Toward an in vitro vasculature: differentiation of mesenchymal stromal cells within an endothelial cell-seeded modular construct in a microfluidic flow chamber

Bone marrow vasculature advanced in vitro models for cancer and cardiovascular research

Cardiometabolic diseases and cancer are among the most common diseases worldwide and are a serious concern to the healthcare system. These conditions, apparently distant, share common molecular and cellular determinants, that can represent targets for preventive and therapeutic approaches. The bone marrow plays an important role in this context as it is the main source of cells involved in cardiovascular regeneration, and one of the main sites of liquid and solid tumor metastasis, both characterized by the cellular trafficking across the bone marrow vasculature. The bone marrow vasculature has been widely studied in animal models, however, it is clear the need for human-specific in vitro models, that resemble the bone vasculature lined by endothelial cells to study the molecular mechanisms governing cell trafficking. In this review, we summarized the current knowledge on in vitro models of bone marrow vasculature developed for cardiovascular and cancer research.

Microfluidic evidence of synergistic effects between mesenchymal stromal cell-derived biochemical factors and biomechanical forces to control endothelial cell function

A functional vascular system is a prerequisite for bone repair as disturbed angiogenesis often causes non-union. Paracrine factors released from human bone marrow derived mesenchymal stromal cells (BMSCs) have angiogenic effects on endothelial cells. However, whether these paracrine factors participate in blood flow dynamics within bone capillaries remains poorly understood. Here, we used two different microfluidic designs to investigate critical steps during angiogenesis and found pronounced effects of endothelial cell proliferation as well as chemotactic and mechanotactic migration induced by BMSC conditioned medium (CM). The application of BMSC-CM in dynamic cultures demonstrates that bioactive factors in combination with fluidic flow-induced biomechanical signals significantly enhanced endothelial cell migration. Transcriptional analyses of endothelial cells demonstrate the induction of a unique gene expression profile related to tricarboxylic acid cycle and energy metabolism by the combination of BMSC-CM factors and shear stress, which opens an interesting avenue to explore during fracture healing. Our results stress the importance of in vivo - like microenvironments simultaneously including biochemical, biomechanical and oxygen levels when investigating key events during vessel repair. STATEMENT OF SIGNIFICANCE: Our results demonstrate the importance of recapitulating in vivo - like microenvironments when investigating key events during vessel repair. Endothelial cells exhibit enhanced angiogenesis characteristics when simultaneous exposing them to hMSC-CM, mechanical forces and biochemical signals simultaneously. The improved angiogenesis may not only result from the direct effect of growth factors, but also by reprogramming of endothelial cell metabolism. Moreover, with this model we demonstrated a synergistic impact of mechanical forces and biochemical factors on endothelial cell behavior and the expression of genes involved in the TCA cycle and energy metabolism, which opens an interesting new avenue to stimulate angiogenesis during fracture healing.

An Easy-to-Fabricate Microfluidic Shallow Trench Induced Three-Dimensional Cell Culturing and Imaging (STICI3D) Platform

Compared to the established monolayer approach of two-dimensional cell cultures, three-dimensional (3D) cultures more closely resemble in vivo models; that is, the cells interact and form clusters mimicking their organization in native tissue. Therefore, the cellular microenvironment of these 3D cultures proves to be more clinically relevant. In this study, we present a novel easy-to-fabricate microfluidic shallow trench induced 3D cell culturing and imaging (STICI3D) platform, suitable for rapid fabrication as well as mass manufacturing. Our design consists of a shallow trench, within which various hydrogels can be formed in situ via capillary action, between and fully in contact with two side channels that allow cell seeding and media replenishment, as well as forming concentration gradients of various molecules. Compared to a micropillar-based burst valve design, which requires sophisticated microfabrication facilities, our capillary-based STICI3D can be fabricated using molds prepared with simple adhesive tapes and razors alone. The simple design supports the easy applicability of mass-production methods such as hot embossing and injection molding as well. To optimize the STICI3D design, we investigated the effect of individual design parameters such as corner radii, trench height, and surface wettability under various inlet pressures on the confinement of a hydrogel solution within the shallow trench using Computational Fluid Dynamics simulations supported with experimental validation. We identified ideal design values that improved the robustness of hydrogel confinement and reduced the effect of end-user dependent factors such as hydrogel solution loading pressure. Finally, we demonstrated cultures of human mesenchymal stem cells and human umbilical cord endothelial cells in the STICI3D to show that it supports 3D cell cultures and enables precise control of cellular microenvironment and real-time microscopic imaging. The easy-to-fabricate and highly adaptable nature of the STICI3D platform makes it suitable for researchers interested in fabricating custom polydimethylsiloxane devices as well as those who are in need of ready-to-use plastic platforms. As such, STICI3Ds can be used in imaging cell-cell interactions, angiogenesis, semiquantitative analysis of drug response in cells, and measurement of transport through cell sheet barriers.

