Extracellular matrix and the mechanics of large artery development

Multiscale insights into postnatal aortic development

Despite its vital importance for establishing proper cardiovascular function, the process through which the vasculature develops and matures postnatally remains poorly understood. From a clinical perspective, an ability to mechanistically model the developmental time course in arteries and veins, as well as to predict how various pathologies and therapeutic interventions alter the affected vessels, promises to improve treatment strategies and long-term clinical outcomes, particularly in pediatric patients suffering from congenital heart defects. In the present study, we conducted a multiscale investigation into the postnatal development of the murine thoracic aorta, examining key allometric relations as well as relationships between in vivo mechanical stresses, collagen and elastin expression, and the gradual accumulation of load-bearing constituents within the aortic wall. Our findings suggest that the production of fibrillar collagens in the developing aorta associates strongly with the ratio of circumferential stresses between systole and diastole, hence emphasizing the importance of a pulsatile mechanobiological stimulus. Moreover, rates of collagen turnover and elastic fiber compaction can be inferred directly by synthesizing transcriptional data and quantitative histological measurements of evolving collagen and elastin content. Consistent with previous studies, we also observed that wall shear stresses acting on the aorta are similar at birth and in maturity, supporting the hypothesis that at least some stress targets are established early in development and maintained thereafter, thus providing a possible homeostatic basis to guide future experiments and inform future predictive modeling.

Developmental origins of mechanical homeostasis in the aorta

BACKGROUND

Mechanical homeostasis promotes proper aortic structure and function. Pathological conditions may arise, in part, from compromised or lost homeostasis. There is thus a need to quantify the homeostatic state and when it emerges. Here we quantify changes in mechanical loading, geometry, structure, and function of the murine aorta from the late prenatal period into maturity.

RESULTS

Our data suggest that a homeostatic set-point is established by postnatal day P2 for the flow-induced shear stress experienced by endothelial cells; this value deviates from its set-point from P10 to P21 due to asynchronous changes in mechanical loading (flow, pressure) and geometry (radius, wall thickness), but is restored thereafter consistent with homeostasis. Smooth muscle contractility also decreases during this period of heightened matrix deposition but is also restored in maturity. The pressure-induced mechanical stress experienced by intramural cells initially remains low despite increasing blood pressure, and then increases while extracellular matrix accumulates.

CONCLUSIONS

These findings suggest that cell-level mechanical homeostasis emerges soon after birth to allow mechanosensitive cells to guide aortic development, with deposition of matrix after P2 increasingly stress shielding intramural cells. The associated tissue-level set-points that emerge for intramural stress can be used to assess and model the aorta that matures biomechanically by P56.

The mechanical characterization of blood vessels and their substitutes in the continuous quest for physiological-relevant performances. A critical review

During the last 50 years, novel biomaterials and tissue engineering techniques have been investigated to produce alternative vascular substitutes that recapitulate the unique elastic mechanical features of blood vessels. A large variation in mechanical characterization, including the test type, protocol, and data analysis, is present in literature which complicates the comparison among studies and prevents the blooming and the advancement of this field. In addition, a limited mechanical assessment of the substitute for the intended application is often provided. In this light, this review presents the mechanical environment of blood vessels, discusses their mechanical behavior responsible for the suited blood flow into the body (non-linearity, anisotropy, hysteresis, and compliance), and compares the mechanical properties reported in literature (obtained with compression, tensile, stress-relaxation, creep, dynamic mechanical analysis, burst pressure, and dynamic compliance tests). This perspective highlights that the mechanical properties extracted through conventional tests are not always suitable indicators of the mechanical performance during the working life of a vascular substitute. The available tests can be then strategically used at different stages of the substitute development, prioritizing the simplicity of the method at early stages, and the physiological pertinence at later stages, following as much as possible ISO standards in the field. A consistent mechanical characterization focused on the behavior to which they will be subdued during real life is one key and missing element in the quest for physiological-like mechanical performance of vascular substitutes.

