Sprouting from arteriovenous shunt vessels with increased blood flow

THE DESIGN OF THE ARTERIOVENOUS VASCULAR LOOP DOES NOT AFFECT ITS PATENCY: EXPERIMENTAL STUDY

OBJECTIVE

To evaluate the effect of the design of a femoral vascular loop with anastomosis in the femoral artery of rabbits on the presence of flow (patency) after seven days.

METHODS

A total of 39 rabbits underwent arteriovenous microanastomosis using the microsurgical technique. Two loop designs were used: one circular and the other angled. The parameters evaluated were presence or absence of flow, signs of hemolysis and hemodynamic changes.

RESULTS

After seven days, flow was present in 68% of the angled loops and 75% of the circular loops (p > 0.05). There was a significant intragroup decrease in pCO₂ and a significant increase in pH. For the other parameters evaluated, no significant differences between the two loop models were found.

CONCLUSIONS

A reproducible vascular loop model was shown. There was no significant difference between the two vascular loop models about the presence of flow after seven days. Level of Evidence V, Animal experimental study.

Material-Induced Venosome-Supported Bone Tubes

The development of alternatives to vascular bone grafts, the current clinical standard for the surgical repair of large segmental bone defects still today represents an unmet medical need. The subcutaneous formation of transplantable bone has been successfully achieved in scaffolds axially perfused by an arteriovenous loop (AVL) and seeded with bone marrow stromal cells or loaded with inductive proteins. Although demonstrating clinical potential, AVL-based approaches involve complex microsurgical techniques and thus are not in widespread use. In this study, 3D-printed microporous bioceramics, loaded with autologous total bone marrow obtained by needle aspiration, are placed around and next to an unoperated femoral vein for 8 weeks to assess the effect of a central flow-through vein on bone formation from marrow in a subcutaneous site. A greater volume of new bone tissue is observed in scaffolds perfused by a central vein compared with the nonperfused negative control. These analyses are confirmed and supplemented by calcified and decalcified histology. This is highly significant as it indicates that transplantable vascularized bone can be grown using dispensable vein and marrow tissue only. This is the first report illustrating the capacity of an intrinsic vascularization by a single vein to support ectopic bone formation from untreated marrow.

Angiogenesis and tissue formation driven by an arteriovenous loop in the mouse

The rapid vascularisation of biomaterials and artificial tissues is a key determinant for their in vivo viability and ultimately for their integration in a host; therefore promoting angiogenesis and maintaining the newly formed vascular beds has become a major goal of tissue engineering. The arteriovenous loop (AVL) has been an extensively studied platform which integrates microsurgery with cells scaffolds and growth factors to form neotissues. Most AVL studies to date are limited to larger animal models, which are surgically easier to perform, but have inherent limits for the understanding and interrogation of the underlying in vivo mechanisms due the paucity of transgenic models. Here, we demonstrate for the first time in a mouse model the utility of the AVL in the de novo production of vascularized tissue. We also present the combined use of the model with 3D printed chambers, which allow us to dictate size and shape of the tissues formed. This novel platform will allow for an understanding of the fundamental mechanisms involved in tissue generation de novo.

MicroRNA-regulated pathways of flow-stimulated angiogenesis and vascular remodeling in vivo

BACKGROUND

Vascular shear stress promotes endothelial cell sprouting in vitro. The impact of hemodynamic forces on microRNA (miRNA) and gene expression within growing vascular networks in vivo, however, remain poorly investigated. Arteriovenous (AV) shunts are an established model for induction of neoangiogenesis in vivo and can serve as a tool for analysis of hemodynamic effects on miRNA and gene expression profiles over time.

METHODS

AV shunts were microsurgically created in rats and explanted on postoperative days 5, 10 and 15. Neoangiogenesis was confirmed by histologic analysis and micro-computed tomography. MiRNA and gene expression profiles were determined in tissue specimens from AV shunts by microarray analysis and quantitative real-time polymerase chain reaction and compared with sham-operated veins by bioinformatics analysis. Changes in protein expression within AV shunt endothelial cells were determined by immunohistochemistry.

