Intact corneal stroma visualization of GFP mouse revealed by multiphoton imaging

Dual-LC PSHG microscopy for imaging collagen type I and type II gels with pixel-resolution analysis

Collagen of type I (Col I) and type II (Col II) are critical for cartilage and connective tissues in the human body, and several diseases may alter their properties. Assessing the identification and quantification of fibrillar collagen without biomarkers is a challenge. Advancements in non-invasive polarization-resolved second-harmonic generation (PSHG) microscopy have provided a method for the non-destructive investigation of collagen molecular level properties. Here we explored an alternative polarization modulated approach, dual-LC PSHG, that is based on two liquid crystal devices (Liquid crystal polarization rotators, LPRs) operating simultaneously with a laser scanning SHG microscope. We demonstrated that this more accessible technology allows the quick and accurate generation of any desired linear and circular polarization state without any mechanical parts. This study demonstrates that this method can aid in improving the ability to quantify the characteristics of both types of collagen, including pitch angle, anisotropy, and circular dichroism analysis. Using this approach, we estimated the effective pitch angle for Col I and Col II to be 49.7° and 51.6°, respectively. The effective peptide pitch angle for Col II gel was first estimated and is similar to the value obtained for Col I gel in the previous studies. Additionally, the difference of the anisotropy parameter of both collagen type gels was assessed to be 0.293, which reflects the different type molecular fibril assembly. Further, our work suggests a potential method for monitoring and differentiating different collagen types in biological tissues, especially cartilage or connective tissue.

Collagen organization of renal cell carcinoma differs between low and high grade tumors

Background

The traditional pathologic grading for human renal cell carcinoma (RCC) has low concordance between biopsy and surgical specimen. There is a need to investigate adjunctive pathology technique that does not rely on the nuclear morphology that defines the traditional grading. Changes in collagen organization in the extracellular matrix have been linked to prognosis or grade in breast, ovarian, and pancreatic cancers, but collagen organization has never been correlated with RCC grade. In this study, we used Second Harmonic Generation (SHG) based imaging to quantify possible differences in collagen organization between high and low grades of human RCC.

Methods

A tissue microarray (TMA) was constructed from RCC tumor specimens. Each TMA core represents an individual patient. A 5 μm section from the TMA tissue was stained with standard hematoxylin and eosin (H&E). Bright field images of the H&E stained TMA were used to annotate representative RCC regions. In this study, 70 grade 1 cores and 51 grade 4 cores were imaged on a custom-built forward SHG microscope, and images were analyzed using established software tools to automatically extract and quantify collagen fibers for alignment and density assessment. A linear mixed-effects model with random intercepts to account for the within-patient correlation was created to compare grade 1 vs. grade 4 measurements and the statistical tests were two-sided.

Results

Both collagen density and alignment differed significantly between RCC grade 1 and RCC grade 4. Specifically, collagen fiber density was greater in grade 4 than in grade 1 RCC (p < 0.001). Collagen fibers were also more aligned in grade 4 compared to grade 1 (p < 0.001).

Conclusions

Collagen density and alignment were shown to be significantly higher in RCC grade 4 vs. grade 1. This technique of biopsy sampling by SHG could complement classical tumor grading approaches. Furthermore it might allow biopsies to be more clinically relevant by informing diagnostics. Future studies are required to investigate the functional role of collagen organization in RCC.

