Visualization of mitochondria in cardiomyocytes by simultaneous harmonic generation and fluorescence microscopy

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.

Resolving arteriolar wall structures in mouse brain in vivo with three-photon microscopy

The brain arteriolar wall is a multilayered structure, whose integrity is of key significance to the brain function. However, resolving these different layers in anmial models in vivo is hampered by the lack of either labeling or imaging technology. Here, we demonstrate that three-photon microscopy (3PM) is an ideal solution. In mouse brain in vivo, excited at the 1700-nm window, label-free third-harmonic generation imaging and three-photon fluorescence (3PF) imaging with Alexa 633 labeling colocalize and resolve the internal elastic lamina. Furthermore, Alexa Fluor 594-conjugated Wheat Germ Agglutinin (WGA-594) shows time-dependent labeling behavior. As time lapses, WGA-594 first labels endothelium, and then vascular smooth muscle cells, which are readily captured and resolved with 3PF imaging. Our results show that 3PM, in combination with proper labeling, is a promising technology for investigating the structures of brain arteriolar wall in vivo.

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.

Perspectives on mitochondrial relevance in cardiac ischemia/reperfusion injury

Cardiovascular disease is the most common cause of death worldwide and in particular, ischemic heart disease holds the most considerable position. Even if it has been deeply studied, myocardial ischemia-reperfusion injury (IRI) is still a side-effect of the clinical treatment for several heart diseases: ischemia process itself leads to temporary damage to heart tissue and obviously the recovery of blood flow is promptly required even if it worsens the ischemic injury. There is no doubt that mitochondria play a key role in pathogenesis of IRI: dysfunctions of these important organelles alter cell homeostasis and survival. It has been demonstrated that during IRI the system of mitochondrial quality control undergoes alterations with the disruption of the complex balance between the processes of mitochondrial fusion, fission, biogenesis and mitophagy. The fundamental role of mitochondria is carried out thanks to the finely regulated connection to other organelles such as plasma membrane, endoplasmic reticulum and nucleus, therefore impairments of these inter-organelle communications exacerbate IRI. This review pointed to enhance the importance of the mitochondrial network in the pathogenesis of IRI with the aim to focus on potential mitochondria-targeting therapies as new approach to control heart tissue damage after ischemia and reperfusion process.

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.

Label-free three-photon imaging of intact human cerebral organoids for tracking early events in brain development and deficits in Rett syndrome

Human cerebral organoids are unique in their development of progenitor-rich zones akin to ventricular zones from which neuronal progenitors differentiate and migrate radially. Analyses of cerebral organoids thus far have been performed in sectioned tissue or in superficial layers due to their high scattering properties. Here, we demonstrate label-free three-photon imaging of whole, uncleared intact organoids (~2 mm depth) to assess early events of early human brain development. Optimizing a custom-made three-photon microscope to image intact cerebral organoids generated from Rett Syndrome patients, we show defects in the ventricular zone volumetric structure of mutant organoids compared to isogenic control organoids. Long-term imaging live organoids reveals that shorter migration distances and slower migration speeds of mutant radially migrating neurons are associated with more tortuous trajectories. Our label-free imaging system constitutes a particularly useful platform for tracking normal and abnormal development in individual organoids, as well as for screening therapeutic molecules via intact organoid imaging.

Disruption of Drosophila larval muscle structure and function by UNC45 knockdown

Background

Proper muscle function is heavily dependent on highly ordered protein complexes. UNC45 is a USC (named since this region is shared by three proteins UNC45/CRO1/She4P) chaperone that is necessary for myosin incorporation into the thick filaments. UNC45 is expressed throughout the entire Drosophila life cycle and it has been shown to be important during late embryogenesis when initial muscle development occurs. However, the effects of UNC45 manipulation at later developmental times, after muscle development, have not yet been studied.

Main results

UNC45 was knocked down with RNAi in a manner that permitted survival to the pupal stage, allowing for characterization of sarcomere organization in the well-studied third instar larvae. Second harmonic generation (SHG) microscopy revealed changes in the striated pattern of body wall muscles as well as a reduction of signal intensity. This observation was confirmed with immunofluorescence and electron microscopy imaging, showing diminished UNC45 signal and disorganization of myosin and z-disk proteins. Concomitant alterations in both synaptic physiology and locomotor function were also found. Both nerve-stimulated response and spontaneous vesicle release were negatively affected, while larval movement was impaired.