RESULTS

Samples from AV shunts exhibited a strong overexpression of proangiogenic cytokines, oxygenation-associated genes (HIF1A, HMOX1), and angiopoetic growth factors. Significant inverse correlations of the expressions of miR-223-3p, miR-130b-3p, miR-19b-3p, miR-449a-5p, and miR-511-3p which were up-regulated in AV shunts, and miR-27b-3p, miR-10b-5p, let-7b-5p, and let-7c-5p, which were down-regulated in AV shunts, with their predicted interacting targets C-X-C chemokine receptor 2 (CXCR2), interleukin-1 alpha (IL1A), ephrin receptor kinase 2 (EPHA2), synaptojanin-2 binding protein (SYNJ2BP), forkhead box C1 (FOXC1) were present. CXCL2 and IL1A overexpression in AV shunt endothelium was confirmed at the protein level by immunohistochemistry.

CONCLUSIONS

Our data indicate that flow-stimulated angiogenesis is determined by an upregulation of cytokines, oxygenation associated genes and miRNA-dependent regulation of FOXC1, EPHA2 and SYNJ2BP.

Endothelial Mechanotransduction, Redox Signaling and the Regulation of Vascular Inflammatory Pathways

The endothelium that lines the interior of blood vessels is directly exposed to blood flow. The shear stress arising from blood flow is “sensed” by the endothelium and is “transduced” into biochemical signals that eventually control vascular tone and homeostasis. Sensing and transduction of physical forces occur via signaling processes whereby the forces associated with blood flow are “sensed” by a mechanotransduction machinery comprising of several endothelial cell elements. Endothelial “sensing” involves converting the physical cues into cellular signaling events such as altered membrane potential and activation of kinases, which are “transmission” signals that cause oxidant production. Oxidants produced are the “transducers” of the mechanical signals? What is the function of these oxidants/redox signals? Extensive data from various studies indicate that redox signals initiate inflammation signaling pathways which in turn can compromise vascular health. Thus, inflammation, a major response to infection or endotoxins, can also be initiated by the endothelium in response to various flow patterns ranging from aberrant flow to alteration of flow such as cessation or sudden increase in blood flow. Indeed, our work has shown that endothelial mechanotransduction signaling pathways participate in generation of redox signals that affect the oxidant and inflammation status of cells. Our goal in this review article is to summarize the endothelial mechanotransduction pathways that are activated with stop of blood flow and with aberrant flow patterns; in doing so we focus on the complex link between mechanical forces and inflammation on the endothelium. Since this “inflammation susceptible” phenotype is emerging as a trigger for pathologies ranging from atherosclerosis to rejection post-organ transplant, an understanding of the endothelial machinery that triggers these processes is very crucial and timely.

Flow-Induced Axial Vascularization: The Arteriovenous Loop in Angiogenesis and Tissue Engineering

Fabrication of a viable vascular network providing oxygen supply is identified as one crucial limiting factor to generate more complex three-dimensional constructs. The arteriovenous loop model provides initial blood supply and has a high angioinductive potency, making it suitable for vascularization of larger, tissue-engineered constructs. Also because of its angiogenic capabilities the arteriovenous loop is recently also used as a model to evaluate angiogenesis in vivo. This review summarizes the history of the arteriovenous loop model in research and its technical and surgical aspects. Through modifications of the isolation chamber and its containing matrices, tissue generation can be enhanced. In addition, matrices can be used as release systems for local application of growth factors, such as vascular endothelial growth factor and basic fibroblast growth factor, to affect vascular network formation. A special focus in this review is set on the assessment of angiogenesis in the arteriovenous loop model. This model provides good conditions for assessment of angiogenesis with the initial cell-free environment of the isolation chamber, which is vascularized by the arteriovenous loop. Because of the angiogenic capabilities of the arteriovenous loop model, different attempts were performed to create functional tissue in the isolation chamber for potential clinical application. Arteriovenous loops in combination with autologous bone marrow aspirate were already used to reconstruct large bone defects in humans.

Numerical modeling of vessel geometry to measure hemodynamics parameters non-invasively in cerebral arteriovenous malformation

Cerebral arteriovenous malformations (CAVM) are congenital lesions that contain a cluster of multiple arteriovenous shunts (NIDUS). Cardiac arrhythmia in CAVM patients causes irregular changes in blood flow and pressure in the NIDUS area. This paper proposes the framework for creating the lumped model of tortuous vessel structure near NIDUS based on radiological images. This lumped model is to analyze flow variations, with various combinations of the transient electrical signals, which simulate similar conditions of cardiac arrhythmia in CAVM patients. This results in flow variation at different nodes of the lumped model. Here we present two AVM patients with evaluation of 150 vessels locations as node points, with an accuracy of 93%, the sensitivity of 95%, and specificity of 94%. The calculated p-value is smaller than the significance level of 0.05.

