Imaging of optically thick specimen using two-photon excitation microscopy

Spatial frequency-based correction of the spherical aberration in living brain imaging

Optical errors, including spherical aberrations, hinder high-resolution imaging of biological samples due to biochemical components and physical properties. We developed the Deep-C microscope system to achieve aberration-free images, employing a motorized correction collar and contrast-based calculations. However, current contrast-maximization techniques, such as the Brenner gradient method, inadequately assess specific frequency bands. The Peak-C method addresses this issue, but its arbitrary neighbor selection and susceptibility to the noise limit its effectiveness. In this paper, we emphasize the importance of a broad spatial frequency range for accurate spherical aberration correction and propose Peak-F. This spatial frequency-based system utilizes a fast Fourier transform as a bandpass filter. This approach overcomes Peak-C's limitations and comprehensively covers the low-frequency domain of image spatial frequencies.

Non-telecentric two-photon microscopy for 3D random access mesoscale imaging

Diffraction-limited two-photon microscopy permits minimally invasive optical monitoring of neuronal activity. However, most conventional two-photon microscopes impose significant constraints on the size of the imaging field-of-view and the specific shape of the effective excitation volume, thus limiting the scope of biological questions that can be addressed and the information obtainable. Here, employing a non-telecentric optical design, we present a low-cost, easily implemented and flexible solution to address these limitations, offering a several-fold expanded three-dimensional field of view. Moreover, rapid laser-focus control via an electrically tunable lens allows near-simultaneous imaging of remote regions separated in three dimensions and permits the bending of imaging planes to follow natural curvatures in biological structures. Crucially, our core design is readily implemented (and reversed) within a matter of hours, making it highly suitable as a base platform for further development. We demonstrate the application of our system for imaging neuronal activity in a variety of examples in zebrafish, mice and fruit flies.

Selective Imaging of Microplastic and Organic Particles in Flow by Multimodal Coherent Anti-Stokes Raman Scattering and Two-Photon Excited Autofluorescence Analysis

Microplastic pollution is an urgent global issue. While spectroscopic techniques have been widely used for the identification of plastics collected from aquatic environments, these techniques are often labor-intensive and time-consuming due to sample collection, preparation, and long measurement times. In this study, a method for the two-dimensional detection and classification of flowing microplastic and organic biotic particles with high spatial and temporal resolutions has been proposed based on the simultaneous detection of coherent anti-Stokes Raman scattering (CARS) and two-photon excited autofluorescence (TPEAF) signals. Poly(methyl methacrylate) (PMMA), polystyrene (PS), and low-density polyethylene (LDPE) particles with sizes ranging from several tens to hundreds of micrometers were selectively detected in flow with an average velocity of 4.17 mm/s by CARS line scanning. With the same flow velocity, flowing PMMA and alga particles were measured using a multimodal system of CARS and TPEAF signals. The average intensities of both PMMA and alga particles in the CARS signals at a frequency of 2940 cm^-1 were higher than the background level, while only algae emitted TPEAF signals. This allowed the classification of PMMA and alga particles to be successfully performed in flow by the simultaneous detection of CARS and TPEAF signals. With the proposed method, the monitoring of microplastics in a continuous water flow without collection or extraction is possible, which is game-changing for the current sampling-based microplastic analysis.

Mitochondrial Targeting of Probes and Therapeutics to the Powerhouse of the Cell

Mitochondria, colloquially known as "the powerhouse of the cell", play important roles in production, but also in processes critical for cellular fate such as cell death, differentiation, signaling, metabolic homeostasis, and innate immunity. Due to its many functions in the cell, the mitochondria have been linked to a variety of human illnesses such as diabetes, cancer, and neurodegenerative diseases. In order to further our understanding and pharmaceutical targeting of this critical organelle, effective strategies must be employed to breach the complex barriers and microenvironment of mitochondria. Here, we summarize advancements in mitochondria-targeted probes and therapeutics.

Fluorescence anisotropy imaging in drug discovery

Non-invasive measurement of drug-target engagement can provide critical insights in the molecular pharmacology of small molecule drugs. Fluorescence polarization/fluorescence anisotropy measurements are commonly employed in protein/cell screening assays. However, the expansion of such measurements to the in vivo setting has proven difficult until recently. With the advent of high-resolution fluorescence anisotropy microscopy it is now possible to perform kinetic measurements of intracellular drug distribution and target engagement in commonly used mouse models. In this review we discuss the background, current advances and future perspectives in intravital fluorescence anisotropy measurements to derive pharmacokinetic and pharmacodynamic measurements in single cells and whole organs.