Conclusions

This study highlights the dependency of normal sarcomere structure on UNC45 expression. We confirm the known role of UNC45 for myosin localization and further show the I-Z-I complex is also disrupted. This suggests a broad need for UNC45 to maintain sarcomere integrity. Newly discovered changes in synaptic physiology reveal the likely presence of a homeostatic response to partially maintain synaptic strength and muscle function.

Mitochondrial Dynamics in Adult Cardiomyocytes and Heart Diseases

Mitochondria are the powerhouse organelles of cells; they participate in ATP generation, calcium homeostasis, oxidative stress response, and apoptosis. Thus, maintenance of mitochondrial function is critical for cellular functions. As highly dynamic organelles, the function of mitochondria is dynamically regulated by their fusion and fission in many cell types, which regulate mitochondrial morphology, number, distribution, metabolism, and biogenesis in cells. Mature rod-shaped cardiomyocytes contain thousands of end-to-end contacted spheroid mitochondria. The movement of mitochondria in these cells is limited, which hinders the impetus for research into mitochondrial dynamics in adult cardiomyocytes. In this review, we discuss the most recent progress in mitochondrial dynamics in mature (adult) cardiomyocytes and the relationship thereof with heart diseases.

Dual- and single-shot susceptibility ratio measurements with circular polarizations in second-harmonic generation microscopy

Polarization-resolved second-harmonic generation (P-SHG) microscopy is a technique capable of characterizing nonlinear optical properties of noncentrosymmetric biomaterials by extracting the nonlinear susceptibility tensor components ratio χzzz2'/χzxx2' , with z-axis parallel and x-axis perpendicular to the C₆ symmetry axis of molecular fiber, such as a myofibril or a collagen fiber. In this paper, we present two P-SHG techniques based on incoming and outgoing circular polarization states for a fast extraction of χzzz2'/χzxx2' : A dual-shot configuration where the SHG circular anisotropy generated using incident right- and left-handed circularly-polarized light is measured; and a single-shot configuration for which the SHG circular anisotropy is measured using only one incident circular polarization state. These techniques are used to extract the χzzz2'/χzxx2' of myosin fibrils in the body wall muscles of Drosophila melanogaster larva. The results are in good agreement with values obtained from the double Stokes-Mueller polarimetry. The dual- and single-shot circular anisotropy measurements can be used for fast imaging that is independent of the in-plane orientation of the sample. They can be used for imaging of contracting muscles, or for high throughput imaging of large sample areas.

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.

Second harmonic generation double stokes Mueller polarimetric microscopy of myofilaments

The experimental implementation of double Stokes Mueller polarimetric microscopy is presented. This technique enables a model-independent and complete polarimetric characterization of second harmonic generating samples using 36 Stokes parameter measurements at different combinations of incoming and outgoing polarizations. The degree of second harmonic polarization and the molecular nonlinear susceptibility ratio are extracted for individual focal volumes of a fruit fly larva wall muscle.

Organized Aggregation of Porphyrins in Lipid Bilayers for Third Harmonic Generation Microscopy

Nonlinear optical microscopy has become a powerful tool for high-resolution imaging of cellular and subcellular composition, morphology, and interactions because of its high spatial resolution, deep penetration, and low photo-damage to tissue. Developing specific harmonic probes is essential for exploiting nonlinear microscopic imaging for biomedical applications. We report an organized aggregate of porphyrins (OAP) that formed within lipidic nanoparticles showing fingerprint spectroscopic properties, structure-associated second harmonic generation, and superradiant third harmonic generation. The OAP facilitated harmonic microscopic imaging of living cells with significantly enhanced contrast. The structure-dependent switch between harmonic (OAP-intact) and fluorescence (OAP-disrupted) generation enabled real-time multi-modality imaging of the cellular fate of nanoparticles. Robustly produced under various conditions and easily incorporated into pre-formed lipid nanovesicles, OAP provides a biocompatible nanoplatform for harmonic imaging.

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.

Characterization of tissue-engineered posterior corneas using second- and third-harmonic generation microscopy

Three-dimensional tissues, such as the cornea, are now being engineered as substitutes for the rehabilitation of vision in patients with blinding corneal diseases. Engineering of tissues for translational purposes requires a non-invasive monitoring to control the quality of the resulting biomaterial. Unfortunately, most current methods still imply invasive steps, such as fixation and staining, to clearly observe the tissue-engineered cornea, a transparent tissue with weak natural contrast. Second- and third-harmonic generation imaging are well known to provide high-contrast, high spatial resolution images of such tissues, by taking advantage of the endogenous contrast agents of the tissue itself. In this article, we imaged tissue-engineered corneal substitutes using both harmonic microscopy and classic histopathology techniques. We demonstrate that second- and third-harmonic imaging can non-invasively provide important information regarding the quality and the integrity of these partial-thickness posterior corneal substitutes (observation of collagen network, fibroblasts and endothelial cells). These two nonlinear imaging modalities offer the new opportunity of monitoring the engineered corneas during the entire process of production.