Microfluidic and Organ-on-a-Chip Approaches to Investigate Cellular and Microenvironmental Contributions to Cardiovascular Function and Pathology

Over the past decade, advances in microfabrication and biomaterials have facilitated the development of microfluidic tissue and organ models to address challenges with conventional animal and cell culture systems. These systems have largely been developed for human disease modeling and preclinical drug development and have been increasingly used to understand cellular and molecular mechanisms, particularly in the cardiovascular system where the characteristic mechanics and architecture are difficult to recapitulate in traditional systems. Here, we review recent microfluidic approaches to model the cardiovascular system and novel insights provided by these systems. Key features of microfluidic approaches include the ability to pattern cells and extracellular matrix (ECM) at cellular length scales and the ability to use patient-derived cells. We focus the review on approaches that have leveraged these features to explore the relationship between genetic mutations and the microenvironment in cardiovascular disease progression. Additionally, we discuss limitations and benefits of the various approaches, and conclude by considering the role further advances in microfabrication technology and biochemistry techniques play in establishing microfluidic cardiovascular disease models as central tools for understanding biological mechanisms and for developing interventional strategies.

Novel Myh11 Dual Reporter Mouse Model Provides Definitive Labeling and Identification of Smooth Muscle Cells-Brief Report

OBJECTIVE

Myh11 encodes a myosin heavy chain protein that is specifically expressed in smooth muscle cells (SMCs) and is important for maintaining vascular wall stability. The goal of this study is to generate a Myh11 dual reporter mouse line for definitive visualization of MYH11+ SMCs in vivo. Approach and Results: We generated a Myh11 knock-in mouse model by inserting LoxP-nlacZ-4XpolyA-LoxP-H2B-GFP-polyA-FRT-Neo-FRT reporter cassette into the Myh11 gene locus. The nuclear (n) lacZ-4XpolyA cassette is flanked by 2 LoxP sites followed by H2B-GFP (histone 2B fused green fluorescent protein). Upon Cre-mediated recombination, nlacZ-stop cassette is removed thereby permitting nucleus localized H2B-GFP expression. Expression of the nuclear localized lacZ or H2B-GFP is under control of the endogenous Myh11 promoter. Nuclear lacZ was expressed specifically in SMCs at embryonic and adult stages. Following germline Cre-mediated deletion of nuclear lacZ, H2B-GFP was specifically expressed in the nuclei of SMCs. Comparison of nuclear lacZ expression with Wnt1Cre and Mef2cCre mediated-H2B-GFP expression revealed heterogenous origins of SMCs from neural crest and second heart field in the great arteries and coronary vessels adjacent to aortic root.

CONCLUSIONS

The Myh11 knock-in dual reporter mouse model offers an exceptional genetic tool to visualize and trace the origins of SMCs in mice.

Insights on the Pathogenesis of Aneurysm through the Study of Hereditary Aortopathies

Thoracic aortic aneurysms (TAA) are permanent and localized dilations of the aorta that predispose patients to a life-threatening risk of aortic dissection or rupture. The identification of pathogenic variants that cause hereditary forms of TAA has delineated fundamental molecular processes required to maintain aortic homeostasis. Vascular smooth muscle cells (VSMCs) elaborate and remodel the extracellular matrix (ECM) in response to mechanical and biochemical cues from their environment. Causal variants for hereditary forms of aneurysm compromise the function of gene products involved in the transmission or interpretation of these signals, initiating processes that eventually lead to degeneration and mechanical failure of the vessel. These include mutations that interfere with transduction of stimuli from the matrix to the actin-myosin cytoskeleton through integrins, and those that impair signaling pathways activated by transforming growth factor-β (TGF-β). In this review, we summarize the features of the healthy aortic wall, the major pathways involved in the modulation of VSMC phenotypes, and the basic molecular functions impaired by TAA-associated mutations. We also discuss how the heterogeneity and balance of adaptive and maladaptive responses to the initial genetic insult might contribute to disease.