Two-Photon Microscopy and Spectroscopy Studies to Determine the Mechanism of Copper Oxide Nanoparticle Uptake by Sweetpotato Roots during Postharvest Treatment

The interaction of engineered nanoparticles with plant tissues is still not well understood. There is a lack of information about the effects of curing (postharvest treatment) and lignin content on copper uptake by sweetpotato roots exposed to copper-based nanopesticides. In this study, Beauregard-14 (lower lignin) and Covington (higher lignin) varieties were exposed to CuO nanoparticles (nCuO), bulk CuO (bCuO), and CuCl₂ at 0, 25, 75, and 125 mg/L. Cured and uncured roots were submerged into copper suspensions/solutions for 30 min. Subsequently, root segments were sliced for imaging with a 2-photon microscope, while other root portions were severed into periderm, cortex, perimedulla, and medulla. They were individually digested and analyzed for Cu content by inductively coupled plasma-optical emission spectroscopy. Microscopy images showed higher fluorescence in periderm and cortex of roots exposed to nCuO, compared with bCuO. At 25 mg/L, only bCuO showed higher Cu concentration in the periderm and cortex of Beauregard-14 (2049 mg/kg and 76 mg/kg before curing; 6769 mg/kg and 354 mg/kg after curing, respectively) and in cortex of Covington (692 mg/kg before curing and 110 mg/kg after curing) compared with controls ( p ≤ 0.05). In medulla, the most internal tissue, only Beauregard-14 exposed to 125 mg bCuO/L showed significantly ( p ≤ 0.05) more Cu before curing (17 mg/kg) and after curing (28 mg/kg), compared with control. This research has shown that the 2-photon microscope can be used to determine CuO particles in nondyed plant tissues. The lack of Cu increase in perimedulla and medulla, even in roots exposed to high CuO concentrations (125 mg/L), suggests that nCuO may represent a good alternative to protect and increase the shelf life of sweetpotato roots, without exposing consumers to excess Cu.

A water-soluble two-photon fluorescence chemosensor for ratiometric imaging of mitochondrial viscosity in living cells

We report a novel water-soluble ratiometric TPEF chemosensor EIN that is specifically responsive and singularly sensitive to mitochondria viscosity in living cells.

An Improved Immunostaining and Imaging Methodology to Determine Cell and Protein Distributions within the Bone Environment

Bone is a dynamic tissue that undergoes multiple changes throughout its lifetime. Its maintenance requires a tight regulation between the cells embedded within the bone matrix, and an imbalance among these cells may lead to bone diseases such as osteoporosis. Identifying cell populations and their proteins within bone is necessary for understanding bone biology. Immunolabeling is one approach used to visualize proteins in tissues. Efficient immunolabeling of bone samples often requires decalcification, which may lead to changes in the structural morphology of the bone. Recently, methyl-methacrylate embedding of non-decalcified tissue followed by heat-induced antigen retrieval has been used to process bone sections for immunolabeling. However, this technique is applicable for bone slices below 50-µm thickness while fixed on slides. Additionally, enhancing epitope exposure for immunolabeling is still a challenge. Moreover, imaging bone cells within the bone environment using standard confocal microscopy is difficult. Here we demonstrate for the first time an improved methodology for immunolabeling non-decalcified bone using a testicular hyaluronidase enzyme-based antigen retrieval technique followed by two-photon fluorescence laser microscopy (TPLM) imaging. This procedure allowed us to image key intracellular proteins in bone cells while preserving the structural morphology of the cells and the bone.

Quantitative analysis of light scattering in polarization-resolved nonlinear microscopy

Polarization resolved nonlinear microscopy (PRNM) is a powerful technique to gain microscopic structural information in biological media. However, deep imaging in a variety of biological specimens is hindered by light scattering phenomena, which not only degrades the image quality but also affects the polarization state purity. In order to quantify this phenomenon and give a framework for polarization resolved microscopy in thick scattering tissues, we develop a characterization methodology based on four wave mixing (FWM) process. More specifically, we take advantage of two unique features of FWM, meaning its ability to produce an intrinsic in-depth local coherent source and its capacity to quantify the presence of light depolarization in isotropic regions inside a sample. By exploring diverse experimental layouts in phantoms with different scattering properties, we study systematically the influence of scattering on the nonlinear excitation and emission processes. The results show that depolarization mechanisms for the nonlinearly generated photons are highly dependent on the scattering center size, the geometry used (epi/forward) and, most importantly, on the thickness of the sample. We show that the use of an un-analyzed detection makes the polarization-dependence read-out highly robust to scattering effects, even in regimes where imaging might be degraded. The effects are illustrated in polarization resolved imaging of myelin lipid organization in mouse spinal cords.