Vascularization strategies for tissue engineers

All tissue-engineered substitutes (with the exception of cornea and cartilage) require a vascular network to provide the nutrient and oxygen supply needed for their survival in vivo. Unfortunately the process of vascular ingrowth into an engineered tissue can take weeks to occur naturally and during this time the tissues become starved of essential nutrients, leading to tissue death. This review initially gives a brief overview of the processes and factors involved in the formation of new vasculature. It then summarizes the different approaches that are being applied or developed to overcome the issue of slow neovascularization in a range of tissue-engineered substitutes. Some potential future strategies are then discussed.

Flow increase is decisive to initiate angiogenesis in veins exposed to altered hemodynamics

Exposing a vein to altered hemodynamics by creating an arteriovenous (AV) shunt evokes considerable vessel formation that may be of therapeutic potential. However, it is unclear whether the introduction of oscillatory flow and/or flow increase is decisive. To distinguish between these mechanical stimuli we grafted a femoral vein into the arterial flow pathway of the contralateral limb in rats creating an arterioarterial (AA) loop (n = 7). Alternatively, we connected the femoral artery and vein using the vein graft, whereby we created an AV-loop (n = 27). Vessel loops were embedded in a fibrin filled chamber and blood flow was measured by means of flow probes immediately after surgery (day 0) and 15 days after loop creation. On day 15, animals were sacrificed and angiogenesis was evaluated using μCT and histological analysis. Mean flow increased from 0.5 to 2.4 mL/min and was elevated throughout the cardiac cycle at day 0 in AV-loops whereas, as expected, it remained unchanged in AA-loops. Flow in AV-loops decreased with time, and was at day 15 not different from untreated femoral vessels or AA-loop grafts. Pulsatile flow oscillations were similar in AV-and AA-loops at day 0. The flow amplitude amounted to ~1.3 mL/min which was comparable to values in untreated arteries. Flow amplitude remained constant in AA-loops, whereas it decreased in AV-loops (day 15: 0.4 mL/min). A large number of newly formed vessels were present in AV-loops at day 15 arising from the grafted vein. In marked contrast, angiogenesis originating from the grafted vein was absent in AA-loops. We conclude that exposure to substantially increased flow is required to initiate angiogenesis in grafted veins, whereas selective enhancement of pulsatile flow is unable to do so. This suggests that indeed flow and most likely wall shear stress is decisive to initiate formation of vessels in this hemodynamically driven angiogenesis model.

Microvascular proliferations in arteriovenous malformations relate to high-flow characteristics, inflammation, and previous therapeutic embolization of the lesion

BACKGROUND

Episodes of microvascular proliferation associated with volume expansion have been observed in arteriovenous malformations (AVMs) of skin and soft tissue.

OBJECTIVE

We sought to investigate the relationship between a microvascular proliferative response and flow velocity in AVMs.

METHODS

Resection specimens of 80 AVMs were clinically categorized as either high- or low-flow lesions, and histopathologically screened for the presence of microvessels, inflammation, thrombosis, or a combination of these. Immunohistochemistry was performed with endoglin (CD105), von Willebrand factor, and fibrinogen antibodies.

RESULTS

Clinically, 37 AVMs were classified as high-flow lesions and 43 as low-flow lesions. In 81% of high-flow lesions microvascular proliferations were seen versus in 14% of low-flow lesions (P < .005). In high-flow lesions, which were embolized before surgery (30% of all), 88% showed microvascular proliferation, 88% inflammation, and 33% thrombosis. However, similar vasoproliferative responses were also observed in nonembolized AVM (69% high-flow and 14% low-flow lesions). Endoglin was more frequently expressed in high-flow lesions. Extracellular von Willebrand factor staining was found in most lesions, irrespective of flow type or presence of microvascular proliferations.

LIMITATIONS

The study was carried out at a single tertiary referral center.

CONCLUSIONS

Microvascular proliferative masses in AVMs appear to be strongly associated with high-flow characteristics. This could be explained to some extent by previous therapeutic embolization and/or inflammation in the lesion. However, occurrence of similar microvascular responses in AVM that were not embolized before surgery suggests that the biomechanical effects of high flow in these lesions may also have an angiogenic effect.