Interferometric backward third harmonic generation microscopy for axial imaging with accuracy beyond the diffraction limit

A new nonlinear microscopy technique based on interference of backward-reflected third harmonic generation (I-THG) from multiple interfaces is presented. The technique is used to measure height variations or changes of a layer thickness with an accuracy of up to 5 nm. Height variations of a patterned glass surface and thickness variations of fibroblasts are visualized with the interferometric epi-THG microscope with an accuracy at least two orders of magnitude better than diffraction limit. The microscopy technique can be broadly applied for measuring distance variations between membranes or multilayer structures inside biological tissue and for surface height variation imaging.

Nonlinear optical microscopy signal processing strategies in cancer

This work reviews the most relevant present-day processing methods used to improve the accuracy of multimodal nonlinear images in the detection of epithelial cancer and the supporting stroma. Special emphasis has been placed on methods of non linear optical (NLO) microscopy image processing such as: second harmonic to autofluorescence ageing index of dermis (SAAID), tumor-associated collagen signatures (TACS), fast Fourier transform (FFT) analysis, and gray level co-occurrence matrix (GLCM)-based methods. These strategies are presented as a set of potential valuable diagnostic tools for early cancer detection. It may be proposed that the combination of NLO microscopy and informatics based image analysis approaches described in this review (all carried out on free software) may represent a powerful tool to investigate collagen organization and remodeling of extracellular matrix in carcinogenesis processes.

Optimization of malaria detection based on third harmonic generation imaging of hemozoin

The pigment hemozoin is a natural by-product of the metabolism of hemoglobin by the parasites which cause malaria. Previously, hemozoin was demonstrated to have a very high nonlinear optical response enabling third harmonic generation (THG) imaging. In this study, we present a complete characterization of the nonlinear THG response of natural hemozoin in malaria-infected red blood cells, as well as in pure isostructural synthesized hematin anhydride, in order to determine optimal imaging parameters for detection. Our study demonstrates the wavelength range for optimal pulsed femtosecond laser excitation of THG from hemozoin crystals. In addition, we show the hemozoin crystal detection as a function of crystal size, incident laser power, and the emission response of the hemozoin crystals to different incident laser polarization states. Our systematic measurements of the nonlinear optical response from hemozoin establish detection limits, which are essential for the optimal design of malaria detection technologies that exploit the THG response of hemozoin.

Myosin filament assembly onto myofibrils in live neonatal cardiomyocytes observed by TPEF-SHG microscopy

AIMS

Understanding myofibrillogenesis is essential for elucidating heart muscle formation, development, and remodelling in response to physiological stimulation. Here, we report the dynamic assembly process of contractile myosin filaments onto myofibrils in a live cardiomyocyte culture during myofibrillogenesis.

METHODS AND RESULTS

Utilizing a custom-built, two-photon excitation fluorescence and second harmonic generation imaging system equipped with an on-stage incubator, we observed new sarcomere additions in rat neonatal cardiomyocytes during 10 h of on-stage incubation. The new sarcomere additions occurred at the side of existing myofibrils, where we observed mature myofibrils acting as templates, or at the interstice of several separated myofibrils.

CONCLUSIONS

During sarcomeric addition, myosin filaments are assembled onto the premyofibril laterally. This lateral addition, which proceeds stepwise along the axial direction, plays an important role in the accumulation of Z-bodies to form mature Z-disks and in the regulation of sarcomeric alignment during maturation.

Characterization of oral squamous cell carcinoma based on higher-harmonic generation microscopy

In vivo higher-harmonic generation microscopy (HGM) performed on healthy human oral mucosa not only provides images with a <500 nm lateral resolution at a 280 μm penetration depth, but also leaves no photodamages in the tissues. These advantages suggest that HGM could serve as an ideal virtual biopsy tool for in vivo, in situ, and immediate histopathological diagnosis of oral cancer. However, translation of such mechanism for clinical cancer diagnosis requires evidence based algorithm capable to differentiate cancerous tissues from normal. It is thus critical to investigate if the endogenous contrast provided by the HGM would be high enough to differentiate cancerous versus normal tissues in human oral mucosa. In this report, ex vivo HGM study was performed on the cancerous mucosa from 10 patients with oral squamous cell carcinoma. Compared with histology, HGM revealed histopathological features including the cytological abnormalities, loss of differentiation, interruption of basement membrane, and irregular epithelial stratification in all 10 specimens. In addition, distinct patterns of collagen fibers and increased distribution area of actin filaments in tumor cells were noted. These results indicate HGM holds great potential for the optical biopsy screening of oral cancer lesions.