Diffraction in a stratified region of a high numerical aperture Fresnel zone plate: a simple and rigorous integral representation

An algorithm for calculating the field distribution of a high numerical aperture Fresnel zone plate (FZP) in stratified media is presented, which is based on the vector angular spectrum method. The diffraction problem of FZP is solved for the case of a multilayer film with planar interfaces perpendicular to the optical axis. The solution is obtained in a rigorous mathematical manner and it satisfies the homogeneous wave equations. The electric strength vector of the transmitted and reflected field in the multilayer media is obtained for any polarized beam normally incident onto a binary phase circular FZP. For radially-, azimuthally- and linearly-polarized beam, the electric field in the focal region can be simplified as double or single integral, which can be readily used for numerical computation.

Design and synthesis of fluorenone-based dyes: two-photon excited fluorescent probes for imaging of lysosomes and mitochondria in living cells

Three fluorenone-based two-photon fluorescent probes for specific targeting of lysosomes and mitochondria in cancer cells.

Investigation of microbial biofilm structure by laser scanning microscopy

Microbial bioaggregates and biofilms are hydrated three-dimensional structures of cells and extracellular polymeric substances (EPS). Microbial communities associated with interfaces and the samples thereof may come from natural, technical, and medical habitats. For imaging such complex microbial communities confocal laser scanning microscopy (CLSM) is the method of choice. CLSM allows flexible mounting and noninvasive three-dimensional sectioning of hydrated, living, as well as fixed samples. For this purpose a broad range of objective lenses is available having different working distance and resolution. By means of CLSM the signals detected may originate from reflection, autofluorescence, reporter genes/fluorescence proteins, fluorochromes binding to specific targets, or other probes conjugated with fluorochromes. Recorded datasets can be used not only for visualization but also for semiquantitative analysis. As a result CLSM represents a very useful tool for imaging of microbiological samples in combination with other analytical techniques.

Monitoring the metabolic state of fungal hyphae and the presence of melanin by nonlinear spectral imaging

Label-free nonlinear spectral imaging microscopy (NLSM) records two-photon-excited fluorescence emission spectra of endogenous fluorophores within the specimen. Here, NLSM is introduced as a novel, minimally invasive method to analyze the metabolic state of fungal hyphae by monitoring the autofluorescence of NAD(P)H and flavin adenine dinucleotide (FAD). Moreover, the presence of melanin was analyzed by NLSM. NAD(P)H, FAD, and melanin were used as biomarkers for freshness of mushrooms of Agaricus bisporus (white button mushroom) that had been stored at 4°C for 0 to 17 days. During this period, the mushrooms did not show changes in morphology or color detectable by eye. In contrast, FAD/NAD(P)H and melanin/NAD(P)H ratios increased over time. For instance, these ratios increased from 0.92 to 2.02 and from 0.76 to 1.53, respectively, at the surface of mushroom caps that had been harvested by cutting the stem. These ratios were lower under the skin than at the surface of fresh mushrooms (0.78 versus 0.92 and 0.41 versus 0.76, respectively), indicative of higher metabolism and lower pigment formation within the fruiting body. Signals were different not only between tissues of the mushroom but also between neighboring hyphae. These data show that NLSM can be used to determine the freshness of mushrooms and to monitor the postharvest browning process at an early stage. Moreover, these data demonstrate the potential of NLSM to address a broad range of fundamental and applied microbiological processes.

Enhancement of imaging depth in turbid media using a wide area detector

The depth of two-photon fluorescence imaging in turbid media can be significantly enhanced by the use of the here described fluorescence detection method that allows to efficiently collect scattered fluorescence photons from a wide area of the turbid sample. By using this detector we were able to perform imaging of turbid samples, simulating brain tissue, at depths up to 3 mm, where the two-photon induced fluorescence signal is too weak to be detected by means used in conventional two-photon microscopy.

Deep tissue fluorescent imaging in scattering specimens using confocal microscopy

In scattering specimens, multiphoton excitation and nondescanned detection improve imaging depth by a factor of 2 or more over confocal microscopy; however, imaging depth is still limited by scattering. We applied the concept of clearing to deep tissue imaging of highly scattering specimens. Clearing is a remarkably effective approach to improving image quality at depth using either confocal or multiphoton microscopy. Tissue clearing appears to eliminate the need for multiphoton excitation for deep tissue imaging.

