Micrometer scaleEx Vivo multiphoton imaging of unstained arterial wall structure

Biomimetic crimped/aligned microstructure to optimize the mechanics of fibrous hybrid materials for compliant vascular grafts

The precise mechanical properties of many tissues are highly dependent on both the composition and arrangement of the nanofibrous extracellular matrix. It is well established that collagen nanofibers exhibit a crimped microstructure in several tissues such as blood vessel, tendon, and heart valve. This collagen fiber arrangement results in the classic non-linear 'J-shaped' stress strain curve characteristic of these tissues. Synthetic biomimetic fibrous materials with a crimped microstructure similar to natural collagen demonstrate similar mechanical properties to natural tissues. The following work describes a nanofabrication method based on electrospinning used to fabricate two component hybrid electrospun fibrous materials that mimic the microstructure and mechanical properties of vascular tissue. The properties of these samples can be precisely and predictably optimized by modifying fabrication parameters. Tubular grafts with biomimetic microstructure were constructed to demonstrate the potential of this fabrication method in vascular graft replacement applications. It was possible to closely match both the overall geometry and the compliance of specific blood vessels by optimizing graft microstructure.

Label-free second harmonic generation imaging of cerebral vascular wall in local ischemia mouse model in vivo

Second harmonic generation (SHG) imaging is label-free and non-invasive, and it has been extensively applied in multiple biological and medical studies, but not in the brain in vivo. In this study, we modified classical two photon excited fluorescence (TPEF) system to perform in vivo simultaneous TPEF and SHG imaging in the local ischemia mouse model. In cerebral vascular walls, we found strong SHG signal, which co-localized with collagen. In the continuous 2 days' in vivo imaging, this SHG signal remained stable in the local ischemic blood vessel in the initial 4 hours, then its signal abruptly increased and got spatially thickened 5 hours after thrombosis, and this tendency continued in the following 48 hours. This study provides direct and precise timeline of rapid collagen change in cerebral vascular walls in vivo, and reveals the subtle but significant temporal-spatial dynamics of this structural signal during local ischemia. Thus, this cerebral in vivo SHG imaging provides a powerful tool to identify the early and subtle pathological change of collagen around clinical key therapeutic time window.

Through the cleared aorta: three-dimensional characterization of mechanical behaviors of rat thoracic aorta under intraluminal pressurization using optical clearing method

The media of aortic wall is characterized by altering layers of elastin and smooth muscle cells (SMCs), along with collagen fibers in both layers, and plays a central role in functional and pathological remodeling such as hypertension and atherosclerosis. Because the arterial function is linked closely to the arterial wall internal structure, it is essential to investigate the alteration of the arterial microstructure during macroscopic deformation to understand cardiovascular pathologies. The present study adopted a tissue clearing method in three-dimensional mechanical characterization of rat thoracic aorta, and successfully observed changes in the structure of each of the three primary components of the aorta under intraluminal pressurization while maintaining tissue mechanical integrity and flexibility. Layers of elastic fibers and SMCs deformed greater on the intimal side than those on the adventitial side. Furthermore, there was a structural agreement in the alignment angle between SMC nuclei and elastic fibers on their intimal side, but not on the adventitial side. This is the first study that changes in the microstructure of three primary components of the aorta were visualized and evaluated through the aorta. The method established here would also be useful to understand tissue mechanics of other load-bearing soft tissues.

Dual-wavelength multimodal multiphoton microscope with SMA-based depth scanning

We report on a multimodal multiphoton microscopy (MPM) system with depth scanning. The multimodal capability is realized by an Er-doped femtosecond fiber laser with dual output wavelengths of 1580 nm and 790 nm that are responsible for three-photon and two-photon excitation, respectively. A shape-memory-alloy (SMA) actuated miniaturized objective enables the depth scanning capability. Image stacks combined with two-photon excitation fluorescence (TPEF), second harmonic generation (SHG), and third harmonic generation (THG) signals have been acquired from animal, fungus, and plant tissue samples with a maximum depth range over 200 µm.

Native extracellular matrix orientation determines multipotent vascular stem cell proliferation in response to cyclic uniaxial tensile strain and simulated stent indentation

Cardiovascular disease is the leading cause of death worldwide, with multipotent vascular stem cells (MVSC) implicated in contributing to diseased vessels. MVSC are mechanosensitive cells which align perpendicular to cyclic uniaxial tensile strain. Within the blood vessel wall, collagen fibers constrain cells so that they are forced to align circumferentially, in the primary direction of tensile strain. In these experiments, MVSC were seeded onto the medial layer of decellularized porcine carotid arteries, then exposed to 10%, 1 Hz cyclic tensile strain for 10 days with the collagen fiber direction either parallel or perpendicular to the direction of strain. Cells aligned with the direction of the collagen fibers regardless of the orientation to strain. Cells aligned with the direction of strain showed an increased number of proliferative Ki67 positive cells, while those strained perpendicular to the direction of cell alignment showed no change in cell proliferation. A bioreactor system was designed to simulate the indentation of a single, wire stent strut. After 10 days of cyclic loading to 10% strain, MVSC showed regions of densely packed, highly proliferative cells. Therefore, MVSC may play a significant role in in-stent restenosis, and this proliferative response could potentially be controlled by controlling MVSC orientation relative to applied strain.

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ECM constrained MVSC align with collagen fibers when cells are strained parallel to collagen.

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Straining MVSC aligned parallel to the direction of strain increased cell proliferation.

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Simulated stent strut indentation showed increased cell density surrounding the indented wire.

Survey of the extracellular matrix architecture across the rat arterial tree

Objective

Methods

Results

Conclusions

Arterial pathologies affect and depend on elastic fibers and collagen. Medial arterial calcification involves mineral deposition onto elastic fibers and smooth muscle cell osteogenesis, which can be induced by elastin degradation. Degradation or remodeling of the extracellular matrix can be a critical component of atherosclerosis and hypertension. Pathologies can also be site-specific. Aneurysms are most common in the abdominal aorta (Ao), followed by the popliteal artery, which shows age-related changes to wall properties comparable to those in central elastic arteries. Visceral artery aneurysms, however, are rare. Location differences in arterial extracellular matrix structure could inform site-specific differences in arterial pathology.