Endothelial Cell Distribution After Flow Exposure With Two Stent Struts Placed in Different Angles

Stent implantation has been a primary treatment for stenosis and other intravascular diseases. However, the struts expansion procedure might cause endothelium lesion and the structure of the struts could disturb the blood flow environment near the wall of the blood vessel. These changes could damage the vascular innermost endothelial cell (EC) layer and pose risks of restenosis and post-deployment thrombosis. This research aims to investigate the effect of flow alterations on EC distribution in the presence of gap between two struts within the parallel flow chamber. To study how the gap presence impacts EC migration and the endothelialization effect on the surface of the struts, two struts were placed with specific orientations and positions on the EC layer in the flow chamber. After a 24-h exposure under wall shear stress (WSS), we observed the EC distribution conditons especially in the gap area. We also conducted computational fluid dynamics (CFD) simulations to calculate the WSS distribution. High EC-concentration areas on the bottom plate corresponded to the high WSS by the presence of gap between the two struts. To find the relation between the WSS and EC distributions on the fluorescence images, WSS condition by CFD simulation could be helpful for the EC distribution. The endothelialization rate, represented by EC density, on the downstream sides of both struts was higher than that on the upstream sides. These observations were made in the flow recirculation at the gap area between two struts. On two side surfaces between the gaps, meaning the downstream at the first and the upstream at the second struts, EC density differences on the downstream surfaces of the first strut were higher than on the upstream surfaces of the second strut. Finally, EC density varied along the struts when the struts were placed at tilted angles. These results indicate that, by the presence of gap between the struts, ECs distribution could be predicted in both perpendicular and tiled positions. And tiled placement affect ECs distribution on the strut side surfaces.

Bioengineering the Bone Marrow Vascular Niche

The bone marrow (BM) tissue is the main physiological site for adult hematopoiesis. In recent years, the cellular and matrix components composing the BM have been defined with unprecedent resolution, both at the molecular and structural levels. With the expansion of this knowledge, the possibility of reproducing a BM-like structure, to ectopically support and study hematopoiesis, becomes a reality. A number of experimental systems have been implemented and have displayed the feasibility of bioengineering BM tissues, supported by cells of mesenchymal origin. Despite being known as an abundant component of the BM, the vasculature has been largely disregarded for its role in regulating tissue formation, organization and determination. Recent reports have highlighted the crucial role for vascular endothelial cells in shaping tissue development and supporting steady state, emergency and malignant hematopoiesis, both pre- and postnatally. Herein, we review the field of BM-tissue bioengineering with a particular focus on vascular system implementation and integration, starting from describing a variety of applicable in vitro models, ending up with in vivo preclinical models. Additionally, we highlight the challenges of the field and discuss the clinical perspectives in terms of adoptive transfer of vascularized BM-niche grafts in patients to support recovering hematopoiesis.

Novel in vitro microfluidic platform for osteocyte mechanotransduction studies

Osteocytes are the major mechanosensing cells in bone remodeling. Current in vitro bone mechanotransduction research use macroscale devices such as flow chambers; however, in vitro microfluidic devices provide an optimal tool to better understand this biological process with its flexible design, physiologically relevant dimensions and high-throughput capabilities. This project aims to design and fabricate a multi-shear stress, co-culture platform to study the interaction between osteocytes and other bone cells under varying flow conditions. Standard microfluidic design utilizing changing geometric parameters is used to induce different flow rates that are directly proportional to the levels of shear stress, with devices fabricated from standard polydimethylsiloxane (PDMS)-based softlithography processes. Each osteocyte channel (OCY) is connected to an adjacent osteoclast channel (OC) by 20-μm perfusion channels for cellular signaling molecule transport. Significant differences in RANKL levels are observed between channels with different shear stress levels, and we observed that pre-osteoclast differentiation was directly affected by adjacent flow-stimulated osteocytes. Significant decrease in the number of differentiating osteoclasts is observed in the OC channel adjacent to the 2-Pa shear stress OCY channel, while differentiation adjacent to the 0.5-Pa shear stress OCY channel is unaffected compared with no-flow controls. Addition of zoledronic acid showed a significant decrease in osteoclast differentiation, compounding to effect instigated by increasing fluid shear stress. Using this platform, we are able to mimic the interaction between osteocytes and osteoclasts in vitro under physiologically relevant bone interstitial fluid flow shear stress. Our novel microfluidic co-culture platform provides an optimal tool for bone cell mechanistic studies and provides a platform for the discovery of potential drug targets for clinical treatments of bone-related diseases.