The effects of spherical aberration on multiphoton fluorescence excitation microscopy

Multiphoton fluorescence excitation microscopy is almost invariably conducted with samples whose refractive index differ from that of the objective immersion medium, conditions that cause spherical aberration. Due to the quadratic nature of multiphoton fluorescence excitation, spherical aberration is expected to profoundly affect the depth dependence of fluorescence excitation. In order to determine the effect of refractive index mismatch in multiphoton fluorescence excitation microscopy, we measured signal attenuation, photobleaching rates and resolution degradation with depth in homogeneous samples with minimal light scattering and absorption over a range of refractive indices. These studies demonstrate that signal levels and resolution both rapidly decline with depth into refractive index mismatched samples. Analyses of photobleaching rates indicate that the preponderance of signal attenuation with depth results from decreased rates of fluorescence excitation, even in a system with a descanned emission collection pathway. Similar results were obtained in analyses of fluorescence microspheres embedded in rat kidney tissue, demonstrating that spherical aberration is an important limiting factor in multiphoton fluorescence excitation microscopy of biological samples.

The effects of refractive index heterogeneity within kidney tissue on multiphoton fluorescence excitation microscopy

Although multiphoton fluorescence excitation microscopy has improved the depth at which useful fluorescence images can be collected in biological tissues, the reach of multiphoton fluorescence excitation microscopy is nonetheless limited by tissue scattering and spherical aberration. Scattering can be reduced in fixed samples by mounting in a medium whose refractive index closely matches that of the fixed material. Using optical 'clearing', the effects of refractive index heterogeneity on signal attenuation with depth are investigated. Quantitative measurements show that by mounting kidney tissue in a high refractive index medium, less than 50% of signal attenuates in 100 μm of depth.

Confocal microscopy of thick specimens

Confocal microscopy is an excellent tool to gain structural information from deep within a biological sample. The depth from which information can be extracted as well as the resolution of the detection system are limited by spherical aberrations in the laser pathway. These spherical aberrations of the visible light can be efficiently canceled by optimizing the refractive index of the immersion media. Another way of cancelling spherical aberrations is by changing tube length, or alternatively, by changing the objective from infinite correction to finite correction, or vice versa, depending on which microscope is used. A combination of these two methods allows for confocal imaging at continuous depths. Presently, confocal microscopes typically operate at a maximum depth of 40 microm in the sample, but with the methods presented here, we show that information can easily be gained from depths up to 100 microm. Additionally, the precision of localization of a single fluorophore in the axial direction, limited by spherical aberrations, can be significantly improved, even if the fluorophore is located deep within the sample. In principle, this method can improve the efficiency of any kind of microscopy based on visible light.

Chemical enhancer induced changes in the mechanisms of transdermal delivery of zinc oxide nanoparticles

The overlapping wavelength of photoluminescence (PL) of zinc oxide nanoparticles (ZnO NPs) and autofluorescence (AF) from the stratum corneum (SC) has for a long time held back researchers from investigating the chemically enhanced penetration pathways of ZnO NPs into the SC lipids. However, the non-linear polarization effect of second harmonic generation (SHG) may be used for ZnO NPs to be distinguished from the AF of the SC. This study combined the SHG of ZnO NPs and the AF of the SC to image the transdermal delivery of ZnO NPs under the chemical enhancer conditions of oleic acid (OA), ethanol (EtOH) and oleic acid-ethanol (OA-EtOH). In addition to qualitative imaging, the microtransport properties of ZnO NPs were quantified to give the enhancements of the vehicle-to-skin partition coefficient (K), the SHG intensity gradient (G) and the effective diffusion path length (L). The results showed that OA, EtOH and OA-EtOH were all capable of enhancing the transdermal delivery of ZnO NPs by increasing the intercellular lipid fluidity or extracting lipids from the SC.

Improving Signal Levels in Intravital Multiphoton Microscopy using an Objective Correction Collar

Multiphoton microscopy has enabled biologists to collect high-resolution images hundreds of microns into biological tissues, including tissues of living animals. While the depth of imaging exceeds that possible from any other form of light microscopy, multiphoton microscopy is nonetheless generally limited to depths of less than a millimeter. Many of the advantages of multiphoton microscopy for deep tissue imaging accrue from the unique nature of multiphoton fluorescence excitation. However, the quadratic relationship between illumination level and fluorescence excitation makes multiphoton microscopy especially susceptible to factors that degrade the illumination focus. Here we examine the effect of spherical aberration on multiphoton microscopy in fixed kidney tissues and in the kidneys of living animals. We find that spherical aberration, as evaluated from axial asymmetry in the point spread function, can be corrected by adjustment of the correction collar of a water immersion objective lens. Introducing a compensatory positive spherical aberration into the imaging system decreased the depth-dependence of signal levels in images collected from living animals, increasing signal by up to 50%.

