Medial collagen organization in human arteries of the heart and brain by polarized light microscopy

Strain-induced collagen denaturation is rate dependent in failure of cerebral arteries

While soft tissues are commonly damaged by mechanical loading, the manifestation of this damage at the microstructural level is not fully understood. Specifically, while rate-induced stiffening has been previously observed in cerebral arteries, associated changes in microstructural damage patterns following high-rate loading are largely undefined. In this study, we stretched porcine middle cerebral arteries to failure at 0.01 and >150 s^-1, both axially and circumferentially, followed by probing for denatured tropocollagen using collagen hybridizing peptide (CHP). We found that collagen fibrils aligned with the loading direction experienced less denaturation following failure tests at high than low rates. Others have demonstrated similar rate dependence in tropocollagen denaturation during soft tissue failure, but this is the first study to quantify this behavior using CHP and to report it for cerebral arteries. These findings may have significant implications for traumatic brain injury and intracranial balloon angioplasty. We additionally observed possible tropocollagen denaturation in vessel layers primarily composed of fibrils transversely aligned to the loading axis. To our knowledge, this is the first observation of collagen denaturation due to transverse loading, but further research is needed to confirm this finding. STATEMENT OF SIGNIFICANCE: Previous work shows that collagen hybridizing peptide (CHP) can be used to identify collagen molecule unfolding and denaturation in mechanically overloaded soft tissues, including the cerebral arteries. But experiments have not explored collagen damage at rates relevant to traumatic brain injury. In this work, we quantified collagen damage in cerebral arteries stretched to failure at both high and low rates. We found that the collagen molecule is less damaged at high than at low rates, suggesting that damage mechanisms of either the collagen molecule or other elements of the collagen superstructure are rate dependent. This work implies that arteries failed at high rates, such as in traumatic brain injury, will have different molecular-level damage patterns than arteries failed at low rates. Consequently, improved understanding of damage characteristics may be expanded in the future to better inform clinically relevant cases of collagen damage such as angioplasty and injury healing.

Single-Cell Transcriptome Analysis of the Circle of Willis in a Mouse Cerebral Aneurysm Model

BACKGROUND

The circle of Willis (CoW) is the most common location for aneurysms to form in humans. Although the major cell types of the intracranial vasculature are well known, the heterogeneity and relative contributions of the different cells in healthy and aneurysmal vessels have not been well characterized. Here, we present the first comprehensive analysis of the lineage heterogeneity and altered transcriptomic profiles of vascular cells from healthy and aneurysmal mouse CoW using single-cell RNA sequencing.

METHODS

Cerebral aneurysms (CAs) were induced in adult male mice using an elastase model. Single-cell RNA sequencing was then performed on CoW samples obtained from animals that either had aneurysms form or rupture 14 days post-induction. Sham-operated animals served as controls.

RESULTS

Unbiased clustering analysis of the transcriptional profiles from >3900 CoW cells identified 19 clusters representing ten cell lineages: vascular smooth muscle cells, endothelial cells fibroblasts, pericytes and immune cells (macrophages, T and B lymphocytes, dendritic cells, mast cells, and neutrophils). The 5 vascular smooth muscle cell subpopulations had distinct transcriptional profiles and were classified as proliferative, stress-induced senescent, quiescent, inflammatory-like, or hyperproliferative. The transcriptional signature of the metabolic pathways of ATP generation was found to be downregulated in 2 major vascular smooth muscle cell clusters when CA was induced. Aneurysm induction led to significant expansion of the total macrophage population, and this expansion was further increased with rupture. Both inflammatory and resolution-phase macrophages were identified, and a massive spike of neutrophils was seen with CA rupture. Additionally, the neutrophil-to-lymphocyte ratio (NLR), which originated from CA induction mirrored what happens in humans.

CONCLUSIONS

Our data identify CA disease-relevant transcriptional signatures of vascular cells in the CoW and is searchable via a web-based R/shiny interface.

Venous Biomechanics of Angioplasty and Stent Placement: Implications of the Poisson Effect

PURPOSE

To characterize the Poisson effect in response to angioplasty and stent placement in veins and identify potential implications for guiding future venous-specific device design.

MATERIALS AND METHODS

In vivo angioplasty and stent placement were performed in 3 adult swine by using an established venous stenosis model. Iron particle endothelium labeling was performed for real-time fluoroscopic tracking of the vessel wall during intervention. A finite-element computational model of a vessel was created with ADINA software (version 9.5) with arterial and venous biomechanical properties obtained from the literature to compare the response to radial expansion.

RESULTS

In vivo angioplasty and stent placement in a venous stenosis animal model with iron particle endothelium labeling demonstrated longitudinal foreshortening that correlated with distance from the center of the balloon (R² = 0.87) as well as adjacent segment narrowing that correlated with the increase in diameter of the treated stenotic segment (R² = 0.89). Finite-element computational analysis demonstrated increased Poisson effect in veins relative to arteries (linear regression coefficient slope comparison, arterial slope 0.033, R² = 0.9789; venous slope 0.204, R² = 0.9975; P < .0001) as a result of greater longitudinal Young modulus in veins compared with arteries.