Engineering transferrable microvascular meshes for subcutaneous islet transplantation

The success of engineered cell or tissue implants is dependent on vascular regeneration to meet adequate metabolic requirements. However, development of a broadly applicable strategy for stable and functional vascularization has remained challenging. We report here highly organized and resilient microvascular meshes fabricated through a controllable anchored self-assembly method. The microvascular meshes are scalable to centimeters, almost free of defects and transferrable to diverse substrates, ready for transplantation. They promote formation of functional blood vessels, with a density as high as ~220 vessels mm-2, in the poorly vascularized subcutaneous space of SCID-Beige mice. We further demonstrate the feasibility of fabricating microvascular meshes from human induced pluripotent stem cell-derived endothelial cells, opening a way to engineer patient-specific microvasculature. As a proof-of-concept for type 1 diabetes treatment, we combine microvascular meshes and subcutaneously transplanted rat islets and achieve correction of chemically induced diabetes in SCID-Beige mice for 3 months.

Human Pluripotent Stem Cells to Engineer Blood Vessels

Development of pluripotent stem cells (PSCs) is a remarkable scientific advancement that allows scientists to harness the power of regenerative medicine for potential treatment of disease using unaffected cells. PSCs provide a unique opportunity to study and combat cardiovascular diseases, which continue to claim the lives of thousands each day. Here, we discuss the differentiation of PSCs into vascular cells, investigation of the functional capabilities of the derived cells, and their utilization to engineer microvascular beds or vascular grafts for clinical application. Graphical Abstract Human iPSCs generated from patients are differentiated toward ECs and perivascular cells for use in disease modeling, microvascular bed development, or vascular graft fabrication.

Engineering in-vitro stem cell-based vascularized bone models for drug screening and predictive toxicology

The production of veritable in-vitro models of bone tissue is essential to understand the biology of bone and its surrounding environment, to analyze the pathogenesis of bone diseases (e.g., osteoporosis, osteoarthritis, osteomyelitis, etc.), to develop effective therapeutic drug screening, and to test potential therapeutic strategies. Dysregulated interactions between vasculature and bone cells are often related to the aforementioned pathologies, underscoring the need for a bone model that contains engineered vasculature. Due to ethical restraints and limited prediction power of animal models, human stem cell-based tissue engineering has gained increasing relevance as a candidate approach to overcome the limitations of animals and to serve as preclinical models for drug testing. Since bone is a highly vascularized tissue, the concomitant development of vasculature and mineralized matrix requires a synergistic interaction between osteogenic and endothelial precursors. A number of experimental approaches have been used to achieve this goal, such as the combination of angiogenic factors and three-dimensional scaffolds, prevascularization strategies, and coculture systems. In this review, we present an overview of the current models and approaches to generate in-vitro stem cell-based vascularized bone, with emphasis on the main challenges of vasculature engineering. These challenges are related to the choice of biomaterials, scaffold fabrication techniques, and cells, as well as the type of culturing conditions required, and specifically the application of dynamic culture systems using bioreactors.

Controlling Differentiation of Stem Cells for Developing Personalized Organ-on-Chip Platforms

Organ-on-chip (OOC) platforms have attracted attentions of pharmaceutical companies as powerful tools for screening of existing drugs and development of new drug candidates. OOCs have primarily used human cell lines or primary cells to develop biomimetic tissue models. However, the ability of human stem cells in unlimited self-renewal and differentiation into multiple lineages has made them attractive for OOCs. The microfluidic technology has enabled precise control of stem cell differentiation using soluble factors, biophysical cues, and electromagnetic signals. This study discusses different tissue- and organ-on-chip platforms (i.e., skin, brain, blood-brain barrier, bone marrow, heart, liver, lung, tumor, and vascular), with an emphasis on the critical role of stem cells in the synthesis of complex tissues. This study further recaps the design, fabrication, high-throughput performance, and improved functionality of stem-cell-based OOCs, technical challenges, obstacles against implementing their potential applications, and future perspectives related to different experimental platforms.