Axially vascularized bone substitutes: a systematic review of literature and presentation of a novel model

INTRODUCTION

The creation of axially vascularized bone substitutes (AVBS) has been successfully demonstrated in several animal models. One prototypical indication is bone replacement in patients with previously irradiated defect sites, such as in the mandibular region. The downside of current clinical practice, when free fibular or scapular grafts are used, is the creation of significant donor site morbidity.

METHODS

Based on our previous experiments, we extended the creation of an arterio-venous loop to generate vascularized bone substitutes to a new defect model in the goat mandibula. In this report, we review the literature regarding different models for axially vascularized bone substitutes and present a novel model demonstrating the feasibility of combining this model with synthetic porous scaffold materials and biological tissue adhesives to grow cells and tissue.

RESULTS

We were able to show the principal possibility to generate axially vascularized bony substitutes in vivo in goat mandibular defects harnessing the regenerative capacity of the living organism and completely avoiding donor site morbidity.

CONCLUSION

From our findings, we conclude that this novel model may well offer new perspectives for orthopedic and traumatic bone defects that might benefit from the reduction of donor site morbidity.

Prefabrication of axial vascularized tissue engineering coral bone by an arteriovenous loop: a better model

The most important problem for the survival of thick 3-dimensional tissues is the lack of vascularization in the context of bone tissue engineering. In this study, a modified arteriovenous loop (AVL) was developed to prefabricate an axial vascularized tissue engineering coral bone in rabbit, with comparison of the arteriovenous bundle (AVB) model. An arteriovenous fistula between rabbit femoral artery and vein was anastomosed to form an AVL. It was placed in a circular side groove of the coral block. The complex was wrapped with an expanded-polytetrafluoroethylene membrane and implanted beneath inguinal skin. After 2, 4, 6 and 8 weeks, the degree of vascularization was evaluated by India ink perfusion, histological examination, vascular casts, and scanning electron microscopy images of vascular endangium. Newly formed fibrous tissues and vasculature extended over the surfaces and invaded the interspaces of entire coral block. The new blood vessels robustly sprouted from the AVL. Those invaginated cavities in the vascular endangium from scanning electron microscopy indicated vessel's sprouted pores. Above indexes in AVL model are all superior to that in AVB model, indicating that the modified AVL model could more effectively develop vascularization in larger tissue engineering bone.

Delineating the angiogenic gene expression profile before pulmonary vascular remodeling in a lamb model of congenital heart disease

Disordered angiogenesis is implicated in pulmonary vascular remodeling secondary to congenital heart diseases (CHD). However, the underlying genes are not well delineated. We showed previously that an ovine model of CHD with increased pulmonary blood flow (PBF, Shunt) has an "angiogenesis burst" between 1 and 4 wk of age. Thus we hypothesized that the increased PBF elicited a proangiogenic gene expression profile before onset of vessel growth. To test this we utilized microarray analysis to identify genes that could be responsible for the angiogenic response. Total RNA was isolated from lungs of Shunt and control lambs at 3 days of age and hybridized to Affymetrix gene chips for microarray analyses (n = 8/group). Eighty-nine angiogenesis-related genes were found to be upregulated and 26 angiogenesis-related genes downregulated in Shunt compared with control lungs (cutting at 1.2-fold difference, P < 0.05). We then confirmed upregulation of proangiogenic genes FGF2, Angiopoietin2 (Angpt2), and Birc5 at mRNA and protein levels and upregulation of ccl2 at mRNA level in 3-day Shunt lungs. Furthermore, we found that pulmonary arterial endothelial cells (PAEC) isolated from fetal lambs exhibited increased expression of FGF2, Angpt2, Birc5, and ccl2 and enhanced angiogenesis when exposed to elevated shear stress (35 dyn/cm²) compared with cells exposed to more physiological shear stress (20 dyn/cm²). Finally, we demonstrated that blocking FGF2, Angpt2, Birc5, or ccl2 signaling with neutralizing antibodies or small interfering RNA (siRNA) significantly decreased the angiogenic response induced by shear stress. In conclusion, we have identified a "proangiogenic" gene expression profile in a lamb model of CHD with increased PBF that precedes onset of pulmonary vascular remodeling. Our data indicate that FGF2, Angpt2, Birc5, and ccl2 may play important roles in the angiogenic response.