CONCLUSIONS

Clinically observed adjacent segment narrowing during venous angioplasty and stent placement is a result of the Poisson effect, with redistribution of radially applied force to the longitudinal direction. The Poisson effect is increased in veins relative to arteries as a result of unique venous biomechanical properties, which may be relevant to consider in the design of future venous interventional devices.

3D Coaxial Bioprinting of Vasculature

Development of a suitable vascular network for an efficient mass exchange is crucial to generate three-dimensional (3D) viable and functional thick construct in tissue engineering. Different technologies have been reported for the fabrication of vasculature conduits, such as decellularized tissues and biomaterial-based blood vessels. Recently, bioprinting has also been considered as a promising method in vascular tissue engineering. In this work, human umbilical vein smooth muscle cells (HUVSMCs) were encapsulated in sodium alginate and printed in the form of vasculature conduits using a coaxial nozzle deposition system. Protocols for cell encapsulation and 3D bioprinting are presented. Investigations including dehydration, swelling, degradation characteristics, and patency, permeability, and mechanical properties were also performed and presented to the reader. In addition, in vitro studies such as cell viability and evaluation of extra cellular matrix deposition were performed.

Intracranial Aneurysms: Pathology, Genetics, and Molecular Mechanisms

Intracranial aneurysms (IA) are local dilatations in cerebral arteries that predominantly affect the circle of Willis. Occurring in approximately 2-5% of adults, these weakened areas are susceptible to rupture, leading to subarachnoid hemorrhage (SAH), a type of hemorrhagic stroke. Due to its early age of onset and poor prognosis, SAH accounts for > 25% of years lost for all stroke victims under the age of 65. In this review, we describe the cerebrovascular pathology associated with intracranial aneurysms. To understand IA genetics, we summarize syndromes with elevated incidence, genome-wide association studies (GWAS), whole exome studies on IA-affected families, and recent research that established definitive roles for Thsd1 (Thrombospondin Type 1 Domain Containing Protein 1) and Sox17 (SRY-box 17) in IA using genetically engineered mouse models. Lastly, we discuss the underlying molecular mechanisms of IA, including defects in vascular endothelial and smooth muscle cells caused by dysfunction in mechanotransduction, Thsd1/FAK (Focal Adhesion Kinase) signaling, and the Transforming Growth Factor β (TGF-β) pathway. As illustrated by THSD1 research, cell adhesion may play a significant role in IA.

Biaxial Stretch Improves Elastic Fiber Maturation, Collagen Arrangement, and Mechanical Properties in Engineered Arteries

Tissue-engineered blood vessels (TEVs) are typically produced using the pulsatile, uniaxial circumferential stretch to mechanically condition and strengthen the arterial grafts. Despite improvements in the mechanical integrity of TEVs after uniaxial conditioning, these tissues fail to achieve critical properties of native arteries such as matrix content, collagen fiber orientation, and mechanical strength. As a result, uniaxially loaded TEVs can result in mechanical failure, thrombus, or stenosis on implantation. In planar tissue equivalents such as artificial skin, biaxial loading has been shown to improve matrix production and mechanical properties. To date however, multiaxial loading has not been examined as a means to improve mechanical and biochemical properties of TEVs during culture. Therefore, we developed a novel bioreactor that utilizes both circumferential and axial stretch that more closely simulates loading conditions in native arteries, and we examined the suture strength, matrix production, fiber orientation, and cell proliferation. After 3 months of biaxial loading, TEVs developed a formation of mature elastic fibers that consisted of elastin cores and microfibril sheaths. Furthermore, the distinctive features of collagen undulation and crimp in the biaxial TEVs were absent in both uniaxial and static TEVs. Relative to the uniaxially loaded TEVs, tissues that underwent biaxial loading remodeled and realigned collagen fibers toward a more physiologic, native-like organization. The biaxial TEVs also showed increased mechanical strength (suture retention load of 303 ± 14.53 g, with a wall thickness of 0.76 ± 0.028 mm) and increased compliance. The increase in compliance was due to combinatorial effects of mature elastic fibers, undulated collagen fibers, and collagen matrix orientation. In conclusion, biaxial stretching is a potential means to regenerate TEVs with improved matrix production, collagen organization, and mechanical properties.

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.

In Vitro Study of Directly Bioprinted Perfusable Vasculature Conduits

The ability to create three dimensional (3D) thick tissues is still a major tissue engineering challenge. It requires the development of a suitable vascular supply for an efficient media exchange. An integrated vasculature network is particularly needed when building thick functional tissues and/or organs with high metabolic activities, such as the heart, liver and pancreas. In this work, human umbilical vein smooth muscle cells (HUVSMCs) were encapsulated in sodium alginate and printed in the form of vasculature conduits using a coaxial deposition system. Detailed investigations were performed to understand the dehydration, swelling and degradation characteristics of printed conduits. In addition, because perfusional, permeable and mechanical properties are unique characteristics of natural blood vessels, for printed conduits these properties were also explored in this work. The results show that cells encapsulated in conduits had good proliferation activities and that their viability increased during prolonged in vitro culture. Deposition of smooth muscle matrix and collagen was observed around the peripheral and luminal surface in long-term cultured cellular vascular conduit through histology studies.