Nicotine Affects Murine Aortic Stiffness and Fatigue Response During Supraphysiological Cycling

Nicotine exposure is a major risk factor for several cardiovascular diseases. Although the deleterious effects of nicotine on aortic remodeling processes have been studied to some extent, the biophysical consequences are not fully elucidated. In this investigation, we applied quasi-static and dynamic loading to quantify ways in which exposure to nicotine affects the mechanical behavior of murine arterial tissue. Segments of thoracic aortas from C57BL/6 mice exposed to 25 mg/kg/day of subcutaneous nicotine for 28 days were subjected to uniaxial tensile loading in an open-circumferential configuration. Comparing aorta segments from nicotine-treated mice relative to an equal number of control counterparts, stiffness in the circumferential direction was nearly twofold higher (377 kPa ± 165 kPa versus 191 kPa ± 65 kPa, n = 5, p = 0.03) at 50% strain. Using a degradative power-law fit to fatigue data at supraphysiological loading, we observed that nicotine-treated aortas exhibited significantly higher peak stress, greater loss of tension, and wider oscillation band than control aortas (p ≤ 0.01 for all three variables). Compared to simple stress relaxation tests, fatigue cycling is shown to be more sensitive and versatile in discerning nicotine-induced changes in mechanical behavior over many cycles. Supraphysiological fatigue cycling thus may have broader potential to reveal subtle changes in vascular mechanics caused by other exogenous toxins or pathological conditions.

Elastic Laminar Reorganization Occurs with Outward Diameter Expansion during Collateral Artery Growth and Requires Lysyl Oxidase for Stabilization

When a large artery becomes occluded, hemodynamic changes stimulate remodeling of arterial networks to form collateral arteries in a process termed arteriogenesis. However, the structural changes necessary for collateral remodeling have not been defined. We hypothesize that deconstruction of the extracellular matrix is essential to remodel smaller arteries into effective collaterals. Using multiphoton microscopy, we analyzed collagen and elastin structure in maturing collateral arteries isolated from ischemic rat hindlimbs. Collateral arteries harvested at different timepoints showed progressive diameter expansion associated with striking rearrangement of internal elastic lamina (IEL) into a loose fibrous mesh, a pattern persisting at 8 weeks. Despite a 2.5-fold increase in luminal diameter, total elastin content remained unchanged in collaterals compared with control arteries. Among the collateral midzones, baseline elastic fiber content was low. Outward remodeling of these vessels with a 10-20 fold diameter increase was associated with fractures of the elastic fibers and evidence of increased wall tension, as demonstrated by the straightening of the adventitial collagen. Inhibition of lysyl oxidase (LOX) function with β-aminopropionitrile resulted in severe fragmentation or complete loss of continuity of the IEL in developing collaterals. Collateral artery development is associated with permanent redistribution of existing elastic fibers to accommodate diameter growth. We found no evidence of new elastic fiber formation. Stabilization of the arterial wall during outward remodeling is necessary and dependent on LOX activity.

Nanofibrous tissue engineering scaffold with nonlinear elasticity created by controlled curvature and porosity

Micro-crimped fibers have been widely used in the field of tissue repair to mimic the natural tissue structure and mechanical properties. However, the electrospun nanofibrous membrane is a kind of dense structure, which cannot meet the requirements of mechanical properties and permeability. In this study, we prepared nanofibrous scaffold with controllable porosity and crimpness by sacrificing fiber components and releasing residual stress. The results show that the crimpness of the fiber is positively related to the porosity, and with the increase of porosity, the fiber crimpness increases greatly. Meanwhile, the scaffold modulus was reduced by 86% and the elongation at break doubled, which is similar to natural blood vessels. Moreover, it is found that the porous micro-crimped fiber scaffold promotes the adhesion and diffusion of endothelial cells, and facilitates the rapid endothelialization of the scaffold, which has a great potential for practical application.

Review of the Techniques Used for Investigating the Role Elastin and Collagen Play in Arterial Wall Mechanics

The arterial wall is characterised by a complex microstructure that impacts the mechanical properties of the vascular tissue. The main components consist of collagen and elastin fibres, proteoglycans, Vascular Smooth Muscle Cells (VSMCs) and ground matrix. While VSMCs play a key role in the active mechanical response of arteries, collagen and elastin determine the passive mechanics. Several experimental methods have been designed to investigate the role of these structural proteins in determining the passive mechanics of the arterial wall. Microscopy imaging of load-free or fixed samples provides useful information on the structure-function coupling of the vascular tissue, and mechanical testing provides information on the mechanical role of collagen and elastin networks. However, when these techniques are used separately, they fail to provide a full picture of the arterial micromechanics. More recently, advances in imaging techniques have allowed combining both methods, thus dynamically imaging the sample while loaded in a pseudo-physiological way, and overcoming the limitation of using either of the two methods separately. The present review aims at describing the techniques currently available to researchers for the investigation of the arterial wall micromechanics. This review also aims to elucidate the current understanding of arterial mechanics and identify some research gaps.

Linear diattenuation imaging of biological tissues with near infrared Mueller scanning microscopy

Among the multitude of optical polarization contrasts that can be observed in complex biological specimens, linear diattenuation (LD) imaging has received little attention. It is indeed challenging to image LD with basic polarizing microscopes because it is often relatively small in comparison with linear retardance (LR). In addition, interpretation of LD images is not straightforward when experiments are conducted in the visible range because LD can be produced by both dichroism and anisotropic scattering. Mueller polarimetry is a powerful implementation of polarization sensing able to differentiate and measure the anisotropies of specimens. In this article, near infrared transmission Mueller scanning microscopy is used to image LD in thin biological specimen sections made of various proteins with unprecedented resolution and sensitivity. The near infrared spectral range makes it possible to lower the contribution of dichroism to the total linear diattenuation in order to highlight anisotropic scattering. Pixel-by-pixel comparison of LD images with LR and multiphoton images demonstrates that LD is produced by under-resolved structures that are not revealed by other means, notably within the sarcomere of skeletal muscles. LD microscopy appears as a powerful tool to provide new insights into the macro-molecular organization of biological specimens at the sub-microscopic scale without labelling.

Functional crosstalk between T cells and monocytes in cancer and atherosclerosis

Monocytes and monocyte-derived cells, including Mϕs and dendritic cells, exhibit a diverse array of phenotypic states that are dictated by their surrounding microenvironment. These cells direct T cell activation and function via cues that range from being immunosuppressive to immunostimulatory. Solid tumors and atherosclerotic plaques represent two pathological niches with distinct immune microenvironments. While monocytes and their progeny possess a phenotypic spectrum found within both disease contexts, most within tumors are pro-tumoral and support evasion of host immune responses by tumor cells. In contrast, monocyte-derived cells within atherosclerotic plaques are usually pro-atherogenic, pro-inflammatory, and predominantly directed against self-antigens. Consequently, cancer immunotherapies strive to enhance the immune response against tumor antigens, whereas atherosclerosis treatments seek to dampen the immune response against lipid antigens. Insights into monocyte-T cell interactions within these niches could thus inform therapeutic strategies for two immunologically distinct diseases. Here, we review monocyte diversity, interactions between monocytes and T cells within tumor and plaque microenvironments, how certain therapies have leveraged these interactions, and novel strategies to assay such associations.