An Integrated Pharmacokinetic and Imaging Evaluation of Vehicle Effects on Solute Human Epidermal Flux and, Retention Characteristics

OBJECTIVE

Our understanding of the differential effects of topically applied vehicles on solute partitioning and diffusion within the skin is presently limited. In this work, in vitro epidermal partitioning, penetration and multiphoton laser scanning microscopy (MPLSM) imaging studies were used to assess the distribution of 2-naphthol across human epidermis.

MATERIALS AND METHODS

Four commonly used liquid vehicles (100% water, 20% propylene glycol (PG)/water, 50% ethanol (EtOH)/water and 100% isopropyl myristate (IPM)) were used.

RESULTS AND DISCUSSION

The maximum flux and membrane retention of 2-naphthol from 50% EtOH/water was almost an order of magnitude or larger than from the other vehicles evaluated whereas IPM resulted in the highest membrane retention and lowest membrane penetration for 2-naphthol than other vehicles. MPLSM studies showed that 2-naphthol solute partitioned favourably into the intercellular lipids and that there was a vehicle-dependent uptake of 2-naphthol into corneocytes.

CONCLUSIONS

The integrated evaluation using in vitro penetration, epidermal retention and MPLSM imaging has shown that vehicle effects on skin penetration occurs by an alteration in the distribution of solutes between the corneocytes and intercellular lipids in addition to the well known mechanisms of altered partitioning into the stratum corneum and enhanced epidermal diffusion.

Spectrally resolved multiphoton imaging of in vivo and excised mouse skin tissues

The deep tissue penetration and submicron spatial resolution of multiphoton microscopy and the high detection efficiency and nanometer spectral resolution of a spectrograph were utilized to record spectral images of the intrinsic emission of mouse skin tissues. Autofluorescence from both cellular and extracellular structures, second-harmonic signal from collagen, and a narrowband emission related to Raman scattering of collagen were detected. Visualization of the spectral images by wavelength-to-RGB color image conversion allowed us to identify and discriminate tissue structures such as epidermal keratinocytes, lipid-rich corneocytes, intercellular structures, hair follicles, collagen, elastin, and dermal cells. Our results also showed morphological and spectral differences between excised tissue section, thick excised tissue, and in vivo tissue samples of mouse skin. Results on collagen excitation at different wavelengths suggested that the origin of the narrowband emission was collagen Raman peaks. Moreover, the oscillating spectral dependency of the collagen second-harmonic intensity was experimentally studied. Overall, spectral imaging provided a wealth of information not easily obtainable with present conventional multiphoton imaging systems.

Femtosecond two-photon high-resolution 3D imaging, spatial-volume rendering and microspectral characterization of immunolocalized MHC-II and mLangerin/CD207 antigens in the mouse epidermis

Langerhans cells (LCs) play a sentinel role by initiating both adaptive and innate immune responses to antigens pertinent to the skin. With the discovery of various LCs markers including antibodies to major histocompatibility complex class II (MHC-II) molecules and CD1a, intracellular presence of racket-shaped "Birbeck granules," and very recently Langerin/CD207, LCs can be readily distinguished from other subsets of dendritic cells. Femtosecond two-photon laser scanning microscopy (TPLSM) in recent years has emerged as an alternative to the single photon-excitation based confocal laser scanning microscope (CLSM), particularly for minimally-invasive deep-tissue 3D and 4D vital as well as nonvital biomedical imaging. We have recently combined high resolution two-photon immunofluorescence (using anti MHC-II and Langerin/CD207 antibodies) imaging with microspectroscopy and advanced image-processing/volume-rendering modalities. In this work, we demonstrate the use of this novel state-of-the-art combinational approach to characterize the steady state 3D organization and spectral features of the mouse epidermis, particularly to identify the spatial distribution of LCs. Our findings provide unequivocal direct evidence that, in the mouse epidermis, the MHC-II and mLangerin/CD207 antigens do indeed manifest a high degree of colocalization around the nucleus of the LCs, while in the distal dendritic processes, mLangerin/CD207 antigens are rather sparsely distributed as punctuate structures. This unique possibility to simultaneously visualize high resolution 3D-resolved spatial distributions of two different immuno-reactive antigens, namely MHC-II and mLangerin/CD207, along with the nuclei of LCs and the adjacent epidermal cells can find interesting applications. These could involve aspects associated with pragmatic analysis of the kinetics of LCs migration as a function of immuno-dermatological responses during (1) human Immunodeficiency virus disease progression, (2) vaccination and targeted gene therapy, (3) skin transplantation/plastic surgery, (4) ultraviolet and other radiation exposure, (5) tissue-engineering of 3D skin constructs, as well as in (6) cosmetic industry, to unravel the influence of cosmeceuticals.