Longitudinal histomechanical heterogeneity of the internal thoracic artery

The internal thoracic artery (ITA) is the principal choice for coronary artery bypass grafting (CABG) due to its mechanical compatibility, histological composition, anti-thrombogenic lumen, and single anastomotic junction. Originating at the subclavian artery, traversing the thoracic cavity, and terminating at the superior epigastric and musculophrenic bifurcation, bilateral ITAs follow a protracted circuitous pathway. The physiological hemodynamics, anatomical configuration, and perivascular changes that occur throughout this length influence the tissue's microstructure and gross mechanical properties. Since histomechanics play a major role in premature graft failure we used inflation-extension testing to quantify the regional material and biaxial mechanical properties at four distinct locations along the left (L) and right (R) ITA and fit the results to a structurally-motivated constitutive model. Our comparative analysis of 44 vessel segments revealed a significant increase in the amount of collagen but not smooth muscle and a significant decrease in elastin and elastic lamellae present with distance from the heart. A subsequent decrease in the total deformation energy and isotropic contribution to the strain energy was present in the LITA but not RITA. Circumferential stress and compliance generally decreased along the length of the LITA while axial stress increased in the RITA. When comparing RITAs to LITAs, some morphological and histological differences were found in proximal sections while distal sections revealed differences predominantly in compliance and axial stress. Overall, this information can be used to better guide graft selection, graft preparation, and xenograft-based tissue-engineering strategies for CABG.

Early impact of moderate preterm birth on the structure, function and gene expression of conduit arteries

NEW FINDINGS

What is the central question of this study? What is the immediate impact of moderate preterm birth on the structure and function of major conduit arteries using a pre-clinical sheep model? What is the main finding and its importance? Postnatal changes in conduit arteries, including a significant decrease in collagen within the thoracic aortic wall (predominately males), narrowed carotid arteries, reduced aortic systolic blood flow, and upregulation of the mRNA expression of cell adhesion and inflammatory markers at 2 days of age in preterm lambs compared to controls, may increase the risk of cardiovascular impairment in later life.

ABSTRACT

The aim of this work was to compare the structure and function of the conduit arteries of moderately preterm and term-born lambs and to determine whether vascular injury-associated genes were upregulated. Time-mated ewes were induced to deliver either preterm (132 ± 1 days of gestation; n = 11 females and n = 10 males) or at term (147 ± 1 days of gestation; n = 10 females and n = 5 males). Two days after birth, ultrasound imaging of the proximal ascending aorta, main, right and left pulmonary arteries, and right and left common carotid arteries was conducted in anaesthetized lambs. Lambs were then killed and segments of the thoracic aorta and left common carotid artery were either snap frozen for real-time PCR analyses or immersion-fixed for histological quantification of collagen, smooth muscle and elastin within the medial layer. Overall there were few differences in vascular structure between moderately preterm and term lambs. However, there was a significant decrease in the proportion of collagen within the thoracic aortic wall (predominantly in males), narrowing of the common carotid arteries and a reduction in peak aortic systolic blood flow in preterm lambs. In addition, there was increased mRNA expression of the cell adhesion marker P-selectin in the thoracic aortic wall and the pro-inflammatory marker IL-1β in the left common carotid artery in preterm lambs, suggestive of postnatal vascular injury. Early postnatal differences in the function and structure of conduit arteries and evidence of vascular injury in moderately preterm offspring may place them at greater risk of cardiovascular impairment later in life.