Multiphoton microscopy for pre-clinical evaluation of flow-diverter stents for treating aneurysms

BACKGROUND

Conventional histological analyses are the gold standard for the study of aneurysms and vascular pathologies in pre-clinical research. Over the past decade, in vivo and ex vivo imaging using multiphoton microscopy have emerged as powerful pre-clinical tools for detailed tissue analyses that can assess morphology, the extracellular matrix (ECM), cell density and vascularisation. Multiphoton microscopy allows for deeper tissue penetration with minor phototoxicity.

OBJECTIVE

The present study aimed to demonstrate the current status of multimodality imaging, including multiphoton microscopy, for detailed analyses of neo-endothelialisation and ECM evolution after flow-diverter stent (FDS) treatment in an experimental rabbit model of aneurysms.

METHODS

Multiphoton microscopy tools for assessing autofluorescence and second harmonic generation (SHG) signals from biological tissues were used to evaluate the endovascular treatment of intracranial aneurysms in an animal model of aneurysms (pig, rabbit). Results from multiphoton microscopy were compared to those from standard histology, electronic and bright field microscopy.

CONCLUSIONS

The present study describes novel evaluation modes based on multiphoton microscopy for visualising tissue morphology (e.g., collagen, elastin, and cells) to qualify and quantify the extent of neo-intimal formation of covered arteries and device integration into the arterial wall using a rabbit model of intracranial aneurysms treated with FDS.

Random laser imaging of bovine pericardium under the uniaxial tensile test

We demonstrate random laser (RL) emission from within bovine pericardium (BP) tissue. The interest in BP relies on its wide use as a valve replacement and as a biological patch. By imaging the emitting tissue, we show that RL emission is mostly generated inside the collagen fibers. Multimode RL operation is thus achieved within the volume of each fiber. Image analysis reveals that the intensity of the RL emission from individual fibers is dependent on the relative orientation to the stress axis. Our results suggest that RL intensity may be used as an indicator of stress concentration in individual fibers.

Chemical imaging of fresh vascular smooth muscle cell response by epi-detected stimulated Raman scattering

An understanding of deformation of cardiovascular tissue under hemodynamic load is crucial for understanding the health and disease of blood vessels. In the present work, an epi-detected stimulated Raman scattering (epi-SRS) imaging platform was designed for in situ functional imaging of vascular smooth muscle cells (VMSCs) in fresh coronary arteries. For the first time, the pressure-induced morphological deformation of fresh VSMCs was imaged with no fixation and in a label-free manner. The relation between the loading pressure and the morphological parameters, including angle and length of the VSMCs, were apparent. The morphological responses of VMSCs to drug treatment were also explored, to demonstrate the capability of functional imaging for VSMCs by this method. The time-course imaging revealed the drug induced change in angle and length of VSMCs. The present study provides a better understanding of the biomechanical framework of blood vessels, as well as their responses to external stimulations, which are fundamental for developing new strategies for cardiovascular disease treatment.

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.

High-Resolution Morphological Approach to Analyse Elastic Laminae Injuries of the Ascending Aorta in a Murine Model of Marfan Syndrome

In Marfan syndrome, the tunica media is disrupted, which leads to the formation of ascending aortic aneurysms. Marfan aortic samples are histologically characterized by the fragmentation of elastic laminae. However, conventional histological techniques using transverse sections provide limited information about the precise location, progression and 3D extension of the microstructural changes that occur in each lamina. We implemented a method using multiphoton excitation fluorescence microscopy and computational image processing, which provides high-resolution en-face images of segmented individual laminae from unstained whole aortic samples. We showed that internal elastic laminae and successive 2^nd laminae are injured to a different extent in murine Marfan aortae; in particular, the density and size of fenestrae changed. Moreover, microstructural injuries were concentrated in the aortic proximal and convex anatomical regions. Other parameters such as the waviness and thickness of each lamina remained unaltered. In conclusion, the method reported here is a useful, unique tool for en-face laminae microstructure assessment that can obtain quantitative three-dimensional information about vascular tissue. The application of this method to murine Marfan aortae clearly shows that the microstructural damage in elastic laminae is not equal throughout the thickness of the tunica media and in the different anatomical regions of the ascending aorta.

High resolution imaging of the fibrous microstructure in bovine common carotid artery using optical polarization tractography

The biomechanical properties of artery are primarily determined by the fibrous structures in the vessel wall. Many vascular diseases are associated with alternations in the orientation and alignment of the fibrous structure in the arterial wall. Knowledge on the structural features of the artery wall is crucial to our understanding of the biology of vascular diseases and the development of novel therapies. Optical coherence tomography (OCT) and polarization-sensitive OCT have shown great promise in imaging blood vessels due to their high resolution, fast acquisition, good imaging depth, and large field of view. However, the feasibility of using OCT based methods for imaging fiber orientation and distribution in the arterial wall has not been investigated. Here we show that the optical polarization tractography (OPT), a technology developed from Jones matrix OCT, can reveal the fiber orientation and alignment in the bovine common carotid artery. The fiber orientation and alignment data obtained in OPT provided a robust contrast marker to clearly resolve the intima and media boundary of the carotid artery wall. Optical polarization tractography can visualize fiber orientation and alignment in carotid artery.

The use of Masson's trichrome staining, second harmonic imaging and two-photon excited fluorescence of collagen in distinguishing intestinal tuberculosis from Crohn's disease

AIM

Differentiation between Crohn's disease (CD) and intestinal tuberculosis (ITB) continues to be difficult. The present study investigated the collagen fibre characteristics of CD and ITB using Masson's trichrome staining, second harmonic generation (SHG) imaging and two-photon excited fluorescence (TPEF) imaging with the aim of distinguishing between them.

METHOD

The characteristics of collagen fibres in intestinal specimens from patients with CD, ITB and healthy controls were compared using Masson's trichrome staining and SHG and TPEF imaging.

RESULTS

Masson's trichrome staining showed that the content of collagen fibre (540.92 [139.61-1681.93] vs 236.17 [72.94-1108.32], P < 0.05) and fibre deposits (888.92 [315.89-3172.9] vs 498.98 [38.82-5802.31], P < 0.05) were both higher in ITB than in CD. The content of collagen fibre (594.677 [139.61-1681.93] vs 107.425 [4.66-988.7], P < 0.05) and fibre deposits (1118.4661 [315.89-5802.31] vs 340.575 [29.62-1188.87], P < 0.05) were significantly higher in lesions with granulomata than in those without. The SHG/TPEF images demonstrated that the percentage of fibrosis in ITB was also significantly higher than in CD (P < 0.05), in both surgical (13.363% ± 5.303% vs 8.322% ± 5.078%, P = 0.044) and endoscopic specimens (mean rank 13.5 vs 7.5, P = 0.023). The SHG/TPEF imaging described different distribution patterns of collagen between CD and ITB; in the former this was irregular in clumps while in ITB the collagen was arranged around caseating granulomata.

