and potential future directions are discussed. As research marches toward even smaller length scales, innovative and engaging visualizations and artistic endeavors will have growing implications on education, communication, policy making, media activism, and public perception of science and technology.

Second harmonic generation microscopy in zebrafish

Modern optical imaging has progressed rapidly with the ability to noninvasively image cellular and subcellular phenomena with high spatial and temporal resolution. In particular, emerging techniques such as second harmonic generation (SHG) microscopy can allow for the monitoring of intrinsic contrast, such as that from collagen, in live and fixed samples. When coupled with multiphoton fluorescence microscopy, SHG can be used to image interactions between cells and the surrounding extracellular environment. There is recent interest in using these approaches to study inflammation and wound healing in zebrafish, an important model for studying these processes. In this chapter we present the practical aspects of using second harmonic generation to image interactions between leukocytes and collagen during wound healing in zebrafish.

Infrared-active quadruple contrast FePt nanoparticles for multiple scale molecular imaging

A single nanomaterial with multiple imaging contrasts and functions is highly desired for multiscale theragnosis. Herein, we demonstrate single 1-1.9 μm infrared-active FePt alloy nanoparticles (FePt NPs) offering unprecedented four-contrast-in-one molecular imaging - computed tomography (CT), magnetic resonance imaging (MRI), photoacoustic (PA) imaging, and high-order multiphoton luminescence (HOMPL) microscopy. The PA response of FePt NPs outperforms that of infrared-active gold nanorods by 3- to 5.6-fold under identical excitation fluence and particle concentrations. HOMPL (680 nm) of an isolated FePt NP renders spatial full-width-at-half-maximum values of 432 nm and 300 nm beyond the optical diffraction limit for 1230-nm and 920-nm excitation, respectively. The in vivo targeting function was successfully visualized using HOMPL, PA imaging, CT, and MRI, thereby validating FePt as a single nanomaterial system covering up to four types (Optical/PA/CT/MRI) of molecular imaging contrast, ranging from the microscopic level to whole-body scale investigation.

Detection of malformations in sea urchin plutei exposed to mercuric chloride using different fluorescent techniques

Embryos of Mediterranean sea urchin Paracentrotus lividus and subtropical Echinometra mathaei were exposed to 5,10, 15 and 20µgL(-1), and to 1, 2, 3 and 4µgL(-1) mercuric chloride (HgCl2), respectively. The effective concentration (EC50) inducing malformation in 50% of 4-arm pluteus stage (P4) was 16.14µgL(-1) for P. lividus and 2.41µgL(-1) for E. mathaei. Two-photon (TP), second (SHG) and third harmonic generation (THG) microscopy techniques, TUNEL staining, propidium iodide (PI) and Hoechst 33342 probes were used to detect light signals or to stain apoptotic and necrotic cells in fixed and alive plutei. Signals were detected differently in the two species: TP fluorescence, commonly associated with apoptotic cells, did not increase with increasing HgCl2 concentrations in P. lividus and in fact, the TUNEL did not reveal induction of apoptosis. PI fluorescence increased in P. lividus in a dose-dependent manner, suggesting a loss of cell permeability. In E. mathaei plutei TP fluorescence increased at increasing HgCl2 concentrations. THG microscopy revealed skeletal rods in both species. Different fluorescent techniques, used in this study, are proposed as early-warning systems to visualize malformations and physiological responses in sea urchin plutei.

Third-harmonic generation susceptibility spectroscopy in free fatty acids

Lipid-correlated disease such as atherosclerosis has been an important medical research topic for decades. Many new microscopic imaging techniques such as coherent anti-Stokes Raman scattering and third-harmonic generation (THG) microscopy were verified to have the capability to target lipids in vivo. In the case of THG microscopy, biological cell membranes and lipid bodies in cells and tissues have been shown as good sources of contrast with a laser excitation wavelength around 1200 nm. We report the THG excitation spectroscopy study of two pure free fatty acids including oleic acid and linoleic acid from 1090 to 1330 nm. Different pure fatty acids presented slightly-different THG χ(3) spectra. The measured peak values of THG third-order susceptibility χ(3) in both fatty acids were surprisingly found not to match completely with the resonant absorption wavelengths around 1190 to 1210 nm, suggesting possible wavelengths selection for enhanced THG imaging of lipids while avoiding laser light absorption. Along with the recent advancement in THG imaging, this new window between 1240 to 1290 nm may offer tremendous new opportunities for sensitive label-free lipid imaging in biological tissues.

