A simple physiologic pulsatile perfusion system for the study of intact vascular tissue

An Arteriovenous Bioreactor Perfusion System for Physiological In Vitro Culture of Complex Vascularized Tissue Constructs

BACKGROUND

The generation and perfusion of complex vascularized tissues in vitro requires sophisticated perfusion techniques. For multiscale arteriovenous networks, not only the arterial, but also the venous, biomechanical and biochemical conditions that physiologically exist in the human body must be accurately emulated. For this, we here present a modular arteriovenous perfusion system for the in vitro culture of a multi-scale bioartificial vascular network.

METHODS

The custom-built perfusion system consisted of two circuits: in the arterial circuit, physiological arterial biomechanical and biochemical conditions were simulated using a modular set-up with a pulsatile peristaltic pump, compliance chambers, and resistors. In the venous circuit, venous conditions were emulated accordingly. In the center of the system, a bioartificial multi-scale vascularized fibrin-based tissue was perfused by both circuits simultaneously under biomimetic arteriovenous conditions. Culture conditions were monitored continuously using a multi-sensor monitoring system.

RESULTS

The physiological arterial and venous pressure- and flow-curves, as well as the microvascular arteriovenous oxygen partial pressure gradient, were accurately emulated in the perfusion system. The multi-sensor monitoring system facilitated live monitoring of the respective parameters and data-logging. In a proof-of-concept experiment, vascularized three-dimensional fibrin tissues showed sustained cell viability and homogenous microvessel formation after culture in the perfusion system.

CONCLUSIONS

The arteriovenous perfusion system facilitated the in vitro culture of a multiscale vascularized tissue under physiological pressure-, flow-, and oxygen-gradient conditions. With that, it presents a promising technique for the in vitro generation and culture of complex large-scale vascularized tissues.

Synergistic coupling between 3D bioprinting and vascularization strategies

Three-dimensional (3D) bioprinting offers promising solutions to the complex challenge of vascularization in biofabrication, thereby enhancing the prospects for clinical translation of engineered tissues and organs. While existing reviews have touched upon 3D bioprinting in vascularized tissue contexts, the current review offers a more holistic perspective, encompassing recent technical advancements and spanning the entire multistage bioprinting process, with a particular emphasis on vascularization. The synergy between 3D bioprinting and vascularization strategies is crucial, as 3D bioprinting can enable the creation of personalized, tissue-specific vascular network while the vascularization enhances tissue viability and function. The review starts by providing a comprehensive overview of the entire bioprinting process, spanning from pre-bioprinting stages to post-printing processing, including perfusion and maturation. Next, recent advancements in vascularization strategies that can be seamlessly integrated with bioprinting are discussed. Further, tissue-specific examples illustrating how these vascularization approaches are customized for diverse anatomical tissues towards enhancing clinical relevance are discussed. Finally, the underexplored intraoperative bioprinting (IOB) was highlighted, which enables the direct reconstruction of tissues within defect sites, stressing on the possible synergy shaped by combining IOB with vascularization strategies for improved regeneration.

3D Printed Bioreactor Enabling the Pulsatile Culture of Native and Angioplastied Large Arteries