CONCLUSION

The evaluation of fibrosis in CD and ITB by Masson's trichrome staining and SHG and TPEF imaging appears to distinguish between these two diseases.

Multiphoton microscopy observations of 3D elastin and collagen fiber microstructure changes during pressurization in aortic media

Elastin and collagen fibers play important roles in the mechanical properties of aortic media. Because knowledge of local fiber structures is required for detailed analysis of blood vessel wall mechanics, we investigated 3D microstructures of elastin and collagen fibers in thoracic aortas and monitored changes during pressurization. Using multiphoton microscopy, autofluorescence images from elastin and second harmonic generation signals from collagen were acquired in media from rabbit thoracic aortas that were stretched biaxially to restore physiological dimensions. Both elastin and collagen fibers were observed in all longitudinal-circumferential plane images, whereas alternate bright and dark layers were observed along the radial direction and were recognized as elastic laminas (ELs) and smooth muscle-rich layers (SMLs), respectively. Elastin and collagen fibers are mainly oriented in the circumferential direction, and waviness of collagen fibers was significantly higher than that of elastin fibers. Collagen fibers were more undulated in longitudinal than in radial direction, whereas undulation of elastin fibers was equibiaxial. Changes in waviness of collagen fibers during pressurization were then evaluated using 2-dimensional fast Fourier transform in mouse aortas, and indices of waviness of collagen fibers decreased with increases in intraluminal pressure. These indices also showed that collagen fibers in SMLs became straight at lower intraluminal pressures than those in EL, indicating that SMLs stretched more than ELs. These results indicate that deformation of the aorta due to pressurization is complicated because of the heterogeneity of tissue layers and differences in elastic properties of ELs, SMLs, and surrounding collagen and elastin.

Multi-modal optical imaging characterization of atherosclerotic plaques

We combined cross-polarization optical coherence tomography (CP OCT) and non-linear microscopy based on second harmonic generation (SHG) and two-photon-excited fluorescence (2PEF) to assess collagen and elastin fibers and other vascular structures in the development of atherosclerosis, including identification of vulnerable plaques, which remains an important clinical problem and imaging application. CP OCT's ability to visualize tissue birefringence and cross-scattering adds new information about the microstructure and composition of the plaque. However its interpretation can be ambiguous, because backscattering contrast may have a similar appearance to the birefringence related fringes. Our results represent a step towards minimally invasive characterization and monitoring of different stages of atherosclerosis, including vulnerable plaques. CP OCT image of intimal thickening in the human coronary artery. The dark stripe in the cross-polarization channel (arrow) is a polarization fringe related to the phase retardation between two eigen polarization states. It is histologically located in the area of the lipid pool, however this stripe is a polarization artifact, rather than direct visualization of the lipid pool.

3D reconstruction of elastin fibres in coronary adventitia

A 3D reconstruction of individual fibres in vascular tissue is necessary to understand the microstructure properties of the vessel wall.  The objective of this study is to determine the 3D microstructure of elastin fibres in the adventitia of coronary arteries.  Quantification of fibre geometry is challenging due to the complex interwoven structure of the fibres.  In particular, accurate linking of gaps remains a significant challenge, and complex features such as long gaps and interwoven fibres have not been adequately addressed by current fibre reconstruction algorithms.  We use a novel line Laplacian deformation method, which better deals with fibre shape uncertainty to reconstruct elastin fibres in the coronary adventitia of five swine. A cost function, based on entropy and Euler Spiral, was used in the shortest path search. We find that mean diameter of elastin fibres is 1.67 ± 1.42 μm and fibre orientation is clustered around two major angles of 8.9˚ and 81.8˚.  Comparing with CT-FIRE, we find that our method gives more accurate estimation of fibre width.  To our knowledge, the measurements obtained using our algorithm represent the first investigation focused on the reconstruction of full elastin fibre length.  Our data provide a foundation for a 3D microstructural model of the coronary adventitia to elucidate the structure-function relationship of elastin fibres.

Microstructure-based biomechanics of coronary arteries in health and disease

Coronary atherosclerosis is the major cause of mortality and disability in developed nations. A deeper understanding of mechanical properties of coronary arteries and hence their mechanical response to stress is significant for clinical prevention and treatment. Microstructure-based models of blood vessels can provide predictions of arterial mechanical response at the macro- and micro-mechanical level for each constituent structure. Such models must be based on quantitative data of structural parameters (constituent content, orientation angle and dimension) and mechanical properties of individual adventitia and media layers of normal arteries as well as change of structural and mechanical properties of atherosclerotic arteries. The microstructural constitutive models of healthy coronary arteries consist of three major mechanical components: collagen, elastin, and smooth muscle cells, while the models of atherosclerotic arteries should account for additional constituents including intima, fibrous plaque, lipid, calcification, etc. This review surveys the literature on morphology, mechanical properties, and microstructural constitutive models of normal and atherosclerotic coronary arteries. It also provides an overview of current gaps in knowledge that must be filed in order to advance this important area of research for understanding initiation, progression and clinical treatment of vascular disease. Patient-specific structural models are highlighted to provide diagnosis, virtual planning of therapy and prognosis when realistic patient-specific geometries and material properties of diseased vessels can be acquired by advanced imaging techniques.

A non-hypocholesterolemic atorvastatin treatment improves vessel elasticity by acting on elastin composition in WHHL rabbits

BACKGROUND AND AIMS

Statins are prescribed for their preventative effects within atherosclerosis development. To our knowledge, no study focusing on very low-dose (non-hypolipidemic effect) and long-term atorvastatin treatment in vivo was available. Our aim was to assess the effect of such atorvastatin treatment on the mechanical and functional characteristics of arteries in the context of primary prevention.

METHODS

An atorvastatin treatment (2.5 mg/kg/day) was tested against controls on 34 male 3 to 12 month-old WHHL rabbits. No effect on total cholesterol, triglycerides, HDL or LDL was observed. The arterial stiffness was evaluated on vigil animals by pulse wave velocity (PWV) measurement. Then, in vitro measurements were made to evaluate (1) the endothelial and vascular smooth muscle function, (2) the elasticity of the arterial wall and (3) the composition in collagen and elastin in the aorta.