Intravital multiphoton microscopic imaging platform for ocular surface imaging

The purpose of this study is to provide an intravital noninvasive multiphoton microscopic platform for long-term ocular imaging in transgenic fluorescent mice with subcellular resolution. A multiphoton microscopic system with tunable laser output was employed. We designed a mouse holder incorporated with stereotaxic motorized stage for in vivo three-dimensional imaging of ocular surface in 3 transgenic mouse line with fluorescent protein (FP) expression to visualize distinct structures. With our imaging platform and the expression of FPs, we obtained the three-dimensional images across the whole cornea from epithelium to endothelium and in conjunctiva with subcellular resolution in vivo. Specified EGFP expression in corneal epithelium of K5-H2B-EGFP mice helped to identify both corneal and limbal epithelial cells while ubiquitous nuclear FP expression in R26R-GR mice allowed us to visualized nuclei of all cell types. Universal membrane-localized FP in mT/mG mice outlined all cell boundaries, nerve fibers, and capillaries. The simultaneously collected second harmonic generation signals from collagenous stroma provided architectural contrast. Time-lapsed recording enabled monitoring the mitotic activity of corneal epithelial cells and limbal epithelial cells. We developed an intravital multiphoton microscopic stereotaxic imaging platform and showed that, by incorporating FP-expressing transgenic mice, this platform enables in vivo 4-dimensional ophthalmic study at subcellular resolution.

Kinetics of corneal leukocytes by intravital multiphoton microscopy

Corneal immune privilege is integral in maintaining the clear avascular window to the foreign world. The presence of distinct populations of corneal leukocytes (CLs) in the normal cornea has been firmly established. However, their precise function and kinetics remain, as of yet, unclear. Through intravital multiphoton microscopy (IV-MPM), allowing the means to accumulate critical spatial and temporal cellular information, we provide details for long-term investigation of CL morphology and kinetics under steady state and following inflammation. Significant alterations in size and morphology of corneal CD11c+ dendritic cells (DCs) were noted following acute sterile inflammation, including cell volume (4364.4 ± 489.6 vs. 1787.6 ± 111.0 μm3, P < 0.001) and sphericity (0.82 ± 0.01 vs. 0.42 ± 0.02, P < 0.001) compared with steady state. Furthermore, IV-MPM analyses revealed alterations in both the CD11c+ DC and major histocompatibility complex class II (MHC)-II+ mature antigen-presenting cell population kinetics during inflammation, including track displacement length (CD11c: 16.57 ± 1.41 vs. 4.64 ± 0.56 μm, P < 0.001; MHC-II: 9.03 ± 0.37 vs. 4.09 ± 0.39, P < 0.001) and velocity (CD11c: 1.91 ± 0.07 μm/min vs. 1.73 ± 0.1302 μm/min; MHC-II: 2.97 ± 0.07 vs. 1.62 ± 0.08, P < 0.001) compared with steady state. Our results reveal in vivo evidence of sessile CL populations exhibiting dendritic morphology under steady state and increased velocity of spherical leukocytes following inflammation. IV-MPM represents a powerful tool to study leukocytes in corneal diseases in context.-Seyed-Razavi, Y., Lopez, M. J., Mantopoulos, D., Zheng, L., Massberg, S., Sendra, V. G., Harris, D. L., Hamrah, P. Kinetics of corneal leukocytes by intravital multiphoton microscopy.

Imaging Collagen in Scar Tissue: Developments in Second Harmonic Generation Microscopy for Biomedical Applications

The ability to respond to injury with tissue repair is a fundamental property of all multicellular organisms. The extracellular matrix (ECM), composed of fibrillar collagens as well as a number of other components is dis-regulated during repair in many organs. In many tissues, scaring results when the balance is lost between ECM synthesis and degradation. Investigating what disrupts this balance and what effect this can have on tissue function remains an active area of research. Recent advances in the imaging of fibrillar collagen using second harmonic generation (SHG) imaging have proven useful in enhancing our understanding of the supramolecular changes that occur during scar formation and disease progression. Here, we review the physical properties of SHG, and the current nonlinear optical microscopy imaging (NLOM) systems that are used for SHG imaging. We provide an extensive review of studies that have used SHG in skin, lung, cardiovascular, tendon and ligaments, and eye tissue to understand alterations in fibrillar collagens in scar tissue. Lastly, we review the current methods of image analysis that are used to extract important information about the role of fibrillar collagens in scar formation.