A Tubular Biomaterial Construct Exhibiting a Negative Poisson's Ratio

Developing functional small-diameter vascular grafts is an important objective in tissue engineering research. In this study, we address the problem of compliance mismatch by designing and developing a 3D tubular construct that has a negative Poisson's ratio νxy (NPR). NPR constructs have the unique ability to expand transversely when pulled axially, thereby resulting in a highly-compliant tubular construct. In this work, we used projection stereolithography to 3D-print a planar NPR sheet composed of photosensitive poly(ethylene) glycol diacrylate biomaterial. We used a step-lithography exposure and a stitch process to scale up the projection printing process, and used the cut-missing rib unit design to develop a centimeter-scale NPR sheet, which was rolled up to form a tubular construct. The constructs had Poisson's ratios of -0.6 ≤ νxy ≤ -0.1. The NPR construct also supports higher cellular adhesion than does the construct that has positive νxy. Our NPR design offers a significant advance in the development of highly-compliant vascular grafts.

Engineering of arteries in vitro

This review will focus on two elements that are essential for functional arterial regeneration in vitro: the mechanical environment and the bioreactors used for tissue growth. The importance of the mechanical environment to embryological development, vascular functionality, and vascular graft regeneration will be discussed. Bioreactors generate mechanical stimuli to simulate biomechanical environment of arterial system. This system has been used to reconstruct arterial grafts with appropriate mechanical strength for implantation by controlling the chemical and mechanical environments in which the grafts are grown. Bioreactors are powerful tools to study the effect of mechanical stimuli on extracellular matrix architecture and mechanical properties of engineered vessels. Hence, biomimetic systems enable us to optimize chemo-biomechanical culture conditions to regenerate engineered vessels with physiological properties similar to those of native arteries. In addition, this article reviews various bioreactors designed especially to apply axial loading to engineered arteries. This review will also introduce and examine different approaches and techniques that have been used to engineer biologically based vascular grafts, including collagen-based grafts, fibrin-gel grafts, cell sheet engineering, biodegradable polymers, and decellularization of native vessels.

Multiaxial mechanical properties and constitutive modeling of human adipose tissue: a basis for preoperative simulations in plastic and reconstructive surgery

A preoperative simulation of soft tissue deformations during plastic and reconstructive surgery is desirable to support the surgeon's planning and to improve surgical outcomes. The current development of constitutive adipose tissue models, for the implementation in multilayer computational frameworks for the simulation of human soft tissue deformations, has proved difficult because knowledge of the required mechanical parameters of fat tissue is limited. Therefore, for the first time, human abdominal adipose tissues were mechanically investigated by biaxial tensile and triaxial shear tests. The results of this study suggest that human abdominal adipose tissues under quasi-static and dynamic multiaxial loadings can be characterized as a nonlinear, anisotropic and viscoelastic soft biological material. The nonlinear and anisotropic features are consequences of the material's collagenous microstructure. The aligned collagenous septa observed in histological investigations causes the anisotropy of the tissue. A hyperelastic model used in this study was appropriate to represent the quasi-static multiaxial mechanical behavior of fat tissue. The constitutive parameters are intended to serve as a basis for soft tissue simulations using the finite element method, which is an apparent method for obtaining promising results in the field of plastic and reconstructive surgery.

Collagen-cellulose composite thin films that mimic soft-tissue and allow stem-cell orientation

Mechanical properties of collagen films are less than ideal for biomaterial development towards musculoskeletal repair or cardiovascular applications. Herein, we present a collagen-cellulose composite film (CCCF) compared against swine small intestine submucosa in regards to mechanical properties, cell growth, and histological analysis. CCCF was additionally characterized by FE-SEM, NMR, mass spectrometry, and Raman Microscopy to elucidate its physical structure, collagen-cellulose composition, and structure activity relationships. Mechanical properties of the CCCF were tested in both wet and dry environments, with anisotropic stress-strain curves that mimicked soft-tissue. Mesenchymal stem cells, human umbilical vein endothelial cells, and human coronary artery smooth muscle cells were able to proliferate on the collagen films with specific cell orientation. Mesenchymal stem cells had a higher proliferation index and were able to infiltrate CCCF to a higher degree than small intestine submucosa. With the underlying biological properties, we present a collagen-cellulose composite film towards forthcoming biomaterial-related applications.

Micropatterned cell sheets with defined cell and extracellular matrix orientation exhibit anisotropic mechanical properties

For an arterial replacement graft to be effective, it must possess the appropriate strength in order to withstand long-term hemodynamic stress without failure, yet be compliant enough that the mismatch between the stiffness of the graft and the native vessel wall is minimized. The native vessel wall is a structurally complex tissue characterized by circumferentially oriented collagen fibers/cells and lamellar elastin. Besides the biochemical composition, the functional properties of the wall, including stiffness, depend critically on the structural organization. Therefore, it will be crucial to develop methods of producing tissues with defined structures in order to more closely mimic the properties of a native vessel. To this end, we sought to generate cell sheets that have specific ECM/cell organization using micropatterned polydimethylsiloxane (PDMS) substrates to guide cell organization and tissue growth. The patterns consisted of large arrays of alternating grooves and ridges. Adult bovine aortic smooth muscle cells cultured on these substrates in the presence of ascorbic acid produced ECM-rich sheets several cell layers thick in which both the cells and ECM exhibited strong alignment in the direction of the micropattern. Moreover, mechanical testing revealed that the sheets exhibited mechanical anisotropy similar to that of native vessels with both the stiffness and strength being significantly larger in the direction of alignment, demonstrating that the microscale control of ECM organization results in functional changes in macroscale material behavior.

