Depth resolved wide field illumination for biomedical imaging and fabrication

Nonlinear microscopic imaging is relatively slow due to the sequential nature of raster scanning. Recently, this limitation was overcome by developing a 3D-resolved wide-field two-photon microscope based on the concept of temporal focusing. The existing temporal focusing systems have poor optical sectioning capability and, due to a shortage of illumination power, low actual frame rate. In this presentation, a comprehensive mathematical model for temporal focusing two-photon microscope will be presented. By optimizing instrument design and the use of high two-photon cross section quantum dots, we demonstrate single quantum dot imaging with submicron resolution at video rate and applied it to study transport processes in cells. Further, we realize that the depth resolved wide field illumination can be used for microfabrication. A prototype three-dimensional lithographic microfabrication system was developed and micropatterning capability based on photobleaching process is demonstrated.

Single-wavelength reflected confocal and multiphoton microscopy for tissue imaging

Both reflected confocal and multiphoton microscopy can have clinical diagnostic applications. The successful combination of both modalities in tissue imaging enables unique image contrast to be achieved, especially if a single laser excitation wavelength is used. We apply this approach for skin and corneal imaging using the 780-nm output of a femtosecond, titanium-sapphire laser. We find that the near-IR, reflected confocal (RC) signal is useful in characterizing refractive index varying boundaries in bovine cornea and porcine skin, while the multiphoton autofluorescence (MAF) and second-harmonic generation (SHG) intensities can be used to image cytoplasm and connective tissues (collagen), respectively. In addition, quantitative analysis shows that we are able to detect MAF from greater imaging depths than with the near-IR RC signal. Furthermore, by performing RC imaging at 488, 543, and 633 nm, we find that a longer wavelength leads to better image contrast for deeper imaging of the bovine cornea and porcine skin tissue. Finally, by varying power of the 780-nm source, we find that comparable RC image quality was achieved in the 2.7 to 10.7-mW range.

Differentiation of normal and cancerous lung tissues by multiphoton imaging

We utilize multiphoton microscopy for the label-free diagnosis of noncancerous, lung adenocarcinoma (LAC), and lung squamous cell carcinoma (SCC) tissues from humans. Our results show that the combination of second-harmonic generation (SHG) and multiphoton excited autofluorescence (MAF) signals may be used to acquire morphological and quantitative information in discriminating cancerous from noncancerous lung tissues. Specifically, noncancerous lung tissues are largely fibrotic in structure, while cancerous specimens are composed primarily of tumor masses. Quantitative ratiometric analysis using MAF to SHG index (MAFSI) shows that the average MAFSI for noncancerous and LAC lung tissue pairs are 0.55+/-0.23 and 0.87+/-0.15, respectively. In comparison, the MAFSIs for the noncancerous and SCC tissue pairs are 0.50+/-0.12 and 0.72+/-0.13, respectively. Our study shows that nonlinear optical microscopy can assist in differentiating and diagnosing pulmonary cancer from noncancerous tissues.

Fibro-C-Index: comprehensive, morphology-based quantification of liver fibrosis using second harmonic generation and two-photon microscopy

We develop a standardized, fully automated, quantification system for liver fibrosis assessment using second harmonic generation microscopy and a morphology-based quantification algorithm. Liver fibrosis is associated with an abnormal increase in collagen as a result of chronic liver diseases. Histopathological scoring is the most commonly used method for liver fibrosis assessment, where a liver biopsy is stained and scored by experienced pathologists. Due to the intrinsic limited sensitivity and operator-dependent variations, there exist high inter- and intraobserver discrepancies. We validate our quantification system, Fibro-C-Index, with a comprehensive animal study and demonstrate its potential application in clinical diagnosis to reduce inter- and intraobserver discrepancies.

Quantitative morphometric measurements using site selective image cytometry of intact tissue

Site selective two-photon tissue image cytometry has previously been successfully applied to measure the number of rare cells in three-dimensional tissue specimens up to cubic millimetres in size. However, the extension of this approach for high-throughput quantification of cellular morphological states has not been demonstrated. In this paper, we report the use of site-selective tissue image cytometry for the study of homologous recombination (HR) events during cell division in the pancreas of transgenic mice. Since HRs are rare events, recombinant cells distribute sparsely inside the organ. A detailed measurement throughout the whole tissue is thus not practical. Instead, the site selective two-photon tissue cytometer incorporates a low magnification, wide field, one-photon imaging subsystem that rapidly identifies regions of interest containing recombinant cell clusters. Subsequently, high-resolution three-dimensional assays based on two-photon microscopy can be performed only in these regions of interest. We further show that three-dimensional morphology extraction algorithms can be used to analyse the resultant high-resolution two-photon image stacks providing information not only on the frequency and the distribution of these recombinant cell clusters and their constituent cells, but also on their morphology.

Cell mechanics at multiple scales

The response of cells to mechanical stresses is a field of growing inquiry. It is well known that both the morphologic and molecular expression of cells depend, in part, on the local mechanical environment, especially for cells such as endothelial cells that experience shear stress, stretch, and pressures. To systematically study the large variety of responses of cells to physical forces (e.g., signaling, adhesion, or stiffness changes), a number of techniques have been developed and used. Here we present methods for three types of cell mechanical studies, from the multicellular to the subcellular scales, and describe the basic principle and main use of each technique along with some design and setup considerations.