Routine interventions such as balloon angioplasty, result in vascular activation and remodeling, often requiring re-intervention. 2D in vitro models and small animal experiments have enabled the discovery of important mechanisms involved in this process, however the clinical translation is often underwhelming. There is a critical need for an ex vivo model representative of the human vascular physiology and encompassing the complexity of the vascular wall and the physical forces regulating its function. Vascular bioreactors for ex vivo culture of large vessels are viable alternatives, but their custom-made design and insufficient characterization often hinders the reproducibility of the experiments. The objective of the study was to design and validate a novel 3D printed cost-efficient and versatile perfusion system, capable of sustaining the viability and functionality of large porcine arteries for 7 days and enabling early post-injury evaluations. MultiJet Fusion 3D printing was used to engineer the EasyFlow insert, converting a conventional 50 ml centrifuge tube into a mini bioreactor. Porcine carotid arteries either left untreated or injured with an angioplasty balloon, were cultured under pulsatile flow for up to 7 days. Pressure, heart rate, medium viscosity and shear conditions were adjusted to resemble arterial in vivo hemodynamics. Tissue viability, cell activation and matrix remodeling were analyzed by immunohistochemistry, and vascular function was monitored by duplex ultrasound. Culture conditions in the EasyFlow bioreactor preserved endothelial coverage and smooth muscle organization and extracellular matrix structure in the vessel wall, as compared to static culture. Injured arteries presented hallmarks of early remodeling, such as intimal denudation, smooth muscle cell disarray and media/adventitia activation in flow culture. Duplex ultrasound confirmed continuous pulsatile blood flow conditions, dose-dependent vasodilator response to nitroglycerin in untreated vessels and impaired dilator response in angioplastied vessels. The scope of this work is to validate a low-cost, robust and reproducible system to explore the culture of native and injured large arteries under pulsatile flow. While the study of vascular pathology is beyond the scope of the present paper, our system enables future investigations and provides a platform to test novel therapies and devices ex vivo, in a patient relevant system.

Survival Rate of Cells Sent by a Low Mechanical Load Tube Pump: The "Ring Pump"

A ring pump (RP) is a useful tool for microchannels and automated cell culturing. We have been developing RPs (a full-press ring pump, FRP; and a mid-press ring pump, MRP). However, damage to cells which were sent by the RP and the MRP was not investigated, and no other studies have compared the damage to cells between RPs and peristaltic pumps (PPs). Therefore, first, we evaluated the damage to cells that were sent by a small size FRP (s-FRP) and small size MRPs (s-MRPs; gap = 25 or 50 μm, respectively). “Small size” means that the s-FRP and the s-MRPs are suitable for microchannel-scale applications. The survival rate of cells sent by the s-MRPs was higher than those sent by the s-FRP, and less damage caused by the former. Second, we compared the survival rate of cells that were sent by a large size FRP (l-FRP), a large size MRP (l-MRP) (gap = 50 μm) and a PP. “Large size” means that the l-FRP and the l-MRP are suitable for automated cell culture system applications. We could not confirm any differences among the cell survival rates. On the other hand, when cells suspended in Dulbecco’s phosphate-buffered saline solution were circulated with the l-MRP (gap = 50 μm) and the PP, we confirmed a difference in cell survival rate, and less damage caused by the former.

Real-time ex vivo perfusion of human lymph nodes invaded by cancer (REPLICANT): a feasibility study

Understanding how breast cancer (BC) grows in axillary lymph nodes (ALNs), and refining how therapies might halt that process, is clinically important. However, modelling the complex ALN microenvironment is difficult, and no human models exist at present. We harvested ALNs from ten BC patients, and perfused them at 37 °C ex vivo for up to 24 h. Controlled autologous testing showed that ALNs remain viable after 24 h of ex vivo perfusion: haematoxylin and eosin-stained histological appearance and proliferation (by Ki67 immunohistochemistry) did not change significantly over time for any perfused ALN compared with a control from time-point zero. Furthermore, targeted gene expression analysis (NanoString PanCancer IO360 panel) showed that only 21/750 genes were differentially expressed between control and perfused ALNs (|log₂ FC| > 1 and q < 0.1): none were involved in apoptosis and metabolism, but rather all 21 genes were involved in immune function and angiogenesis. During perfusion, tissue acid-base balance remained stable. Interestingly, the flow rate increased (p < 0.001) in cancer-replaced (i.e. metastasis occupied more than 90% of the surface area on multiple levels) compared to cancer-free nodes (i.e. nodes with no metastasis on multiple sections). CXCL11 transcripts were significantly more abundant in cancer-replaced nodes, while CXCL12 transcripts were significantly more abundant in cancer-free nodes. These cytokines were also detected in the circulating perfusate. Monoclonal antibodies (nivolumab and trastuzumab) were administered into a further three ALNs to confirm perfusion efficacy. These drugs saturated the nodes; nivolumab even induced cancer cell death. Normothermic ALN perfusion is not only feasible but sustains the tumour microenvironment ex vivo for scientific investigation. This model could facilitate the identification of actionable immuno-oncology targets. © 2019 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Endothelialization of cardiovascular devices