Three-dimensional culture of small-diameter vascular grafts

Analysis of efforts to engineer 3D small-diameter (<6 mm) vascular grafts, indicating the importance of stem cells, co-culture, and pulsatile flow.

A novel high-speed production process to create modular components for the bottom-up assembly of large-scale tissue-engineered constructs

To replace damaged or diseased tissues, large tissue-engineered constructs can be prepared by assembling modular components in a bottom-up approach. However, a high-speed method is needed to produce sufficient numbers of these modules for full-sized tissue substitutes. To this end, a novel production technique is devised, combining air shearing and a plug flow reactor-style design to rapidly produce large quantities of hydrogel-based (here type I collagen) cylindrical modular components with tunable diameters and length. Using this technique, modules containing NIH 3T3 cells show greater than 95% viability while endothelial cell surface attachment and confluent monolayer formation are demonstrated. Additionally, the rapidly produced modules are used to assemble large tissue constructs (>1 cm(3) ) in vitro. Module building blocks containing luciferase-expressing L929 cells are packed in full size adult rat-liver-shaped bioreactors and perfused with cell medium, to demonstrate the capacity to build organ-shaped constructs; bioluminescence demonstrates sustained viability over 3 d. Cardiomyocyte-embedded modules are also used to assemble electrically stimulatable contractile tissue.

The role of mechanical stimuli in the vascular differentiation of mesenchymal stem cells

Mesenchymal stem cells (MSCs) are among the most promising and suitable stem cell types for vascular tissue engineering. Substantial effort has been made to differentiate MSCs towards vascular cell phenotypes, including endothelial cells and smooth muscle cells (SMCs). The microenvironment of vascular cells not only contains biochemical factors that influence differentiation, but also exerts hemodynamic forces, such as shear stress and cyclic strain. Recent evidence has shown that these forces can influence the differentiation of MSCs into endothelial cells or SMCs. In this Commentary, we present the main findings in the area with the aim of summarizing the mechanisms by which shear stress and cyclic strain induce MSC differentiation. We will also discuss the interactions between these mechanical cues and other components of the microenvironment, and highlight how these insights could be used to maintain differentiation.

Vascular targeting of nanocarriers: perplexing aspects of the seemingly straightforward paradigm

Targeted nanomedicine holds promise to find clinical use in many medical areas. Endothelial cells that line the luminal surface of blood vessels represent a key target for treatment of inflammation, ischemia, thrombosis, stroke, and other neurological, cardiovascular, pulmonary, and oncological conditions. In other cases, the endothelium is a barrier for tissue penetration or a victim of adverse effects. Several endothelial surface markers including peptidases (e.g., ACE, APP, and APN) and adhesion molecules (e.g., ICAM-1 and PECAM) have been identified as key targets. Binding of nanocarriers to these molecules enables drug targeting and subsequent penetration into or across the endothelium, offering therapeutic effects that are unattainable by their nontargeted counterparts. We analyze diverse aspects of endothelial nanomedicine including (i) circulation and targeting of carriers with diverse geometries, (ii) multivalent interactions of carrier with endothelium, (iii) anchoring to multiple determinants, (iv) accessibility of binding sites and cellular response to their engagement, (v) role of cell phenotype and microenvironment in targeting, (vi) optimization of targeting by lowering carrier avidity, (vii) endocytosis of multivalent carriers via molecules not implicated in internalization of their ligands, and (viii) modulation of cellular uptake and trafficking by selection of specific epitopes on the target determinant, carrier geometry, and hydrodynamic factors. Refinement of these aspects and improving our understanding of vascular biology and pathology is likely to enable the clinical translation of vascular endothelial targeting of nanocarriers.