Determination of the layer-specific distributed collagen fibre orientations in human thoracic and abdominal aortas and common iliac arteries

The established method of polarized microscopy in combination with a universal stage is used to determine the layer-specific distributed collagen fibre orientations in 11 human non-atherosclerotic thoracic and abdominal aortas and common iliac arteries (63 ± 15.3 years, mean ± s.d.). A dispersion model is used to quantify over 37 000 recorded fibre angles from tissue samples. The study resulted in distinct fibre families, fibre directions, dispersion and thickness data for each layer and all vessels investigated. Two fibre families were present for the intima, media and adventitia in the aortas, with often a third and sometimes a fourth family in the intima in the respective axial and circumferential directions. In all aortas, the two families were almost symmetrically arranged with respect to the cylinder axis, closer to the axial direction in the adventitia, closer to the circumferential direction in the media and in between in the intima. The same trend was found for the intima and adventitia of the common iliac arteries; however, there was only one preferred fibre alignment present in the media. In all locations and layers, the observed fibre orientations were always in the tangential plane of the walls, with no radial components and very small dispersion through the wall thickness. A wider range of in-plane fibre orientations was present in the intima than in the media and adventitia. The mean total wall thickness for the aortas and the common iliac artery was 1.39 and 1.05 mm, respectively. For the aortas, a slight thickening of the intima and a thinning of the media in increasingly distal regions were observed. A clear intimal thickening was present distal to the branching of the celiac arteries. All data, except for the media of the common iliac arteries, showed two prominent collagen fibre families for all layers so that two-fibre family models seem most appropriate.

Remodeling of the collagen fiber architecture due to compaction in small vessels under tissue engineered conditions

Mechanical loading protocols in tissue engineering (TE) aim to improve the deposition of a properly organized collagen fiber network. In addition to collagen remodeling, these conditioning protocols can result in tissue compaction. Tissue compaction is beneficial to tissue collagen alignment, yet it may lead to a loss of functionality of the TE construct due to changes in geometry after culture. Here, a mathematical model is presented to relate the changes in collagen architecture to the local compaction within a TE small blood vessel, assuming that under static conditions, compaction is the main factor responsible for collagen fiber organization. An existing structurally based model is extended to incorporate volumetric tissue compaction. Subsequently, the model is applied to describe the collagen architecture of TE constructs under either strain based or stress based stimulus functions. Our computations indicate that stress based simulations result in a helical collagen fiber distribution along the vessel wall. The helix pitch angle increases from a circumferential direction in the inner wall, over about 45 deg in the middle vessel layer, to a longitudinal direction in the outer wall. These results are consistent with experimental data from TE small diameter blood vessels. In addition, our results suggest a stress dependent remodeling of the collagen, suggesting that cell traction is responsible for collagen orientation. These findings may be of value to design improved mechanical conditioning protocols to optimize the collagen architecture in engineered tissues.

3D constitutive modeling of the biaxial mechanical response of intact and layer-dissected human carotid arteries

Human arteries with non-atherosclerotic intimal thickening consist of three distinct layers: adventitia, media and intima. From a series of axial extension and inflation tests on intact and layer-dissected human carotid arteries (adventitia and media-intima composite), a 3D structurally-based strain-energy function (SEF) is calibrated, and a set of five material parameters is identified which is not yet available in the literature. The zero-stress states of the artery tubes investigated are considered in the calibration process, and the related kinematics for the finite deformation of the individual continuum are described in detail. The SEF employed is capable of describing the different mechanical properties of the intact and layer-dissected tissue tubes (arterial segments) investigated at different pressure domains and axial stretches. The correlation coefficients and error measures determined indicate good correlation between the model and the experimental data for all tested tubes. Mean values of each individual material parameter provide a kind of 'master model' that characterizes the mean response of all mechanical data obtained from the human carotid arteries. The material parameters and the 3D constitutive model serve as a foundation for finite element simulations, and hence the analysis of more complex patient-specific boundary-value problems in the human carotid physiology and pathology.

A structural constitutive model considering angular dispersion and waviness of collagen fibres of rabbit facial veins

Background

Structural constitutive models of vascular wall integrate information on composition and structural arrangements of tissue. In blood vessels, collagen fibres are arranged in coiled and wavy bundles and the individual collagen fibres have a deviation from their mean orientation. A complete structural constitutive model for vascular wall should incorporate both waviness and orientational distribution of fibres. We have previously developed a model, for passive properties of vascular wall, which considers the waviness of collagen fibres. However, to our knowledge there is no structural model of vascular wall which integrates both these features.

Methods

In this study, we have suggested a structural strain energy function that incorporates not only the waviness but also the angular dispersion of fibres. We studied the effect of parameters related to the orientational distribution on macro-mechanical behaviour of tissue during inflation-extension tests. The model was further applied on experimental data from rabbit facial veins.

Results

Our parametric study showed that the model is less sensitive to the orientational dispersion when fibres are mainly oriented circumferentially. The macro-mechanical response is less sensitive to changes in the mean orientation when fibres are more dispersed. The model accurately fitted the experimental data of veins, while not improving the quality of the fit compared to the model without dispersion. Our results showed that the orientational dispersion of collagen fibres could be compensated by a less abrupt and shifted to higher strain collagen engagement pattern. This should be considered when the model is fitted to experimental data and model parameters are used to study structural modifications of collagen fibre network in physiology and disease.