Ultrafast optical pulse delivery with fibers for nonlinear microscopy

Nonlinear microscopies including multiphoton excitation fluorescence microscopy and multiple-harmonic generation microscopy have recently gained popularity for cellular and tissue imaging. The optimization of these imaging methods for minimally invasive use requires optical fibers to conduct light into tight space, where free-space delivery is difficult. The delivery of high-peak power laser pulses with optical fibers is limited by dispersion resulting from nonlinear refractive index responses. In this article, we characterize a variety of commonly used optical fibers in terms of how they affect pulse profile and imaging performance of nonlinear microscopy; the following parameters are quantified: spectral bandwidth and temporal pulse width, two-photon excitation efficiency, and optical resolution. A theoretical explanation for the measured performance of these fibers is also provided.

Multiscale structural analysis of mouse lingual myoarchitecture employing diffusion spectrum magnetic resonance imaging and multiphoton microscopy

The tongue consists of a complex, multiscale array of myofibers that comprise the anatomical underpinning of lingual mechanical function. 3-D myoarchitecture was imaged in mouse tongues with diffusion spectrum magnetic resonance imaging (DSI) at 9.4 T (b(max) 7000 smm, 150-microm isotropic voxels), a method that derives the preferential diffusion of water/voxel, and high-throughput (10 fps) two-photon microscope (TPM). Net fiber alignment was represented for each method in terms of the local maxima of an orientational distribution function (ODF) derived from the local diffusion (DSI) and 3-D structural autocorrelation (TPM), respectively. Mesoscale myofiber tracts were generated by alignment of the principal orientation vectors of the ODFs. These data revealed a consistent relationship between the properties of the respective ODFs and the virtual superimposition of the distributed mesoscale myofiber tracts. The identification of a mesoscale anatomical construct, which specifically links the microscopic and macroscopic spatial scales, provides a method for relating the orientation and distribution of cells and subcellular components with overall tissue morphology, thus contributing to the development of multiscale methods for mechanical analysis.

Fast fluorescence laser tracking microrheometry, II: quantitative studies of cytoskeletal mechanotransduction

Fluorescence laser tracking microrheometry (FLTM) is what we believe to be a novel method able to assess the local, frequency-dependent mechanical properties of living cells with nanometer spatial sensitivity at speeds up to 50 kHz. In an earlier article, we described the design, development, and optimization phases of the FLTM before reporting its performances in a variety of viscoelastic materials. In the work presented here, we demonstrate the suitability of FLTM to study local cellular rheology and obtain values for the storage and loss moduli G'(omega) and G''(omega) of fibroblasts consistent with past literature. We further establish that chemically induced cytoskeletal disruption is accompanied by reduced cellular stiffness and viscosity. Next, we provide a systematic study of some experimental variables that may critically influence microrheology measurements. First, we interrogate and justify the relevance of bead endocytosis as a method of cellular internalization of 1-microm probes in FLTM. Second, we show that as sample temperature increases, FLTM findings are elevated toward higher frequencies. Third, we confirm that relevant bead sizes (1 and 2 microm) have no effect on FLTM measurements. Fourth, we report the lack of influence of bead coatings (antiintegrin, antitransferrin, antidystroglycan, or uncoated tracers were surveyed) on their rheological readouts. Finally, we demonstrate the potential of FLTM in studying how substratum rigidity regulates cellular rheological properties. Interestingly, multiple, coupled strain relaxation mechanisms can be observed separated by two plateau moduli. Although these observations can be partly explained by rheological theories describing entangled actin filaments, there is a clear need to extend existing microrheology models to the cytoskeleton, including potentially important factors such as network geometry and remodeling.

Three-dimensional tissue cytometer based on high-speed multiphoton microscopy

Image cytometry technology has been extended to 3D based on high-speed multiphoton microscopy. This technique allows in situ study of tissue specimens preserving important cell-cell and cell-extracellular matrix interactions. The imaging system was based on high-speed multiphoton microscopy (HSMPM) for 3D deep tissue imaging with minimal photodamage. Using appropriate fluorescent labels and a specimen translation stage, we could quantify cellular and biochemical states of tissues in a high throughput manner. This approach could assay tissue structures with subcellular resolution down to a few hundred micrometers deep. Its throughput could be quantified by the rate of volume imaging: 1.45 mm(3)/h with high resolution. For a tissue containing tightly packed, stratified cellular layers, this rate corresponded to sampling about 200 cells/s. We characterized the performance of 3D tissue cytometer by quantifying rare cell populations in 2D and 3D specimens in vitro. The measured population ratios, which were obtained by image analysis, agreed well with the expected ratios down to the ratio of 1/10(5). This technology was also applied to the detection of rare skin structures based on endogenous fluorophores. Sebaceous glands and a cell cluster at the base of a hair follicle were identified. Finally, the 3D tissue cytometer was applied to detect rare cells that had undergone homologous mitotic recombination in a novel transgenic mouse model, where recombination events could result in the expression of enhanced yellow fluorescent protein in the cells. 3D tissue cytometry based on HSMPM demonstrated its screening capability with high sensitivity and showed the possibility of studying cellular and biochemical states in tissues in situ. This technique will significantly expand the scope of cytometric studies to the biomedical problems where spatial and chemical relationships between cells and their tissue environments are important.

