Collagen fiber arrangement in normal and diseased cartilage studied by polarization sensitive nonlinear microscopy

Evaluation of the parallel coupling constitutive model for biomaterials using a fully coupled network-matrix model

In this article we discuss the effective properties of composites containing a crosslinked athermal fiber network embedded in a continuum elastic matrix, which are representative for a broad range of biological materials. The goal is to evaluate the accuracy of the widely used biomechanics parallel coupling model in which the tissue response is defined as the additive superposition of the network and matrix contributions, and the interaction of the two components is neglected. To this end, explicit, fully coupled models are used to evaluate the linear and non-linear response of the composite. It is observed that in the small strain, linear regime the parallel model leads to errors when the ratio of the individual stiffnesses of the two components is in the range 0.1-10, and the error increases as the matrix approaches the incompressible limit. The data presented can be used to correct the parallel model to improve the accuracy of the overall stiffness prediction. In the non-linear large deformation regime linear superposition does not apply. The data shows that the matrix reduces the stiffening rate of the network, and the response is softer than that predicted by the parallel model. The correction proposed for the linear regime mitigates to a large extent the error in the non-linear regime as well, provided the matrix Poisson ratio is not close to 0.5. The special case in which the matrix is rendered auxetic is also evaluated and it is seen that the auxeticity of the matrix may compensate the stiffening introduced by the network, leading to a composite with linear elastic response over a broad range of strains.

Multimodal 2D and 3D microscopic mapping of growth cartilage by computational imaging techniques - a short review including new research

Being able to image the microstructure of growth cartilage is important for understanding the onset and progression of diseases such as osteochondrosis and osteoarthritis, as well as for developing new treatments and implants. Studies of cartilage using conventional optical brightfield microscopy rely heavily on histological staining, where the added chemicals provide tissue-specific colours. Other microscopy contrast mechanisms include polarization, phase- and scattering contrast, enabling non-stained or 'label-free' imaging that significantly simplifies the sample preparation, thereby also reducing the risk of artefacts. Traditional high-performance microscopes tend to be both bulky and expensive.Computational imagingdenotes a range of techniques where computers with dedicated algorithms are used as an integral part of the image formation process. Computational imaging offers many advantages like 3D measurements, aberration correction and quantitative phase contrast, often combined with comparably cheap and compact hardware. X-ray microscopy is also progressing rapidly, in certain ways trailing the development of optical microscopy. In this study, we first briefly review the structures of growth cartilage and relevant microscopy characterization techniques, with an emphasis on Fourier ptychographic microscopy (FPM) and advanced x-ray microscopies. We next demonstrate with our own results computational imaging through FPM and compare the images with hematoxylin eosin and saffron (HES)-stained histology. Zernike phase contrast, and the nonlinear optical microscopy techniques of second harmonic generation (SHG) and two-photon excitation fluorescence (TPEF) are explored. Furthermore, X-ray attenuation-, phase- and diffraction-contrast computed tomography (CT) images of the very same sample are presented for comparisons. Future perspectives on the links to artificial intelligence, dynamic studies andin vivopossibilities conclude the article.

Articular Cartilage-From Basic Science Structural Imaging to Non-Invasive Clinical Quantitative Molecular Functional Information for AI Classification and Prediction

This review presents the changes that the imaging of articular cartilage has undergone throughout the last decades. It highlights that the expectation is no longer to image the structure and associated functions of articular cartilage but, instead, to devise methods for generating non-invasive, function-depicting images with quantitative information that is useful for detecting the early, pre-clinical stage of diseases such as primary or post-traumatic osteoarthritis (OA/PTOA). In this context, this review summarizes (a) the structure and function of articular cartilage as a molecular imaging target, (b) quantitative MRI for non-invasive assessment of articular cartilage composition, microstructure, and function with the current state of medical diagnostic imaging, (c), non-destructive imaging methods, (c) non-destructive quantitative articular cartilage live-imaging methods, (d) artificial intelligence (AI) classification of degeneration and prediction of OA progression, and (e) our contribution to this field, which is an AI-supported, non-destructive quantitative optical biopsy for early disease detection that operates on a digital tissue architectural fingerprint. Collectively, this review shows that articular cartilage imaging has undergone profound changes in the purpose and expectations for which cartilage imaging is used; the image is becoming an AI-usable biomarker with non-invasive quantitative functional information. This may aid in the development of translational diagnostic applications and preventive or early therapeutic interventions that are yet beyond our reach.

Polarimetric second harmonic generation microscopy of partially oriented fibers II: Imaging study

Polarimetric second harmonic generation (SHG) microscopy imaging is employed to investigate the ultrastructural organization of biological and biomimetic partially oriented fibrillar structures. The linear polarization-in polarization-out SHG microscopy measurements are conducted with rat tail tendon, rabbit cornea, pig cartilage, and biomimetic meso-tetra(4-sulfonatophenyl)porphine (TPPS4) cylindrical aggregates, which represent different two- and three-dimensional (2D and 3D) configurations of C6 symmetry fibril structures in the focal volume (voxel) of the microscope. The polarization-in polarization-out imaging of rat tail tendon reveals that SHG intensity is affected by parallel/antiparallel arrangements of the fibers, and achiral (R) and chiral (C) susceptibility component ratio values change by tilting the tendon fibers out of image plane. The R ratio changes for the 2D crossing fibers observed in cornea tissue. The 3D crossing of fibers also affects R ratio in cartilage tissue. The distinctly different dependence of R on crossing and tilting of fibers is demonstrated in collagen and TPPS4 aggregates, due to the achiral molecular susceptibility ratio having values below and above 3, respectively. The polarimetric microscopy results correspond well with the analytical expressions of amplitude and R and C ratios dependence on the crossing angle of the fibers. The experimentally measured SHG intensity and R and C ratio maps are consistent with the computational modeling of various fiber configurations presented in the preceding article. The demonstrated SHG intensity and R and C ratio dependencies on fibril configurations provide the basis for interpreting polarimetric SHG microscopy images in terms of 3D ultrastructural organization of fibers in each voxel of the samples.

Impact of experimental setup parameters on the measurement of articular cartilage optical properties in the visible and short near-infrared spectral bands

There is increasing research on the potential application of diffuse optical spectroscopy and hyperspectral imaging for characterizing the health of the connective tissues, such as articular cartilage, during joint surgery. These optical techniques facilitate the rapid and objective diagnostic assessment of the tissue, thus providing unprecedented information toward optimal treatment strategy. Adaption of optical techniques for diagnostic assessment of musculoskeletal disorders, including osteoarthritis, requires precise determination of the optical properties of connective tissues such as articular cartilage. As every indirect method of tissue optical properties estimation consists of a measurement step followed by a computational analysis step, there are parameters associated with these steps that could influence the estimated values of the optical properties. In this study, we report the absorption and reduced scattering coefficients of articular cartilage in the spectral band of 400-1400 nm. We assess the impact of the experimental setup parameters, including surrounding medium, sample volume, and scattering anisotropy factor on the reported optical properties. Our results suggest that the absorption coefficient of articular cartilage is sensitive to the variation in the surrounding medium, whereas its reduced scattering coefficient is invariant to the experimental setup parameters.