Blood-contacting surfaces of cardiovascular devices are not biocompatible for creating an endothelial layer on them. Numerous research studies have mainly sought to modify these surfaces through physical, chemical and biological means to ease early endothelial cell (EC) adhesion, migration and proliferation, and eventually to build an endothelial layer on the surfaces. The first priority for surface modification is inhibition of protein adsorption that leads to inhibition of platelet adhesion to the device surfaces, which may favor EC adhesion. Surface modification through surface texturing, if applicable, can bring some hopeful outcomes in this regard. Surface modifications through chemical and/or biological means may play a significant role in easy endothelialization of cardiovascular devices and inhibit smooth muscle cell proliferation. Cellular engineering of cells relevant to endothelialization can boost the positive outcomes obtained through surface engineering. This review briefly summarizes recent developments and research in early endothelialization of cardiovascular devices. STATEMENT OF SIGNIFICANCE: Endothelialization of cardiovascular implants, including heart valves, vascular stents and vascular grafts is crucial to solve many problems in our health care system. Numerous research efforts have been made to improve endothelialization on the surfaces of cardiovascular implants, mainly through surface modifications in three ways - physically, chemically and biologically. This review is intended to highlight comprehensive research studies to date on surface modifications aiming for early endothelialization on the blood-contacting surfaces of cardiovascular implants. It also discusses future perspectives to help guide endothelialization strategies and inspire further innovations.

Study of Hepatic Vascular Dynamics Based on Symmetrical Pulsating Perfusion

Background

The traditionally used perfusion method is constant flow. This study proposes a novel method called Symmetric Pulsating Flow (SPF) and verified that this method is applicable.

Material/Methods

The fluid dynamic behavior of perfusate in the vessel, the shear stress, and the vascular deformation were simulated based on the bi-directional fluid-structure interaction. The differences of the fluid dynamic behaviors and the mechanical characteristics of vascular wall were studied and compared between the 2 methods during the process of hepatic perfusion. The simulations and comparisons were carried out on 3 different vascular models.

Results

Utilizing the constant flow perfusion, a double vortex clearly appeared at the rear end of the foreign matter and reflux retention can be caused by the double vortex. The reflux retention caused lower shear stress against the vascular wall and thus brought new accumulation of foreign matter. The SPF perfusion, however, prevented the double vortex, and avoided such reflux retention during the vascular perfusion. In addition, the SPF can clean the vascular wall better with a slower speed, which causes less injury to the vessel, and the pulsating effect can reduce the accumulation of new foreign matter.

Conclusions

The SPF perfusion can clean the vascular wall more thoroughly with less injury.

Ex vivo lymphatic perfusion system for independently controlling pressure gradient and transmural pressure in isolated vessels

In addition to external forces, collecting lymphatic vessels intrinsically contract to transport lymph from the extremities to the venous circulation. As a result, the lymphatic endothelium is routinely exposed to a wide range of dynamic mechanical forces, primarily fluid shear stress and circumferential stress, which have both been shown to affect lymphatic pumping activity. Although various ex vivo perfusion systems exist to study this innate pumping activity in response to mechanical stimuli, none are capable of independently controlling the two primary mechanical forces affecting lymphatic contractility: transaxial pressure gradient, [Formula: see text], which governs fluid shear stress; and average transmural pressure, [Formula: see text], which governs circumferential stress. Hence, the authors describe a novel ex vivo lymphatic perfusion system (ELPS) capable of independently controlling these two outputs using a linear, explicit model predictive control (MPC) algorithm. The ELPS is capable of reproducing arbitrary waveforms within the frequency range observed in the lymphatics in vivo, including a time-varying [Formula: see text] with a constant [Formula: see text], time-varying [Formula: see text] and [Formula: see text], and a constant [Formula: see text] with a time-varying [Formula: see text]. In addition, due to its implementation of syringes to actuate the working fluid, a post-hoc method of estimating both the flow rate through the vessel and fluid wall shear stress over multiple, long (5 s) time windows is also described.