Label-free tetra-modal molecular imaging of living cells with CARS, SHG, THG and TSFG (coherent anti-Stokes Raman scattering, second harmonic generation, third harmonic generation and third-order sum frequency generation)

We have developed a new multimodal molecular imaging system that combines CARS (coherent anti-Stokes Raman scattering), SHG (second harmonic generation), THG (third harmonic generation) and multiplex TSFG (third-order sum frequency generation) using a subnanosecond white-light laser source. Molecular composition and their distribution in living cells are clearly visualized with different contrast enhancements through different mechanisms of CARS, SHG, THG and TSFG. A correlation image of CARS and TSF reveals that the TSF signal is generated predominantly from lipid droplets inside a cell as well as the peripheral cell wall.

Myofibrillogenesis in live neonatal cardiomyocytes observed with hybrid two-photon excitation fluorescence-second harmonic generation microscopy

We developed a hybrid two-photon excitation fluorescence-second harmonic generation (TPEF-SHG) imaging system with an on-stage incubator for long-term live-cell imaging. Using the imaging system, we observed the addition of new sarcomeres during myofibrillogenesis while a cardiomyocyte was spreading on the substrate. The results suggest that the TPEF-SHG imaging system with an on-stage incubator is an effective tool for investigation of dynamic myofibrillogenesis.

In vivo optical virtual biopsy of human oral mucosa with harmonic generation microscopy

Recent clinical studies on human skin indicated that in vivo multi-harmonic generation microscopy (HGM) can achieve sub-micron resolution for histopathological analysis with a high penetration depth and leave no energy or photodamages in the interacted tissues. It is thus highly desired to apply HGM for in vivo mucosa histopathological diagnosis. In this paper, the first in vivo optical virtual biopsy of human oral mucosa by using epi-HGM is demonstrated. We modified an upright microscope to rotate the angle of objective for in vivo observation. Our clinical study reveals the capability of HGM to in vivo image cell distributions in human oral mucosa, including epithelium and lamina propria with a high penetration depth greater than 280 μm and a high spatial resolution better than 500 nm. We also found that the third-harmonic-generation (THG) contrast on nucleus depends strongly on its thicknesses, in agreement with a numerical simulation. Besides, 4% acetic acid was found to be able to enhance the THG contrast of nucleus in oral mucosa, while such enhancement was found to decay due to the metabolic clearance of the contrast enhancer by the oral mucosa. Our clinical study indicated that, the combined epi-THG and epi-second-harmonic-generation (SHG) microscopy is a promising imaging tool for in vivo noninvasive optical virtual biopsy and disease diagnosis in human mucosa.

In vivo imaging of myelin in the vertebrate central nervous system using third harmonic generation microscopy

Loss of myelin in the central nervous system (CNS) leads to debilitating neurological deficits. High-resolution optical imaging of myelin in the CNS of animal models is limited by a lack of in vivo myelin labeling strategies. We demonstrated that third harmonic generation (THG) microscopy-a coherent, nonlinear, dye-free imaging modality-provides micrometer resolution imaging of myelin in the mouse CNS. In fixed tissue, we found that THG signals arose from white matter tracts and were colocalized with two-photon excited fluorescence (2PEF) from a myelin-specific dye. In vivo, we used simultaneous THG and 2PEF imaging of the mouse spinal cord to resolve myelin sheaths surrounding individual fluorescently-labeled axons, and followed myelin disruption after spinal cord injury. Finally, we suggest optical mechanisms that underlie the myelin specificity of THG. These results establish THG microscopy as an ideal tool for the study of myelin loss and recovery.

Improving sensitivity in nonlinear Raman microspectroscopy imaging and sensing

Nonlinear Raman microspectroscopy based on a broadband coherent anti-Stokes Raman scattering is an emerging technique for noninvasive, chemically specific, microscopic analysis of tissues and large population of cells and particles. The sensitivity of this imaging is a critical aspect of a number of the proposed biomedical application. It is shown that the incident laser power is the major parameter controlling this sensitivity. By careful optimizing the laser system, the high-quality vibrational spectra acquisition at the multi-kHz rate becomes feasible.