Instant polarized light microscopy pi (IPOLπ) for quantitative imaging of collagen architecture and dynamics in ocular tissues

Collagen architecture determines the biomechanical environment in the eye, and thus characterizing collagen fiber organization and biomechanics is essential to fully understand eye physiology and pathology. We recently introduced instant polarized light microscopy (IPOL) that encodes optically information about fiber orientation and retardance through a color snapshot. Although IPOL allows imaging collagen at the full acquisition speed of the camera, with excellent spatial and angular resolutions, a limitation is that the orientation-encoding color is cyclic every 90 degrees (π/2 radians). In consequence, two orthogonal fibers have the same color and therefore the same orientation when quantified by color-angle mapping. In this study, we demonstrate IPOLπ, a new variation of IPOL, in which the orientation-encoding color is cyclic every 180 degrees (π radians). Herein we present the fundamentals of IPOLπ, including a framework based on a Mueller-matrix formalism to characterize how fiber orientation and retardance determine the color. The improved quantitative capability of IPOLπ enables further study of essential biomechanical properties of collagen in ocular tissues, such as fiber anisotropy and crimp. We present a series of experimental calibrations and quantitative procedures to visualize and quantify ocular collagen orientation and microstructure in the optic nerve head, a region in the back of the eye. There are four important strengths of IPOLπ compared to IPOL. First, IPOLπ can distinguish the orientations of orthogonal collagen fibers via colors, whereas IPOL cannot. Second, IPOLπ requires a lower exposure time than IPOL, thus allowing faster imaging speed. Third, IPOLπ allows visualizing non-birefringent tissues and backgrounds from tissue absorption, whereas both appear dark in IPOL images. Fourth, IPOLπ is cheaper and less sensitive to imperfectly collimated light than IPOL. Altogether, the high spatial, angular, and temporal resolutions of IPOLπ enable a deeper insight into ocular biomechanics and eye physiology and pathology.

Instant polarized light microscopy pi (IPOLπ) for quantitative imaging of collagen architecture and dynamics in ocular tissues

Collagen architecture determines the biomechanical environment in the eye, and thus characterizing collagen fiber organization and biomechanics is essential to fully understand eye physiology and pathology. We recently introduced instant polarized light microscopy (IPOL) that encodes optically information about fiber orientation and retardance through a color snapshot. Although IPOL allows imaging collagen at the full acquisition speed of the camera, with excellent spatial and angular resolutions, a limitation is that the orientation-encoding color is cyclic every 90 degrees (π/2 radians). In consequence, two orthogonal fibers have the same color and therefore the same orientation when quantified by color-angle mapping. In this study, we demonstrate IPOLπ, a new variation of IPOL, in which the orientation-encoding color is cyclic every 180 degrees (π radians). Herein we present the fundamentals of IPOLπ, including a framework based on a Mueller-matrix formalism to characterize how fiber orientation and retardance determine the color. The improved quantitative capability of IPOLπ enables further study of essential biomechanical properties of collagen in ocular tissues, such as fiber anisotropy and crimp. We present a series of experimental calibrations and quantitative procedures to visualize and quantify ocular collagen orientation and microstructure in the optic nerve head, a region in the back of the eye. There are four important strengths of IPOLπ compared to IPOL. First, IPOLπ can distinguish the orientations of orthogonal collagen fibers via colors, whereas IPOL cannot. Second, IPOLπ requires a lower exposure time than IPOL, thus allowing faster imaging speed. Third, IPOLπ allows visualizing non-birefringent tissues and backgrounds from tissue absorption, whereas both appear dark in IPOL images. Fourth, IPOLπ is cheaper and less sensitive to imperfectly collimated light than IPOL. Altogether, the high spatial, angular, and temporal resolutions of IPOLπ enable a deeper insight into ocular biomechanics and eye physiology and pathology.

Characterization of collagen response to bone fracture healing using polarization-SHG

In this study, we extend on the three parameter analysis approach of utilizing a noninvasive dual-liquid-crystal-based polarization-resolved second harmonic generation (SHG) microscopy to facilitate the quantitative characterization of collagen types I and II in fracture healing tissues. The SHG images under various linear and circular polarization states are analyzed and quantified in terms of the peptide pitch angle (PA), SHG-circular dichroism (CD), and anisotropy parameter (AP). The results show that the collagen PA has a value of 49.26° after 2 weeks of fracture healing (collagen type II domination) and 49.05° after 4 weeks (collagen type I domination). Moreover, the SHG-CD and AP values of the different collagen types differ by 0.05. The change tendencies of the extracted PA, SHG-CD, and AP parameters over the healing time are consistent with the collagen properties of healthy nonfractured bone. Thus, the feasibility of the proposed dual-liquid-crystal-based polarization-SHG method for differentiating between collagen types I and II in bone fracture healing tissue is confirmed.

A Nonparametric Approach for Estimating Three-Dimensional Fiber Orientation Distribution Functions (ODFs) in Fibrous Materials

Many biological tissues contain an underlying fibrous microstructure that is optimized to suit a physiological function. The fiber architecture dictates physical characteristics such as stiffness, diffusivity, and electrical conduction. Abnormal deviations of fiber architecture are often associated with disease. Thus, it is useful to characterize fiber network organization from image data in order to better understand pathological mechanisms. We devised a method to quantify distributions of fiber orientations based on the Fourier transform and the Qball algorithm from diffusion MRI. The Fourier transform was used to decompose images into directional components, while the Qball algorithm efficiently converted the directional data from the frequency domain to the orientation domain. The representation in the orientation domain does not require any particular functional representation, and thus the method is nonparametric. The algorithm was verified to demonstrate its reliability and used on datasets from microscopy to show its applicability. This method increases the ability to extract information of microstructural fiber organization from experimental data that will enhance our understanding of structure-function relationships and enable accurate representation of material anisotropy in biological tissues.

Polarisation optics for biomedical and clinical applications: a review

Many polarisation techniques have been harnessed for decades in biological and clinical research, each based upon measurement of the vectorial properties of light or the vectorial transformations imposed on light by objects. Various advanced vector measurement/sensing techniques, physical interpretation methods, and approaches to analyse biomedically relevant information have been developed and harnessed. In this review, we focus mainly on summarising methodologies and applications related to tissue polarimetry, with an emphasis on the adoption of the Stokes-Mueller formalism. Several recent breakthroughs, development trends, and potential multimodal uses in conjunction with other techniques are also presented. The primary goal of the review is to give the reader a general overview in the use of vectorial information that can be obtained by polarisation optics for applications in biomedical and clinical research.

3D printing of patient-specific implants for osteochondral defects: workflow for an MRI-guided zonal design

Abstract

Magnetic resonance imaging (MRI) is a common clinical practice to visualize defects and to distinguish different tissue types and pathologies in the human body. So far, MRI data have not been used to model and generate a patient-specific design of multilayered tissue substitutes in the case of interfacial defects. For orthopedic cases that require highly individual surgical treatment, implant fabrication by additive manufacturing holds great potential. Extrusion-based techniques like 3D plotting allow the spatially defined application of several materials, as well as implementation of bioprinting strategies. With the example of a typical multi-zonal osteochondral defect in an osteochondritis dissecans (OCD) patient, this study aimed to close the technological gap between MRI analysis and the additive manufacturing process of an implant based on different biomaterial inks. A workflow was developed which covers the processing steps of MRI-based defect identification, segmentation, modeling, implant design adjustment, and implant generation. A model implant was fabricated based on two biomaterial inks with clinically relevant properties that would allow for bioprinting, the direct embedding of a patient’s own cells in the printing process. As demonstrated by the geometric compatibility of the designed and fabricated model implant in a stereolithography (SLA) model of lesioned femoral condyles, a novel versatile CAD/CAM workflow was successfully established that opens up new perspectives for the treatment of multi-zonal (osteochondral) defects.