Conclusions

The presented model incorporates structural features related to waviness and orientational distribution of collagen fibres and thus offers possibilities to better understand the relation between structure and function in the vascular wall. Also, the model can be used to further study mechanically induced collagen remodelling in vascular tissue in health and disease.

Influence of medial collagen organization and axial in situ stretch on saccular cerebral aneurysm growth

A model for saccular cerebral aneurysm growth, proposed by Kroon and Holzapfel (2007, "A Model for Saccular Cerebral Aneurysm Growth in a Human Middle Cerebral Artery," J. Theor. Biol., 247, pp. 775-787; 2008, "Modeling of Saccular Aneurysm Growth in a Human Middle Cerebral Artery," ASME J. Biomech. Eng., 130, p. 051012), is further investigated. A human middle cerebral artery is modeled as a two-layer cylinder where the layers correspond to the media and the adventitia. The immediate loss of media in the location of the aneurysm is taken to be responsible for the initiation of the aneurysm growth. The aneurysm is regarded as a development of the adventitia, which is composed of several distinct layers of collagen fibers perfectly aligned in specified directions. The collagen fibers are the only load-bearing constituent in the aneurysm wall; their production and degradation depend on the stretch of the wall and are responsible for the aneurysm growth. The anisotropy of the surrounding media was modeled using the strain-energy function proposed by Holzapfel et al. (2000, "A New Constitutive Framework for Arterial Wall Mechanics and a Comparative Study of Material Models," J. Elast., 61, pp. 1-48), which is valid for an elastic material with two families of fibers. It was shown that the inclusion of fibers in the media reduced the maximum principal Cauchy stress and the maximum shear stress in the aneurysm wall. The thickness increase in the aneurysm wall due to material growth was also decreased. Varying the fiber angle in the media from a circumferential direction to a deviation of 10 deg from the circumferential direction did, however, only show a little effect. Altering the axial in situ stretch of the artery had a much larger effect in terms of the steady-state shape of the aneurysm and the resulting stresses in the aneurysm wall. The peak values of the maximum principal stress and the thickness increase both became significantly higher for larger axial stretches.

Effects of olmesartan and enalapril at low or high doses on cardiac, renal and vascular interstitial matrix in spontaneously hypertensive rats

We have evaluated the effects of different doses of an angiotensin-converting enzyme (ACE) inhibitor, enalapril (ENA) and of an angiotensin II type 1 receptor blocker olmesartan (OLM), on extracellular matrix of the heart, kidney, aorta and mesenteric artery of spontaneously hypertensive rats (SHR). Forty SHR and eight Wistar-Kyoto controls (WKY) were included in the study. Eight SHR were treated with high-dose OLM 15 mg/kg per day, eight with high-dose ENA 25 mg/kg per day, eight with low-dose OLM 1 mg/kg per day and eight with low-dose ENA (2 mg/kg per day). Eight SHR and eight WKY were kept untreated as controls. Treatment was from age 4 to 12 weeks. Systolic blood pressure (SBP) was measured non-invasively every week. The left ventricular weight to body weight (RLVM) was measured, and the cardiac, aortic and glomerular interstitial collagen content was evaluated using Sirius red staining and image analysis. Mesenteric small arteries were dissected and mounted on a micromyograph, and the media:lumen ratio (M/L) was calculated. Collagen subtypes were evaluated by polarized light microscopy. The SHR treated with high-dose OLM or ENA showed a normalization of SBP. The RLVM was significantly increased in untreated SHR compared with untreated WKY, whereas significantly lower values were observed in the groups of SHR treated with high-dose OLM or ENA. A significant increase in cardiac and glomerular collagen content was observed in untreated SHR. Both high- or low-dose OLM and ENA normalized collagen content in the heart and the kidney. Both high-dose OLM and high-dose ENA normalized M/L ratio; however, OLM proved to be more effective than ENA in normalizing collagen pattern. In fact, aortic collagen content was normalized by both high-dose and low-dose OLM, but only by high-dose ENA. In conclusion, both OLM and ENA were significantly and equally effective in the prevention of cardiac and renal damage in SHR, whereas OLM was more effective than ENA in terms of effects on vascular extracellular matrix.

A theoretical model for fibroblast-controlled growth of saccular cerebral aneurysms

A new theoretical model for the growth of saccular cerebral aneurysms is proposed by extending the recent constitutive framework of Kroon and Holzapfel [2007a. A model for saccular cerebral aneurysm growth by collagen fibre remodelling. J. Theor. Biol. 247, 775-787]. The continuous turnover of collagen is taken to be the driving mechanism in aneurysmal growth. The collagen production rate depends on the magnitude of the cyclic deformation of fibroblasts, caused by the pulsating blood pressure during the cardiac cycle. The volume density of fibroblasts in the aneurysmal tissue is taken to be constant throughout the growth process. The growth model is assessed by considering the inflation of an axisymmetric membranous piece of aneurysmal tissue, with material characteristics representative of a cerebral aneurysm. The diastolic and systolic states of the aneurysm are computed, together with its load-free state. It turns out that the value of collagen pre-stretch, that determines growth speed and stability of the aneurysm, is of pivotal importance. The model is able to predict aneurysms with typical berry-like shapes observed clinically, and the predicted wall stresses correlate well with the experimentally obtained ultimate stresses of this type of tissue. The model predicts that aneurysms should fail when reaching a size of about 1.2-3.6mm, which is smaller than what has been clinically observed. With some refinements, the model may, however, be used to predict future growth of diagnosed aneurysms.