From nano to macro: studying the hierarchical structure of the corneal extracellular matrix

In this review, we discuss current methods for studying ocular extracellular matrix (ECM) assembly from the 'nano' to the 'macro' levels of hierarchical organization. Since collagen is the major structural protein in the eye, providing mechanical strength and controlling ocular shape, the methods presented focus on understanding the molecular assembly of collagen at the nanometre level using X-ray scattering through to the millimetre to centimetre level using non-linear optical (NLO) imaging of second harmonic generated (SHG) signals. Three-dimensional analysis of ECM structure is also discussed, including electron tomography, serial block face scanning electron microscopy (SBF-SEM) and digital image reconstruction. Techniques to detect non-collagenous structural components of the ECM are also presented, and these include immunoelectron microscopy and staining with cationic dyes. Together, these various approaches are providing new insights into the structural blueprint of the ocular ECM, and in particular that of the cornea, which impacts upon our current understanding of the control of corneal shape, pathogenic mechanisms underlying ectatic disorders of the cornea and the potential for corneal tissue engineering.

Second-harmonic generation imaging of cancer

The last 30 years has seen great advances in optical microscopy with the introduction of sophisticated fluorescence-based imaging methods such as confocal and multiphoton laser scanning microscopy. There is increasing interest in using these methods to quantitatively examine sources of intrinsic biological contrast including autofluorescent endogenous proteins and light interactions such as second-harmonic generation (SHG) in collagen. In particular, SHG-based microscopy has become a widely used quantitative modality for imaging noncentrosymmetric proteins such as collagen in a diverse range of tissues. Due to the underlying physical origin of the SHG signal, it is highly sensitive to collagen fibril/fiber structure and, importantly, to collagen-associated changes that occur in diseases such as cancer, fibrosis, and connective tissue disorders. An overview of SHG physics background and technologies is presented with a focused review on applications of SHG primarily as applied to cancer.

Precise, motion-free polarization control in Second Harmonic Generation microscopy using a liquid crystal modulator in the infinity space

Second Harmonic Generation (SHG) microscopy coupled with polarization analysis has great potential for use in tissue characterization, as molecular and supramolecular structural details can be extracted. Such measurements are difficult to perform quickly and accurately. Here we present a new method that uses a liquid crystal modulator (LCM) located in the infinity space of a SHG laser scanning microscope that allows the generation of any desired linear or circular polarization state. As the device contains no moving parts, polarization can be rotated accurately and faster than by manual or motorized control. The performance in terms of polarization purity was validated using Stokes vector polarimetry, and found to have minimal residual polarization ellipticity. SHG polarization imaging characteristics were validated against well-characterized specimens having cylindrical and/or linear symmetries. The LCM has a small footprint and can be implemented easily in any standard microscope and is cost effective relative to other technologies.

Second harmonic generation microscopy for quantitative analysis of collagen fibrillar structure

Second-harmonic generation (SHG) microscopy has emerged as a powerful modality for imaging fibrillar collagen in a diverse range of tissues. Because of its underlying physical origin, it is highly sensitive to the collagen fibril/fiber structure, and, importantly, to changes that occur in diseases such as cancer, fibrosis and connective tissue disorders. We discuss how SHG can be used to obtain more structural information on the assembly of collagen in tissues than is possible by other microscopy techniques. We first provide an overview of the state of the art and the physical background of SHG microscopy, and then describe the optical modifications that need to be made to a laser-scanning microscope to enable the measurements. Crucial aspects for biomedical applications are the capabilities and limitations of the different experimental configurations. We estimate that the setup and calibration of the SHG instrument from its component parts will require 2-4 weeks, depending on the level of the user's experience.

Determination of temporal and spatial concentration gradients in hydrogel beads using multiphoton microscopy techniques

Multiphoton microscopy is a promising technique to detect spatially and temporally resolved concentration gradients of chemical compounds, e.g., reactants in hydrogel-encapsulated biocatalysts. In contrast to current techniques, the improved spatial and temporal resolution of this method in data acquisition and its ability to measure hydrogel beads facilitates the identification of various kinetic phenomena. To our knowledge, multiphoton microscopy is used here for the first time to examine diffusion, mass transfer, and reaction in immobilized hydrogel systems. In a first step, the phenomena of diffusion and diffusion-coupled mass transfer through the phase interface are investigated in the bead center. Finally, the complete system--consisting of diffusion, mass transfer, and enzymatic reaction--is observed by measuring concentration gradients along the bead radius with temporal and spatial resolution. This metrology enables a subsequent mechanistic model identification, which in turn leads to an enhanced knowledge of reaction kinetics and supports the design of biotechnological processes. This task was only possible due to excellent spatial (25 microm) and temporal (5 s) resolution and the accuracy (+/-1%) achieved by using a multiphoton microscopy setup.