Spectrally resolved fluorescence correlation spectroscopy based on global analysis

Multicolor fluorescence correlation spectroscopy has been recently developed to study chemical interactions of multiple chemical species labeled with spectrally distinct fluorophores. In the presence of spectral overlap, there exists a lower detectability limit for reaction products with multicolor fluorophores. In addition, the ability to separate bound product from reactants allows thermodynamic properties such as dissociation constants to be measured for chemical reactions. In this report, we utilize a spectrally resolved two-photon microscope with single-photon counting sensitivity to acquire spectral and temporal information from multiple chemical species. Further, we have developed a global fitting analysis algorithm that simultaneously analyzes all distinct auto- and cross-correlation functions from 15 independent spectral channels. We have demonstrated that the global analysis approach allows the concentration and diffusion coefficients of fluorescent particles to be resolved despite the presence of overlapping emission spectra.

Dual-Channel Two-Photon Microscopy Study of Transdermal Transport in Skin Treated with Low-Frequency Ultrasound and a Chemical Enhancer

Visualization of transdermal permeant pathways is necessary to substantiate model-based conclusions drawn using permeability data. The aim of this investigation was to visualize the transdermal delivery of sulforhodamine B (SRB), a fluorescent hydrophilic permeant, and of rhodamine B hexyl ester (RBHE), a fluorescent hydrophobic permeant, using dual-channel two-photon microscopy (TPM) to better understand the transport pathways and the mechanisms of enhancement in skin treated with low-frequency ultrasound (US) and/or a chemical enhancer (sodium lauryl sulfate--SLS) relative to untreated skin (the control). The results demonstrate that (1) both SRB and RBHE penetrate beyond the stratum corneum and into the viable epidermis only in discrete regions (localized transport regions--LTRs) of US treated and of US/SLS-treated skin, (2) a chemical enhancer is required in the coupling medium during US treatment to obtain two significant levels of increased penetration of SRB and RBHE in US-treated skin relative to untreated skin, and (3) transcellular pathways are present in the LTRs of US treated and of US/SLS-treated skin for SRB and RBHE, and in SLS-treated skin for SRB. In summary, the skin is greatly perturbed in the LTRs of US treated and US/SLS-treated skin with chemical enhancers playing a significant role in US-mediated transdermal drug delivery.

Multifocal multiphoton microscopy based on multianode photomultiplier tubes

Multifocal multiphoton microscopy (MMM) enhances imaging speed by parallelization. It is not well understood why the imaging depth of MMM is significantly shorter than conventional single-focus multiphoton microscopy (SMM). In this report, we show that the need for spatially resolved detectors in MMM results in a system that is more sensitive to the scattering of emission photons with reduced imaging depth. For imaging depths down to twice the scattering mean free path length of emission photons (2xl (s) (em)), the emission point spread function (PSF(em)) is found to consist of a narrow, diffraction limited distribution from ballistic emission photons and a broad, relatively low amplitude distribution from scattered photons. Since the scattered photon distribution is approximately 100 times wider than that of the unscattered photons at 2xl (s) (em), image contrast and depth are degraded without compromising resolution. To overcome the imaging depth limitation of MMM, we present a new design that replaces CCD cameras with multi-anode photomultiplier tubes (MAPMTs) allowing more efficient collection of scattered emission photons. We demonstrate that MAPMT-based MMM has imaging depth comparable to SMM with equivalent sensitivity by imaging tissue phantoms, ex vivo human skin specimens based on endogenous fluorophores, and green fluorescent protein (GFP) expressing neurons in mouse brain slices.

Geometric confinement influences cellular mechanical properties I -- adhesion area dependence

Interactions between the cell and the extracellular matrix regulate a variety of cellular properties and functions, including cellular rheology. In the present study of cellular adhesion, area was controlled by confining NIH 3T3 fibroblast cells to circular micropatterned islands of defined size. The shear moduli of cells adhering to islands of well defined geometry, as measured by magnetic microrheometry, was found to have a significantly lower variance than those of cells allowed to spread on unpatterned surfaces. We observe that the area of cellular adhesion influences shear modulus. Rheological measurements further indicate that cellular shear modulus is a biphasic function of cellular adhesion area with stiffness decreasing to a minimum value for intermediate areas of adhesion, and then increasing for cells on larger patterns. We propose a simple hypothesis: that the area of adhesion affects cellular rheological properties by regulating the structure of the actin cytoskeleton. To test this hypothesis, we quantified the volume fraction of polymerized actin in the cytosol by staining with fluorescent phalloidin and imaging using quantitative 3D microscopy. The polymerized actin volume fraction exhibited a similar biphasic dependence on adhesion area. Within the limits of our simplifying hypothesis, our experimental results permit an evaluation of the ability of established, micromechanical models to predict the cellular shear modulus based on polymerized actin volume fraction. We investigated the "tensegrity", "cellular-solids", and "biopolymer physics" models that have, respectively, a linear, quadratic, and 5/2 dependence on polymerized actin volume fraction. All three models predict that a biphasic trend in polymerized actin volume fraction as a function of adhesion area will result in a biphasic behavior in shear modulus. Our data favors a higher-order dependence on polymerized actin volume fraction. Increasingly better experimental agreement is observed for the tensegrity, the cellular solids, and the biopolymer models respectively. Alternatively if we postulate the existence of a critical actin volume fraction below which the shear modulus vanishes, the experimental data can be equivalently described by a model with an almost linear dependence on polymerized actin volume fraction; this observation supports a tensegrity model with a critical actin volume fraction.