Graphic abstract

Micro-mechanical damage of needle puncture on bovine annulus fibrosus fibrils studied using polarization-resolved Second Harmonic Generation(P-SHG) microscopy

Needle injection has been widely used in spinal therapeutic or diagnostic processes, such as discography. The use of needles has been suspected in causing mild disc degeneration which can lead to long-term back pain. However, the localised microscopic damage caused by needles has not been well studied. The local progressive damage on a microscopic level caused by needle punctures on the surface of bovine annulus fibrosus was investigated. Four different sizes of needle were used for the puncture and twenty-nine bovine intervertebral discs were studied. Polarization-resolved second harmonic generation and fluorescent microscopy were used to study the local microscopic structural changes in collagen and cell nuclei due to needle damage. Repeated 70 cyclic loadings at ±5% of axial strain were applied after the needle puncture in order to assess progressive damage caused by the needle. Puncture damage on annulus fibrosus were observed either collagen fibre bundles being pushed aside, being cut through or combination of both with part being lift or pushed in. The progressive damage was found less relevant to the needle size and more progressive damage was only observed using the larger needle. Two distinct populations of collagen, in which one was relatively more organised than the other population, were observed especially after the puncture from skewed distribution of polarization-SHG analysis. Cell shape was found rounder near the puncture site where collagen fibres were damaged.

Polarization-resolved SHG imaging as a fast screening method for collagen alterations during aging: Comparison with light and electron microscopy

Our previous study on rat skin showed that cumulative oxidative pressure induces profound structural and ultrastructural alterations in both rat skin epidermis and dermis during aging. Here, we aimed to investigate the biophotonic properties of collagen as a main dermal component in the function of chronological aging. We used second harmonic generation (SHG) and two-photon excited fluorescence (TPEF) on 5 μm thick skin paraffin sections from 15-day-, 1-month- and 21-month-old rats, respectively, to analyze collagen alterations, in comparison to conventional light and electron microscopy methods. Obtained results show that polarization-resolved SHG (PSHG) images can detect collagen fiber alterations in line with chronological aging and that this method is consistent with light and electron microscopy. Moreover, the β coefficient calculated from PSHG images points out that delicate alterations lead to a more ordered structure of collagen molecules due to oxidative damage. The results of this study also open the possibility of successfully applying this fast and label-free method to previously fixed samples.

Comparison of concentrated fresh mononuclear cells and cultured mesenchymal stem cells from bone marrow for bone regeneration

Autologous bone marrow mononuclear cell (BMMNC) transplantation has been widely studied in recent years. The fresh cell cocktail in BMMNCs, without going through the in vitro culture process, helps to establish a stable microenvironment for osteogenesis, and each cell type may play a unique role in bone regeneration. Our study compared the efficacy of concentrated fresh BMMNCs and cultured bone marrow‐derived mesenchymal stem cells (BMSCs) in Beagle dogs for the first time. Fifteen‐millimeter segmental bone defects were created in the animals' tibia bones. In BMMNCs group, the defects were repaired with concentrated fresh BMMNCs combined with β‐TCP (n = 5); in cultured BMSC group, with in vitro cultured and osteo‐induced BMSCs combined with β‐TCP (n = 5); in scaffold‐only group, with a β‐TCP graft alone (n = 5); and in blank group, nothing was grafted (n = 3). The healing process was monitored by X‐rays and single photon emission computed tomography. The animals were sacrificed 12 months after surgery and their tibias were harvested and analyzed by microcomputed tomography and hard tissue histology. Moreover, the microstructure, chemical components, and microbiomechanical properties of the regenerated bone tissue were explored by multiphoton microscopy, Raman spectroscopy and nanoindentation. The results showed that BMMNCs group promoted much more bone regeneration than cultured BMSC group. The grafts in BMMNCs group were better mineralized, and they had collagen arrangement and microbiomechanical properties similar to the contralateral native tibia bone. These results indicate that concentrated fresh bone marrow mononuclear cells may be superior to in vitro expanded stem cells in segmental bone defect repair.

Will Tissue-Engineering Strategies Bring New Hope for the Reconstruction of Nasal Septal Cartilage?

The nasal septal cartilage plays an important role in the growth of midface and as a vertical strut preventing the collapse of the nasal bones. The repair of nasal cartilage defects remains a major challenge in reconstructive surgery. The tissue engineering strategy in the development of tissue has opened a new perspective to generate functional tissue for transplantation. Given the poor regenerative properties of cartilage and a limited amount of autologous cartilage availability, intense interest has evoked for tissue engineering approaches for cartilage development to provide better outcomes for patients who require nasal septal reconstruction. Despite numerous attempts to substitute the shapely hyaline cartilage in the nasal cartilages, many significant challenges remained unanswered. The aim of this research was to carry out a critical review of the literature on research work carried out on the development of septal cartilage using a tissue engineering approach, concerning different cell sources, scaffolds and growth factors, as well as its clinical pathway and trials have already been carried out.

Role of scattering and birefringence in phase retardation revealed by locus of Stokes vector on Poincaré sphere

SIGNIFICANCE

Biological tissues are typically characterized by high anisotropic scattering and may also exhibit linear form birefringence. Both scattering and birefringence bias the phase shift between transverse electric field components of polarized light. These phase alterations are associated with particular structural malformations in the tissue. In fact, the majority of polarization-based techniques are unable to distinguish the nature of the phase shift induced by birefringence or scattering of light.

AIM

We explore the distinct contributions of scattering and birefringence in the phase retardation of circularly polarized light propagated in turbid tissue-like scattering medium.

APPROACH

The circularly polarized light in frame of Stokes polarimetry approach is used for the screening of biotissue phantoms and chicken skin samples. The change of optical properties in chicken skin is accomplished by optical clearing, which reduces scattering, and mechanical stretch, which induces birefringence. The change of optical properties of skin tissue is confirmed by spectrophotometric measurements and second-harmonic generation imaging.

RESULTS

The contributions of scattering and birefringence in the phase retardation of circularly polarized light propagated in biological tissues are distinguished by the locus of the Stokes vector mapped on the Poincaré sphere. The phase retardation of circularly polarized light due to scattering alterations is assessed. The value of birefringence in chicken skin is estimated as 0.3  ×  10^-3, which agrees with alternative studies. The change of birefringence of skin tissue due to mechanical stretch in the order of 10^-6 is detected.

CONCLUSIONS

While the polarimetric parameters on their own do not allow distinguishing the contributions of scattering and birefringence, the resultant Stokes vector trajectory on the Poincaré sphere reveals the role of scattering and birefringence in the total phase retardation. The described approach, applied independently or in combination with Mueller polarimetry, can be beneficial for the advanced characterization of various types of malformations within biological tissues.

Maturation of the Meniscal Collagen Structure Revealed by Polarization-Resolved and Directional Second Harmonic Generation Microscopy

We report Polarization-resolved Second Harmonic Generation (P-SHG) and directional SHG (forward and backward, F/B) measurements of equine foetal and adult collagen in meniscus, over large field-of-views using sample-scanning. Large differences of collagen structure and fibril orientation with maturation are revealed, validating the potential for this novel methodology to track such changes in meniscal structure. The foetal menisci had a non-organized and more random collagen fibrillar structure when compared with adult using P-SHG. For the latter, clusters of homogeneous fibril orientation (inter-fibrillar areas) were revealed, separated by thick fibers. F/B SHG showed numerous different features in adults notably, in thick fibers compared to interfibrillar areas, unlike foetal menisci that showed similar patterns for both directions. This work confirms previous studies and improves the understanding of meniscal collagen structure and its maturation, and makes F/B and P-SHG good candidates for future studies aiming at revealing structural modifications to meniscus due to pathologies.