A bioreactor system for interfacial culture and physiological perfusion of vascularized tissue equivalents

A pivotal requirement for the generation of vascularized tissue equivalents is the development of culture systems that provide a physiological perfusion of the vasculature and tissue-specific culture conditions. Here, we present a bioreactor system that is suitable to culture vascularized tissue equivalents covered with culture media and at the air-medium interface, which is a vital stimulus for skin tissue. For the perfusion of the vascular system a new method was integrated into the bioreactor system that creates a physiological pulsatile medium flow between 80 and 120 mmHg to the arterial inflow of the equivalent's vascular system. Human dermal microvascular endothelial cells (hDMECs) were injected into the vascular system of a biological vascularized scaffold based on a decellularized porcine jejunal segment and cultured in the bioreactor system for 14 days. Histological analysis and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) staining revealed that the hDMECs were able to recolonize the perfused vascular structures and expressed endothelial cell specific markers such as platelet endothelial cell adhesion molecule and von Willebrand factor. These results indicate that our bioreactor system can serve as a platform technology to generate advanced bioartificial tissues with a functional vasculature for future clinical applications.

NUMERICAL INVESTIGATION OF PULSATILE FLOW IN AN S-TYPE BYPASS GRAFT

Intimal hyperplasia developed at the end-to-side anastomosis of artery bypass is closely related to unphysiological hemodynamics. The helical flow as a normal physiological phenomenon in arteries is beneficial to endothelial damage repair. To deeply understand the physiological flow properties in a S-type bypass (StB) graft, four end-to-side bypass models including 30°, 45°, 60° conventional bypasses and a 45° StB were compared numerically under physiological pulsatile flow. The results showed that strong helical flow was observed at the distal anastomosis of StB. The distribution of hemodynamic parameters such as helicity, average wall shear stress and oscillating shear index, etc. were significantly improved at the S-type anastomosis as compared with those of three conventional models. The area-averaged normalized helicity in StB reached maxima at the moments of maximum flow rate and systolic deceleration. The hemodynamic performance in a 45° StB was improved as compared with a 30° conventional model. It is concluded that large StB anastomosis angle can be taken to achieve good hemodynamic performance while much smaller anastomosis angle has to be adopted for conventional bypass. As such, a S-type anastomosis should be a feasible choice of clinical artery bypass grafting due to its significant improvement in hemodynamic performance.

Microparticle formation after co-culture of human whole blood and umbilical artery in a novel in vitro model of flow

Cardiovascular disease (CVD) is now the largest killer in western society, and the importance of interactions between vascular endothelium and circulating blood components in disease pathogenesis is well established. Microparticles are a heterogeneous population of <1 μm blood borne particles that arise from blebbing or shedding of cell membranes. The microparticle population includes several classes of apoptotic bodies; however, increased numbers of procoagulant microparticles have been described in plasma from people with CVD. We have previously demonstrated that interactions of monocytes and platelets with isolated inflamed endothelial cells lead to production of pro-coagulant tissue factor bearing microparticles under laminar flow conditions. Here we have investigated microparticle production after perfusion of human whole blood through intact inflamed human umbilical artery. When blood was perfused through umbilical arteries which had been pre-stimulated with tumour necrosis factor (TNFα) for 18 h under flow conditions, there was significantly increased production of microparticles from both platelet and non-platelet sources, in particular from erythrocytes. To determine whether microparticles generated during interactions with inflamed endothelium could induce a pro-inflammatory response in trans, we isolated microparticles by centrifugation after co-culture and incubated with isolated quiescent endothelial cells followed by measurement of reactive oxygen species formation. Microparticles derived from co-culture with inflamed endothelium induced significantly enhanced levels of reactive oxygen species (ROS). These data suggest that presence of an inflamed endothelium causes release of pro-inflammatory microparticles from circulating blood cells, which could contribute to prolonged endothelial activation and subsequent atherosclerotic changes in blood vessels subjected to inflammatory insult.