RESULTS

The PWV increasing observed with age in control group was canceled by treatment, creating a significance difference between groups at 12 months (5.17 ± 0.50 vs 2.14 ± 0.34 m s(-1) in control and treated groups respectively). Vasoreactivity modifications can't explain this result but maintain of elasticity with treatment in large arteries was confirm by a static tensile test. A first possible explanation is the change of wall composition with treatment, validated by the percentage of elastin at 12 months, 4.4% lower in the control group compared to the treated group (p < 0.05).

CONCLUSIONS

This study shows that a non-hypocholesterolemic statin treatment could improve vessel elasticity in the atherosclerotic WHHL model. The great novelty of this work is the vessel wall composition changing associated. This first approach in animal opens the reflection on the use of these low doses in humans. This could be interesting in the context of arterial stiffening with aging, non-hyperlipidemic atherosclerosis or with cholesterol reduce by another therapy or lifestyle modification.

Ultrahigh-resolution optical coherence elastography

Visualizing stiffness within the local tissue environment at the cellular and subcellular level promises to provide insight into the genesis and progression of disease. In this Letter, we propose ultrahigh-resolution optical coherence elastography (UHROCE), and demonstrate 3D imaging of local axial strain of tissues undergoing compressive loading. We combine optical coherence microscopy (OCM) and phase-sensitive detection of local tissue displacement to produce strain elastograms with resolution (x,y,z) of 2×2×15  μm. We demonstrate this performance on a freshly excised mouse aorta and reveal the mechanical heterogeneity of vascular smooth muscle cells and elastin sheets, otherwise unresolved in a typical, lower resolution optical coherence elastography (OCE) system.

Vascular Smooth Muscle Sirtuin-1 Protects Against Aortic Dissection During Angiotensin II-Induced Hypertension

Background

Sirtuin-1 (SirT1), a nicotinamide adenine dinucleotide⁺–dependent deacetylase, is a key enzyme in the cellular response to metabolic, inflammatory, and oxidative stresses; however, the role of endogenous SirT1 in the vasculature has not been fully elucidated. Our goal was to evaluate the role of vascular smooth muscle SirT1 in the physiological response of the aortic wall to angiotensin II, a potent hypertrophic, oxidant, and inflammatory stimulus.

Methods and Results

Mice lacking SirT1 in vascular smooth muscle (ie, smooth muscle SirT1 knockout) had drastically high mortality (70%) caused by aortic dissection after angiotensin II infusion (1 mg/kg per day) but not after an equipotent dose of norepinephrine, despite comparable blood pressure increases. Smooth muscle SirT1 knockout mice did not show any abnormal aortic morphology or blood pressure compared with wild-type littermates. Nonetheless, in response to angiotensin II, aortas from smooth muscle SirT1 knockout mice had severely disorganized elastic lamellae with frequent elastin breaks, increased oxidant production, and aortic stiffness compared with angiotensin II–treated wild-type mice. Matrix metalloproteinase expression and activity were increased in the aortas of angiotensin II–treated smooth muscle SirT1 knockout mice and were prevented in mice overexpressing SirT1 in vascular smooth muscle or with use of the oxidant scavenger tempol.

Conclusions

Endogenous SirT1 in aortic smooth muscle is required to maintain the structural integrity of the aortic wall in response to oxidant and inflammatory stimuli, at least in part, by suppressing oxidant-induced matrix metalloproteinase activity. SirT1 activators could potentially be a novel therapeutic approach to prevent aortic dissection and rupture in patients at risk, such as those with hypertension or genetic disorders, such as Marfan’s syndrome.

Detection of plaque structure and composition using OCT combined with two-photon luminescence (TPL) imaging

BACKGROUND AND OBJECTIVES

Atherosclerosis and plaque rupture leads to myocardial infarction and stroke. A novel hybrid optical coherence tomography (OCT) and two-photon luminescence (TPL) fiber-based imaging system was developed to characterize tissue constituents in the context of plaque morphology.

STUDY DESIGN/MATERIALS AND METHODS

Ex vivo coronary arteries (34 regions of interest) from three human hearts with atherosclerotic plaques were examined by OCT-TPL imaging. Histological sections (4 μm in thickness) were stained with Oil Red O for lipid, Von Kossa for calcium, and Verhoeff-Masson Tri-Elastic for collagen/elastin fibers and compared with imaging results.

RESULTS

Biochemical components in plaques including lipid, oxidized-LDL, and calcium, as well as a non-tissue component (metal) are distinguished by multi-channel TPL images with statistical significance (P < 0.001). TPL imaging provides complementary optical contrast to OCT (two-photon absorption/emission vs scattering). Merged OCT-TPL images demonstrate the distribution of lipid deposits in registration with detailed plaque surface profile.

CONCLUSIONS

Results suggest that multi-channel TPL imaging can effectively identify lipid sub-types and different plaque components. Furthermore, fiber-based hybrid OCT-TPL imaging simultaneously detects plaque structure and composition, improving the efficacy of vulnerable plaque detection and characterization.

Layer-resolved colorectal tissues using nonlinear microscopy

In this work, multiphoton microscopy (MPM), based on the nonlinear optical processes two-photon excited fluorescence (TPEF) and second harmonic generation (SHG), was extended to evaluate the feasibility of using MPM to distinguish layers of the bowel wall. It was found that MPM has the ability to identify the four-layer microstructures of colorectal tissues including mucosa, submucosa, muscularis propria, and serosa as there are many intrinsic signal sources in each layer. Our results also showed the capability of using the quantitative analyses of MPM images for quantifying some feature parameters including the nuclear area, nuclear-to-cytoplasmic ratio, and optical redox ratio. This work demonstrates that MPM has the potential in noninvasively monitoring the development and progression of colorectal diseases and then guiding effective treatment.

Dual-modality fiber-based OCT-TPL imaging system for simultaneous microstructural and molecular analysis of atherosclerotic plaques

New optical imaging techniques that provide contrast to study both the anatomy and composition of atherosclerotic plaques can be utilized to better understand the formation, progression and clinical complications of human coronary artery disease. We present a dual-modality fiber-based optical imaging system for simultaneous microstructural and molecular analysis of atherosclerotic plaques that combines optical coherence tomography (OCT) and two-photon luminescence (TPL) imaging. Experimental results from ex vivo human coronary arteries show that OCT and TPL optical contrast in recorded OCT-TPL images is complimentary and in agreement with histological analysis. Molecular composition (e.g., lipid and oxidized-LDL) detected by TPL imaging can be overlaid onto plaque microstructure depicted by OCT, providing new opportunities for atherosclerotic plaque identification and characterization.