Two-photon fluorescence and second harmonic generation characterization of extracellular matrix remodeling in post-injury murine temporomandibular joint osteoarthritis

End stage temporomandibular joint osteoarthritis (TMJ-OA) is characterized by fibrillations, fissures, clefts, and erosion of the mandibular condylar cartilage. The goal of this study was to define changes in pericellular and interterritorial delineations of the extracellular matrix (ECM) that occur preceding and concurrent with the development of this end stage degeneration in a murine surgical instability model. Two-photon fluorescence (TPF) and second harmonic generation (SHG) microscopy was used to evaluate TMJ-OA mediated changes in the ECM. We illustrate that TPF/SHG microscopy reconstructs the three-dimensional network of key fibrillar and micro-fibrillar collagens altered during the progression of TMJ-OA. This method not only generates spatially distinct pericellular and interterritorial delineations of the ECM but distinguishes early and end stage TMJ-OA by signal organization, orientation, and composition. Early stage TMJ-OA at 4- and 8-weeks post-injury is characterized by two structurally distinct regions containing dense, large fiber collagens and superficial, small fiber collagens rich in types I, III, and VI collagen oriented along the mesiodistal axis of the condyle. At 8-weeks post-injury, type VI collagen is locally diminished on the central and medial condyle, but the type I/III rich superficial layer is still present. Twelve- and 16-weeks post-injury mandibular cartilage is characteristic of end-stage disease, with hypocellularity and fibrillations, fissures, and clefts in the articular layer that propagate along the mediolateral axis of the MCC. We hypothesize that the localized depletion of interterritorial and pericellular type VI collagen may signify an early marker for the transition from early to end stage TMJ-OA, influence the injury response of the tissue, and underlie patterns of degeneration that follow attritional modes of failure.

Pre-Clinical Translation of Second Harmonic Microscopy of Meniscal and Articular Cartilage Using a Prototype Nonlinear Microendoscope

Previous studies using nonlinear microscopy have demonstrated that osteoarthritis (OA) is characterized by the gradual replacement of Type II collagen with Type I collagen. The objective of this study was to develop a prototype nonlinear laser scanning microendoscope capable of resolving the structural differences of collagen in various orthopaedically relevant cartilaginous surfaces. The current prototype developed a miniaturized femtosecond laser scanning instrument, mounted on an articulated positioning system, capable of both conventional arthroscopy and second-harmonic laser-scanning microscopy. Its optical system includes a multi-resolution optical system using a gradient index objective lens and a customized multi-purpose fiber optic sheath to maximize the collection of backscattered photons or provide joint capsule illumination. The stability and suitability of the prototype arthroscope to approach and image cartilage were evaluated through preliminary testing on fresh, minimally processed, and partially intact porcine knee joints. Image quality was sufficient to distinguish between hyaline cartilage and fibrocartilage through unique Type I and Type II collagen-specific characteristics. Imaging the meniscus revealed that the system was able to visualize differences in the collagen arrangement between the superficial and lamellar layers. Such detailed in vivo imaging of the cartilage surfaces could obviate the need to perform biopsies for ex vivo histological analysis in the future, and provide an alternative to conventional external imaging to characterize and diagnose progressive and degenerative cartilage diseases such as OA. Moreover, this system is readily customizable and may provide a suitable and modular platform for developing additional tools utilizing femtosecond lasers for tissue cutting within the familiar confines of two or three portal arthroscopy techniques.

Composition, structure and tensile biomechanical properties of equine articular cartilage during growth and maturation

Articular cartilage undergoes structural and biochemical changes during maturation, but the knowledge on how these changes relate to articular cartilage function at different stages of maturation is lacking. Equine articular cartilage samples of four different maturation levels (newborn, 5-month-old, 11-month-old and adult) were collected (N = 25). Biomechanical tensile testing, Fourier transform infrared microspectroscopy (FTIR-MS) and polarized light microscopy were used to study the tensile, biochemical and structural properties of articular cartilage, respectively. The tensile modulus was highest and the breaking energy lowest in the newborn group. The collagen and the proteoglycan contents increased with age. The collagen orientation developed with age into an arcade-like orientation. The collagen content, proteoglycan content, and collagen orientation were important predictors of the tensile modulus (p < 0.05 in multivariable regression) and correlated significantly also with the breaking energy (p < 0.05 in multivariable regression). Partial least squares regression analysis of FTIR-MS data provided accurate predictions for the tensile modulus (r = 0.79) and the breaking energy (r = 0.65). To conclude, the composition and structure of equine articular cartilage undergoes changes with depth that alter functional properties during maturation, with the typical properties of mature tissue reached at the age of 5-11 months.

Bioluminescence and second harmonic generation imaging reveal dynamic changes in the inflammatory and collagen landscape in early osteoarthritis

Osteoarthritis (OA) is a leading cause of chronic disability whose mechanism of pathogenesis is largely elusive. Local inflammation is thought to play a key role in OA progression, especially in injury-associated OA. While multiple inflammatory cytokines are detected, the timing and extent of overall inflammatory activities in early OA and the manner by which joint inflammation correlates with cartilage structural damage are still unclear. We induced OA via destabilization of the medial meniscus (DMM) in NFκB luciferase reporter mice, whose bioluminescent signal reflects the activity of NFκB, a central mediator of inflammation. Bioluminescence imaging data showed that DMM and sham control joints had a similar surge of inflammation at 1-week post-surgery, but the DMM joint exhibited a delay in resolution of inflammation in subsequent weeks. A similar trend was observed with synovitis, which we found to be mainly driven by synovial cell density and inflammatory infiltration rather than synovial lining thickness. Interestingly, an association between synovitis and collagen structural damage was observed in early OA. Using Second Harmonic Generation (SHG) imaging, we analyzed collagen fiber organization in articular cartilage. Zonal differences in collagen fiber thickness and organization were observed as soon as OA initiated after DMM surgery, and persisted over time. Even at 1-week post-surgery, the DMM joint showed a decrease in collagen fiber thickness in the deep zone and an increase in collagen fiber disorganization in the superficial zone. Since we were able detect and quantify collagen structural changes very early in OA development by SHG imaging, we concluded that SHG imaging is a highly sensitive tool to evaluate pathological changes in OA. In summary, this study uncovered a dynamic profile of inflammation and joint cartilage damage during OA initiation and development, providing novel insights into OA pathology.

Chronic cigarette smoke exposure drives spiral ganglion neuron loss in mice

Tobacco use is associated with an increased risk of hearing loss in older individuals, suggesting cigarette smoke (CS) exposure may target the peripheral auditory organs. However, the effects of CS exposure on general cochlear anatomy have not previously been explored. Here we compare control and chronic CS exposed cochleae from adult mice to assess changes in structure and cell survival. Two-photon imaging techniques, including the imaging of second harmonic generation (SHG) and two-photon excitation fluorescence (TPEF) from native molecules, were used to probe the whole cochlear organ for changes. We found evidence for fibrillar collagen accumulation in the spiral ganglion and organ of Corti, consistent with fibrosis. Quantitative TPEF indicated that basal CS-exposed spiral ganglion neurons experienced greater oxidative stress than control neurons, which was confirmed by histological staining for lipid peroxidation products. Cell counts confirmed that the CS-exposed spiral ganglion also contained fewer basal neurons. Taken together, these data support the premise that CS exposure induces oxidative stress in cochlear cells. They also indicate that two-photon techniques may screen cochlear tissues for oxidative stress.