The role of physiological mechanical cues on mesenchymal stem cell differentiation in an airway tract-like dense collagen-silk fibroin construct

Airway tracts serve as a conduit of transport in the respiratory system. Architecturally, these are composed of cartilage rings that offer flexibility and prevent collapse during normal breathing. To this end, the successful regeneration of an airway tract requires the presence of differentiated chondrocytes and airway smooth muscle cells. This study investigated the role of physiological dynamic mechanical stimulation, in vitro, on the differentiation of mesenchymal stem cells (MSCs), three-dimensionally seeded within a tubular dense collagen matrix construct-reinforced with rings of electrospun silk fibroin mat (TDC-SFC). In particular, the role of either shear stress supplied by laminar fluid flow or cyclic shear stress in combination with circumferential strain, provided by pulsatile flow, on the chondrogenic differentiation, and contractile lineage of MSCs, and their effects on TDC-SFC morphology and mechanical properties were analysed. Chondrogenic differentiation of MSCs was observed in the presence of chondrogenic supplements under both static and laminar flow cultures. In contrast, physiological pulsatile flow resulted in preferential cellular orientation within TDC-SFC, as dictated by dynamic circumferential strain, and induced MSC contractile phenotype expression. In addition, pulsatile flow decreased MSC-mediated collagen matrix remodelling and increased construct circumferential strength. Therefore, TDC-SFC demonstrated the central role of a matrix in the delivery of mechanical stimuli over chemical factors, by providing an in vitro niche to control MSC differentiation, alignment and its capacity to remodel the matrix.

Del-1 overexpression in endothelial cells increases vascular density in tissue-engineered implants containing endothelial cells and adipose-derived mesenchymal stromal cells

We used a combination of strategies to stimulate the vascularization of tissue-engineered constructs in vivo including a modular approach to build larger tissues from individual building blocks ("modules") mixed together. Each building block included vascular cells by design; modules were submillimeter-sized collagen gels with an outer layer of endothelial cells (ECs), and with embedded adipose-derived mesenchymal stromal cells (adMSCs) to support EC survival and blood vessel maturation in vivo. We transduced the ECs that coat the modules with a lentiviral construct to overexpress the angiogenic extracellular matrix (ECM) protein Developmental endothelial locus-1 (Del-1). Upon injection of modules in a subcutaneous SCID/Bg mouse model, there was an increase in the number of blood vessels for implants with ECs transduced to overexpress Del-1 compared with control implants (with enhanced green fluorescent protein [eGFP]-transduced ECs) over the 21-day duration of the study. The greatest difference between Del-1 and eGFP implants and the highest number of blood vessels were observed 7 days after transplantation. The day-7 Del-1 implants also had increased SMA+ staining compared with control, suggesting increased blood vessel maturation through recruitment of SMA+ smooth muscle cells or pericytes to stabilize the newly formed blood vessels. Perfusion studies (microcomputed tomography, ultrasound imaging, and systemic injection of fluorescent UEA-1 or dextran) showed that some of the newly formed blood vessels (both donor derived and host derived, in both Del-1 and eGFP implants) were perfused and connected to the host vasculature as early as 7 days after transplantation, and at later time points as well. Nevertheless, perfusion of the implants was limited in some cases, suggesting that further improvements are necessary to normalize the vasculature at the implant site.

Cotransplantation of adipose-derived mesenchymal stromal cells and endothelial cells in a modular construct drives vascularization in SCID/bg mice

A modular approach to adipose tissue engineering was explored by embedding adipose-derived mesenchymal stromal cells (adMSC) in sub-mm-sized collagen rods or "modules" and coating with human microvascular endothelial cells (HMEC). After subcutaneous injection into a SCID/Bg mouse, HMEC on modules containing embedded adMSC appeared to detach from the modules to form vessels as early as day 3, as confirmed by the human EC-specific UEA-1 lectin stain, and these vessels persisted for up to 90 days. Vessel numbers decreased over 14 days, but vessel size increased suggesting a maturing of the vasculature. Vessel perfusion with the host was confirmed at 21 days by microCT. HMEC on modules without embedded adMSC remained attached to the module surface at day 3 and UEA-1 staining disappeared over 14 days suggesting cell death. It appeared that cotransplantation with adMSC had an anti-apoptotic and proangiogenic effect on HMEC. The early revascularization strategy may be successful in supporting adMSC viability and differentiation, as a preliminary study suggests progressive fat accumulation in the HMEC+adMSC implants: ∼60% of the implant area stained positive for Oil Red O by day 90. adMSC-embedded modules without HMEC surface coating did not show similar levels of Oil Red O staining. All implant volumes decreased over the time course of the experiment, yet HMEC+adMSC module implants were larger than adMSC-only implants at day 90. Collagen gel is mechanically weak and contracts in vivo making it unsuitable as a biomaterial for adipose tissue engineering where volume maintenance is critical. When combined with an appropriate biomaterial, the modular approach to adipose tissue engineering may represent a successful strategy to engineer soft tissue substitutes of clinical relevance.