Geometric confinement influences cellular mechanical properties II -- intracellular variances in polarized cells

During migration, asymmetrically polarized cells achieve motion by coordinating the protrusion and retraction of their leading and trailing edges, respectively. Although it is well known that local changes in the dynamics of actin cytoskeleton remodeling drive these processes, neither the cytoskeletal rheological properties of these migrating cells are well quantified nor is it understand how these rheological properties are regulated by underlying molecular processes. In this report, we have used soft lithography to create morphologically polarized cells in order to examine rheological differences between the front and rear zone of an NIH 3T3 cell posed for migration. In addition, we trapped superparamagnetic beads with optical tweezers and precisely placed them at specific locations on the immobilized cells. The beads were then allowed to endocytose overnight before magnetic tweezers experiments were performed to measure the local rheological properties of the leading and trailing edges. Our results indicate that the leading edge has an approximately 1.9 times higher shear modulus than the trailing edge and that this increase in shear modulus correlates with a greater density of filamentous actin, as measured by phalloidin-staining observed through quantitative 3D microscopy.

Two-dimensional standing wave total internal reflection fluorescence microscopy: superresolution imaging of single molecular and biological specimens

The development of high resolution, high speed imaging techniques allows the study of dynamical processes in biological systems. Lateral resolution improvement of up to a factor of 2 has been achieved using structured illumination. In a total internal reflection fluorescence microscope, an evanescence excitation field is formed as light is total internally reflected at an interface between a high and a low index medium. The <100 nm penetration depth of evanescence field ensures a thin excitation region resulting in low background fluorescence. We present even higher resolution wide-field biological imaging by use of standing wave total internal reflection fluorescence (SW-TIRF). Evanescent standing wave (SW) illumination is used to generate a sinusoidal high spatial frequency fringe pattern on specimen for lateral resolution enhancement. To prevent thermal drift of the SW, novel detection and estimation of the SW phase with real-time feedback control is devised for the stabilization and control of the fringe phase. SW-TIRF is a wide-field superresolution technique with resolution better than a fifth of emission wavelength or approximately 100 nm lateral resolution. We demonstrate the performance of the SW-TIRF microscopy using one- and two-directional SW illumination with a biological sample of cellular actin cytoskeleton of mouse fibroblast cells as well as single semiconductor nanocrystal molecules. The results confirm the superior resolution of SW-TIRF in addition to the merit of a high signal/background ratio from TIRF microscopy.

Investigation of the point spread function of surface plasmon-coupled emission microscopy

Surface plasmon-coupled emission microscopy (SPCEM) was proposed as a high sensitivity technique that makes use of a thin layer of metal deposited on glass slides to efficiently excite fluorophores and to collect the emission light. However, the theoretical aspect of SPCEM imaging has not been well studied. We propose a model for SPCEM and show, through theoretical analysis and empirical results, that the point spread function of SPCEM is irregular and has an annular-like structure, significantly different from the familiar point spread function of the conventional wide-field microscopy. This result is due to the highly polarized and anisotropic emission caused by the metal layer.

Intact corneal stroma visualization of GFP mouse revealed by multiphoton imaging

The aim of this work is to demonstrate that multiphoton microscopy is a preferred technique to investigate intact cornea structure without slicing and staining. At the micron resolution, multiphoton imaging can provide both large morphological features and detailed structure of epithelium, corneal collagen fibril bundles and keratocytes. A large area multiphoton cross-section across an intact eye excised from a GFP mouse was obtained by a homebuilt multiphoton microscope. The broadband multiphoton fluorescence (435-700 nm) and second harmonic generation (SHG, 360-400 nm) signals were generated by the 760 nm output of a femtosecond titanium-sapphire laser. A water immersion objective (Fluor, 40X, NA 0.8; Nikon) was used to facilitate imaging the curve ocular surface. The multiphoton image over entire cornea provides morphological information of epithelial cells, keratocytes, and global collagen orientation. Specifically, our planar, large area multiphoton image reveals a concentric pattern of the stroma collagen, indicative of the laminar collagen organization throughout the stroma. In addition, the green fluorescence protein (GFP) labeling contributed to fluorescence contrast of cellular area and facilitated visualizing of inactive keratocytes. Our results show that multiphoton imaging of GFP labeled mouse cornea manifests both morphological significance and structural details. The second harmonic generation imaging reveals the collagen orientation, while the multiphoton fluorescence imaging indicates morphology and distribution of cells in cornea. Our results support that multiphoton microscopy is an appropriate technology for further in vivo investigation and diagnosis of cornea.

High-resolution whole organ imaging using two-photon tissue cytometry

Three-dimensional (3-D) tissue imaging offers substantial benefits to a wide range of biomedical investigations from cardiovascular biology, diabetes, Alzheimer's disease to cancer. Two-photon tissue cytometry is a novel technique based on high-speed multiphoton microscopy coupled with automated histological sectioning, which can quantify tissue morphology and physiology throughout entire organs with subcellular resolution. Furthermore, two-photon tissue cytometry offers all the benefits of fluorescence-based approaches including high specificity and sensitivity and appropriateness for molecular imaging of gene and protein expression. We use two-photon tissue cytometry to image an entire mouse heart at subcellular resolution to quantify the 3-D morphology of cardiac microvasculature and myocyte morphology spanning almost five orders of magnitude in length scales.