Captopril treatment during development alleviates mechanically induced aortic remodeling in newborn elastin knockout mice

Deposition of elastin and collagen in the aorta correlates with increases in blood pressure and flow during development, suggesting that the aorta adjusts its mechanical properties in response to hemodynamic stresses. Elastin knockout (Eln^-/-) mice have high blood pressure and pathological remodeling of the aorta and die soon after birth. We hypothesized that decreasing blood pressure in Eln^-/- mice during development may reduce hemodynamic stresses and alleviate pathological remodeling of the aorta. We treated Eln^+/+ and Eln^-/- mice with the anti-hypertensive medication captopril throughout embryonic development and then evaluated left ventricular (LV) pressure and aortic remodeling at birth. We found that captopril treatment decreased Eln^-/- LV pressure to values near Eln^+/+ mice and alleviated the wall thickening and changes in mechanical behavior observed in untreated Eln^-/- aorta. The changes in thickness and mechanical behavior in captopril-treated Eln^-/- aorta were not due to alterations in measured elastin or collagen amounts, but may have been caused by alterations in smooth muscle cell (SMC) properties. We used a constitutive model to understand how changes in stress contributions of each wall component could explain the observed changes in composite mechanical behavior. Our modeling results show that alterations in the collagen natural configuration and SMC properties in the absence of elastin may explain untreated Eln^-/- aortic behavior and that partial rescue of the SMC properties may account for captopril-treated Eln^-/- aortic behavior.

Potential injurious effects of the fine particulate PM2.5 on the progression of atherosclerosis in apoE-deficient mice by activating platelets and leukocytes

INTRODUCTION

Exposure to the fine particulate matter PM2.5 is strongly associated with atherosclerotic diseases, creating considerable public concern. Nevertheless, the mechanisms have not been fully elucidated. We exposed atherosclerosis-prone apoE-deficient mice to PM2.5 to begin investigating these mechanisms.

MATERIAL AND METHODS

Thirty-two 8-week-old male apoE^-/- mice were divided to two groups fed with high-fat diet: a control group instilled with 0.9% saline, and an experimental group instilled with PM2.5 (30 mg/kg/day) for 8 weeks. We measured PM2.5 in whole blood by the ICP-MS method, and lipids and inflammatory factors by standard methods. The whole descending arteries were stained with oil red O; Aortic roots were stained with Movat, Sirius Red and immunohistochemical stains for pathological analysis; Brachiocephalic arteries for scanning electron microscopy, the descending arteries for Q-PCR. Echocardiography was used to evaluate cardiac function.

RESULTS

In PM2.5 group, we observed elevated heavy metal components, consistent with higher amounts of platelets in total blood. The PM2.5 group also had elevated serum inflammatory factor levels. Finally, the PM2.5 group showed larger atherosclerotic plaques (p = 0.0231), higher numbers of lesion macrophages (p = 0.0183), greater injury to endothelial layers with greater adherence of platelets and leukocytes, elevated inflammatory factor levels, the NAD(P)H oxidase subunits p22phox and p47phox (p = 0.0079 and p = 0.0294), the M1/M2 associated markers IL-6, TNF-α (p = 0.0291, p = 0.0286), iNOS, IL-12 (p = 0.0122 and p = 0.0280) and arginase-1, and CD206 (p = 0.0216 and p = 0.0317).

CONCLUSIONS

PM2.5 exposure activated circulating leukocytes, platelets and associated inflammatory factors, contributing to the progression of atherosclerosis in apoE^-/- mice.

MicroCT imaging reveals differential 3D micro-scale remodelling of the murine aorta in ageing and Marfan syndrome

Aortic wall remodelling is a key feature of both ageing and genetic connective tissue diseases, which are associated with vasculopathies such as Marfan syndrome (MFS). Although the aorta is a 3D structure, little attention has been paid to volumetric assessment, primarily due to the limitations of conventional imaging techniques. Phase-contrast microCT is an emerging imaging technique, which is able to resolve the 3D micro-scale structure of large samples without the need for staining or sectioning.

Methods: Here, we have used synchrotron-based phase-contrast microCT to image aortae of wild type (WT) and MFS Fbn1^C1039G/+ mice aged 3, 6 and 9 months old (n=5). We have also developed a new computational approach to automatically measure key histological parameters.