Characterization of cellular and matrix alterations in the early pathogenesis of osteochondritis dissecans in pigs using second harmonic generation and two-photon excitation fluorescence microscopy

Osteochondritis dissecans is a joint disease that is observed in several species. The disease can develop as a cause of ischemic chondronecrosis in the epiphyseal growth cartilage. Some lesions of chondronecrosis undergo spontaneous resolution, but it is not possible to predict whether a lesion will resolve or progress and require intervention. Proliferation of cells into clusters occurs at the lesion margin, but it is unclear if the clusters have a repair function. The aims of the current study were to examine clusters and potential matrix changes in response to ischemic chondronecrosis in the distal femur of 10 pigs aged 70-180 days using advanced microscopy based on two-photon excitation fluorescence and second harmonic generation. These microscopy techniques can perform 3D imaging of cells and collagen without staining. The results indicated a lower collagen density in the chondronecrotic areas compared to the normal growth cartilage, and fissures and breaks in the matrix integrity were demonstrated that potentially can propagate and cause osteochondritis dissecans. A higher number of cells in clusters was correlated with reduction in collagen density in the lesions. Some of the cells in the clusters had a morphology similar to progenitor cells, suggesting a potential repair role of the clusters. The study has shed further light on the secondary responses after initial lesion formation, which information can be of potential use to create models that can predict lesion progression and that may hence avoid unnecessary interventions in the future. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Quantitative Morphometry for Osteochondral Tissues Using Second Harmonic Generation Microscopy and Image Texture Information

Osteoarthritis (OA) is a chronic joint disorder involving degeneration of articular cartilage and subchondral bone in joints. We previously established a second harmonic generation (SHG) imaging technique for evaluating degenerative changes to articular cartilage in an OA mouse model. SHG imaging, an optical label-free technique, enabled observation of collagen fibrils, and characterized critical changes in the collagenous patterns of the joints. However, it still remains to be determined how morphological changes in the organization of tissue collagen fibrils should be quantified. In this study, we addressed this issue by employing an approach based on texture analysis. Image texture analysis using the gray level co-occurrence matrix was explored to extract image features. We investigated an image patch-based strategy, in which texture features were extracted on individual patches derived from original images to capture local structural patterns in them. We verified that this analysis enables discrimination of cartilaginous and osseous tissues in mouse joints. Moreover, we applied this method to OA cartilage pathology assessment, and observed improvements in the performance results compared with those obtained using an existing feature descriptor. The proposed approach can be applied to a wide range of conditions associated with collagen remodeling and diseases of cartilage and bone.

Poisson's Contraction and Fiber Kinematics in Tissue: Insight From Collagen Network Simulations

Connective tissue mechanics is highly nonlinear, exhibits a strong Poisson's effect, and is associated with significant collagen fiber re-arrangement. Although the general features of the stress-strain behavior have been discussed extensively, the Poisson's effect received less attention. In general, the relationship between the microscopic fiber network mechanics and the macroscopic experimental observations remains poorly defined. The objective of the present work is to provide additional insight into this relationship. To this end, results from models of random collagen networks are compared with experimental data on reconstructed collagen gels, mouse skin dermis, and the human amnion. Attention is devoted to the mechanism leading to the large Poisson's effect observed in experiments. The results indicate that the incremental Poisson's contraction is directly related to preferential collagen orientation. The experimentally observed downturn of the incremental Poisson's ratio at larger strains is associated with the confining effect of fibers transverse to the loading direction and contributing little to load bearing. The rate of collagen orientation increases at small strains, reaches a maximum, and decreases at larger strains. The peak in this curve is associated with the transition of the network deformation from bending dominated, at small strains, to axially dominated, at larger strains. The effect of fiber tortuosity on network mechanics is also discussed, and a comparison of biaxial and uniaxial loading responses is performed.

Structural and Mechanical Comparison of Human Ear, Alar, and Septal Cartilage

Supplemental Digital Content is available in the text.

Background:

In the human ear and nose, cartilage plays a key role in establishing its form and function. Interestingly, there is a noticeable paucity on biochemical, structural, and mechanical studies focused on facial cartilage. Such studies are needed to provide elementary knowledge that is fundamental to tissue engineering of cartilage. Therefore, in this study, a comparison is made of the biochemical, structural, and mechanical differences between ear, ala nasi, and septum on the extracellular matrix (ECM) level.

Methods:

Cartilage samples were harvested from 10 cadaveric donors. Each sample was indented 10 times with a nanoindenter to determine the effective Young’s modulus. Structural information of the cartilage was obtained by multiple-photon laser scanning microscopy capable of revealing matrix components at subcellular resolution. Biochemistry was performed to measure glycosaminoglycan (GAG), DNA, elastin, and collagen content.

Results:

Significant differences were seen in stiffness between ear and septal cartilage (P = 0.011) and between ala nasi and septal cartilage (P = 0.005). Elastin content was significantly higher in ear cartilage. Per cartilage subtype, effective Young’s modulus was not significantly correlated with cell density, GAG, or collagen content. However, in septal cartilage, low elastin content was associated with higher stiffness. Laser microscopy showed a distinct difference between ear cartilage and cartilage of nasal origin.

Conclusion:

Proposed methods to investigate cartilage on the ECM level provided good results. Significant differences were seen not only between ear and nasal cartilage but also between the ala nasi and septal cartilage. Albeit its structural similarity to septal cartilage, the ala nasi has a matrix stiffness comparable to ear cartilage.

Parametric imaging of collagen structural changes in human osteoarthritic cartilage using optical polarization tractography

Collagen degeneration is an important pathological feature of osteoarthritis. The purpose of this study is to investigate whether the polarization-sensitive optical coherence tomography (PSOCT)-based optical polarization tractography (OPT) can be useful in imaging collagen structural changes in human osteoarthritic cartilage samples. OPT eliminated the banding artifacts in conventional PSOCT by calculating the depth-resolved local birefringence and fiber orientation. A close comparison between OPT and PSOCT showed that OPT provided improved visualization and characterization of the zonal structure in human cartilage. Experimental results obtained in this study also underlined the importance of knowing the collagen fiber orientation in conventional polarized light microscopy assessment. In addition, parametric OPT imaging was achieved by quantifying the surface roughness, birefringence, and fiber dispersion in the superficial zone of the cartilage. These quantitative parametric images provided complementary information on the structural changes in cartilage, which can be useful for a comprehensive evaluation of collagen damage in osteoarthritic cartilage.

Techniques to assess bone ultrastructure organization: orientation and arrangement of mineralized collagen fibrils

Bone's remarkable mechanical properties are a result of its hierarchical structure. The mineralized collagen fibrils, made up of collagen fibrils and crystal platelets, are bone's building blocks at an ultrastructural level. The organization of bone's ultrastructure with respect to the orientation and arrangement of mineralized collagen fibrils has been the matter of numerous studies based on a variety of imaging techniques in the past decades. These techniques either exploit physical principles, such as polarization, diffraction or scattering to examine bone ultrastructure orientation and arrangement, or directly image the fibrils at the sub-micrometre scale. They make use of diverse probes such as visible light, X-rays and electrons at different scales, from centimetres down to nanometres. They allow imaging of bone sections or surfaces in two dimensions or investigating bone tissue truly in three dimensions, in vivo or ex vivo, and sometimes in combination with in situ mechanical experiments. The purpose of this review is to summarize and discuss this broad range of imaging techniques and the different modalities of their use, in order to discuss their advantages and limitations for the assessment of bone ultrastructure organization with respect to the orientation and arrangement of mineralized collagen fibrils.