Multifunctional Imaging of Endogenous Contrast by Simultaneous Nonlinear and Optical Coherence Microscopy of Thick Tissues

A variety of high resolution optical microscopy techniques have been developed in recent years for basic and clinical studies of biological systems. We demonstrate a trimodal microscope combining optical coherence microscopy (OCM) with two forms of nonlinear microscopy, namely two-photon excited fluorescence (2PF) and second harmonic generation (SHG), for imaging turbid media. OCM combines the advantages of confocal detection and coherence gating for structural imaging in highly scattering tissues. Nonlinear microscopy enables the detection of biochemical species, such as elastin, NAD(P)H, and collagen. While 2PF arises from nonlinear excitation of fluorescent species, SHG is a form of nonlinear scattering observed in materials that lack a center of inversion symmetry, such as type I collagen. Characterization of the microscope showed nearly diffraction-limited spatial resolution in all modalities. Images were obtained in fish scales and excised human skin samples. The primary endogenous sources of contrast in the dermis were due to elastin autofluorescence and collagen SHG. Multimodal microscopy allows the simultaneous visualization of structural and functional information of biological systems.

Frequency domain lifetime and spectral imaging microscopy

In the femtoliter observation volume of a two-photon microscope, multiple fluorophores can be present and complex photophysics can take place. Combined detection of the fluorescence emission spectra and lifetimes can provide deeper insight into specimen properties than these two imaging modalities taken separately. Therefore, we have developed a detection scheme based on a frequency-modulated multichannel photomultiplier, which measures simultaneously the spectrum and the lifetime of the emitted fluorescence. Experimentally, the efficiency of the frequency domain lifetime measurement was compared to a time domain set-up. The performance of this spectrally and lifetime-resolved microscope was evaluated on reference specimens and living cells labeled with three different stains targeting the membrane, the mitochondria, and the nucleus.

Detecting Single Quantum Dot Motion With Nanometer Resolution for Applications in Cell Biology

Quantum dots (QDs), semiconductor particles of nanometer dimension, have emerged as excellent fluorescent analogs in tracer experiments with single molecule sensitivity for bioassays. Cell imaging greatly benefits from the remarkable optical and physical properties of these inorganic nanocrystals: QDs are much brighter and exhibit a higher resistance to photobleaching than traditional fluorophores, and their narrow emission spectrum and flexible surface chemistry make them particularly suitable for multiplex imaging. Here, we have demonstrated the achievement of a nanometer spatial resolution on the position of a single QD in a simple optomechanical instrument using a high-sensitivity low-noise detector, an intensified CCD camera. Furthermore, nanometer variations in the amplitude of a QD's sinusoidal oscillations could be quantitatively distinguished after fast Fourier transform (FFT) based data processing. As confirmed by experiments where QDs were attached to the surface of bovine aortic endothelial cells, this method can be exploited in biology to assess molecular and subcellular contributions to responses such as motility, intracellular trafficking, and mechanotransduction, with high resolution and minimal disturbance to cells.

Investigating mechanisms of collagen thermal denaturation by high resolution second-harmonic generation imaging

We apply the technique of second-harmonic generation (SHG) microscopy to obtain large area submicron resolution image of Type I collagen from rat tail tendon as it is heated from 40 degrees C to 70 degrees C for 0-180 min. The change in the collagen structure as reflected in its SHG image is observed at length scales from submicron to hundreds of microns. We observed that heating the tendon below the temperature of 54 degrees C does not produce any change in the averaged SHG intensity. At the heating temperature of 54 degrees C and above, we find that increasing the heating temperature and time leads to decreasing SHG intensity. As the tendon is heated above 54 degrees C, the regions where the SHG signal vanish and form a tiger-tail like pattern. In addition, a decrease in the SHG signal occurs uniformly throughout the tendon. By comparing the relative SHG intensities in small and large areas, we found that the denaturation process responsible for forming the tiger-tail like pattern occurs at a higher rate than the global denaturation process occurring throughout the tendon. We also measured the fibril spacing and found that it remains constant at 1.61 +/- 0.04 micron for all heating temperature and times. The constant fibril density shows that the global denaturation process occurs at a length scale smaller than the size of the fibril. Our results show that second-harmonic generation microscopy is effective in monitoring the thermal damage to collagen and has potential applications in biomedicine.

Comparing the quantification of Forster resonance energy transfer measurement accuracies based on intensity, spectral, and lifetime imaging

The measurement of Forster resonance energy transfer (FRET) in microscopes can be realized by different imaging modalities. In the present work, reference FRET constructs are developed to allow the comparison of FRET microscopy measurements using intensity, spectral, and lifetime imaging. Complimentary DNA strands are respectively labeled with Oregon Green 488 (OG488) or tetramethylrhodamine (TMR). The OG488 dye is fixed at the 5(') end of one strand, and the TMR label position is allowed to vary along the complimentary strand. Since OG488 and TMR are FRET pairs, the FRET efficiency can be determined theoretically from the distance separating the two dyes of the double-stranded DNA molecules. Microscopic images are formed by imaging microcapillaries containing various mixtures of oligonucleotides labeled with the FRET fluorophore pair, only the donor, or only acceptor. Traditional two-channel intensity measurements are compared with spectrally resolved imaging and fluorescence lifetime imaging by calculating a FRET index. The latter proves to be the best method to quantify FRET efficiency in the image. More importantly, the intensity fraction of molecules undergoing FRET can be quantitatively measured in each pixel of the image.