Results: This analysis revealed that WT mice undergo age-dependent aortic remodelling characterised by increases in ascending aorta diameter, tunica media thickness and cross-sectional area. The MFS aortic wall was subject to comparable remodelling, but the magnitudes of the changes were significantly exacerbated, particularly in 9 month-old MFS mice with ascending aorta wall dilations. Moreover, this morphological remodelling in MFS aorta included internal elastic lamina surface breaks that extended throughout the MFS ascending aorta and were already evident in animals who had not yet developed aneurysms.

Conclusions: Our 3D microCT study of the sub-micron wall structure of whole, intact aorta reveals that histological remodelling of the tunica media in MFS could be viewed as an accelerated ageing process, and that phase-contrast microCT combined with computational image analysis allows the visualisation and quantification of 3D morphological remodelling in large volumes of unstained vascular tissues.

Comparative gene array analyses of severe elastic fiber defects in late embryonic and newborn mouse aorta

Elastic fibers provide reversible elasticity to the large arteries and are assembled during development when hemodynamic forces are increasing. Mutations in elastic fiber genes are associated with cardiovascular disease. Mice lacking expression of the elastic fiber genes elastin ( Eln-/-), fibulin-4 ( Efemp2-/-), or lysyl oxidase ( Lox-/-) die at birth with severe cardiovascular malformations. All three genetic knockout models have elastic fiber defects, aortic wall thickening, and arterial tortuosity. However, Eln-/- mice develop arterial stenoses, while Efemp2-/- and Lox-/- mice develop ascending aortic aneurysms. We performed comparative gene array analyses of these three genetic models for two vascular locations and developmental stages to determine differentially expressed genes and pathways that may explain the common and divergent phenotypes. We first examined arterial morphology and wall structure in newborn mice to confirm that the lack of elastin, fibulin-4, or lysyl oxidase expression provided the expected phenotypes. We then compared gene expression levels for each genetic model by three-way ANOVA for genotype, vascular location, and developmental stage. We found three genes upregulated by genotype in all three models, Col8a1, Igfbp2, and Thbs1, indicative of a common response to severe elastic fiber defects in developing mouse aorta. Genes that are differentially regulated by vascular location or developmental stage in all three models suggest mechanisms for location or stage-specific disease pathology. Comparison of signaling pathways enriched in all three models shows upregulation of integrins and matrix proteins involved in early wound healing, but not of mature matrix molecules such as elastic fiber proteins or fibrillar collagens.

Arterial smooth muscle dynamics in development and repair

Arterial vasculature distributes blood from early embryonic development and provides a nutrient highway to maintain tissue viability. Atherosclerosis, peripheral artery diseases, stroke and aortic aneurysm represent the most frequent causes of death and are all directly related to abnormalities in the function of arteries. Vascular intervention techniques have been established for the treatment of all of these pathologies, yet arterial surgery can itself lead to biological changes in which uncontrolled arterial wall cell proliferation leads to restricted blood flow. In this review we describe the intricate cellular composition of arteries, demonstrating how a variety of distinct cell types in the vascular walls regulate the function of arteries. We provide an overview of the developmental origin of arteries and perivascular cells and focus on cellular dynamics in arterial repair. We summarize the current knowledge of the molecular signaling pathways that regulate vascular smooth muscle differentiation in the embryo and in arterial injury response. Our review aims to highlight the similarities as well as differences between cellular and molecular mechanisms that control arterial development and repair.