Two-photon microscopy of deep intravital tissues and its merits in clinical research

Multiphoton excitation laser scanning microscopy, relying on the simultaneous absorption of two or more photons by a molecule, is one of the most exciting recent developments in biomedical imaging. Thanks to its superior imaging capability of deeper tissue penetration and efficient light detection, this system becomes more and more an inspiring tool for intravital bulk tissue imaging. Two-photon excitation microscopy including 2-photon fluorescence and second harmonic generated signal microscopy is the most common multiphoton microscopic application. In the present review we take diverse ocular tissues as intravital samples to demonstrate the advantages of this approach. Experiments with registration of intracellular 2-photon fluorescence and extracellular collagen second harmonic generated signal microscopy in native ocular tissues are focused. Data show that the in-tandem combination of 2-photon fluorescence and second harmonic generated signal microscopy as two-modality microscopy allows for in situ co-localization imaging of various microstructural components in the whole-mount deep intravital tissues. New applications and recent developments of this high technology in clinical studies such as 2-photon-controlled drug release, in vivo drug screening and administration in skin and kidney, as well as its uses in tumourous tissues such as melanoma and glioma, in diseased lung, brain and heart are additionally reviewed. Intrinsic emission two-modal 2-photon microscopy/tomography, acting as an efficient and sensitive non-injurious imaging approach featured by high contrast and subcellular spatial resolution, has been proved to be a promising tool for intravital deep tissue imaging and clinical studies. Given the level of its performance, we believe that the non-linear optical imaging technique has tremendous potentials to find more applications in biomedical fundamental and clinical research in the near future.

Direct backward third-harmonic generation in nanostructures

Direct-backward third harmonic generation (DBTHG) has been regarded as negligible or even inexistent due to the large value of wave-vector mismatch. In the past, BTHG signals were often interpreted as back-reflected or back-scattered forward-THG (FTHG). In this paper, we theoretically and experimentally demonstrate that backward third harmonic waves can be directly generated, and that their magnitude can be comparable with FTHG in nanostructures. Experimental data of DBTHG from ZnO thin films, CdSe quantum dots and Fe(3)O(4) nanoparticles agree well with simulation results based on the Green's function. An integral equation was also derived for fast computation of DBTHG in nano films. Our investigation suggests that DBTHG can be a potentially powerful tool in nano-science research, especially when combined with FTHG measurements.

The potential of biophotonic techniques in stem cell tracking and monitoring of tissue regeneration applied to cardiac stem cell therapy

The use of injected stem cells, leading to regeneration of ischemic heart tissue, for example, following coronary artery occlusion, has emerged as a major new option for managing 'heart attack' patients. While some clinical trials have been encouraging, there have also been failures and there is little understanding of the multiplicity of factors that lead to the outcome. In this overview paper, the opportunities and challenges in applying biophotonic techniques to regenerative medicine, exemplified by the challenge of stem cell therapy of ischemic heart disease, are considered. The focus is on optical imaging to track stem cell distribution and fate, and optical spectroscopies and/or imaging to monitor the structural remodeling of the tissue and the resulting functional changes. The scientific, technological, and logistics issues involved in moving some of these techniques from pre-clinical research mode ultimately into the clinic are also highlighted.

Optical microscopy in photosynthesis

Emerging as well as the most frequently used optical microscopy techniques are reviewed and image contrast generation methods in a microscope are presented, focusing on the nonlinear contrasts such as harmonic generation and multiphoton excitation fluorescence. Nonlinear microscopy presents numerous advantages over linear microscopy techniques including improved deep tissue imaging, optical sectioning, and imaging of live unstained samples. Nonetheless, with the exception of multiphoton excitation fluorescence, nonlinear microscopy is in its infancy, lacking protocols, users and applications; hence, this review focuses on the potential of nonlinear microscopy for studying photosynthetic organisms. Examples of nonlinear microscopic imaging are presented including isolated light-harvesting antenna complexes from higher plants, starch granules, chloroplasts, unicellular alga Chlamydomonas reinhardtii, and cyanobacteria Leptolyngbya sp. and Anabaena sp. While focusing on nonlinear microscopy techniques, second and third harmonic generation and multiphoton excitation fluorescence microscopy, other emerging nonlinear imaging modalities are described and several linear optical microscopy techniques are reviewed in order to clearly describe their capabilities and to highlight the advantages of nonlinear microscopy.