Evaluation of tubular poly(trimethylene carbonate) tissue engineering scaffolds in a circulating pulsatile flow system

Tubular scaffolds (internal diameter approximately 3 mm and wall thickness approximately 0.8 mm) with a porosity of approximately 83% and an average pore size of 116 µm were prepared from flexible poly(trimethylene carbonate) (PTMC) polymer by dip-coating and particulate leaching methods. PTMC is a flexible and biocompatible polymer that crosslinks upon irradiation; porous network structures were obtained by irradiating the specimens in vacuum at 25 kGy before leaching soluble salt particles. To assess the suitability of these scaffolds in dynamic cell culturing for cardiovascular tissue engineering, the scaffolds were coated with a thin (0.1 to 0.2 mm) non-porous PTMC layer and its performance was evaluated in a closed pulsatile flow system (PFS). For this, the PFS was operated at physiological conditions at liquid flows of 1.56 ml/s with pressures varying from 80-120 mmHg at a frequency of 70 pulsations per minute. The mechanical properties of these coated porous PTMC scaffolds were not significantly different than non-coated scaffolds. Typical tensile strengths in the radial direction were 0.15 MPa, initial stiffness values were close to 1.4 MPa. Their creep resistance in cyclic deformation experiments was excellent. In the pulsatile flow setup, the distention rates of these flexible and elastic scaffolds were approximately 0.10% per mmHg, which is comparable to that of a porcine carotid artery (0.11% per mmHg). The compliance and stiffness index values were close to those of natural arteries.?In long-term deformation studies, where the scaffolds were subjected to physiological pulsatile pressures for one week, the morphology and mechanical properties of the PTMC scaffolds did not change. This suggests their suitability for application in a dynamic cell culture bioreactor.

Effects of Axial Stretch on Cell Proliferation and Intimal Thickness in Arteries in Organ Culture

Intimal hyperplasia (IH) remains the major cause of intermediate and long-term failure of vascular grafts and endovascular interventions. Arteries are subjected to a significant longitudinal stress in addition to the shear stress and tensile stress from the blood flow. The aim of this study was to determine the effect of axial stretch on cell proliferation and IH in arteries. Porcine carotid arteries, intact or endothelial cell (EC) denudated, were maintained ex vivo at different stretch ratios (1.3, 1.5, and 1.8) and flow rates (16 or 160 mL/min) while remaining at physiologic pressure for 7 days. The viability of the arteries was verified with norepinephrine, carbachol, and sodium nitroprusside stimulations, and the cell proliferation was detected using bromodeoxyuridine labeling and immunostaining. Our results showed that the axial stretch ratio did not significantly affect intimal thickness and cell proliferation in normal arteries. However, axial stretch increased the neointimal thickness in EC denudated arteries cultured under low flow conditions. The cell proliferation increased significantly in the intima and inner half of the media of the EC denudated arteries under normal or elevated axial stretch in comparison to intact arteries at the same stretch ratio. These results demonstrated that axial stretch with EC denudation and low flow increases neointimal formation and cell proliferation in the arteries.

Vascular smooth cell proliferation in perfusion culture of porcine carotid arteries

Objective of this study was to develop a novel in vitro artery culture system to study vascular smooth muscle cell (SMC) proliferation of porcine carotid arteries in response to injury, basic fibroblast growth factor (FGF2), and FGF2 conjugated with cytotoxin saporin (SAP). Perfusion-cultured porcine carotid arteries remained contractile in response to norepinephrine and relaxant to acetylcholine for up to 96 h. SMC proliferation of cultured arteries was detected by bromodeoxyuridine incorporation in both non-injured and balloon-injured arteries. In the inner layer of the vessel wall near the lumen, SMC proliferation were less than 10% in uninjured vessels, 66% in injured vessels, 80% in injured vessels with FGF2 treatment, and 5% in injured vessels with treatment of FGF2-SAP. Thus, the cultured porcine carotid arteries were viable; and the injury stimulated SMC proliferation, which was significantly enhanced by FGF2 and inhibited by FGF2-SAP.