Multiphoton autofluorescence and second-harmonic generation imaging of the ex vivo porcine eye

PURPOSE

The purpose of this work was to demonstrate the use of the combined imaging modality of multiphoton autofluorescence and second-harmonic generation (SHG) microscopy in obtaining spectrally resolved morphologic features of the cornea, limbus, conjunctiva, and sclera in whole, ex vivo porcine eyes.

METHODS

The 780-nm output of a femtosecond, titanium-sapphire laser was used to induce broadband autofluorescence (435-700 nm) and SHG (390 nm) from various regions of the surface of ex vivo porcine eyes. A water-immersion objective was used for convenient imaging of the curved surface of the eye.

RESULTS

Multiphoton autofluorescence was useful in identifying cellular structures of the different domains of the ocular surface, and the SHG signal can be used to resolve collagen organization within the cornea stroma and sclera of ex vivo porcine eyes.

CONCLUSIONS

Multiphoton autofluorescence and SHG microscopy have been demonstrated to be an effective technique for resolving, respectively, the cellular and collagen structures within the ocular surface of ex vivo porcine eyes. SHG imaging resolved the difference in structural orientations between corneal and sclera collagen fibers. Specifically, the corneal collagen is organized in a depth-dependent fashion, whereas the scleral collagen is randomly packed. Because this technique does not require histologic preparation procedures, it has the potential to be applied for in vivo studies with minimal disturbance to the eye.

Extended resolution wide-field optical imaging: objective-launched standing-wave total internal reflection fluorescence microscopy

Standing-wave total-internal-reflection fluorescence (SW-TIRF) microscopy uses a super-diffraction-limited standing evanescent wave to extract the high-spatial-frequency content of an object through a diffraction-limited optical imaging system. The effective point-spread function is better than a quarter of the emission wavelength. With a 1.45 numerical aperture objective and 532 nm excitation wavelength, a Rayleigh resolution of approximately 100 nm can be achieved, which is better than twice the resolution of conventional TIRF microscopy. This first experimental realization of SW-TIRF in an objective-launched geometry demonstrates the potential for extended resolution imaging at high speed by using wide-field microscopy.

Single-photon counting multicolor multiphoton fluorescence microscope

We present a multicolor multiphoton fluorescence microscope with single-photon counting sensitivity. The system integrates a standard multiphoton fluorescence microscope, an optical grating spectrograph operating in the UV-Vis wavelength region, and a 16-anode photomultiplier tube (PMT). The major technical innovation is in the development of a multichannel photon counting card (mC-PhCC) for direct signal collection from multi-anode PMTs. The electronic design of the mC-PhCC employs a high-throughput, fully-parallel, single-photon counting scheme along with a high-speed electrical or fiber-optical link interface to the data acquisition computer. There is no electronic crosstalk among the detection channels of the mC-PhCC. The collected signal remains linear up to an incident photon rate of 10(8) counts per second. The high-speed data interface offers ample bandwidth for real-time readout: 2 MByte lambda-stacks composed of 16 spectral channels, 256 x 256 pixel image with 12-bit dynamic range can be transferred at 30 frames per second. The modular design of the mC-PhCC can be readily extended to accommodate PMTs of more anodes. Data acquisition from a 64-anode PMT has been verified. As a demonstration of system performance, spectrally resolved images of fluorescent latex spheres and ex-vivo human skin are reported. The multicolor multiphoton microscope is suitable for highly sensitive, real-time, spectrally-resolved three-dimensional imaging in biomedical applications.

Multi-Photon Excitation of Intrinsic Protein Fluorescence and Its Application to Pharmaceutical Drug Screening

The majority of proteins contain intrinsic fluorophores as natural sensors of molecular structures, dynamics, and interactions. The intrinsic protein fluorescence signal allows for the label-free and, hence, undisturbed and rapid study of protein-ligand interactions. Ultraviolet-based drug screening is hampered by the background, photobleaching, light scattering, inner filter effects, and interfering assay compounds. Such problems can be overcome by means of molecular three-photon excitation (3PE) with infrared femtosecond light pulses since longer excitation wavelengths result in less Raleigh scattering, and the subfemtoliter (confocal-like) 3PE volume minimizes out-of-focus photobleaching, background generation, and inner filter effects. We demonstrate the general feasibility of 3PE for protein spectroscopy and illustrate the technique's excellent potential for high-throughput screening. By using the intrinsic fluorescence intensity of a protein-substrate, we were able to discriminate between ligands of different affinities in binding assays.

Two-photon 3-D mapping of ex vivo human skin endogenous fluorescence species based on fluorescence emission spectra

Spectral resolved tissue imaging has a broad range of biomedical applications such as the minimally invasive diagnosis of diseases and the study of wound healing and tissue engineering processes. Two-photon microscopy imaging of endogenous fluorescence has been shown to be a powerful method for the quantification of tissue structure and biochemistry. While two-photon excited autofluorescence is observed ubiquitously, the identities and distributions of endogenous fluorophores have not been completely characterized in most tissues. We develop an image-guided spectral analysis method to analyze the distribution of fluorophores in human skin from 3-D resolved two-photon images. We identify five factors that contribute to most of the luminescence signals from human skin. Luminescence species identified include tryptophan, NAD(P)H, melanin, and elastin, which are autofluorescent, and collagen that contributes to a second harmonic signal.