Invited review article: Imaging techniques for harmonic and multiphoton absorption fluorescence microscopy

We review the current state of multiphoton microscopy. In particular, the requirements and limitations associated with high-speed multiphoton imaging are considered. A description of the different scanning technologies such as line scan, multifoci approaches, multidepth microscopy, and novel detection techniques is given. The main nonlinear optical contrast mechanisms employed in microscopy are reviewed, namely, multiphoton excitation fluorescence, second harmonic generation, and third harmonic generation. Techniques for optimizing these nonlinear mechanisms through a careful measurement of the spatial and temporal characteristics of the focal volume are discussed, and a brief summary of photobleaching effects is provided. Finally, we consider three new applications of multiphoton microscopy: nonlinear imaging in microfluidics as applied to chemical analysis and the use of two-photon absorption and self-phase modulation as contrast mechanisms applied to imaging problems in the medical sciences.

Image analysis in nonlinear microscopy

The ability to automatically extract quantitative data from nonlinear microscopy images is here explored, taking nonlinear and coherent effects into account. Objects of different degrees of complexity were investigated: theoretical images of spherical objects, experimentally collected coherent anti-Stokes Raman scattering images of polystyrene spheres in background-generating agar, well-separated lipid droplets in living yeast cells, and conglomerations of lipid droplets in living C. elegans nematodes. The in linear microscopy useful measure of full width at half-maximum (FWHM) was shown to provide inadequate measures of object size due to the nonlinear density dependence of the signal. Instead, the capability of four state-of-the-art image analysis algorithms was evaluated. Among these, local thresholding was found to be the widest applicable segmentation algorithm.

Higher harmonic generation microscopy of in vitro cultured mammal oocytes and embryos

Oocyte and embryo selection governs the success of assisted reproductive technologies. The imaging tools applied for selecting embryos may need to contain several key properties: noninvasiveness, high 3D resolution, and the contrast capability to provide as much information about the embryos as possible, such as spindle fibers, zona pellucida, and organelles. Currently adopted imaging techniques can only provide one or two of these desired properties and are with limited contrast of the embryos. Some image techniques can even damage the embryos. Previous studies have shown that harmonic generation microscopy (HGM), a virtual-transition based technology, can provide noninvasive imaging in zebrafish embryos with a sub-cellular 3D resolution and a millimeter penetration depth, and thus could be a suitable tool for future oocyte and embryo selection of assisted reproductive technologies. However to evaluate HGM in clinical use, the intrinsic contrast origin of the second harmonic generation (SHG) and third harmonic generation (THG) inside the mammal embryos has to be studied. In this work we performed HGM studies on the in vitro cultured mouse oocytes and embryos by combining the SHG and THG modalities, with a focus on the contrast origin evaluation. Through the noninvasive HGM imaging, we can clearly identify various structures in the whole oocytes and embryos, including spindle fibers, zona pellucida, polar bodies, cell membranes, and the laminated organelles in the cells. The origin of the THG contrast was further confirmed through the standard staining studies. Through SHG signals, we could not only observe the spindle fibers when the oocytes were arrested at metaphase II or during the cleavage of the embryos, but can also distinguish and analyze the thickness of the three layers of the zona pellucida. Combining two different higher-harmonic generation modalities, SHG and THG, HGM successfully revealed the sub-cellular structures of the whole mouse embryos with a high 3D spatial resolution.

Intermyofilament dynamics of myocytes revealed by second harmonic generation microscopy

Drosophila melanogaster larva myocytes are imaged with second harmonic generation (SHG) microscopy undergoing forced stretching and rhythmic contractions to determine the nature of the SHG signal. During stretching, double peaked SHG profiles of the anisotropic (A-) bands evolve into single peaks with a higher SHG intensity. The dip in the intensity profile at the center of the A-band is attributed to destructive interference from out-of-phase second harmonic radiating myosin molecules that, in the central region of myofilaments, are arranged antiparallel. An intensity increase at the center of the A-band appears during forced stretching due to a small, less than 100 nm, intermyofilament separation of the antiparallel myosin molecules leading to constructive interference of the SHG radiation. In addition, the same phenomenon occurs during periodic contractions of the myocyte, where an SHG intensity increase with the lengthening of sarcomeres is observed. The SHG intensity dependence on sarcomere length can be used for imaging myocyte contractions with low resolution microscopy, and can be applied for the development of diagnostic tools where monitoring of muscle contraction dynamics is required.