Tripling the maximum imaging depth with third-harmonic generation microscopy

The growing interest in performing high-resolution, deep-tissue imaging has galvanized the use of longer excitation wavelengths and three-photon-based techniques in nonlinear imaging modalities. This study presents a threefold improvement in maximum imaging depth of ex vivo porcine vocal folds using third-harmonic generation (THG) microscopy at 1552-nm excitation wavelength compared to two-photon microscopy (TPM) at 776-nm excitation wavelength. The experimental, analytical, and Monte Carlo simulation results reveal that THG improves the maximum imaging depth observed in TPM significantly from 140 to 420 μm in a highly scattered medium, reaching the expected theoretical imaging depth of seven extinction lengths. This value almost doubles the previously reported normalized imaging depths of 3.5 to 4.5 extinction lengths using three-photon-based imaging modalities. Since tissue absorption is substantial at the excitation wavelength of 1552 nm, this study assesses the tissue thermal damage during imaging by obtaining the depth-resolved temperature distribution through a numerical simulation incorporating an experimentally obtained thermal relaxation time (τ). By shuttering the laser for a period of 2τ, the numerical algorithm estimates a maximum temperature increase of ∼2°C at the maximum imaging depth of 420 μm. The paper demonstrates that THG imaging using 1552 nm as an illumination wavelength with effective thermal management proves to be a powerful deep imaging modality for highly scattering and absorbing tissues, such as scarred vocal folds.

Label-free imaging of thick tissue at 1550 nm using a femtosecond optical parametric generator

We have developed a simple wavelength-tunable optical parametric generator (OPG), emitting broadband ultrashort pulses with peak wavelengths at 1530-1790 nm, for nonlinear label-free microscopy. The OPG consists of a periodically poled lithium niobate crystal, pumped at 1064 nm by a ultrafast Yb:fiber laser with high pulse energy. We demonstrate that this OPG can be used for label-free imaging, by third-harmonic generation, of nuclei of brain cells and blood vessels in a >150 μm thick brain tissue section, with very little decay of intensity with imaging depth and no visible damage to the tissue at an incident average power of 15 mW.

Power and challenges of using zebrafish as a model for skeletal tissue imaging

The zebrafish (Danio rerio) is now a widely used model organism in biomedical research. The species is also increasingly used for studying skeletal development and regeneration and for understanding human skeletal diseases. The small size of this model organism is an advantage and an extreme challenge for visualizing and diagnosing the animals' skeleton. This applies especially to early stages of skeletal development. Similar challenges arise for the analysis of the skeleton of other small fish species, such as medaka (Oryzias latipes). High quality histological preparations and knowledge about the special quality of the zebrafish skeleton remain prerequisites for a correct analysis. In addition, new methods for fast and high-resolution 2D and 3D skeletal tissue screening are required for a maximal understanding of skeletal development. We, in this study, review advantages and limitations of adapting current visualization techniques for zebrafish skeletal research. We discuss the methods for in toto visualization, such as X-raying, micro-CT, Alizarin red staining and optical projection tomography. Techniques for in vivo imaging, such as second harmonic generation microscopy and two-photon excitation fluorescence, are also discussed. Finally, we explore the possibilities of light-sheet microscopy for the analysis of the zebrafish skeleton.

Third-harmonic generation microscopy reveals dental anatomy in ancient fossils

Fossil teeth are primary tools in the study of vertebrate evolution, but standard imaging modalities have not been capable of providing high-quality images in dentin, the main component of teeth, owing to small refractive index differences in the fossilized dentin. Our first attempt to use third-harmonic generation (THG) microscopy in fossil teeth has yielded significant submicrometer level anatomy, with an unexpectedly strong signal contrasting fossilized tubules from the surrounding dentin. Comparison between fossilized and extant teeth of crocodilians reveals a consistent evolutionary signature through time, indicating the great significance of THG microscopy in the evolutionary studies of dental anatomy in fossil teeth.