Modeling of Saccular Aneurysm Growth in a Human Middle Cerebral Artery

Saccular aneurysm growth in a human middle cerebral artery is modeled. The aneurysm growth model was presented in a companion paper by Kroon and Holzapfel (“A Model for Saccular Cerebral Aneurysm Growth by Collagen Fibre Remodelling,” J. Theor. Biol., in press) and was assessed there for axisymmetric growth. The aneurysm growth model is now evaluated for a more realistic setting. The middle cerebral artery is modeled as a two-layered cylinder, where the layers correspond to the media and the adventitia. An instant loss of the media in a region of the artery wall initiates the growth of the saccular aneurysm. The aneurysm wall is assumed to be a development of the adventitia of the original healthy artery, and collagen is assumed to be the only load-bearing constituent in the adventitia and in the aneurysm wall. The collagen is organized in a number of distinct layers where fibers in a specific layer are perfectly aligned in a certain fiber direction. The production of new collagen is taken to depend on the stretching of the aneurysm wall, and the continuous remodeling of the collagen fibers is responsible for the aneurysm growth. The general behavior of the growth model is investigated and also the influence of the structural organization of the collagen fabric. The analysis underlines the fact that the material behavior of aneurysmal tissue cannot be expected to be isotropic. The model predictions agree well with clinical and experimental results, for example, in terms of aneurysm size and shape, wall stress levels, and wall thickness.

A thermoresponsive, microtextured substrate for cell sheet engineering with defined structural organization

The proper function of many tissues depends critically on the structural organization of the cells and matrix of which they are comprised. Therefore, in order to engineer functional tissue equivalents that closely mimic the unique properties of native tissues it is necessary to develop strategies for reproducing the complex, highly organized structure of these tissues. To this end, we sought to develop a simple method for generating cell sheets that have defined ECM/cell organization using microtextured, thermoresponsive polystyrene substrates to guide cell organization and tissue growth. The patterns consisted of large arrays of alternating grooves and ridges (50 microm wide, 5 microm deep). Vascular smooth muscle cells cultured on these substrates produced intact sheets consisting of cells that exhibited strong alignment in the direction of the micropattern. These sheets could be readily transferred from patterned substrates to non-patterned substrates without the loss of tissue organization. Ultimately, such sheets will be layered to form larger tissues with defined ECM/cell organization that spans multiple length scales.

An ultrastructural analysis of collagen in tissue engineered arteries

Collagen is the structural molecule that is most correlated with strength in blood vessels. In this study, we compared the properties of collagen in engineered and native blood vessels. Transmission electron microscopy (TEM) was used to image sections of engineered and native arteries. Band periodicities of engineered and native collagen fibrils indicated that spacing between collagen molecules was similar in engineered and native tissues. Engineered arteries, however, had thinner collagen fibrils and fibers than native arteries. Further, collagen fibrils were more loosely packed within collagen fibers in engineered arteries than in native arteries. The sensitivity of TEM analysis allowed measurement of the relative frequency of observation for alignment of collagen. These observations showed that collagen in both engineered and native arteries was aligned circumferentially, helically, and axially, but that engineered arteries had less circumferential collagen and more axial collagen than native arteries. Given that collagen is primarily responsible for dictating the ultimate mechanical properties of arterial tissue, future efforts should focus on using relative frequency of observation for alignment of collagen as a descriptive input for models of the mechanical properties of engineered or native tissues.

A model for saccular cerebral aneurysm growth by collagen fibre remodelling

The first structural model for saccular cerebral aneurysm growth is proposed. It is assumed that the development of the aneurysm is accompanied by a loss of the media, and that only collagen fibres provide load-bearing capacity to the aneurysm wall. The aneurysm is modelled as an axisymmetric multi-layered membrane, exposed to an inflation pressure. Each layer is characterized by an orientation angle, which changes between different layers. The collagen fibres and fibroblasts within a specific layer are perfectly aligned. The growth and the morphological changes of the aneurysm are accomplished by the turnover of collagen. Fibroblasts are responsible for collagen production, and the related deformations are assumed to govern the collagen production rate. There are four key parameters in the model: a normalized pressure, the number of layers in the wall, an exponent in the collagen mass production rate law, and the pre-stretch under which the collagen is deposited. The influence of the model parameters on the aneurysmal response is investigated, and a stability analysis is performed. The model is able to predict clinical observations and mechanical test results, for example, in terms of predicted aneurysm size, shape, wall stress and wall thickness.

Small-Diameter Artificial Arteries Engineered In Vitro

Although the need for a functional arterial replacement is clear, the lower blood flow velocities of small-diameter arteries like the coronary artery have led to the failure of synthetic materials that are successful for large-diameter grafts. Although autologous vessels remain the standard for small diameter grafts, many patients do not have a vessel suitable for use because of vascular disease, amputation, or previous harvest. As a result, tissue engineering has emerged as a promising approach to address the shortcomings of current therapies. Investigators have explored the use of arterial tissue cells or differentiated stem cells combined with various types of natural and synthetic scaffolds to make tubular constructs and subject them to chemical and/or mechanical stimulation in an attempt to develop a functional small-diameter arterial replacement graft with varying degrees of success. Here, we review the progress in all these major facets of the field.