Effect of Low Shear Stress on Permeability and Occludin Expression in Porcine Artery Endothelial Cells

INTRODUCTION

Although both fluid shear stress and mass transport of atherogenic substances into the vascular wall are known to be important factors in atherogenesis, there has been little research on the effect of shear stress on vascular permeability. Therefore, the effects of shear stress on the permeability of arteries and the expression of the endothelial cell tight junction molecule occludin, an important regulator of vascular permeability, were investigated.

METHODS

Porcine carotid arteries were perfusion cultured ex vivo with low (1.5 dyne/cm(2)) or physiologic (15 dyne/cm(2)) shear stress and 100 mmHg pressure for 24 hours. Subsequently, 20 nm gold particles in solution were infused into the lumen of vessels at 100 mmHg for 30 minutes. Frozen sections were then cut and stained for gold particles. Image analysis was used to determine the density of the particles in the vessel walls. The expression of endothelial cell occludin mRNA and protein were determined using reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting, respectively.

RESULTS

Permeability results showed a 2.8-fold increase in the apparent permeability of vessels cultured with low versus physiologic shear stress. RT-PCR and Western blotting results showed significant decreases in occludin mRNA and protein expression at 12 and 24 hours in vessels cultured with low versus physiologic shear stress.

CONCLUSIONS

These results demonstrate that low shear stress increases vascular permeability in porcine carotid arteries, possibly owing to decreased occludin expression. These results may have implications in the preferential formation of atherosclerotic vascular disease adjacent to branches and bifurcations where low mean shear stresses may occur.

Modeling, design and validation of a novel microfluidic sensor for in-vitro isotonic measurement of microvessel contraction/dilation

The paper presents an innovative fluidic microdevice for in-vitro isotonic measurements of microvessel contraction/dilation. The proposed microdevice is based on the well-known fluid dynamic principle which correlates a geometrical change of a solid body immersed in a constant flow, with the differential pressure change induced in the flow itself. Indeed, a biochemically-induced change of microvessel diameter lead to a change of its hydraulic resistance which can be in-vitro measured by a differential pressure sensor. The novel microfluidic sensor has been modeled, designed and fabricated in order to properly implement this working principle. The fluidic scheme of the sensor consists of two pressure-sensorized chambers which are connected by a calibrated channel where the microvessel is placed. Experimental tests have been performed by using three microvessel passive simulators with different inner diameter (300, 600 and 800 microm), in order to validate the expected sensor functioning characterized by two sensitivities: the pressure drop S (PL) and the overall S 2-3 ones. Their measured values (S (PL,m)=2.132 mV/V/Pa, S (2-3,m)=0.787 mV/V/(mm3/s)/mm) confirm the validity of the proposed working principle and permit to be confident in a future application of the sensor in a clinical context.

Physical characterization of vascular grafts cultured in a bioreactor

Tubular scaffolds of collagen and elastin (weight ratio 1:1) with interconnected pores were prepared by freeze drying and crosslinked with N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) in the presence or absence of a Jeffamine spacer (poly(propylene glycol)-bis-(2-aminopropyl ether), J230). The crosslinked and uncrosslinked matrices had porosities of 90% and average pore sizes of 131-151 microm. Smooth muscle cells (SMC) were cultured in the crosslinked and uncrosslinked tubular scaffolds under pulsatile flow conditions (mean flow rate 9.6 ml/min, 120 beats/min, pressure 80-120 mmHg). All the constructs could withstand cyclic mechanical strain in the absence of any mechanical support without cracking or suffering permanent deformation. After 7d, SMC were homogeneously distributed throughout the uncrosslinked and EDC/NHS crosslinked constructs, whereas hardly any cell was observed on the luminal side of J230/EDC/NHS crosslinked matrices. Considering the better mechanical performance of EDC/NHS crosslinked matrices compared to non-crosslinked constructs after 7d of culture, SMC were dynamically cultured in the former scaffolds for 14d. During this period, the high strain stiffness of the constructs increased more than two-fold to 38+/-2 kPa, whereas the low strain stiffness doubled to 8+/-2 kPa. The yield stress and yield strain were 30+/-10 kPa and 120+/-20%, respectively. SMC were homogeneously distributed throughout the EDC/NHS crosslinked collagen/elastin constructs and collagen fibres tended to orient in the circumferential direction.