Imaging the zebrafish dentition: from traditional approaches to emerging technologies

The zebrafish, a model organism for which a plethora of molecular and genetic techniques exists, has a lifelong replacing dentition of 22 pharyngeal teeth. This is in contrast to the mouse, which is the key organism in dental research but whose teeth are never replaced. Employing the zebrafish as the main organism to elucidate the mechanisms of continuous tooth replacement, however, poses at least one major problem, related to the fact that all teeth are located deep inside the body. Investigating tooth replacement thus relies on conventional histological methods, which are often laborious, time-consuming and can cause tissue deformations. In this review, we investigate the advantages and limitations of adapting current visualization techniques to dental research in zebrafish. We discuss techniques for fast sectioning, such as vibratome sectioning and high-resolution episcopic microscopy, and methods for in toto visualization, such as Alizarin red staining, micro-computed tomography, and optical projection tomography. Techniques for in vivo imaging, such as two-photon excitation fluorescence and second harmonic generation microscopy, are also covered. Finally, the possibilities of light sheet microscopy are addressed.

Compact multimode fiber beam-shaping system based on GPU accelerated digital holography

Real-time, on-demand, beam shaping at the end of the multimode fiber has recently been made possible by exploiting the computational power of rapidly evolving graphics processing unit (GPU) technology [Opt. Express 22, 2933 (2014)]. However, the current state-of-the-art system requires the presence of an acousto-optic deflector (AOD) to produce images at the end of the fiber without interference effects between neighboring output points. Here, we present a system free from the AOD complexity where we achieve the removal of the undesired interference effects computationally using GPU implemented Gerchberg-Saxton and Yang-Gu algorithms. The GPU implementation is two orders of magnitude faster than the CPU implementation which allows video-rate image control at the distal end of the fiber virtually free of interference effects.

Third Harmonic Generation microscopy as a reliable diagnostic tool for evaluating lipid body modification during cell activation: the example of BV-2 microglia cells

Nonlinear optical processes have found widespread applications in fields ranging from fundamental physics to biomedicine. In this study, we attempted to evaluate cell activation by using the Third Harmonic Generation (THG) imaging microscopy as a new diagnostic tool. The BV-2 microglia cell line with or without activation by lipopolysaccharide was chosen as a representative biological model. The results showed that THG imaging could discriminate between the control versus activated state of BV-2 cells not only as to THG signal intensity but also as to THG signal area, while verifying once more that the majority of the intracellular detected signal corresponds to lipid bodies. Since THG imaging is a real time, non-destructive modality and does not require any prior cell processing and staining, the results presented here provide an important tool for normal versus activated cell discrimination, which could be proved very useful in the study of inflammation.

Mitigating phototoxicity during multiphoton microscopy of live Drosophila embryos in the 1.0-1.2 µm wavelength range

Light-induced toxicity is a fundamental bottleneck in microscopic imaging of live embryos. In this article, after a review of photodamage mechanisms in cells and tissues, we assess photo-perturbation under illumination conditions relevant for point-scanning multiphoton imaging of live Drosophila embryos. We use third-harmonic generation (THG) imaging of developmental processes in embryos excited by pulsed near-infrared light in the 1.0-1.2 µm range. We study the influence of imaging rate, wavelength, and pulse duration on the short-term and long-term perturbation of development and define criteria for safe imaging. We show that under illumination conditions typical for multiphoton imaging, photodamage in this system arises through 2- and/or 3-photon absorption processes and in a cumulative manner. Based on this analysis, we derive general guidelines for improving the signal-to-damage ratio in two-photon (2PEF/SHG) or THG imaging by adjusting the pulse duration and/or the imaging rate. Finally, we report label-free time-lapse 3D THG imaging of gastrulating Drosophila embryos with sampling appropriate for the visualisation of morphogenetic movements in wild-type and mutant embryos, and long-term multiharmonic (THG-SHG) imaging of development until hatching.