Cell tracking and detection of molecular expression in live cells using lipid-enclosed CdSe quantum dots as contrast agents for epi-third harmonic generation microscopy

We demonstrated that lipid-enclosed CdSe quantum dots (LEQDs) can function as versatile contrast agents in epi-detection third harmonic generation (THG) microscopy for biological applications in vivo. With epi-THG intensities 20 times stronger than corresponding fluorescence intensities from the same LEQDs under the same conditions of energy absorption, such high brightness LEQDs were proved for the abilities of cell tracking and detection of specific molecular expression in live cancer cells. Using nude mice as an animal model, the distribution of LEQD-loaded tumor cells deep in subcutaneous tissues were imaged with high THG contrast. This is the first demonstration that THG contrast can be manipulated in vivo with nanoparticles. By linking LEQDs with anti-Her2 antibodies, the expression of Her2/neu receptors in live breast cancer cells could also be easily detected through THG. Compared with fluorescence modalities, the THG modality also provides the advantage of no photobleaching and photoblinkin g effects. Combined with a high penetration 1230 nm laser, these novel features make LEQDs excellent THG contrast agents for in vivo deep-tissue imaging in the future.

Imaging fluorescence detected linear dichroism of plant cell walls in laser scanning confocal microscope

Anisotropy carries important information on the molecular organization of biological samples. Its determination requires a combination of microscopy and polarization spectroscopy tools. The authors constructed differential polarization (DP) attachments to a laser scanning microscope in order to determine physical quantities related to the anisotropic distribution of molecules in microscopic samples; here the authors focus on fluorescence-detected linear dichroism (FDLD). By modulating the linear polarization of the laser beam between two orthogonally polarized states and by using a demodulation circuit, the authors determine the associated transmitted and fluorescence intensity-difference signals, which serve the basis for LD (linear dichroism) and FDLD, respectively. The authors demonstrate on sections of Convallaria majalis root tissue stained with Acridin Orange that while (nonconfocal) LD images remain smeared and weak, FDLD images recorded in confocal mode reveal strong anisotropy of the cell wall. FDLD imaging is suitable for mapping the anisotropic distribution of transition dipoles in 3 dimensions. A mathematical model is proposed to account for the fiber-laminate ultrastructure of the cell wall and for the intercalation of the dye molecules in complex, highly anisotropic architecture.

Monitoring of lipid storage in Caenorhabditis elegans using coherent anti-Stokes Raman scattering (CARS) microscopy

Better understanding of the fundamental mechanisms behind metabolic diseases requires methods to monitor lipid stores on single-cell level in vivo. We have used Caenorhabditis elegans as a model organism to demonstrate the limitations of fluorescence microscopy for imaging of lipids compared with coherent anti-Stokes Raman scattering (CARS) microscopy, the latter allowing chemically specific and label-free imaging in living organisms. CARS microscopy was used to quantitatively monitor the impact of genetic variations in metabolic pathways on lipid storage in 60 specimens of C. elegans. We found that the feeding-defective mutant pha-3 contained a lipid volume fraction one-third of that found in control worms. In contrast, mutants (daf-2, daf-4 dauer) with deficiencies in the insulin and transforming growth factors (IGF and TGF-beta) signaling pathways had lipid volume fractions that were 1.4 and 2 times larger than controls, respectively. This was observed as an accumulation of small-sized lipid droplets in the hypodermal cells, hosting as much as 40% of the total lipid volume in contrast to the 9% for the wild-type larvae. Spectral CARS microscopy measurements indicated that this is accompanied by a shift in the ordering of the lipids from gel to liquid phase. We conclude that the degree of hypodermal lipid storage and the lipid phase can be used as a marker of lipid metabolism shift. This study shows that CARS microscopy has the potential to become a sensitive and important tool for studies of lipid storage mechanisms, improving our understanding of phenomena underlying metabolic disorders.

Influence of semicrystalline order on the second-harmonic generation efficiency in the anisotropic bands of myocytes

The influence of semicrystalline order on the second-harmonic generation (SHG) efficiency in the anisotropic bands of Drosophila melanogaster sarcomeres from larval and adult muscle has been investigated. Differences in the semicrystalline order were obtained by using wild-type and mutant strains containing different amounts of headless myosin. The reduction in semicrystalline order without altering the chemical composition of myofibrils was achieved by observing highly stretched sarcomeres and by inducing a loss of viability in myocytes. In all cases the reduction of semicrystalline order in anisotropic bands of myocytes resulted in a substantial decrease in SHG. Second-harmonic imaging during periodic contractions of myocytes revealed higher intensities when sarcomeres were in the relaxed state compared with the contracted state. This study demonstrates that an ordered semicrystalline arrangement of anisotropic bands plays a determining role in the efficiency of SHG in myocytes.