Mitigating thermal mechanical damage potential during two-photon dermal imaging

Two-photon excitation fluorescence microscopy allows in vivo high-resolution imaging of human skin structure and biochemistry with a penetration depth over 100 microm. The major damage mechanism during two-photon skin imaging is associated with the formation of cavitation at the epidermal-dermal junction, which results in thermal mechanical damage of the tissue. In this report, we verify that this damage mechanism is of thermal origin and is associated with one-photon absorption of infrared excitation light by melanin granules present in the epidermal-dermal junction. The thermal mechanical damage threshold for selected Caucasian skin specimens from a skin bank as a function of laser pulse energy and repetition rate has been determined. The experimentally established thermal mechanical damage threshold is consistent with a simple heat diffusion model for skin under femtosecond pulse laser illumination. Minimizing thermal mechanical damage is vital for the potential use of two-photon imaging in noninvasive optical biopsy of human skin in vivo. We describe a technique to mitigate specimen thermal mechanical damage based on the use of a laser pulse picker that reduces the laser repetition rate by selecting a fraction of pulses from a laser pulse train. Since the laser pulse picker decreases laser average power while maintaining laser pulse peak power, thermal mechanical damage can be minimized while two-photon fluorescence excitation efficiency is maximized.

A fast global fitting algorithm for fluorescence lifetime imaging microscopy based on image segmentation

Global fitting algorithms have been shown to improve effectively the accuracy and precision of the analysis of fluorescence lifetime imaging microscopy data. Global analysis performs better than unconstrained data fitting when prior information exists, such as the spatial invariance of the lifetimes of individual fluorescent species. The highly coupled nature of global analysis often results in a significantly slower convergence of the data fitting algorithm as compared with unconstrained analysis. Convergence speed can be greatly accelerated by providing appropriate initial guesses. Realizing that the image morphology often correlates with fluorophore distribution, a global fitting algorithm has been developed to assign initial guesses throughout an image based on a segmentation analysis. This algorithm was tested on both simulated data sets and time-domain lifetime measurements. We have successfully measured fluorophore distribution in fibroblasts stained with Hoechst and calcein. This method further allows second harmonic generation from collagen and elastin autofluorescence to be differentiated in fluorescence lifetime imaging microscopy images of ex vivo human skin. On our experimental measurement, this algorithm increased convergence speed by over two orders of magnitude and achieved significantly better fits.

Cell stiffness and receptors: evidence for cytoskeletal subnetworks

Viscoelastic models of cells often treat cells as homogeneous objects. However, studies have demonstrated that cellular properties are local and can change dramatically on the basis of the location probed. Because membrane receptors are linked in various ways to the intracellular space, with some receptors linking to the cytoskeleton and others diffusing freely without apparent linkages, the cellular physical response to mechanical stresses is expected to depend on the receptor engaged. In this study, we tested the hypothesis that cellular mechanical stiffness as measured via cytoskeletally linked receptors is greater than stiffness measured via receptors that are not cytoskeletally linked. We used a magnetic micromanipulator to apply linear stresses to magnetic beads attached to living cells via selected receptors. One of the receptor classes probed, the dystroglycan receptors, is linked to the cytoskeleton, while the other, the transferrin receptors, is not. Fibronectin-coated beads were used to test cellular mechanical properties of the cytoskeleton without membrane dependence by allowing the beads to endocytose. For epithelial cells, transferrin-dependent stiffness and endocytosed bead-dependent stiffness were similar, while dystroglycan-dependent stiffness was significantly lower. For smooth muscle cells, dystroglycan-dependent stiffness was similar to the endocytosed bead-dependent stiffness, while the transferrin-dependent stiffness was lower. The conclusion of this study is that the measured cellular stiffness is critically influenced by specific receptor linkage and by cell type and raises the intriguing possibility of the existence of separate cytoskeletal networks with distinct mechanical properties that link different classes of receptors.

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.

Mechanotransduction through growth-factor shedding into the extracellular space

Physical forces elicit biochemical signalling in a diverse array of cells, tissues and organisms, helping to govern fundamental biological processes. Several hypotheses have been advanced that link physical forces to intracellular signalling pathways, but in many cases the molecular mechanisms of mechanotransduction remain elusive. Here we find that compressive stress shrinks the lateral intercellular space surrounding epithelial cells, and triggers cellular signalling via autocrine binding of epidermal growth factor family ligands to the epidermal growth factor receptor. Mathematical analysis predicts that constant rate shedding of autocrine ligands into a collapsing lateral intercellular space leads to increased local ligand concentrations that are sufficient to account for the observed receptor signalling; direct experimental comparison of signalling stimulated by compressive stress versus exogenous soluble ligand supports this prediction. These findings establish a mechanism by which mechanotransduction arises from an autocrine ligand-receptor circuit operating in a dynamically regulated extracellular volume, not requiring induction of force-dependent biochemical processes within the cell or cell membrane.