Review: application of stem cells for vascular tissue engineering

As the prevalence of vascular disease has continued to expand, the need for a suitable arterial replacement has prompted researchers to look beyond synthetic and autologous grafts toward the field of tissue engineering. Advances in vascular tissue engineering have utilized both mesenchymal and hematopoietic stem cells as a cell source in an attempt to create a fully engineered small-diameter graft. Stem cells offer enormous potential as a cell source because of their proliferative and growth potential, and the application of stem cell technology has far-reaching implications for future applications. The innovative use of stem cells for vascular tissue engineering has opened new possibilities for a fully engineered blood vessel. The purpose of this review is to summarize the current perspective on the use of stem cells for vascular tissue engineering. It focuses principally on the classes of stem cells used, techniques for differentiation scaffolding technology, and the successes and failures of models.

Novel Pulse Duplicating Bioreactor System for Tissue-Engineered Vascular Construct

Cell culture in a biomimetic environment is known to improve the mechanical endurance of tissue-engineered cardiovascular components. Our goal was to generate a bioreactor that can reproduce a wide range of pulsatile flows with a completely physiological pressure profile. The morphology and biochemical properties of tissue-engineered products were also studied to test the usefulness of this novel bioreactor. The combination of an outflow valve, compliance chamber, and resistant clamps together with a balloon pumping system was able to successfully reproduce both physiological systolic and diastolic pressures. The compliance chamber was especially effective in transforming the original peaky pressure waveform into a physiological pressure profile. The tissues, cultured under a physiological pressure waveform with pulsatile flow, presented widely distributed cells in close contact with each other. They also showed significantly higher cell numbers, total protein content, and proteoglycan-glycosaminoglycan content than cultured tissues under a peaky pressure wave or under static conditions. This new bioreactor system is suitable for evaluating a favorable environment for tissue-engineered cardiovascular components.

Basic Fibroblast Growth Factor Coating and Endothelial Cell Seeding of a Decellularized Heparin-coated Vascular Graft

The objective of this study was to determine the effect of basic fibroblast growth factor (bFGF) coating on endothelial cell seeding and proliferation on a decellularized heparin coated vascular graft and to determine the retention of seeded cells on the graft under flow conditions. Disks of heparin coated decellularized grafts were incubated for 24 h as controls or with bFGF. Human microvascular endothelial cells (HMECs) or canine peripheral blood endothelial progenitor cells (CEPC) were seeded onto the disks and incubated for 96 h or 48 h, respectively. HMECs were also seeded onto the luminal surfaces of two heparin-coated decellularized grafts for 3 h. One graft was placed in a perfusion culture system and cultured for an additional 6 h with flow and pressure. After culturing, there were 4.7 +/- 1.4 cells/mm(2) HMECs on control grafts and 11.4 +/- 1.4 cells/mm(2) in bFGF treated grafts (P < 0.05). Likewise, with CEPCs, there were 14.8 +/- 4.8 cells/mm(2) in control grafts and 33.3 +/- 7.3 cells/mm(2) in bFGF treated grafts. After only 3 h of cell attachment, 60% of HMECs were retained in the intact graft exposed flow relative to the static control graft, which is an acceptable level. These data demonstrate that bFGF coating on the heparin bound decellularized grafts significantly increases both HMEC and dog EPC proliferation and that seeded cells are stable under perfusion conditions.