Deficiencies in estrogen-mediated regulation of cerebrovascular homeostasis may contribute to an increased risk of cerebral aneurysm pathogenesis and rupture in menopausal and postmenopausal women

Despite the catastrophic consequence of ruptured intracranial aneurysms, very little is understood regarding their pathogenesis, and there are no reliable predictive markers for identifying at-risk individuals. Few studies have addressed the molecular pathological basis and mechanisms of intracranial aneurysm formation, growth, and rupture. The pathogenesis and rupture of cerebral aneurysms have been associated with inflammatory processes, and these have been implicated in the digestion and breakdown of vascular wall matrix. Epidemiological data indicate that the risk of cerebral aneurysm pathogenesis and rupture in women rises during and after menopause as compared to premenopausal women, and has been attributed to hormonal factors. Moreover, experimental evidence supports a role for estrogen in the modulation of each phase of the inflammatory response implicated in cerebral aneurysm pathogenesis and rupture. While the risk of aneurysm rupture in men also increases with age, this increased risk has been attributed to other recognized risk factors including cigarette smoking, use of alcohol, and history of hypertension, all of which are more common in men than women. We hypothesize, therefore, that decreases in both circulating estrogen levels and cerebrovascular estrogen receptor density may contribute to an increased risk of cerebral aneurysm pathogenesis and rupture in women during and after menopause. To test our hypothesis, experiments are needed to identify genes regulated by estrogen and to evaluate gene expression and intracellular mechanisms in cells/tissues exposed to varying concentrations and duration of treatment with estrogen, metabolites of estrogen, and selective estrogen receptor modulators (SERMs). Furthermore, it is not likely that the regulation of cerebrovascular homeostasis is due to the actions of estrogen alone, but rather the interplay of estrogen and other hormones and their associated receptor expression. The potential interactions of these hormones in the maintenance of normal cerebrovascular tone need to be elucidated. Additional studies are needed to define the role that estrogen and other sex hormones may play in the cerebrovascular circulation and the pathogenesis and rupture of cerebral aneurysms. Efforts directed at understanding the basic pathophysiological mechanisms of aneurysm pathogenesis and rupture promise to yield dividends that may have important therapeutic and clinical implications. The development of non-invasive tools such as molecular MRI for the detection of specific cells, molecular markers, and tissues may facilitate early diagnosis of initial pathophysiological changes that are undetectable by clinical examination or other diagnostic tools, and can also be used to evaluate the state of activity of cerebral aneurysm pathogenesis before, during, and after treatment.

Effects of different immersion media in multiphoton imaging of the epithelium and dermis of human skin

In this work, we compared the performance of objectives with similar numerical aperture of 0.75 but different immersion media of air, water, glycerin, and oil in the imaging of human skin epithelium and dermis. In general, we found that the oil immersion objective recorded the strongest intensity at the same mechanical depth. We also characterized the focal shifts and found that with decreasing refractive index, the focal shift becomes increasingly more negative (for both the epithelium and dermis). In imaging the dermis, we estimated the image resolution at the depths of 18.8 and 30.2 microm, and found that the image resolution were comparable at these depths under the four types of immersion conditions. Our results demonstrate that by changing the immersion media, the main microscopic imaging effects are the recorded axial intensities and the focal shifts. The effects on the image resolution are negligible.

Effects of objective numerical apertures on achievable imaging depths in multiphoton microscopy

Multiphoton microscopy is a powerful technique for achieving three-dimensional submicron imaging in biological specimens. However, specimen optical parameters such as refractive indices and scattering coefficients can result in the loss of image resolution and decreased signal in depth. These factors are coupled to the focusing objective's numerical aperture (NA) in limiting the achievable imaging depths. In this work, we performed multiphoton imaging on aqueous fluorescent solution, human skin, and rat tail tendon to show that, under the same immersion condition, lower NA objectives can examine more deeply into biological specimens and should be used when optimal imaging depths is desired.