Label-free structural characterization of mitomycin C-modulated wound healing after photorefractive keratectomy by the use of multiphoton microscopy

We applied multiphoton autofluorescence (MAF) and second-harmonic generation (SHG) microscopy to monitor corneal wound healing after photorefractive keratectomy (PRK). Our results show that keratocyte activation can be observed by an increase in its MAF, while SHG imaging of corneal stroma can show the depletion of Bowman's layer after PRK and the reticular collagen deposition in the wound healing stage. Furthermore, quantification of the keratocyte activation and collagen deposition in conjunction with immunohistochemistry and histological images demonstrate the effectiveness of mitomycin C (MMC) in suppressing myofibroblast proliferation and collagen regeneration in the post-PRK wound healing process.

Multimodal, multiphoton microscopy and image correlation analysis for characterizing corneal thermal damage

We used the combination of multiphoton autofluorescence (MAF), forward second-harmonic generation (FWSHG), and backward second-harmonic generation (BWSHG) imaging for the qualitative and quantitative characterization of thermal damage of ex vivo bovine cornea. We attempt to characterize the structural alterations by qualitative MAF, FWSHG, and BWSHG imaging in the temperature range of 37 to 90 degrees C. In addition to measuring the absolute changes in the three types of signals at the stromal surface, we also performed image correlation analysis between FWSHG and BWSHG and demonstrate that with increasing thermal damage, image correlation between FWSHG and BWSHG significantly increases. Our results show that while MAF and BWSHG intensities may be used as preliminary indicators of the extent of corneal thermal damage, the most sensitive measures are provided by the decay in FWSHG intensity and the convergence of FWSHG and BWSHG images.

In vivo imaging of the immune response in the eye

The immune system is governed by dynamic events involving in part direct intercellular interactions between an immune cell and other cells or the cell's environment. Owing to its unique optical characteristics, the eye offers remarkable opportunities for the analysis of the immune system by intravital microscopy. In this review, we present a brief overview of the current state of knowledge of leukocyte trafficking in each of three anatomically distinct and medically important regions of the eye (cornea, iris, retina) as determined by the application of intravital microscopy to animal models of disease. Additionally, we discuss the use of ocular imaging in patients and volunteers. Finally, we examine the future prospects for this field in terms of its potential for impacting our understanding of fundamental immunological phenomena.

Two-photon microscopy: shedding light on the chemistry of vision

Two-photon microscopy (TPM) has come to occupy a prominent place in modern biological research with its ability to resolve the three-dimensional distribution of molecules deep inside living tissue. TPM can employ two different types of signals, fluorescence and second harmonic generation, to image biological structures with subcellular resolution. Two-photon excited fluorescence imaging is a powerful technique with which to monitor the dynamic behavior of the chemical components of tissues, whereas second harmonic imaging provides novel ways to study their spatial organization. Using TPM, great strides have been made toward understanding the metabolism, structure, signal transduction, and signal transmission in the eye. These include the characterization of the spatial distribution, transport, and metabolism of the endogenous retinoids, molecules essential for the detection of light, as well as the elucidation of the architecture of the living cornea. In this review, we present and discuss the current applications of TPM for the chemical and structural imaging of the eye. In addition, we address what we see as the future potential of TPM for eye research. This relatively new method of microscopy has been the subject of numerous technical improvements in terms of the optics and indicators used, improvements that should lead to more detailed biochemical characterizations of the eyes of live animals and even to imaging of the human eye in vivo.