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.

Polarization resolved second harmonic microscopy

Second harmonic (SH) microscopy has proven to be a powerful imaging modality over the past years due to its intrinsic advantages as a multiphoton process with endogenous contrast specificity, which allows pinhole-less optical sectioning, non-invasive observation, deep tissue penetration, and the possibility of easier signal detection at visible wavelengths. Depending on the relative orientation between the polarization of the incoming light and the second-order susceptibility of non-centrosymmetric structures, SH microscopy provides the unique capacity to probe the absolute molecular structure of a broad variety of biological tissues without the necessity for additional labeling. In addition, SH microscopy, when working with polarimetry, provides clear and in-depth insights on the details of molecular orientation and structural symmetry. In this review, the working principles of the polarization resolving techniques and the corresponding implements of SH microscopy are elucidated, with focus on Stokes vector based polarimetry. An overview of the advancements on SH anisotropy measurements are also presented. Specifically, the recent progresses on the following three topics in polarization resolved SH microscopy will be elucidated, which include Stokes vector resolving for imaging molecular structure and orientation, 3-D structural chirality by SH circular dichroism, and correlation with fluorescence lifetime imaging (FLIM) for in vivo wound healing diagnosis. The potentials and challenges for future researches in exploring complex biological tissues are also discussed.

Noninvasive assessment of articular cartilage surface damage using reflected polarized light microscopy

Articular surface damage occurs to cartilage during normal aging, osteoarthritis, and in trauma. A noninvasive assessment of cartilage microstructural alterations is useful for studies involving cartilage explants. This study evaluates polarized reflectance microscopy as a tool to assess surface damage to cartilage explants caused by mechanical scraping and enzymatic degradation. Adult bovine articular cartilage explants were scraped, incubated in collagenase, or underwent scrape and collagenase treatments. In an additional experiment, cartilage explants were subject to scrapes at graduated levels of severity. Polarized reflectance parameters were compared with India ink surface staining, features of histological sections, changes in explant wet weight and thickness, and chondrocyte viability. The polarized reflectance signal was sensitive to surface scrape damage and revealed individual scrape features consistent with India ink marks. Following surface treatments, the reflectance contrast parameter was elevated and correlated with image area fraction of India ink. After extensive scraping, polarized reflectance contrast and chondrocyte viability were lower than that from untreated explants. As part of this work, a mathematical model was developed and confirmed the trend in the reflectance signal due to changes in surface scattering and subsurface birefringence. These results demonstrate the effectiveness of polarized reflectance microscopy to sensitively assess surface microstructural alterations in articular cartilage explants.

Cartilage Tissue Engineering by the 3D Bioprinting of iPS Cells in a Nanocellulose/Alginate Bioink

Cartilage lesions can progress into secondary osteoarthritis and cause severe clinical problems in numerous patients. As a prospective treatment of such lesions, human-derived induced pluripotent stem cells (iPSCs) were shown to be 3D bioprinted into cartilage mimics using a nanofibrillated cellulose (NFC) composite bioink when co-printed with irradiated human chondrocytes. Two bioinks were investigated: NFC with alginate (NFC/A) or hyaluronic acid (NFC/HA). Low proliferation and phenotypic changes away from pluripotency were seen in the case of NFC/HA. However, in the case of the 3D-bioprinted NFC/A (60/40, dry weight % ratio) constructs, pluripotency was initially maintained, and after five weeks, hyaline-like cartilaginous tissue with collagen type II expression and lacking tumorigenic Oct4 expression was observed in 3D -bioprinted NFC/A (60/40, dry weight % relation) constructs. Moreover, a marked increase in cell number within the cartilaginous tissue was detected by 2-photon fluorescence microscopy, indicating the importance of high cell densities in the pursuit of achieving good survival after printing. We conclude that NFC/A bioink is suitable for bioprinting iPSCs to support cartilage production in co-cultures with irradiated chondrocytes.

Bovine and degenerated human annulus fibrosus: a microstructural and micromechanical comparison

The complex structure of the annulus fibrosus is strongly related to its mechanical properties. Recent work showed that it is possible to observe the relative movement of fibre bundles in loaded cow tail annulus; the aim of this work was to describe and quantify annulus fibrosus micromechanics in degenerated human disc, and compare it with cow tail annulus, an animal model often used in the literature. Second harmonic generation was used to image the collagen matrix in twenty strips of annulus fibrosus harvested from intervertebral disc of seven patients undergoing surgery. Samples were loaded to 6% tensile strain in 1% steps. Elastic modulus was calculated from loading curves, and micromechanical strains were calculated from the images using custom software. The same protocol was applied to twenty strips of annulus harvested from cow tail discs. Significant morphological differences were found between human and cow tail samples, the most striking being the lack of collagen fibre crimp in the former. Fibres were also observed bending and running from one lamella to the other, forming a strong flexible interface. Interdigitation of fibre bundles was also present at this interface. Quantitative results show complex patterns of inter-bundle and inter-lamellar behaviour, with inter-bundle sliding being the main strain mechanism. Elastic modulus was similar between species, and it was not affected by the degree of degeneration. This work gives an insight into the complex structure and mechanical function of the annulus fibrosus, which should be accounted for in disc numerical modelling.

Noninvasive surface damage assessment of bovine articular cartilage explants by reflected polarized light microscopy

Articular surface damage is a hallmark of cartilage degeneration. Noninvasive assessment of cartilage microstructural alterations has potential clinical value. In this study, we use bovine patellofemoral articular cartilage explants treated with mechanical scraping and collagenase to create cartilage surface disruption, and use polarized reflectance microscopy to quantify alterations to surface and sub-surface microstructure. Reflected polarized signal was sensitive to mild damage to the cartilage surface, and highlighted disruptive alterations. The results indicate the efficacy of reflected polarized light microscopy in assessing the microstructural status of superficial articular cartilage.

Unified Theory for Polarization Analysis in Second Harmonic and Sum Frequency Microscopy

A unified theoretical framework for the recovery of second-order nonlinear susceptibility tensors and sample orientations from polarization-dependent second harmonic generation and sum frequency generation microscopy was developed. Jones formalism was extended to nonlinear optics and was used to bridge the experimental observables and the local-frame tensor elements. Four commonly used experimental architectures were explicitly explored, including polarization rotation with no postsample optics, polarization-in polarization-out measurement, and polarization modulation with and without postsample optics. Polarization-dependent second harmonic generation measurement was performed on Z-cut quartz and the local-frame tensor elements were calculated. The recovered tensor elements agree with the expected values dictated by symmetry.

The Impact of Collagen Fibril Polarity on Second Harmonic Generation Microscopy

In this work, we report the implementation of interferometric second harmonic generation (SHG) microscopy with femtosecond pulses. As a proof of concept, we imaged the phase distribution of SHG signal from the complex collagen architecture of juvenile equine growth cartilage. The results are analyzed in respect to numerical simulations to extract the relative orientation of collagen fibrils within the tissue. Our results reveal large domains of constant phase together with regions of quasi-random phase, which are correlated to respectively high- and low-intensity regions in the standard SHG images. A comparison with polarization-resolved SHG highlights the crucial role of relative fibril polarity in determining the SHG signal intensity. Indeed, it appears that even a well-organized noncentrosymmetric structure emits low SHG signal intensity if it has no predominant local polarity. This work illustrates how the complex architecture of noncentrosymmetric scatterers at the nanoscale governs the coherent building of SHG signal within the focal volume and is a key advance toward a complete understanding of the structural origin of SHG signals from tissues.