Human Dupuytren's Ex Vivo Culture for the Study of Myofibroblasts and Extracellular Matrix Interactions

Organ fibrosis or "scarring" is known to account for a high death toll due to the extensive amount of disorders and organs affected (from cirrhosis to cardiovascular diseases). There is no effective treatment and the in vitro tools available do not mimic the in vivo situation rendering the progress of the out of control wound healing process still enigmatic. To date, 2D and 3D cultures of fibroblasts derived from DD patients are the main experimental models available. Primary cell cultures have many limitations; the fibroblasts derived from DD are altered by the culture conditions, lack cellular context and interactions, which are crucial for the development of fibrosis and weakly represent the derived tissue. Real-time PCR analysis of fibroblasts derived from control and DD samples show that little difference is detectable. 3D cultures of fibroblasts include addition of extracellular matrix that alters the native conditions of these cells. As a way to characterize the fibrotic, proliferative properties of these resection specimens we have developed a 3D culture system, using intact human resections of the nodule part of the cord. The system is based on transwell plates with an attached nitrocellulose membrane that allows contact of the tissue with the medium but not with the plastic, thus, preventing the alteration of the tissue. No collagen gel or other extracellular matrix protein substrate is required. The tissue resection specimens maintain their viability and proliferative properties for 7 days. This is the first "organ" culture system that allows human resection specimens from DD patients to be grown ex vivo and functionally tested, recapitulating the in vivo situation.

Real-time histology in liver disease using multiphoton microscopy with fluorescence lifetime imaging

Conventional histology with light microscopy is essential in the diagnosis of most liver diseases. Recently, a concept of real-time histology with optical biopsy has been advocated. In this study, live mice livers (normal, with fibrosis, steatosis, hepatocellular carcinoma and ischemia-reperfusion injury) were imaged by MPM-FLIM for stain-free real-time histology. The acquired MPM-FLIM images were compared with conventional histological images. MPM-FLIM imaged subsurface cellular and subcellular histopathological hallmarks of live liver in mice models at high resolution. Additional information such as distribution of stellate cell associated autofluorescence and fluorescence lifetime changes was also gathered by MPM-FLIM simultaneously, which cannot be obtained from conventional histology. MPM-FLIM could simultaneously image and quantify the cellular morphology and microenvironment of live livers without conventional biopsy or fluorescent dyes. We anticipate that in the near future MPM-FLIM will be evaluated from bench to bedside, leading to real-time histology and dynamic monitoring of human liver diseases.

Label-free, three-dimensional multiphoton microscopy of the connective tissue in the anterior vaginal wall

INTRODUCTION AND HYPOTHESIS

Multiphoton microscopy (MPM) is a nonlinear, high-resolution laser scanning technique and a powerful approach for analyzing the spatial architecture within tissues. To demonstrate the potential of this technique for studying the extracellular matrix of the pelvic organs, we aimed to establish protocols for the detection of collagen and elastin in the vagina and to compare the MPM density of these fibers to fibers detected using standard histological methods.

METHODS

Samples of the anterior vaginal wall were obtained from nine patients undergoing a hysterectomy or cystocele repair. Samples were shock frozen, fixed with formaldehyde or Thiel's solution, or left untreated. Samples were imaged with MPM to quantify the amount of collagen and elastin via second harmonic generation and autofluorescence, respectively. In six patients, sample sections were also histologically stained and imaged with brightfield microscopy. The density of the fibers was quantified using the StereoInvestigator and Cavalieri software.

RESULTS

With MPM, collagen and elastin could be visualized to a depth of 100 μm, and no overlap of signals was detected. The different tissue processing protocols used did not result in significantly different fiber counts after MPM. MPM-based fiber quantifications are comparable to those based on conventional histological stains. However, MPM provided superior resolution, particularly of collagen fibers.

CONCLUSIONS

MPM is a robust, rapid, and label-free method that can be used to quantify the collagen and elastin content in thick specimens of the vagina. It is an excellent tool for future three-dimensional studies of the extracellular matrix in patients with pelvic organ prolapse.

IL-6 regulates extracellular matrix remodeling associated with aortic dilation in a fibrillin-1 hypomorphic mgR/mgR mouse model of severe Marfan syndrome

Background

Development of thoracic aortic aneurysms is the most significant clinical phenotype in patients with Marfan syndrome. An inflammatory response has been described in advanced stages of the disease. Because the hallmark of vascular inflammation is local interleukin‐6 (IL‐6) secretion, we explored the role of this proinflammatory cytokine in the formation of aortic aneurysms and rupture in hypomorphic fibrillin‐deficient mice (mgR/mgR).

Methods and Results

MgR/mgR mice developed ascending aortic aneurysms with significant dilation of the ascending aorta by 12 weeks (2.7±0.1 and 1.3±0.1 for mgR/mgR versus wild‐type mice, respectively; P<0.001). IL‐6 signaling was increased in mgR/mgR aortas measured by increases in IL‐6 and SOCS3 mRNA transcripts (P<0.05) and in cytokine secretion of IL‐6, MCP‐1, and GM‐CSF (P<0.05). To investigate the role of IL‐6 signaling, we generated mgR homozygous mice with IL‐6 deficiency (DKO). The extracellular matrix of mgR/mgR mice showed significant disruption of elastin and the presence of dysregulated collagen deposition in the medial‐adventitial border by second harmonic generation multiphoton autofluorescence microscopy. DKO mice exhibited less elastin and collagen degeneration than mgR/mgR mice, which was associated with decreased activity of matrix metalloproteinase‐9 and had significantly reduced aortic dilation (1.0±0.1 versus 1.6±0.2 mm change from baseline, DKO versus mgR/mgR, P<0.05) that did not affect rupture and survival.

Conclusion

Activation of IL‐6‐STAT3 signaling contributes to aneurysmal dilation in mgR/mgR mice through increased MMP‐9 activity, aggravating extracellular matrix degradation.

Biaxial deformation of collagen and elastin fibers in coronary adventitia

The microstructural deformation-mechanical loading relation of the blood vessel wall is essential for understanding the overall mechanical behavior of vascular tissue in health and disease. We employed simultaneous mechanical loading-imaging to quantify in situ deformation of individual collagen and elastin fibers on unstained fresh porcine coronary adventitia under a combination of vessel inflation and axial extension loading. Specifically, the specimens were imaged under biaxial loads to study microscopic deformation-loading behavior of fibers in conjunction with morphometric measurements at the zero-stress state. Collagen fibers largely orientate in the longitudinal direction, while elastin fibers have major orientation parallel to collagen, but with additional orientation angles in each sublayer of the adventitia. With an increase of biaxial load, collagen fibers were uniformly stretched to the loading direction, while elastin fibers gradually formed a network in sublayers, which strongly depended on the initial arrangement. The waviness of collagen decreased more rapidly at a circumferential stretch ratio of λθ = 1.0 than at λθ = 1.5, while most collagen became straightened at λθ = 1.8. These microscopic deformations imply that the longitudinally stiffer adventitia is a direct result of initial fiber alignment, and the overall mechanical behavior of the tissue is highly dependent on the corresponding microscopic deformation of fibers. The microstructural deformation-loading relation will serve as a foundation for micromechanical models of the vessel wall.