Antecedents of two-photon excitation laser scanning microscopy

In 1931, Maria Göppert-Mayer published her doctoral dissertation on the theory of two-photon quantum transitions (two-photon absorption and emission) in atoms. This report describes and analyzes the theoretical and experimental work on nonlinear optics, in particular two-photon excitation processes, that occurred between 1931 and the experimental implementation of two-photon excitation microscopy by the group of Webb in 1990. In addition to Maria Göppert-Mayer's theoretical work, the invention of the laser has a key role in the development of two-photon microscopy. Nonlinear effects were previously observed in different frequency domains (low-frequency electric and magnetic fields and magnetization), but the high electric field strength afforded by lasers was necessary to demonstrate many nonlinear effects in the optical frequency range. In 1978, the first high-resolution nonlinear microscope with depth resolution was described by the Oxford group. Sheppard and Kompfner published a study in Applied Optics describing microscopic imaging based on second-harmonic generation. In their report, they further proposed that other nonlinear optical effects, such as two-photon fluorescence, could also be applied. However, the developments in the field of nonlinear optical stalled due to a lack of a suitable laser source. This obstacle was removed with the advent of femtosecond lasers in the 1980s. In 1990, the seminal study of Denk, Strickler, and Webb on two-photon laser scanning fluorescence microscopy was published in Science. Their paper clearly demonstrated the capability of two-photon excitation microscopy for biology, and it served to convince a wide audience of scientists of the potential capability of the technique.

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.

Evaluation of Fluorescent Probe Surface Intensities as an Indicator of Transdermal Permeant Distributions Using Wide-Area Two-Photon Fluorescence Microscopy

The application of high-speed two-photon fluorescence microscopy (HTPM) to examine transdermal transport processes has enabled the noninvasive visualization of permeant spatial distributions over a larger, more clinically relevant wide area of the skin. Earlier studies demonstrated that the transdermal fluorescent probe distribution over a 2 x 2 mm skin area was well represented by a significantly reduced sampling of the 400 microscale skin sites (100 x 100 microm) constituting the wide area. In the present study, the 400 microscale skin sites are considered individually, and the site-to-site variability in permeant distributions is used as a model to reflect the range in experimentally measured skin permeabilities resulting from the inherent stratum corneum structural heterogeneity. The correlation established between the permeant surface intensity and the corresponding permeant intensity gradient at each skin site provides an indication of the potential for screening transdermal permeant distributions solely based on the evaluation of microscale permeant surface intensities. The strong linear correlation between the intensity gradient and the surface intensity for the hydrophilic model permeant, sulforhodamine B, demonstrated that surface intensities provide a robust indicator of the corresponding transdermal probe distributions at the microscale. For the hydrophobic model permeant, rhodamine B hexyl ester, however, weak correlations were observed between these two parameters. This result suggests that the stratum corneum microscale surface intensity does not validly capture the corresponding intensity gradients for the entire range of skin permeabilities typically encountered as a result of the inherent stratum corneum heterogeneity.

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.

Topographic heterogeneity in transdermal transport revealed by high-speed two-photon microscopy: determination of representative skin sample sizes

A novel application of high-speed two-photon microscopy was utilized to determine the optimum number of skin sites required to accurately determine the changes in transdermal transport properties incurred globally, over a clinically relevant area of skin. In contrast to the four to six skin sites (100 microm by 100 mirom area per site) examined previously, this study accounted for the fluorescent probe distributions at 400 consecutive skin sites, covering a total skin area of 2 mm by 2 mm. The oleic-acid-induced changes in the transdermal transport properties of the model hydrophobic probe, rhodamine B hexyl ester, and of the model hydrophilic probe, sulforhodamine B, for this 400-skin-site study exhibited different dependencies on sample size for each probe. Whereas the examination of six skin sites captures the relative changes in the global transdermal transport properties of the hydrophobic probe, the valid assessment of these changes for the hydrophilic probe requires a significantly larger sample size of at least 24 skin sites.

Three-dimensional cellular deformation analysis with a two-photon magnetic manipulator workstation

The ability to apply quantifiable mechanical stresses at the microscopic scale is critical for studying cellular responses to mechanical forces. This necessitates the use of force transducers that can apply precisely controlled forces to cells while monitoring the responses noninvasively. This paper describes the development of a micromanipulation workstation integrating two-photon, three-dimensional imaging with a high-force, uniform-gradient magnetic manipulator. The uniform-gradient magnetic field applies nearly uniform forces to a large cell population, permitting statistical quantification of select molecular responses to mechanical stresses. The magnetic transducer design is capable of exerting over 200 pN of force on 4.5-microm-diameter paramagnetic particles and over 800 pN on 5.0-microm ferromagnetic particles. These forces vary within +/-10% over an area 500 x 500 microm2. The compatibility with the use of high numerical aperture (approximately 1.0) objectives is an integral part of the workstation design allowing submicron-resolution, three-dimensional, two-photon imaging. Three-dimensional analyses of cellular deformation under localized mechanical strain are reported. These measurements indicate that the response of cells to large focal stresses may contain three-dimensional global deformations and show the suitability of this workstation to further studying cellular response to mechanical stresses.

Lateral resolution enhancement with standing evanescent waves

A high-resolution fluorescence microscopy technique has been developed that achieves a lateral resolution of better than one sixth of the emission wavelength (FWHM). By use of a total-internal-reflection geometry, standing evanescent waves are generated that spatially modulate the excitation of the sample. An enhanced two-dimensional image is formed from a weighted sum of images taken at different phases and directions of the standing wave. The performance of such a system is examined through theoretical calculations of both the point-spread function and the optical transfer function.