Morphometry of Medial Gaps of Human Brain Artery Branches

Background and Purpose— The bifurcation regions of the major human cerebral arteries are vulnerable to the formation of saccular aneurysms. A consistent feature of these bifurcations is a discontinuity of the tunica media at the apex of the flow divider. The objective was to measure the 3-dimensional geometry of these medial gaps or “medial defects.”

Methods— Nineteen bifurcations and 2 junctions of human cerebral arteries branches (from 4 male and 2 female subjects) were formalin-fixed at physiological pressure and processed for longitudinal serial sectioning. The apex and adjacent regions were examined and measurements were made from high-magnification photomicrographs, or projection microscope images, of the gap dimensions at multiple levels through the bifurcation.

Results— Plots were made of the width of the media as a function of distance from the apex. The media at each edge of the medial gap widened over a short distance, reaching the full width of the media of the contiguous daughter vessel. Medial gap dimensions were compared with the planar angle of the bifurcation, and a strong negative correlation was found, ie, the acute angled branches have the more prominent medial gaps.

Conclusions— A discontinuity of the media at the apex was seen in all the bifurcations examined and was also found in the junction regions of brain arteries. We determined that the gap width is continuous with well-defined dimensions throughout its length and average length-to-width ratio of 6.9. The gaps were generally centered on the prominence of the apical ridge.

Engineered alignment in media equivalents: magnetic prealignment and mandrel compaction

We predicted and measured the evolution of smooth muscle cell (SMC) orientation in media-equivalents (MEs) for four fabrication conditions (F-, M-, F+, M+) under Free or Mandrel compaction (F/M) with and without magnetic prealignment of the collagen fibrils in the circumferential direction (+/-). Mandrel compaction refers to SMC-induced compaction of the ME that is constrained by having a nonadhesive mandrel placed in the ME lumen. Predictions were made using our anisotropic biphasic theory (ABT) for tissue-equivalent mechanics. Successful prediction of trends of the SMC orientation data for all four fabrication cases was obtained: maintenance of the initial isotropic state for F-, loss of initial circumferential alignment for F+, development of circumferential alignment for M-, and enhancement of initial circumferential alignment for M+. These results suggest two mechanisms by which the presence of the mandrel leads to much greater mechanical stiffness in the circumferential direction reported for mandrel compacted MEs relative to free compacted MEs: (1) by inducing an increasing circumferential alignment of the SMC and collagen, and (2) by inducing a large stress on the SMC, resulting in secretion and accumulation of stiffening components.

Collagen organization in the branching region of human brain arteries

BACKGROUND AND PURPOSE

Unruptured saccular aneurysms are relatively common, occurring in 4% to 9% of autopsies. Their development at the apex region of brain artery bifurcations is attributed to a combination of structural factors and the effect of blood pressure. Collagen is a primary tension-bearing fabric of the vessel wall, and our purpose was to examine its 3-dimensional alignment at arterial branches.

METHODS

Sixteen segments of arteries from the circle of Willis, including bifurcations, were pressure distended, fixed, and sectioned in 1 of 3 orthogonal planes. We measured the 3-dimensional organization of collagen at the flow divider by using the polarized light microscope. An electron microscopy study performed in tandem provided measurements on the collagen fibril diameters and packing density.

RESULTS

Orientation data of the collagen fabric were obtained from sections from 3 different cutting planes. The tunica media of all bifurcations had an alignment that was primarily circumferential, and the medial gap (medial defect) was distinguishable at the apex of all bifurcations. The subendothelial layer was thin at the apex but thicker and more disorganized distally. Adventitial collagen showed little organization except for a high degree of alignment along the apex. Results from the electron microscopy study showed densely packed collagen fibrils of uniform diameter at the apex, compared with slightly smaller and less densely packed fibrils nearby.

CONCLUSIONS

In the region of the medial gap, a narrow band of highly aligned tendonlike collagen running in the direction of the ridge of the flow divider was a consistent finding. This structure would provide strength and stability to the vessel and is inconsistent with the concept of an inherent defect in the structure of bifurcations.

The layered fabric of cerebral artery fenestrations

BACKGROUND AND PURPOSE

Intravascular bridges, resulting from developmental anomalies of brain arteries, are now better known as arterial fenestrations. Their tendency to develop aneurysms, similar to arterial bifurcations, makes their anatomy and microstructure important for study.

METHODS

Six segments of artery, each including a fenestration (five from the vertebrobasilar junction and one from the middle cerebral artery), were pressure distended, fixed, and sectioned. We made three-dimensional orientation measurements of smooth muscle and collagen, stained to enhance their birefringence, using the polarized light microscope.