Perfusion bioreactor for vascular tissue engineering with capacities for longitudinal stretch

Arterial growth during embryonic vascular development is associated with longitudinal strain. The longitudinal strain is an important element of the embryonic vascular mechanical environment (EVME). Thus, a perfusion bioreactor for vascular tissue engineered constructs must include the functional capacity for longitudinal strain. To accomplish this goal, a perfusion bioreactor with the capacity for longitudinal strain was developed. The bioreactor includes two media perfusion systems: one for the inside perfusion and one for the outside perfusion of the cardiovascular engineered tubular construct (CETC). The watertight perfusion chamber allows periodic changing of longitudinal strain of the construct during mechanical conditioning. The range of the longitudinal strain is 0% to 200%. The biomechanical properties of the CETC are controlled by a pressure transducer and a digital TV camera. The pressure transducer and TV camera are connected to a computer. This allows the recording of a relationship between the radius of the VTEG and pressure in both static and dynamic regimens. This bioreactor can perform biomechanical conditioning with longitudinal strain.

Nicotine and cotinine up-regulate vascular endothelial growth factor expression in endothelial cells

Cigarette smoking is an important risk factor for both vascular disease and various forms of cancer. Vascular endothelial growth factor (VEGF) is an endothelial-specific mitogen that is normally expressed only in low levels in normal arteries but may be involved in the progression of both vascular disease and cancer. Some clinical evidence suggests that cigarette smoking may increase plasma VEGF levels, but there is a lack of basic science studies investigating this possibility. We show here, using an intact porcine common carotid artery perfusion culture model, that nicotine and cotinine, the major product of nicotine metabolism, cause a significant increase in endothelial cell VEGF expression. VEGF mRNA levels were compared between groups using reverse transcriptase-polymerase chain reaction, whereas protein level changes were demonstrated with Western blotting and immunohistochemistry. Our results showed significant increases in endothelial cell VEGF mRNA and protein levels because of nicotine and cotinine at concentrations representative of plasma concentrations seen in habitual smokers. VEGF immunostaining also paralleled these results. These findings may give a clue as to the mechanisms by which nicotine and cotinine from cigarette smoking increase vascular disease progression and tumor growth and metastasis.

Shear stress regulates occludin and VEGF expression in porcine arterial endothelial cells

BACKGROUND

One of the initiating factors of atherosclerosis is the accumulation of low-density lipoprotein in the intima. Despite the correlation between low shear stress and vascular lesion formation, there is little research on the effects of shear stress on the molecular regulators of endothelial cell permeability. In this study, the effects of shear stress on the expression of occludin and vascular endothelial growth factor (VEGF), two important regulators of endothelial permeability, were investigated.

METHODS

Porcine carotid arteries were cultured in perfusion culture systems for 24 h with 100 mm Hg pressure and low or physiologic shear stress. Subsequently, vessel sections were taken for histology and endothelial cells were isolated for RNA and protein extraction. Reverse transcription polymerase chain reaction (RT-PCR) was used to determine occludin and VEGF mRNA levels. Western blotting and immunohistochemistry were performed to examine occludin and VEGF protein levels.

RESULTS

RT-PCR showed that endothelial cells from vessels cultured with low shear stress had an 11% decrease in occludin/GAPDH band density ratio (P < 0.05) and a 16% increase in VEGF/beta-actin band density ratio (P < 0.05) relative to the physiologic shear stress group. Western blot showed a 50% decrease in occludin protein expression (P < 0.01) and a 95% increase in VEGF protein expression in endothelial cells from vessels cultured with low shear stress relative to the physiologic shear stress group. Immunoreactivity of occludin and VEGF in vessels also reflected these changes.

CONCLUSIONS

These results demonstrate that low shear stress both decreases endothelial cell occludin mRNA and protein expression and increases endothelial cell VEGF mRNA and protein expression. These changes may suggest a possible molecular mechanism for increased endothelial permeability due to low shear stress.