Second harmonic generation imaging reveals a distinct organization of collagen fibrils in locations associated with cartilage growth

PURPOSE

The articular-epiphyseal cartilage complex (AECC) is responsible for the expansion of the bone ends and serves the function of the articular cartilage in juvenile mammals. Bundles of collagen fibrils surrounding cells were in the literature observed more frequently near the articular surface of the AECC. The articular surface, the perichondrium, and cartilage canals are interfaces where appositional growth of the AECC has been demonstrated. The current study aimed to evaluate the potential of second harmonic generation (SHG) to locate the collagen fibril bundles near the articular surface and to examine whether a comparable collagen fibril organization could be observed near the other interfaces of the AECC.

MATERIALS AND METHODS

The study included the femoral condyle of four piglets aged 82-141 days. The forward and backward scattered SHG, and their ratio, was analyzed across the AECC using objectives with different numerical aperture. Two-photon-excited fluorescence was used to visualize cells.

RESULTS

A similar pattern of collagen fibril organization was observed near the articular surface, around cartilage canals, and adjacent to the perichondrium. The pattern consisted of a higher ratio of forward to backward scattered SHG that increased relative to the surrounding matrix at lower numerical aperture. This was interpreted to reflect collagen fibril bundles in the territorial matrix of cells in these areas.

CONCLUSIONS

The observed arrangement of collagen fibrils was suggested to be related to the presumed different growth activity in these areas and indicated that SHG may be used as an indirect and label-free marker for cartilage matrix growth.

Estimation of Jones matrix, birefringence and entropy using Cloude-Pottier decomposition in polarization-sensitive optical coherence tomography

Estimation of polarimetric parameters has been a fundamental issue to assess biological tissues that have form birefringence or polarization scrambling in polarization-sensitive optical coherence tomography (PS-OCT). We present a mathematical framework to provide a maximum likelihood estimation of the target covariance matrix and its incoherent target decomposition to estimate a Jones matrix of a dominant scattering mechanism, called Cloude-Pottier decomposition, thereby deriving the phase retardation and the optic axis of the sample. In addition, we introduce entropy that shows the randomness of the polarization property. Underestimation of the entropy at a low sampling number is mitigated by asymptotic quasi maximum likelihood estimator. A bias of the entropy from random noises is corrected to show only the polarization property inherent in the sample. The theory is validated with experimental measurements of a glass plate and waveplates, and applied to the imaging of a healthy human eye anterior segment as an image filter.

Probing the 3D structure of cornea-like collagen liquid crystals with polarization-resolved SHG microscopy

This work aims at characterizing the three-dimensional organization of liquid crystals composed of collagen, in order to determine the physico-chemical conditions leading to highly organized structures found in biological tissues such as cornea. To that end, we use second-harmonic generation (SHG) microscopy, since aligned collagen structures have been shown to exhibit intrinsic SHG signals. We combine polarization-resolved SHG experiments (P-SHG) with the theoretical derivation of the SHG signal of collagen molecules tilted with respect to the focal plane. Our P-SHG images exhibit striated patterns with variable contrast, as expected from our analytical and numerical calculations for plywood-like nematic structures similar to the ones found in the cornea. This study demonstrates the benefits of P-SHG microscopy for in situ characterization of highly organized biopolymers at micrometer scale, and the unique sensitivity of this nonlinear optical technique to the orientation of collagen molecules.

Polarized light interaction with tissues

This tutorial-review introduces the fundamentals of polarized light interaction with biological tissues and presents some of the recent key polarization optical methods that have made possible the quantitative studies essential for biomedical diagnostics. Tissue structures and the corresponding models showing linear and circular birefringence, dichroism, and chirality are analyzed. As the basis for a quantitative description of the interaction of polarized light with tissues, the theory of polarization transfer in a random medium is used. This theory employs the modified transfer equation for Stokes parameters to predict the polarization properties of single- and multiple-scattered optical fields. The near-order of scatterers in tissues is accounted for to provide an adequate description of tissue polarization properties. Biomedical diagnostic techniques based on polarized light detection, including polarization imaging and spectroscopy, amplitude and intensity light scattering matrix measurements, and polarization-sensitive optical coherence tomography are described. Examples of biomedical applications of these techniques for early diagnostics of cataracts, detection of precancer, and prediction of skin disease are presented. The substantial reduction of light scattering multiplicity at tissue optical clearing that leads to a lesser influence of scattering on the measured intrinsic polarization properties of the tissue and allows for more precise quantification of these properties is demonstrated.

Lamellar and fibre bundle mechanics of the annulus fibrosus in bovine intervertebral disc

The intervertebral disc is a multicomposite structure, with an outer fibrous ring, the annulus fibrosus, retaining a gel-like core, the nucleus pulposus. The disc presents complex mechanical behaviour, and it is of high importance for spine biomechanics. Advances in multiscale modelling and disc repair raised a need for new quantitative data on the finest details of annulus fibrosus mechanics. In this work we explored inter-lamella and inter-bundle behaviour of the outer annulus using micromechanical testing and second harmonic generation microscopy. Twenty-one intervertebral discs were dissected from cow tails; the nucleus and inner annulus were excised to leave a ring of outer annulus, which was tested in circumferential loading while imaging the tissue's collagen fibres network with sub-micron resolution. Custom software was developed to determine local tissue strains through image analysis. Inter-bundle linear and shear strains were 5.5 and 2.8 times higher than intra-bundle strains. Bundles tended to remain parallel while rotating under loading, with large slipping between them. Inter-lamella linear strain was almost 3 times the intra-lamella one, but no slipping was observed at the junction between lamellae. This study confirms that outer annulus straining is mainly due to bundles slipping and rotating. Further development of disc multiscale modelling and repair techniques should take into account this modular behaviour of the lamella, rather than considering it as a homogeneous fibre-reinforced matrix.

STATEMENT OF SIGNIFICANCE

The intervertebral disc is an organ tucked between each couple of vertebrae in the spine. It is composed by an outer fibrous layer retaining a gel-like core. This organ undergoes severe and repeated loading during everyday life activities, since it is the compliant component that gives the spine its flexibility. Its properties are affected by pathologies such as disc degeneration, a major cause of back pain. In this article we explored the micromechanical behaviour of the disc's outer layer using second harmonic generation, a technique which allowed us to visualize, with unprecedented detail, how bundles of collagen fibres slide relative to each other when loaded. Our results will help further the development of new multiscale numerical models and repairing techniques.