Optical assessment of the cardiac rhythm of contracting cardiomyocytes in vitro and a pulsating heart in vivo for pharmacological screening

Our quest in the pathogenesis and therapies targeting human heart diseases requires assessment of the contractile dynamics of cardiac models of varied complexity, such as isolated cardiomyocytes and the heart of a model animal. It is hence beneficial to have an integral means that can interrogate both cardiomyocytes in vitro and a heart in vivo. Herein we report an application of dual-beam optical reflectometry to determine noninvasively the rhythm of two representative cardiac models-chick embryonic cardiomyocytes and the heart of zebrafish. We probed self-beating cardiomyocytes and revealed the temporally varying contractile frequency with a short-time Fourier transform. Our unique dual-beam setup uniquely records the atrial and ventricular pulsations of zebrafish simultaneously. To minimize the cross talk between signals associated with atrial and ventricular chambers, we particularly modulated the two probe beams at distinct frequencies and extracted the signals specific to individual cardiac chambers with phase-sensitive detection. With this setup, we determined the atrio-ventricular interval, a parameter that is manifested by the electrical conduction from the atrium to the ventricle. To demonstrate pharmacological applications, we characterized zebrafish treated with various cardioactive and cardiotoxic drugs, and identified abnormal cardiac rhythms and atrioventricular (AV) blocks of varied degree. In light of its potential capability to assess cardiac models both in vitro and in vivo and to screen drugs with cardioactivity or toxicity, we expect this approach to have broad applications ranging from cardiopharmacology to developmental biology.

Strong guided mode resonant local field enhanced visible harmonic generation in an azo-polymer resonant waveguide grating

Guided mode resonance (GMR) enhanced second- and third-harmonic generation (SHG and THG) is demonstrated in an azo-polymer resonant waveguide grating (RWG), comprised of a poled azo-polymer layer on top of a textured SU8 substrate with a thin intervening layer of TiO2. Strong SHG and THG outputs are observed by matching either in-coming fundamental- or out-going harmonic-wavelength to the GMR wavelengths of the azo-polymer RWG. Without the azo-polymer coating, pure TiO2 RWGs, do not generate any detectable SHG using a fundamental beam peak intensity of 2 MW/cm(2). Without the textured TiO2 layer, a planar poled azo-polymer layer results in 3650 times less SHG than the full nonlinear RWG structure under identical excitation conditions. Rigorous coupled-wave analysis calculations confirm that this enhancement of the nonlinear conversion is due to strong local electric fields that are generated at the interfaces of the TiO2 and azo-polymer layers when the RWG is excited at resonant wavelengths associated with both SHG and THG conversion processes.

Virtual spatial overlap modulation microscopy for resolution improvement

High spatial and temporal resolutions are important advantages of optical imaging over other modalities. The recently developed spatial overlap modulation (SPOM) microscopy enables high resolution imaging by spatial modulation of double-beam overlap. However, SPOM suffers from bad temporal resolution and high system complexity. In this paper, we re-formulate the SPOM resolution theory and develop Virtual SPOM (vSPOM) microscopy. By one-way oversampling and convolution with differential filters, vSPOM not only realizes the same factor of spatial resolution improvement as SPOM, but overcome SPOM's major drawbacks. We demonstrated vSPOM on in vivo clinical images and find that the Gabor filter, which represents two-beam vSPOM, is the most effective among all vSPOM filters. The development of vSPOM enables easy incorporation of SPOM into any imaging system, and extends the use of SPOM to real-time in vivo applications.

Towards 3D in silico modeling of the sea urchin embryonic development

Embryogenesis is a dynamic process with an intrinsic variability whose understanding requires the integration of molecular, genetic, and cellular dynamics. Biological circuits function over time at the level of single cells and require a precise analysis of the topology, temporality, and probability of events. Integrative developmental biology is currently looking for the appropriate strategies to capture the intrinsic properties of biological systems. The "-omic" approaches require disruption of the function of the biological circuit; they provide static information, with low temporal resolution and usually with population averaging that masks fast or variable features at the cellular scale and in a single individual. This data should be correlated with cell behavior as cells are the integrators of biological activity. Cellular dynamics are captured by the in vivo microscopy observation of live organisms. This can be used to reconstruct the 3D + time cell lineage tree to serve as the basis for modeling the organism's multiscale dynamics. We discuss here the progress that has been made in this direction, starting with the reconstruction over time of three-dimensional digital embryos from in toto time-lapse imaging. Digital specimens provide the means for a quantitative description of the development of model organisms that can be stored, shared, and compared. They open the way to in silico experimentation and to a more theoretical approach to biological processes. We show, with some unpublished results, how the proposed methodology can be applied to sea urchin species that have been model organisms in the field of classical embryology and modern developmental biology for over a century.