Inflammation and immune system interactions in atherosclerosis

Cardiovascular disease (CVD) is the leading cause of mortality worldwide, accounting for 16.7 million deaths each year. The underlying cause of the majority of CVD is atherosclerosis. In the past, atherosclerosis was considered to be the result of passive lipid accumulation in the vessel wall. Today's picture is far more complex. Atherosclerosis is considered a chronic inflammatory disease that results in the formation of plaques in large and mid-sized arteries. Both cells of the innate and the adaptive immune system play a crucial role in its pathogenesis. By transforming immune cells into pro- and anti-inflammatory chemokine- and cytokine-producing units, and by guiding the interactions between the different immune cells, the immune system decisively influences the propensity of a given plaque to rupture and cause clinical symptoms like myocardial infarction and stroke. In this review, we give an overview on the newest insights in the role of different immune cells and subtypes in atherosclerosis.

Nonlinear optical microscopy in decoding arterial diseases

Pathological understanding of arterial diseases is mainly attributable to histological observations based on conventional tissue staining protocols. The emerging development of nonlinear optical microscopy (NLOM), particularly in second-harmonic generation, two-photon excited fluorescence and coherent Raman scattering, provides a new venue to visualize pathological changes in the extracellular matrix caused by atherosclerosis progression. These techniques in general require minimal tissue preparation and offer rapid three-dimensional imaging. The capability of label-free microscopic imaging enables disease impact to be studied directly on the bulk artery tissue, thus minimally perturbing the sample. In this review, we look at recent progress in applications related to arterial disease imaging using various forms of NLOM.

Characterizing the collagen fiber orientation in pericardial leaflets under mechanical loading conditions

When implanted inside the body, bioprosthetic heart valve leaflets experience a variety of cyclic mechanical stresses such as shear stress due to blood flow when the valve is open, flexural stress due to cyclic opening and closure of the valve, and tensile stress when the valve is closed. These types of stress lead to a variety of failure modes. In either a natural valve leaflet or a processed pericardial tissue leaflet, collagen fibers reinforce the tissue and provide structural integrity such that the very thin leaflet can stand enormous loads related to cyclic pressure changes. The mechanical response of the leaflet tissue greatly depends on collagen fiber concentration, characteristics, and orientation. Thus, understating the microstructure of pericardial tissue and its response to dynamic loading is crucial for the development of more durable heart valve, and computational models to predict heart valves' behavior. In this work, we have characterized the 3D collagen fiber arrangement of bovine pericardial tissue leaflets in response to a variety of different loading conditions under Second-Harmonic Generation Microscopy. This real-time visualization method assists in better understanding of the effect of cyclic load on collagen fiber orientation in time and space.

Dynamic T cell-APC interactions sustain chronic inflammation in atherosclerosis

Atherosclerosis is a chronic inflammatory disease of large and medium-sized arteries characterized by leukocyte accumulation in the vessel wall. Both innate and adaptive immune responses contribute to atherogenesis, but the identity of atherosclerosis-relevant antigens and the role of antigen presentation in this disease remain poorly characterized. We developed live-cell imaging of explanted aortas to compare the behavior and role of APCs in normal and atherosclerotic mice. We found that CD4+ T cells were capable of interacting with fluorescently labeled (CD11c-YFP+) APCs in the aortic wall in the presence, but not the absence, of cognate antigen. In atherosclerosis-prone Apoe-/-CD11c-YFP+ mice, APCs extensively interacted with CD4+ T cells in the aorta, leading to cell activation and proliferation as well as secretion of IFN-γ and TNF-α. These cytokines enhanced uptake of oxidized and minimally modified LDL by macrophages. We conclude that antigen presentation by APCs to CD4+ T cells in the arterial wall causes local T cell activation and production of proinflammatory cytokines, which promote atherosclerosis by maintaining chronic inflammation and inducing foam cell formation.

Combined two-photon luminescence microscopy and OCT for macrophage detection in the hypercholesterolemic rabbit aorta using plasmonic gold nanorose

BACKGROUND AND OBJECTIVES

The macrophage is an important early cellular marker related to risk of future rupture of atherosclerotic plaques. Two-channel two-photon luminescence (TPL) microscopy combined with optical coherence tomography (OCT) was used to detect, and further characterize the distribution of aorta-based macrophages using plasmonic gold nanorose as an imaging contrast agent.

STUDY DESIGN/MATERIALS AND METHODS

Nanorose uptake by macrophages was identified by TPL microscopy in macrophage cell culture. Ex vivo aorta segments (8 × 8 × 2 mm(3) ) rich in macrophages from a rabbit model of aorta inflammation were imaged by TPL microscopy in combination with OCT. Aorta histological sections (5 µm in thickness) were also imaged by TPL microscopy.

RESULTS

Merged two-channel TPL images showed the lateral and depth distribution of nanorose-loaded macrophages (confirmed by RAM-11 stain) and other aorta components (e.g., elastin fiber and lipid droplet), suggesting that nanorose-loaded macrophages are diffusively distributed and mostly detected superficially within 20 µm from the luminal surface of the aorta. Moreover, OCT images depicted detailed surface structure of the diseased aorta.

CONCLUSIONS

Results suggest that TPL microscopy combined with OCT can simultaneously reveal macrophage distribution with respect to aorta surface structure, which has the potential to detect vulnerable plaques and monitor plaque-based macrophages overtime during cardiovascular interventions.

The effects of angiotensin II on the coupled microstructural and biomechanical response of C57BL/6 mouse aorta

RATIONALE

Abdominal aortic aneurysm (AAA) is a complex disease that leads to a localized dilation of the infrarenal aorta, the rupture of which is associated with significant morbidity and mortality. Animal models of AAA can be used to study how changes in the microstructural and biomechanical behavior of aortic tissues develop as disease progresses in these animals. We chose here to investigate the effect of angiotensin II (AngII) in C57BL/6 mice as a first step towards understanding how such changes occur in the established ApoE(-/-) AngII infused mouse model of AAA.

OBJECTIVE

The objective of this study was to utilize a recently developed device in our laboratory to determine how the microstructural and biomechanical properties of AngII-infused C57BL/6 wildtype mouse aorta change following 14 days of AngII infusion.