Vascular Smooth Muscle Cells and Arterial Stiffening: Relevance in Development, Aging, and Disease

The cushioning function of large arteries encompasses distension during systole and recoil during diastole which transforms pulsatile flow into a steady flow in the microcirculation. Arterial stiffness, the inverse of distensibility, has been implicated in various etiologies of chronic common and monogenic cardiovascular diseases and is a major cause of morbidity and mortality globally. The first components that contribute to arterial stiffening are extracellular matrix (ECM) proteins that support the mechanical load, while the second important components are vascular smooth muscle cells (VSMCs), which not only regulate actomyosin interactions for contraction but mediate also mechanotransduction in cell-ECM homeostasis. Eventually, VSMC plasticity and signaling in both conductance and resistance arteries are highly relevant to the physiology of normal and early vascular aging. This review summarizes current concepts of central pressure and tensile pulsatile circumferential stress as key mechanical determinants of arterial wall remodeling, cell-ECM interactions depending mainly on the architecture of cytoskeletal proteins and focal adhesion, the large/small arteries cross-talk that gives rise to target organ damage, and inflammatory pathways leading to calcification or atherosclerosis. We further speculate on the contribution of cellular stiffness along the arterial tree to vascular wall stiffness. In addition, this review provides the latest advances in the identification of gene variants affecting arterial stiffening. Now that important hemodynamic and molecular mechanisms of arterial stiffness have been elucidated, and the complex interplay between ECM, cells, and sensors identified, further research should study their potential to halt or to reverse the development of arterial stiffness.

Impact of early- and late-onset preeclampsia on features of placental and newborn vascular health

INTRODUCTION

Offspring exposed to preeclampsia (PE) show an increased risk of cardiovascular disease in adulthood. We hypothesize that this is mediated by a disturbed vascular development of the placenta, umbilical cord and fetus. Therefore, we investigated associations between early-onset PE (EOPE), late-onset PE (LOPE) and features of placental and newborn vascular health.

METHODS

We performed a nested case-control study in The Rotterdam Periconceptional Cohort, including 30 PE pregnancies (15 EOPE, 15 LOPE) and 218 control pregnancies (164 uncomplicated controls, 54 complicated controls including 28 fetal growth restriction, 26 preterm birth) and assessed macroscopic and histomorphometric outcomes of the placenta and umbilical cord.

RESULTS

A significant association was observed between PE and a smaller umbilical vein area and wall thickness, independent of gestational age and birth weight. In EOPE we observed significant associations with a lower weight, length and width of the placenta, length of the umbilical cord, and thickness and wall area of the umbilical vein and artery. These associations attenuated after gestational age and birth weight adjustment. In LOPE a significant association with a larger placental width and smaller umbilical vein wall thickness was shown, independent of gestational age and birth weight.

DISCUSSION

Our study suggests that PE is associated with a smaller umbilical cord vein area and wall thickness, independent of gestational age and birth weight, which may serve as a proxy of disturbed cardiovascular development in the newborn. Follow-up studies are needed to ultimately predict and lower the risk of cardiovascular disease in offspring exposed to PE.

Electrospun nanofibrous sheets of collagen/elastin/polycaprolactone improve cardiac repair after myocardial infarction

Electrospun nanofibrous sheets get increasing attention in myocardial infarction (MI) treatment due to their good cytocompatibility to deliver transplanted stem cells to infarcted areas and due to mechanical characteristics to support damaged tissue. Cardiac extracellular matrix is essential for implanted cells since it provides the cardiac microenvironment. In this study, we hypothesized high concentrations of cardiac nature protein (NP), namely elastin and collagen, in hybrid polycaprolactone (PCL) electrospun nanofibrous sheets could be effective as cardiac-mimicking patch. Optimal ratio of elastin and collagen with PCL in electrospun sheets (80% NP/PCL) was selected based on cytocompatibility and mechanical characteristics. Bone-marrow (BM) c-kit(+) cells anchoring onto NP/PCL sheets exhibited increased proliferative capacity compared with those seeded on PCL in vitro. Moreover, we examined the improvement of cardiac function in MI mice by cell-seeded cardiac patch. Green Fluorescent Protein (GFP)-labeled BM c-kit(+) cells were loaded on 80% NP/PCL sheets which was transplanted into MI mice. Both 80% NP/PCL and c-kit(+)-seeded 80% NP/PCL effectively improved cardiac function after 4 weeks of transplantation, with reduced infarction area and restricted LV remodeling. C-kit(+)-seeded 80% NP/PCL was even superior to the 80% NP/PCL alone and both superior to PCL. GFP(+) cells were identified both in the sheets and local infarcted area where transplanted cells underwent cardiac differentiation after 4 weeks. To the best of our knowledge, this is the first report that sheets with high concentrations of nature proteins loaded with BM c-kit(+) cells might be a novel promising candidate for tissue-engineered cardiac patch to improve cardiac repair after MI.