Bioreactors to influence stem cell fate: augmentation of mesenchymal stem cell signaling pathways via dynamic culture systems

BACKGROUND

Mesenchymal stem cells (MSCs) are a promising cell source for bone and cartilage tissue engineering as they can be easily isolated from the body and differentiated into osteoblasts and chondrocytes. A cell based tissue engineering strategy using MSCs often involves the culture of these cells on three-dimensional scaffolds; however the size of these scaffolds and the cell population they can support can be restricted in traditional static culture. Thus dynamic culture in bioreactor systems provides a promising means to culture and differentiate MSCs in vitro.

SCOPE OF REVIEW

This review seeks to characterize key MSC differentiation signaling pathways and provides evidence as to how dynamic culture is augmenting these pathways. Following an overview of dynamic culture systems, discussion will be provided on how these systems can effectively modify and maintain important culture parameters including oxygen content and shear stress. Literature is reviewed for both a highlight of key signaling pathways and evidence for regulation of these signaling pathways via dynamic culture systems.

MAJOR CONCLUSIONS

The ability to understand how these culture systems are affecting MSC signaling pathways could lead to a shear or oxygen regime to direct stem cell differentiation. In this way the efficacy of in vitro culture and differentiation of MSCs on three-dimensional scaffolds could be greatly increased.

GENERAL SIGNIFICANCE

Bioreactor systems have the ability to control many key differentiation stimuli including mechanical stress and oxygen content. The further integration of cell signaling investigations within dynamic culture systems will lead to a quicker realization of the promise of tissue engineering and regenerative medicine. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

Bone marrow-derived mesenchymal stromal cells enhance chimeric vessel development driven by endothelial cell-coated microtissues

Adding bone marrow-derived mesenchymal stromal cells (bmMSCs) to endothelialized collagen gel modules resulted in mature vessel formation, presumably caused in part by the observed display of pericyte-like behavior for the transplanted GFP(+) bmMSCs. A previous study determined that rat aortic endothelial cells (RAECs) delivered on the surface of small (∼0.8 mm long×0.5 mm diameter) collagen gel cylinders (microtissues, modular tissue engineering) formed vessels after transplantation into immunosuppressed Sprague-Dawley (SD) rats. Although the RAECs formed vessels in this allogeneic transplant model, there was a robust inflammatory response and the vessels that formed were leaky as shown by microcomputed tomography (microCT) perfusion studies. In vitro assays showed that SD rat bmMSCs embedded into the collagen gel modules increased the extent of EC proliferation and enhanced EC sprouting. In vivo, although vessel number was not affected, the new vessels formed by the bmMSCs and RAECs were more stable and leaked less in the microCT perfusion analysis than vessels formed by implanted RAECs alone. Addition of the bmMSCs also decreased the total number of CD68(+) macrophages that infiltrated the implant and changed the distribution of CD163(+) (M2) macrophages so that they were found within the newly developed vascularized tissue. Most interestingly, the bmMSCs became smooth muscle actin positive and migrated to surround the EC layer of the vessel, which is the location typical of pericytes. The combination of these two effects was presumed to be the cause of improved vascularity when bmMSCs were embedded in the EC-coated modules. Further exploration of these observations is warranted to exploit modular tissue engineering as a means of forming large vascularized functional tissues using microtissue components.

Endothelialized biomaterials for tissue engineering applications in vivo

Rebuilding tissues involves the creation of a vasculature to supply nutrients and this in turn means that the endothelial cells (ECs) of the resulting endothelium must be a quiescent non-thrombogenic blood contacting surface. Such ECs are deployed on biomaterials that are composed of natural materials such as extracellular matrix proteins or synthetic polymers in the form of vascular grafts or tissue-engineered constructs. Because EC function is influenced by their origin, biomaterial surface chemistry and hemodynamics, these issues must be considered to optimize implant performance. In this review, we examine the recent in vivo use of endothelialized biomaterials and discuss the fundamental issues that must be considered when engineering functional vasculature.