Cartilage Tissue Engineering: Preventing Tissue Scaffold Contraction Using a 3D-Printed Polymeric Cage

Scaffold contraction is a common but underestimated problem in the field of tissue engineering. It becomes particularly problematic when creating anatomically complex shapes such as the ear. The aim of this study was to develop a contraction-free biocompatible scaffold construct for ear cartilage tissue engineering. To address this aim, we used three constructs: (i) a fibrin/hyaluronic acid (FB/HA) hydrogel, (ii) a FB/HA hydrogel combined with a collagen I/III scaffold, and (iii) a cage construct containing (ii) surrounded by a 3D-printed poly-ɛ-caprolactone mold. A wide range of different cell types were tested within these constructs, including chondrocytes, perichondrocytes, adipose-derived mesenchymal stem cells, and their combinations. After in vitro culturing for 1, 14, and 28 days, all constructs were analyzed. Macroscopic observation showed severe contraction of the cell-seeded hydrogel (i). This could be prevented, in part, by combining the hydrogel with the collagen scaffold (ii) and prevented in total using the 3D-printed cage construct (iii). (Immuno)histological analysis, multiphoton laser scanning microscopy, and biomechanical analysis showed extracellular matrix deposition and increased Young's modulus and thereby the feasibility of ear cartilage engineering. These results demonstrated that the 3D-printed cage construct is an adequate model for contraction-free ear cartilage engineering using a range of cell combinations.

In situ time-series monitoring of collagen fibers produced by standing-cultured osteoblasts using a second-harmonic-generation microscope

In bone tissue engineering and regeneration, there is a considerable need for an unstained method of monitoring collagen fibers produced by osteoblasts. This is because collagen fibers play an important role as a bone matrix and continuous monitoring of their temporal dynamics is important in clarifying the organization process toward forming bone tissue. In the work described here, using a second-harmonic-generation (SHG) microscope, we performed in situ time-series monitoring of collagen fibers produced by cultured osteoblasts without the need for staining. Use of the 19 fs near-infrared pulsed light enables us to visualize the temporal dynamics in a thin layer of collagen fibers produced by a single layer of osteoblasts in high-contrast SHG images. While the collagen fibers were produced and stored inside the osteoblasts at an early stage of culturing, the network structure of collagen fibers was formed and locally condensed at a late stage. Furthermore, we extracted a quantitative parameter of collagen maturity degree in the cultured sample by use of image analysis based on a two-dimensional Fourier transform of the SHG image. The proposed method will be useful for in situ quality and quantity control of collagen fibers in bone tissue engineering and regeneration.

Imaging of subchondral bone by optical coherence tomography upon optical clearing of articular cartilage

Optical clearing is an effective method to reduce light scattering of biological tissues that provides significant enhancement of light penetration into the biological tissues making non-invasive diagnosis more feasible. In current report Optical Coherence Tomography (OCT) in conjunction with optical clearing is applied for assessment of deep cartilage layers and cartilage-bone interface. The solution of Iohexol in water has been used as an optical clearing agent. The cartilage-bone boundary becomes visible after 15 min of optical clearing that enabling non-invasive estimation of its roughness: Sa = 10 ± 1 µm. The results show that for 0.9 mm thick cartilage optical clearing is stopped after 50 min with an increase of refractive index from 1.386 ± 0.008 to 1.510 ± 0.009. Current approach enables more reliable detection of arthroscopically inaccessible regions, including cartilage-bone boundary and subchondral bone, and potentially improves accuracy of the osteoarthritis diagnosis.

Fast interferometric second harmonic generation microscopy

We report the implementation of fast Interferometric Second Harmonic Generation (I-SHG) microscopy to study the polarity of non-centrosymmetric structures in biological tissues. Using a sample quartz plate, we calibrate the spatially varying phase shift introduced by the laser scanning system. Compensating this phase shift allows us to retrieve the correct phase distribution in periodically poled lithium niobate, used as a model sample. Finally, we used fast interferometric second harmonic generation microscopy to acquire phase images in tendon. Our results show that the method exposed here, using a laser scanning system, allows to recover the polarity of collagen fibrils, similarly to standard I-SHG (using a sample scanning system), but with an imaging time about 40 times shorter.

Analysis of forward and backward Second Harmonic Generation images to probe the nanoscale structure of collagen within bone and cartilage

Collagen ultrastructure plays a central role in the function of a wide range of connective tissues. Studying collagen structure at the microscopic scale is therefore of considerable interest to understand the mechanisms of tissue pathologies. Here, we use second harmonic generation microscopy to characterize collagen structure within bone and articular cartilage in human knees. We analyze the intensity dependence on polarization and discuss the differences between Forward and Backward images in both tissues. Focusing on articular cartilage, we observe an increase in Forward/Backward ratio from the cartilage surface to the bone. Coupling these results to numerical simulations reveals the evolution of collagen fibril diameter and spatial organization as a function of depth within cartilage.

Maintaining polarization in polarimetric multiphoton microscopy

Polarimetric measurements in multiphoton microscopy can reveal information about the local molecular order of a sample. However, the presence of a dichroic through which the excitation beam propagates will generally scramble its polarization. We propose a simple scheme whereby a second properly-oriented compensation dichroic is used to negate any alteration regardless of the wavelength and the initial polarization. We demonstrate how this robust and rapid approach simplifies polarimetric measurements in second-harmonic generation, two-photon excited fluorescence and coherent anti-Stokes Raman scattering. Illustration of the polarization maintaining strategy with the compensating dichroic oriented such that its s- and p-axes are interchanged with these of the primary dichroic.

The micromechanics of the superficial zone of articular cartilage

OBJECTIVE

To investigate the relationships between the unique mechanical and structural properties of the superficial zone of articular cartilage on the microscopic scale.

DESIGN

Fresh unstained equine metacarpophalangeal cartilage samples were mounted on tensile and compressive loading rigs on the stage of a multiphoton microscope. Sequential image stacks were acquired under incremental loads together with simultaneous measurements of the applied stress and strain. Second harmonic generation was used to visualise the collagen fibre network, while two photon fluorescence was used to visualise elastin fibres and cells. The changes visualised by each modality were tracked between successive loads.

RESULTS

The deformation of the cartilage matrix was heterogeneous on the microscopic length scale. This was evident from local strain maps, which showed shearing between different regions of collagen under tensile strain, corrugations in the articular surface at higher tensile strains and a non-uniform distribution of compressive strain in the axial direction. Chondrocytes elongated and rotated under tensile strain and were compressed in the axial direction under compressive load. The magnitude of deformation varied between cells, indicating differences in either load transmission through the matrix or the mechanical properties of individual cells. Under tensile loading the reorganisation of the elastin network differed from a homogeneous elastic response, indicating that it forms a functional structure.

CONCLUSIONS

This study highlights the complexity of superficial zone mechanics and demonstrates that the response of the collagen matrix, elastin fibres and chondrocytes are all heterogeneous on the microscopic scale.

Nonlinear optical microscopy of early stage (ICRS Grade-I) osteoarthritic human cartilage

In a synovial joint, the articular cartilage is directly affected during the progression of Osteoarthritis (OA). The characterization of early stage modification in extra-cellular matrix of cartilage is essential for detection as well as understanding the progression of disease. The objective of this study is to demonstrate the potential and capability of nonlinear optical microscopy for the morphological investigation of early stage osteoarthritic cartilage. ICRS Grade-I cartilage sections were obtained from the femoral condyle of the human knee. The surface of articular cartilage was imaged by second harmonic generation and two-photon excited fluorescence microscopy. Novel morphological features like microsplits and wrinkles were observed, which would otherwise not be visible in other clinical imaging modalities (e.g., CT, MRI, ultrasound and arthroscope. The presence of superficial layer with distinct collagen fibrils parallel to the articular surface in 4 specimens out of 14 specimens, indicates that different phases of OA within ICRS Grade-I can be detected by SHG microscopy. All together, the observed novel morphologies in early stage osteoarthritic cartilage indicates that SHG microscopy might be a significant tool for the assessment of cartilage disorder.