Vascular Mechanics in Decellularized Aortas and Coronary Resistance Microvessels in Type 2 Diabetic db/db Mice

We previously reported differences in stiffness between macro- and micro-vessels in type 2 diabetes (T2DM). The aim of this study was to define the mechanical properties of the ECM independent of vascular cells in coronary resistance micro-vessels (CRMs) and macro-vessels (aorta) in control Db/db and T2DM db/db mice. Passive vascular remodeling and mechanics were measured in both intact and decellularized CRMs and aortas from 0 to 125 mmHg. We observed no differences in intact control and diabetic aortic diameters, wall thicknesses, or stiffnesses (p > 0.05). Aortic decellularization caused a significant increase in internal and external diameters and incremental modulus over a range of pressures that occurred to a similar degree in T2DM. Differences in aortic diameters due to decellularization occurred at lower pressures (0-75 mmHg) and converged with intact aortas at higher, physiological pressures (100-125 mmHg). In contrast, CRM decellularization caused increased internal diameter and incremental modulus only in the db/db mice, but unlike the aorta, the intact and decellularized CRM curves were more parallel. These data suggest that (1) micro-vessels may be more sensitive to early adverse consequences of diabetes than macro-vessels and (2) the ECM is a structural limit in aortas, but not CRMs.

Investigating developmental cardiovascular biomechanics and the origins of congenital heart defects

Innovative research on the interactions between biomechanical load and cardiovascular (CV) morphogenesis by multiple investigators over the past 3 decades, including the application of bioengineering approaches, has shown that the embryonic heart adapts both structure and function in order to maintain cardiac output to the rapidly growing embryo. Acute adaptive hemodynamic mechanisms in the embryo include the redistribution of blood flow within the heart, dynamic adjustments in heart rate and developed pressure, and beat to beat variations in blood flow and vascular resistance. These biomechanically relevant events occur coincident with adaptive changes in gene expression and trigger adaptive mechanisms that include alterations in myocardial cell growth and death, regional and global changes in myocardial architecture, and alterations in central vascular morphogenesis and remodeling. These adaptive mechanisms allow the embryo to survive these biomechanical stresses (environmental, maternal) and to compensate for developmental errors (genetic). Recent work from numerous laboratories shows that a subset of these adaptive mechanisms is present in every developing multicellular organism with a “heart” equivalent structure. This chapter will provide the reader with an overview of some of the approaches used to quantify embryonic CV functional maturation and performance, provide several illustrations of experimental interventions that explore the role of biomechanics in the regulation of CV morphogenesis including the role of computational modeling, and identify several critical areas for future investigation as available experimental models and methods expand.

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.