Evaluation of microscopic techniques (epifluorescence microscopy, CLSM, TPE-LSM) as a basis for the quantitative image analysis of activated sludge

Microscopic techniques ranging from epifluorescence microscopy to confocal laser scanning microscopy (CLSM) and two photon excitation laser scanning microscopy (TPE-LSM) combined with fluorescent stains can help to evaluate complex microbial aggregates such as activated sludge flocs. To determine the application limits of these microscopic techniques, activated sludge samples from three different sources were evaluated after staining with a fluorescent viability indicator (Baclight Bacterial Viability Kit, Molecular Probes). Image analysis routines were developed to quantify overall amounts of red and green stained cells, location of stained cells within the flocs, and the spatial organization in clusters and filaments. It was found that the selection of the appropriate microscopic technique depends strongly on the type of microbial aggregates being analyzed. For flocs with high cell density, the use of TPE-LSM is preferred, since it provides a clearer image of the internal structure of the aggregate. Epifluorescence microscopy did not allow to reliably quantify red stained cells in dense aggregates. CLSM did not adequately image the internal filamentous structure and the location of stained cells within dense flocs. However, for typical activated sludge flocs epifluorescence and CLSM proved adequate.

Two-photon confocal microscopy: a nondestructive method for studying wound healing

Two-photon confocal microscopy is a new technology useful in nondestructive analysis of tissue. The pattern generated from laser-excited autofluorescence and second harmonic signals can be analyzed to construct a three-dimensional, microanatomical, structural image. The healing of full-thickness guinea pig skin wounds was studied over a period of 28 days using two-photon confocal microscopy. Three-dimensional data were rendered from two-dimensional images and compared with conventional, en face, histologic sections. Two-photon confocal microscopy images show resolution of muscle, fascia fibers, collagen fibers, inflammatory cells, blood vessels, and hair. Although these images do not currently have the resolution of standard histology, the ability to noninvasively acquire three-dimensional images of skin promises to be an important tool in wound-healing studies.

Performances of high numerical aperture water and oil immersion objective in deep-tissue, multi-photon microscopic imaging of excised human skin

Multi-photon fluorescence microscopy (MPFM) is a powerful technique for imaging scattering, biological specimens in depth. In addition to the sectioning effect generated by the point-like excitation volume, the near-infrared wavelengths used for multi-photon excitation allow deeper penetration into optically turbid specimens. In physiological specimens, the optical properties such as the scattering coefficients and refractive indices are often heterogeneous. In these specimens, it is not clear which type of immersion objective can provide optimized images in-depth. In particular, in-depth dermatological imaging applications using MPFM requires such optimization to obtain qualitative and quantitative information from the skin specimens. In this work, we address this issue by comparing the performances of two common types of high numerical aperture (NA) objectives: water-immersion and oil-immersion. A high-quality water-immersion objective (Zeiss, 40 x C-Apochromat, NA 1.2) and a comparable oil-immersion objective (Zeiss, 40 x Fluar, NA 1.25) were used for in-depth imaging of autofuorescent excised human skin and sulforhodamine B treated human skin specimens. Our results show that in the epidermal layers, the two types of immersion objectives perform comparably. However, in the dermis, multi-photon imaging using the oil immersion objective results in stronger fluorescence detection. These observations are most likely due to the degraded point-spread-function (PSF) caused by refractive index mismatch between the epidermis and the dermis.

Whole-mount sections displaying microvascular and glandular structures in human uterus using multiphoton excitation microscopy

There has been considerable interest over many years in the precise structural relationships between microvessels and secretory glands in human endometrium. However, microcirculatory networks have rarely been studied in three-dimensions (3D) using modern computerised technologies, this has been partly due to the late arrival of suitable endothelial cell markers. This study was designed to develop a technique to visualize and to reveal the relationships between microvessels, their glandular environment and epithelial boundaries in 3D, using endometrium from human hysterectomy biopsies. Specimens were carefully selected from women with conditions unlikely to affect the microvascular networks. Monoclonal antibodies (mouse anti-human CD 34 and goat anti-mouse fluorescein (FITC)) were used to visualize the microvessels, and polyclonal antibodies (rabbit anti-human keratin and goat anti-rabbit tetramethylrhodamine (TRITC)) were used to visualize the glandular structures. The samples were studied with a Leica multiphoton system using a titanium-sapphire laser (excitation 800 nm with pulses in the 200 fs range) to obtain a stack of two-dimensional (2D) images to a minimal focus depth of 120 microm. The initial data sets acquired were volume rendered using the integrated software of the Leica system to produce 3D images. This software allowed for the acquisition of data sets from the microscope and for an observational morphological assessment to be made, but was limited in preparing the data for any quantitative analysis. The additional use of ImarisBasic 3.1 visualization software allowed for an observational morphological assessment but also included numerous tools for data manipulation.