Automatic cell segmentation and nuclear-to-cytoplasmic ratio analysis for third harmonic generated microscopy medical images

Traditional biopsy procedures require invasive tissue removal from a living subject, followed by time-consuming and complicated processes, so noninvasive in vivo virtual biopsy, which possesses the ability to obtain exhaustive tissue images without removing tissues, is highly desired. Some sets of in vivo virtual biopsy images provided by healthy volunteers were processed by the proposed cell segmentation approach, which is based on the watershed-based approach and the concept of convergence index filter for automatic cell segmentation. Experimental results suggest that the proposed algorithm not only reveals high accuracy for cell segmentation but also has dramatic potential for noninvasive analysis of cell nuclear-to-cytoplasmic ratio (NC ratio), which is important in identifying or detecting early symptoms of diseases with abnormal NC ratios, such as skin cancers during clinical diagnosis via medical imaging analysis.

Third-harmonic generation microscopy with Bessel beams: a numerical study

We study theoretically and numerically third-harmonic generation (THG) from model geometries (interfaces, slabs, periodic media) illuminated by Bessel beams produced by focusing an annular intensity profile. Bessel beams exhibit a phase and intensity distribution near focus different from Gaussian beams, resulting in distinct THG phase matching properties and coherent scattering directions. Excitation wave vectors are controlled by adjusting the bounding aperture angles of the Bessel beam. In addition to extended depth-of-field imaging, this opens interesting perspectives for coherent nonlinear microscopy, such as extracting sample spatial frequencies in the λ/8 - λ range in the case of organized media.

Imaging granularity of leukocytes with third harmonic generation microscopy

Using third harmonic generation (THG) microscopy, we demonstrate that granularity differences of leukocytes can be revealed without a label. Excited by a 1230 nm femtosecond laser, THG signals were generated at a significantly higher level in neutrophils than other mononuclear cells, whereas signals in agranular lymphocytes were one order of magnitude smaller. Interestingly, the characteristic THG features can also be observed in vivo to track the newly recruited leukocytes following lipopolysaccharide (LPS) challenge. These results suggest that label-free THG imaging may provide timely tracking of leukocyte movement without disturbing the normal cellular or physiological status.

3D+t morphological processing: applications to embryogenesis image analysis

We propose to directly process 3D + t image sequences with mathematical morphology operators using a new classification of the 3D+t structuring elements. Several methods (filtering, tracking, segmentation) dedicated to the analysis of 3D + t datasets of zebrafish embryogenesis are introduced and validated through a synthetic dataset. Then, we illustrate the application of these methods to the analysis of datasets of zebrafish early development acquired with various microscopy techniques. This processing paradigm produces spatio-temporal coherent results as it benefits from the intrinsic redundancy of the temporal dimension and minimizes the needs for human intervention in semi-automatic algorithms.

Quantitative in vivo imaging of embryonic development: opportunities and challenges

Animal models are critically important for a mechanistic understanding of embryonic morphogenesis. For decades, visualizing these rapid and complex multidimensional events has relied on projection images and thin section reconstructions. While much insight has been gained, fixed tissue specimens offer limited information on dynamic processes that are essential for tissue assembly and organ patterning. Quantitative imaging is required to unlock the important basic science and clinically relevant secrets that remain hidden. Recent advances in live imaging technology have enabled quantitative longitudinal analysis of embryonic morphogenesis at multiple length and time scales. Four different imaging modalities are currently being used to monitor embryonic morphogenesis: optical, ultrasound, magnetic resonance imaging (MRI), and micro-computed tomography (micro-CT). Each has its advantages and limitations with respect to spatial resolution, depth of field, scanning speed, and tissue contrast. In addition, new processing tools have been developed to enhance live imaging capabilities. In this review, we analyze each type of imaging source and its use in quantitative study of embryonic morphogenesis in small animal models. We describe the physics behind their function, identify some examples in which the modality has revealed new quantitative insights, and then conclude with a discussion of new research directions with live imaging.