Circular polarization biomicroscopy: a method for determining human corneal stromal lamellar organization in vivo

The theory of polarization biomicroscopy is explored using Stokes vectors and Mueller matrices. It is established that circular polarization can be used to simultaneously detect birefringent elements at any orientation unlike orientation-sensitive techniques using linear polarized light alone. A method of biomicroscopy using circular polarized light is described and tested in a physical model. The method is then used to investigate the lamellar structure of human corneas in vivo in pairs of eyes of 38 subjects. An approximate confocal elliptic/hyperbolic distribution of stromal fibrils, presumed to be collagen, is clearly identified within central and intermediate areas of the cornea. All subjects tested demonstrate approximate mirror symmetry between pairs of eyes typically with a preferred orientation of central fibrils at approximately 15 degrees to the horizontal in a superotemporal-inferonasal direction.

Second harmonic imaging and scoring of collagen in fibrotic tissues

We compare second harmonic generation (SHG) to histological and immunohistochemical techniques for the visualization and scoring of collagen in biological tissues. We show that SHG microscopy is highly specific for fibrillar collagens and that combined SHG and two-photon excited fluorescence (2PEF) imaging can provide simultaneous three-dimensional visualization of collagen synthesis and assembly sites in transgenic animal models expressing GFP constructs. Finally, we propose several scores for characterizing collagen accumulation based on SHG images and appropriate for different types of collagen distributions. We illustrate the sensitivity of these scores in a murine model of renal fibrosis using a morphological segmentation of the tissue based on endogenous 2PEF signals.

Application of second harmonic imaging microscopy to assess structural changes in optic nerve head structure ex vivo

Glaucoma represents the second leading cause of blindness worldwide. While both age and intraocular pressure (IOP) are well-recognized risk factors for this disease, the underlying pathologic process involves the accelerated death of retinal ganglion cells (RGCs) that is associated with progressive loss of vision. The loss of RGCs has been postulated to occur primarily by injury to axons in the optic nerve head (ONH) due to its anatomic features and the mechanical vulnerability of the lamina cribrosa, the specialized ONH zone comprised of collagen beams that define the channels or pores through which axon bundles exit the eye. Recent advances in multiphoton microscopy using femtosecond lasers that generate second harmonic (SH) signals from collagen allows for direct optical imaging of the lamina cribrosa. We assess the application of SH generated microscopy (SHG) to the study of the ONH, and test the general hypothesis that increasing intraocular pressure in the same eye results in the movement of ONH collagen beams leading to distortion of the lamina cribrosa channels and compression of the axon bundles.

Noninvasive assessment of collagen gel microstructure and mechanics using multiphoton microscopy

Multiphoton microscopy of collagen hydrogels produces second harmonic generation (SHG) and two-photon fluorescence (TPF) images, which can be used to noninvasively study gel microstructure at depth ( approximately 1 mm). The microstructure is also a primary determinate of the mechanical properties of the gel; thus, we hypothesized that bulk optical properties (i.e., SHG and TPF) could be used to predict bulk mechanical properties of collagen hydrogels. We utilized polymerization temperature (4-37 degrees C) and glutaraldehyde to manipulate collagen hydrogel fiber diameter, space-filling properties, and cross-link density. Multiphoton microscopy and scanning electron microscopy reveal that as polymerization temperature decreases (37-4 degrees C) fiber diameter and pore size increase, whereas hydrogel storage modulus (G', from 23 +/- 3 Pa to 0.28 +/- 0.16 Pa, respectively, mean +/- SE) and mean SHG decrease (minimal change in TPF). In contrast, glutaraldehyde significantly increases the mean TPF signal (without impacting the SHG signal) and the storage modulus (16 +/- 3.5 Pa before to 138 +/- 40 Pa after cross-linking, mean +/- SD). We conclude that SHG and TPF can characterize differential microscopic features of the collagen hydrogel that are strongly correlated with bulk mechanical properties. Thus, optical imaging may be a useful noninvasive tool to assess tissue mechanics.