RESULTS

The general contour of the fenestrations is streamlined with a thickened layered subendothelium at the trailing or distal edge, structurally similar to the region of convergence of major brain arteries. Defects of the medial layer were found at both proximal and distal edges of all the fenestrations. Results included regional mean orientations of individual layers, with circular SDs. The medial layer was found to be coherently aligned perpendicular to the direction of blood flow, with a mean circular SD of 12 degrees. The adventitia was less coherent (mean circular SD, 16 degrees) with the same average orientation, and the multilayered subendothelium had layers of obliquely oriented fibers with a wide range of coherence for individual fiber groups. Layers of the side regions were analogous to those in segments of brain artery and differed significantly from the proximal and distal edges of the fenestration structure.

CONCLUSIONS

The plasticity of form of the fenestrations at both the proximal and distal edges is in response to hemodynamic forces and is analogous to branching regions of brain arteries. Medial defects, a common feature in both brain arteries and fenestrations, may predispose the arterial fenestration to aneurysm formation.

Molecular and cellular concepts in atherosclerosis

Atherosclerosis is a complex disease of uncertain cause. Its pathobiology is believed to represent an abnormal expression of the processes of vascular healing. Etiologic models derive from a 'response to injury' paradigm and can be divided into three separate disease stages: endothelial dysfunction, smooth muscle proliferation and architectural disruption. The initiating event of endothelial dysfunction is unknown, but is believed to be related to low-density lipoproteins and/or their oxidized derivatives. Endothelial injury is signalled to the smooth muscle cells of the media by three routes: direct cell-cell interaction, secretion of soluble growth factors and monocyte-derived cytokines. Monocytes are recruited by the endothelium and invade the subintimal space by a complex interaction of a variety of adhesion proteins and receptors on both cell types. Smooth muscle cell proliferation is initiated by a change in phenotype expression from 'contractile' to 'synthetic' resulting from the binding of fibronectin to specific integrin receptors. Three functionally distinct activities may represent separate subtypes of the 'synthetic phenotype': migration from the media to the intima, increased proliferation and inappropriate extracellular matrix synthesis. The loss of normal regulatory control and anchorage independence of proliferation suggest a relationship to oncogenic transformation. Both migration and proliferation result from the binding of platelet-derived growth factor-like factors to smooth muscle cell receptors, which initiates a cascade of intracellular molecular events leading either to cytoskeletal locomotory restructuring or cell cycle activation. Both pathways also appear to be coregulated by integrin receptors and both depend upon phosphorylation of cell membrane, cytosolic and nuclear regulatory proteins. Clinical expression of atherosclerosis may follow sudden loss of architectural integrity of the intimal plaque by three different mechanisms: plaque fissuring, intraluminal plaque rupture or intramural hemorrhage related to abnormal vessel wall stress and/or biochemistry.

Layered collagen fabric of cerebral aneurysms quantitatively assessed by the universal stage and polarized light microscopy

We evaluated the effectiveness of the Universal stage, an instrument for measuring three-dimensional orientation of birefringent materials, for studying the collagen fabric in the wall of brain aneurysms. Vessels from autopsy were fixed at normal arterial distending pressure with 10% formalin, and prepared for polarized light microscopy, with paraffin embedding and staining with picrosirius red for birefringent enhancement. Quantitative data were obtained from tangential and oblique sections (7 microns thickness) of an intact 8 mm aneurysm, a 1.5 mm aneurysm, and a tangential section (3 microns thickness) of a cerebral artery. Sections of full-size aneurysms seen through the microscope, adjusted either for plane or circularly polarized light, revealed distinctive layers of collagen across the aneurysmal wall, which at higher magnification were further subdivided. Three-dimensional measurements, numbering 1,082, were made by use of the Universal stage attachment to the polarizing microscope. They were plotted by computer-controlled graphics on Lambert projections and analyzed by circular statistics. When assessed layer by layer, the collagen spanned a full range of orientations relative to the tangential plane. The circular standard deviation, a measure of the spread of alignment about the mean, was as low as 10 degrees for coherently organized collagen and as high as 40 degrees for the least coherently organized collagen, values characteristic of either the organized tunica media, or the least organized tunica adventitia of cerebral arteries. Although there was a marked thinning of the wall of one aneurysm, there was no evidence of structural weakness based only on the directional organization assessed by our measurements.

Fabric organization of the subendothelium of the human brain artery by polarized-light microscopy

The thickened subendothelium of brain arteries that is characteristic of atherosclerosis was assessed for the directional organization of the two main birefringent components, smooth muscle cells and collagen. Thirty-three arteries from 16 autopsy cases were pressure fixed at 30, 60, 110, and 200 mm Hg, sectioned at a thickness of 7 microns , and stained with silver impregnation to enhance tissue birefringence. The intended focus of the study was on muscle organization, but it also included the collagen among the cells because of the coalignment of the two tissues and their similar staining properties for polarized-light microscopy. The birefringent medial fabric at all pressures was circumferentially oriented, with a mean deviation of the 33 sections of 1.4 degrees from circumferential with an average circular standard deviation of 3.5 degrees, thereby showing remarkable coherence. In contrast, the subendothelium showed great variability both in thickness and in organization. Many arteries had no measurable subendothelium, and others had as much as 100%, with some atherosclerotic lesions as much as 300% of the medial width. Measurements from the subendothelium revealed a helical arrangement of tissue, often divided into separate regions, with a balance of left- and right-handed helical components and generally with lower pitch angles in the layers adjacent to the lumen. The average circular standard deviation within individual subendothelial layers was 14.5 degrees.