METHODS

C57BL/6 wildtype mice were infused with either saline or AngII for 14 day. Aortas were excised and tested using a device capable of simultaneously characterizing the biaxial mechanical response and load-dependent (unfixed, unfrozen) extracellular matrix organization of mouse aorta (using multiphoton microscopy). Peak strains and stiffness values were compared across experimental groups, and both datasets were fit to a Fung-type constitutive model. The mean mode and full width at half maximum (FWHM) of fiber histograms from two photon microscopy were quantified in order to assess the preferred fiber distribution and degree of fiber splay, respectively.

RESULTS

The axial stiffness of all mouse aorta was found to be an order of magnitude larger than the circumferential stiffness. The aortic diameter was found to be significantly increased for the AngII infused mice as compared to saline infused control (p=0.026). Aneurysm, defined as a percent increase in maximum diameter of 30% (defined with respect to saline control), was found in 3 of the 6 AngII infused mice. These three mice displayed adventitial collagen that lacked characteristic fiber crimp. The biomechanical response in the AngII infused mice showed significantly reduced circumferential compliance. We also noticed that the ability of the adventitial collagen fibers in AngII infused mice to disperse in reaction to circumferential loading was suppressed.

CONCLUSIONS

Collagen remodeling is present following 14 days of AngII infusion in C57BL/6 mice. Aneurysmal development occurred in 50% of our AngII infused mice, and these dilatations were accompanied with adventitial collagen remodeling and decreased circumferential compliance.

How dendritic cells shape atherosclerosis

Atherosclerosis is an inflammatory disease of the arteries, which results in major morbidity and mortality. Immune cells initiate and sustain local inflammation. Here, we focus on how dendritic cell (DC)-mediated processes might be relevant to atherosclerosis. Although only small numbers of DCs are detected in healthy arteries, these numbers dramatically increase during atherosclerosis development. In the earliest fatty streaks, DCs are found next to the vascular endothelium. During plaque growth, new DCs are actively recruited, and their egress from the vessel wall is dampened. In the adventitia next to mature atherosclerotic lesions, tertiary lymphoid organs develop, which also contain DCs. Thus, DCs probably participate in all stages of atherosclerosis from fatty streaks to mature lesions.

Polarization state studies in second harmonic generation signals to trace atherosclerosis lesions

We have performed multi-photon image reconstructions as well as polarization state analyses inside an artery wall affected by atherosclerosis to investigate the changes in collagen structure. Mice, either healthy or affected by spontaneous atherosclerosis, have been used for this purpose. A two-photon imaging system has been used to investigate atherosclerotic lesions in the ascending aorta of mice. Second harmonic imaging has been performed alternatively on healthy samples and on affected region. The reconstructed images show that the spatial distribution of the collagen network seems disorganized by the disease. The polarization state studies reveal however that the apparent disorganization of the collagen is related to its spatially diffuse distribution and that the internal structure of the collagen fibers is not affected by the disease. In addition, a theoretical simulation of the second harmonic polarization states shows that they are consistent with the known 3D structure of the collagen network.

Adaptation of a planar microbiaxial optomechanical device for the tubular biaxial microstructural and macroscopic characterization of small vascular tissues

Murine models of disease are a powerful tool for researchers to gain insight into disease formation, progression, and therapies. The biomechanical indicators of diseased tissue provide a unique insight into some of these murine models, since the biomechanical properties in scenarios such as aneurysm and Marfan syndrome can dictate tissue failure and mortality. Understanding the properties of the tissue on the macroscopic scale has been shown to be important, as one can then understand the tissue's ability to withstand the high stresses seen in the cardiac pulsatile cycle. Alterations in the biomechanical response can foreshadow prospective mechanical failure of the tissue. These alterations are often seen on the microstructural level, and obtaining detailed information on such changes can offer a better understanding of the phenomena seen on the macroscopic level. Unfortunately, mouse models present problems due to the size and delicate features in the mechanical testing of such tissues. In addition, some smaller arteries in large-animal studies (e.g., coronary and cerebral arteries) can present the same issues, and are sometimes unsuitable for planar biaxial testing. The purpose of this paper is to present a robust method for the investigation of the mechanical properties of small arteries and the classification of the microstructural orientation and degree of fiber alignment. This occurs through the cost-efficient modification of a planar biaxial tester that works in conjunction with a two-photon nonlinear microscope. This system provides a means to further investigate how microstructure and mechanical properties are modified in diseased transgenic animals where the tissue is in small tube form. Several other hard-to-test tubular specimens such as cerebral aneurysm arteries and atherosclerotic coronary arteries can also be tested using the described modular device.

Design and demonstration of a microbiaxial optomechanical device for multiscale characterization of soft biological tissues with two-photon microscopy

The biomechanical response of tissues serves as a valuable marker in the prediction of disease and in understanding the related behavior of the body under various disease and age states. Alterations in the macroscopic biomechanical response of diseased tissues are well documented; however, a thorough understanding of the microstructural events that lead to these changes is poorly understood. In this article we introduce a novel microbiaxial optomechanical device that allows two-photon imaging techniques to be coupled with macromechanical stimulation in hydrated planar tissue specimens. This allows that the mechanical response of the microstructure can be quantified and related to the macroscopic response of the same tissue sample. This occurs without the need to fix tissue in strain states that could introduce a change in the microstructural configuration. We demonstrate the passive realignment of fibrous proteins under various types of loading, which demonstrates the ability of tissue microstructure to reinforce itself in periods of high stress. In addition, the collagen and elastin response of tissue during viscoelastic behavior is reported showing interstitial fluid movement and fiber realignment potentially responsible for the temporal behavior. We also demonstrate that nonhomogeneities in fiber strain exist over biaxial regions of assumed homogeneity.

Polarization-resolved Second Harmonic microscopy in anisotropic thick tissues

We thoroughly analyze the linear propagation effects that affect polarization-resolved Second Harmonic Generation imaging of thick anisotropic tissues such as collagenous tissues. We develop a theoretical model that fully accounts for birefringence and diattenuation along the excitation propagation, and polarization scrambling upon scattering of the harmonic signal. We obtain an excellent agreement with polarizationresolved SHG images at increasing depth within a rat-tail tendon for both polarizations of the forward SHG signal. Most notably, we observe interference fringes due to birefringence in the SHG depth profile when excited at π/4 angle from the tendon axis. We also measure artifactual decrease of ρ = Χxxx/Χxyy with depth due to diattenuation of the excitation. We therefore derive a method that proves reliable to determine both ρ and the tendon birefringence and diattenuation.