Quantitative characterization of biological liquids for third-harmonic generation microscopy

Third-harmonic generation (THG) microscopy provides images of unstained biological samples based on spatial variations in third-order nonlinear susceptibility, refractive index, and dispersion. In this study, we establish quantitative values for the third-order nonlinear susceptibilities of several solvents (water, ethanol, glycerol), physiological aqueous (ions, amino acids, polypeptides, bovine serum albumin, glucose) and lipid (triglycerides, cholesterol) solutions as a function of solute concentration in the 1.05-1.25 microm excitation range. We use these data in conjunction with imaging experiments to show that THG imaging with approximately 1.2 microm excitation lacks specificity and sensitivity to detect physiological ion concentration changes, and that nonaqueous structures such as lipid bodies provide a more robust source of signal. Finally, we illustrate the impact of index-matching liquids in THG images. These data provide a basis for interpreting biological THG images and for developing additional applications.

Noninvasive harmonics optical microscopy for long-term observation of embryonic nervous system development in vivo

Nervous system development is a complicated dynamic process, and many mechanisms remain unknown. By utilizing endogenous second-harmonic-generation as the contrast of polarized nerve fibers and third-harmonic-generation (THG) to reveal morphological changes, we have successfully observed the vertebrate embryonic nervous development from the very beginning based on a 1230-nm light source. The dynamic development of the nerve system within a live zebrafish embryo can be recorded continuously more than 20 hr without fluorescence markers. Since the THG process is not limited by the time of gene expression and differentiation as fluorescence-based techniques are, the observable stages can be advanced to the very beginning of the development process. The complete three-dimensional brain development from a neural plate to a neural tube can be uncovered with a submicron lateral resolution. We have, for the first time, also reported the generation of SHG from myelinated nerve fibers and the outer segment of the photoreceptors with a stacked membrane structure. Our study clearly indicates the fact that higher-harmonics-based optical microscopy has the strong potential to long-term in vivo study of the nervous system, including genetic disorders of the nervous system, axon pathfinding, neural regeneration, neural repair, and neural stem cell development.

Corneal multiphoton microscopy and intratissue optical nanosurgery by nanojoule femtosecond near-infrared pulsed lasers

Multiphoton microscopy including multiphoton autofluorescence imaging (MAI) and second-harmonic generation (SHG) is being used as a novel diagnostic tool to perform tissue nonlinear optical tomography with submicron resolution. The three-dimensional corneal ultrastructure of whole depth has been viewed without any staining or mechanical slicing. Compared with photodisruptive surgical effects occurring at TW/cm2 light intensity, multiphoton imaging can be induced at MW-GW/ cm2 photon intensity. The intratissue surgical effect including nanojoule (nJ) femtosecond laser ablation and flap generation was induced through multiphoton nonlinear absorption at a wavelength of 800 nm and ascertained by the histological outcomes. More interesting, the multiphoton microscopy based on nonlinear absorption of femtosecond laser pulses at the wavelength of 715-930 nm emitted from solid-state Ti:sapphire system is acting as a precise non-invasive monitoring tool to determine the interest of region, to visualize and verify the outcomes in in vivo intrastromal laser nanosurgery. Overall, these data suggest that multiphoton microscopy is a highly sensitive and promising technique for studying the morphometric and biomechanical properties of biological tissues and that the nJ ultrashort Lasers can be used as a ultra-precise nanoscalpel for performing intratissue surgery.

In vivo optical biopsy of hamster oral cavity with epi-third-harmonic-generation microscopy

The first in vivo optical virtual biopsy based on epi-third-harmonic-generation (THG) microscopy is successfully demonstrated using Syrian hamster oral mucosa as a model system. Without complex physical biopsy procedures, epi-THG microscopy can provide high spatial resolution dynamic images of oral mucosa and sub-mucosa in all three dimensions. The demonstrated intra-vital epi-THG microscopy provide high resolution observation of blood flow in the capillary and could be a promising tool to image angiogenesis, which is an important feature for many human diseases including malignancies. The system setup of epi-THG microscopy can be easily integrated with other nonlinear optical microscopy such as second-harmonic generation and multi-photon fluorescence microscopy by using the same laser system to provide better integrated molecular and structural information for future clinical diagnosis. By adding 6% acetic acid solution on the mucosa, THG contrast on the borders of nuclei was found to be greatly enhanced due to the alterations of their linear and nonlinear THG susceptibilities. With a virtual-transition-based technology without using fluorescence, the optical epi-THG biopsy we demonstrated shows promise for future noninvasive in vivo diseases examinations.