Differential mechanical response and microstructural organization between non-human primate femoral and carotid arteries

Unique anatomic locations and physiologic functions predispose different arteries to varying mechanical responses and pathologies. However, the underlying causes of these mechanical differences are not well understood. The objective of this study was to first identify structural differences in the arterial matrix that would account for the mechanical differences between healthy femoral and carotid arteries and second to utilize these structural observations to perform a microstructurally motivated constitutive analysis. Femoral and carotid arteries were subjected to cylindrical biaxial loading and their microstructure was quantified using two-photon microscopy. The femoral arteries were found to be less compliant than the carotid arteries at physiologic loads, consistent with previous studies, despite similar extracellular compositions of collagen and elastin ([Formula: see text]). The femoral arteries exhibited significantly less circumferential dispersion of collagen fibers ([Formula: see text]), despite a similar mean fiber alignment direction as the carotid arteries. Elastin transmural distribution, in vivo axial stretch, and opening angles were also found to be distinctly different between the arteries. Lastly, we modeled the arteries' mechanical behaviors using a microstructural-based, distributed collagen fiber constitutive model. With this approach, the material parameters of the model were solved using the experimental microstructural observations. The findings of this study support an important role for microstructural organization in arterial stiffness.

Extracellular matrix synthesis in vascular disease: hypertension, and atherosclerosis

Extracellular matrix (ECM) within the vascular network provides both a structural and regulatory role. The ECM is a dynamic composite of multiple proteins that form structures connecting cells within the network. Blood vessels are distended by blood pressure and, therefore, require ECM components with elasticity yet with enough tensile strength to resist rupture. The ECM is involved in conducting mechanical signals to cells. Most importantly, ECM regulates cellular function through chemical signaling by controlling activation and bioavailability of the growth factors. Cells respond to ECM by remodeling their microenvironment which becomes dysregulated in vascular diseases such hypertension, restenosis and atherosclerosis. This review examines the cellular and ECM components of vessels, with specific emphasis on the regulation of collagen type I and implications in vascular disease.

A finite element study on variations in mass transport in stented porcine coronary arteries based on location in the coronary arterial tree

Drug-eluting stents have a significant clinical advantage in late-stage restenosis due to the antiproliferative drug release. Understanding how drug transport occurs between coronary arterial locations can better help guide localized drug treatment options. Finite element models with properties from specific porcine coronary artery sections (left anterior descending (LAD), right (RCA); proximal, middle, distal regions) were created for stent deployment and drug delivery simulations. Stress, strain, pore fluid velocity, and drug concentrations were exported at different time points of simulation (0-180 days). Tests indicated that the highest stresses occurred in LAD sections. Higher-than-resting homeostatic levels of stress and strain existed at upwards of 3.0 mm away from the stented region, whereas concentration of species only reached 2.7 mm away from the stented region. Region-specific concentration showed 2.2 times higher concentrations in RCA artery sections at times corresponding to vascular remodeling (peak in the middle segment) compared to all other segments. These results suggest that wall transport can occur differently based on coronary artery location. Awareness of peak growth stimulators and where drug accumulation occurs in the vasculature can better help guide local drug delivery therapies.

Elastogenesis at the onset of human cardiac valve development

Semilunar valve leaflets have a well-described trilaminar histoarchitecture, with a sophisticated elastic fiber network. It was previously proposed that elastin-containing fibers play a subordinate role in early human cardiac valve development; however, this assumption was based on data obtained from mouse models and human second and third trimester tissues. Here, we systematically analyzed tissues from human fetal first (4-12 weeks) and second (13-18 weeks) trimester, adolescent (14-19 years) and adult (50-55 years) hearts to monitor the temporal and spatial distribution of elastic fibers, focusing on semilunar valves. Global expression analyses revealed that the transcription of genes essential for elastic fiber formation starts early within the first trimester. These data were confirmed by quantitative PCR and immunohistochemistry employing antibodies that recognize fibronectin, fibrillin 1, 2 and 3, EMILIN1 and fibulin 4 and 5, which were all expressed at the onset of cardiac cushion formation (~week 4 of development). Tropoelastin/elastin protein expression was first detectable in leaflets of 7-week hearts. We revealed that immature elastic fibers are organized in early human cardiovascular development and that mature elastin-containing fibers first evolve in semilunar valves when blood pressure and heartbeat accelerate. Our findings provide a conceptual framework with the potential to offer novel insights into human cardiac valve development and disease.