Autonomously vascularized cellular constructs in tissue engineering: opening a new perspective for biomedical science

In tissue engineering cell cultures play a crucial role besides the matrix materials for the end of substituting lost tissue functions. The cell itself is situated at the cross-roads leading to different orders of scale, from molecule to organism and different levels of function, from biochemistry to macrophysiology. Extensive in vitro investigations have dissected a vast amount of cellular phenomena and the role of a number of bioactive substances has been elucidated in the past. Further, recombinant DNA technologies allow modulation of the expression profiles of virtually all kinds of cells. However, issues of vascularization in vivo limit transferability of these observations and restrict upscaling into clinical applications. Novel in vivo models of vascularization have evolved inspired from reconstructive microsurgical concepts and they encompass axial neovascularization by means of vascular induction. This work represents a brief description of latest developments and potential applications of neovascularization and angiogenesis in tissue engineering.

The venous graft as an effector of early angiogenesis in a fibrin matrix

The arteriovenous loop (AV loop) model is gaining importance as a means of initiating and sustaining perfusion in tissue engineering constructs in vivo. This study represents an attempt to dissect the morphology of early arterialization and angiogenesis in the AV loop in a fibrin matrix with special focus on the interpositional venous graft (IVG) segment. An AV loop was constructed in 30 rats using the femoral vessels and an IVG. The AV loop was encased in an isolation chamber filled with a fibrin matrix. Evaluation methods included scanning electron microscopy (SEM) of corrosion casts, immune histology and micro magnetic resonance angiography (MRA). Direct luminal neovascular sprouting was evident between day 10 and day 14 from the vein and the IVG but not from the arterial segment. Arterialization of the IVG manifested itself on the corrosion casts as a gradual reduction in luminal caliber with onset after day 7. Microdissection of the microvascular replicas could demonstrate for the first time the presence of direct luminal sprouts from the IVG. MRA was used to display the shunt pattern of perfusion in the patent AV loop. From the three segments of the vascular axis in the AV loop the IVG is the most versatile for applications in the clinical as well as the experimental setting. Kinetics of angiogenesis warrant further investigation in the IVG.

Achieving the ideal properties for vascular bypass grafts using a tissue engineered approach: a review

The multiple demands placed on small calibre cardiovascular bypass grafts have meant that a synthetic prosthesis with good long-term patency has not been developed. A tissue-engineered graft could fulfil the ideal characteristics present in an artery. However, the great disadvantage of such a conduit is the time necessary for maturation leading to unacceptable delays once the decision to intervene surgically has been made. This maturation process is essential to produce a graft which can withstand haemodynamic stress. Once implanted, the tissue-engineered graft can contract in response to immediate haemodynamic conditions and remodel in the long term. We review the latest tissue engineering approaches used to give the favourable properties of mechanical strength, arterial compliance, low thrombogenicity, long-term resistance towards biodegradation as well as technological advances which shorten the time required for production of an implantable graft.

How does blood cell concentration modulate cardiovascular parameters in developing zebrafish (Danio rerio)?

Blood flow and shear forces are considered to be important parameters possibly stimulating angiogenesis or cardiovascular remodeling. The main objective of this study was to test the hypothesis that a significant reduction in shear forces as a consequence of a significant isovolemic anemia induced by microsurgical techniques during early larval development of the zebrafish might induce a compensatory stimulation of erythropoiesis and/or induce a modification of cardiac activity or even the formation of the heart and may influence the shaping of the vascular bed. Blood from 2 day old zebrafish larvae was withdrawn and replaced by zebrafish Ringer's solution, so that the blood cell concentration was reduced by at least 75%. At 5 days post fertilization (dpf) a partial recovery in blood cell concentration was observed and reached a value of 814.55+/-85.42 cells/nL, while in control animals blood cell concentration amounted to 1856.00+/-131.59 cells/nL. At 7 dpf the value of blood cell concentration was 1023.89+/-95.75 cells/nL versus 1701.54+/-146.03 cells/nL in control animals. Compared to control animals, heart rate and cardiac output were significantly reduced in anemic animals and alterations in the formation of the vascular bed were also observed. A significant decrease in the end-diastolic volume suggested that ventricular volume was reduced. Thus, within a few days zebrafish larvae were nearly able to compensate for an isovolemic anemia by an enhanced erythropoiesis. However, several changes in cardiovascular system indicated that phenotypic plasticity is established even at an early developmental stage.