Characterizing point spread functions of two-photon fluorescence microscopy in turbid medium

In recent years, fluorescence microscopy based on two-photon excitation has become a popular tool for biological and biomedical imaging. Among its advantages is the enhanced depth penetration permitted by fluorescence excitation with the near-infrared photons, which is particularly attractive for deep-tissue imaging. To fully utilize two-photon fluorescence microscopy as a three-dimensional research technique in biology and medicine, it is important to characterize the two-photon imaging parameters in a turbid medium. We investigated the two-photon point spread functions (PSFs) in a number of scattering samples. Gel samples containing 0.1-microm fluorescent microspheres and Liposyn III were used as phantoms mimicking the turbid environment often found in tissue. A full characterization of the two-photon PSFs of a water and oil immersion objective was completed in samples composed of 0, 0.25, 0.5, 1, and 2% Liposyn III. Our results show that up to depths of about 100 (oil) and 200 microm (water), the presence of scatterers (up to 2% Liposyn III) does not appreciably degrade the PSF widths of the objectives.

Visualization of oleic acid-induced transdermal diffusion pathways using two-photon fluorescence microscopy

In a novel application of dual-channel high-speed two-photon fluorescence microscopy, the skin autofluores-cence and the transdermal fluorescent model drug spatial distributions were imaged simultaneously over precisely the same spatial coordinates. The dual channels enable the detection of the fluorescence emission wavelengths characteristic of the endogenous (intrinsic) skin fluorophores, as well as of the rhodamine-based model drug intensity emission at a different wavelength range of the fluorescence emission spectrum. These fluorescent model drugs delineate the oleic acid induced changes in permeant diffusion with respect to the skin structural features over the 0.3 mm by 0.3 mm skin area imaged per skin sample. The dual-channel high-speed two-photon fluorescence microscopy studies presented here provide evidence for the existence of intracorneocyte diffusion in addition to the commonly cited lipid multilamellar transdermal pathway. The image quantification analysis methodology introduced in this paper reveals that intracorneocyte diffusion exists for the hydrophobic (rhodamine B hexyl ester) and for the hydrophilic (sulforhodamine B) model drugs, in the absence of oleic acid chemical enhancer action. The mechanism of oleic acid chemical enhancer action, however, depends on the model drug physicochemical properties, where the oleic acid induces hydrophobic model drug localization to the lipid multilamellar region, while increasing the hydrophilic model drug lipid to corneocyte partitioning.

Two-photon microscopy in brain tissue: parameters influencing the imaging depth

Light scattering by tissue limits the imaging depth of two-photon microscopy and its use for functional brain imaging in vivo. We investigate the influence of scattering on both fluorescence excitation and collection, and identify tissue and instrument parameters that limit the imaging depth in the brain. (i) In brain slices, we measured that the scattering length at lambda=800 nm is a factor 2 higher in juvenile cortical tissue (P14-P18) than in adult tissue (P90). (ii) In a detection geometry typical for in vivo imaging, we show that the collected fraction of fluorescence drops at large depths, and that it is proportional to the square of the effective angular acceptance of the detection optics. Matching the angular acceptance of the microscope to that of the objective lens can result in a gain of approximately 3 in collection efficiency at large depths (>500 microm). A low-magnification (20x), high-numerical aperture objective (0.95) further increases fluorescence collection by a factor of approximately 10 compared with a standard 60x-63x objective without compromising the resolution. This improvement should allow fluorescence measurements related to neuronal or vascular brain activity at >100 microm deeper than with standard objectives.

Visualization of meiotic events in intact living anthers by means of two-photon microscopy

In this paper we describe the application of two-photon microscopy (2 PM) to the study of meiosis in plants. Fresh, unfixed anthers of Agapanthus umbelatus were briefly incubated on a minimal medium containing the DNA fluorophore DAPI. DAPI incorporation took place in about 30 min and nuclei and other DNA-containing organelles kept their fluorescence for more than 24 h. Using PM it was possible to optically section the whole, unfixed anthers to a depth of approximately 200 microm. This was up to the mid sagital section and into the sporogenic tissue. Several meiotic figures were observed with unparalleled resolution. Sequences of nuclear dynamics and division were occasionally observed in the surrounding tissues and epidermal layer of cells. However we could not optimize the procedures up to the level of observing the dynamics of division on the meiotic nuclei as well. We hypothesize that either (1) meiotic cells are sensitive to the reasonably high excitation levels of infrared light needed to attain such penetration in the tissue, or (2) that our incubation procedures are not sufficiently non-invasive for meiosis to remain unperturbed. To the best of our knowledge this is the first report on direct observation of living meiotic cells in plants and establishes the potential of 2 PM for intact organ research.