Therapeutic neovascularization: contributions from bioengineering

Musculoskeletal tissues-on-a-chip: role of natural polymers in reproducing tissue-specific microenvironments

Over the past years, 3D in vitro models have been widely employed in the regenerative medicine field. Among them, organ-on-a-chip technology has the potential to elucidate cellular mechanism exploiting multichannel microfluidic devices to establish 3D co-culture systems that offer control over the cellular, physico-chemical and biochemical microenvironments. To deliver the most relevant cues to cells, it is of paramount importance to select the most appropriate matrix for mimicking the extracellular matrix of the native tissue. Natural polymers-based hydrogels are the elected candidates for reproducing tissue-specific microenvironments in musculoskeletal tissue-on-a-chip models owning to their interesting and peculiar physico-chemical, mechanical and biological properties. Despite these advantages, there is still a gap between the biomaterials complexity in conventional tissue engineering and the application of these biomaterials in 3D in vitro microfluidic models. In this review, the aim is to suggest the adoption of more suitable biomaterials, alternative crosslinking strategies and tissue engineered-inspired approaches in organ-on-a-chip to better mimic the complexity of physiological musculoskeletal tissues. Accordingly, after giving an overview of the musculoskeletal tissue compositions, the properties of the main natural polymers employed in microfluidic systems are investigated, together with the main musculoskeletal tissues-on-a-chip devices.

Applications of electrospun scaffolds with enlarged pores in tissue engineering

Despite electrospinning has multiple advantages over other methods such as creating materials with superfine fiber diameter, high specific surface area, and good mechanical properties, the pore diameter of scaffolds prepared...

Novel Human Placenta-Based Extract for Vascularization Strategies in Tissue Engineering

There is critical unmet need for new vascularized tissues to support or replace injured tissues and organs. Various synthetic and natural materials were already established for use of two-dimensional (2D) and three-dimensional (3D) in vitro neovascularization assays, however, they still cannot mimic the complex functions of the sum of the extracellular matrix (ECM) in native intact tissue. Currently, this issue is only addressed by artificial products such as Matrigel^™, which comprises a complex mixture of ECM proteins, extracted from animal tumor tissue. Despite its outstanding bioactivity, the isolation from tumor tissue hinders its translation into clinical applications. Since nonhuman ECM proteins may cause immune reactions, as are frequently observed in clinical trials, human ECM proteins represent the best option when aiming for clinical applications. Here, we describe an effective method of isolating a human placenta substrate (hpS) that induces the spontaneous formation of an interconnected network of green fluorescence-labeled human umbilical vein endothelial cells (gfpHUVECs) in vitro. The substrate was biochemically characterized by using a combination of bicinchoninic acid (BCA) assay, DNA, and glycosaminoglycan (GAG) content assays, sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS-PAGE) analysis and Western blot, angiogenesis arrays, chromatographic thrombin detection, high performance liquid chromatography (HPLC)-based amino acid quantification analysis, and assessment of antimicrobial properties. 2D in vitro cell culture experiments have been performed to determine the vasculogenic potential of hpS, which demonstrated that cell networks developed on hpS show a significantly higher degree of complexity (number of tubules/junctions; total/mean tube length) when compared with Matrigel. As 3D cell culture techniques represent a more accurate representation of the in vivo condition, the substrate was 3D solidified using various natural polymers. 3D in vitro vasculogenesis assays have been performed by seeding gfpHUVECs in an hpS-fibrinogen clot. In conclusion, hpS provides a potent human/material-based alternative to xenogenic-material-based biomaterials for vascularization strategies in tissue engineering.

Advances in Injectable and Self-healing Polysaccharide Hydrogel Based on the Schiff Base Reaction

Injectable hydrogel possesses great application potential in disease treatment and tissue engineering, but damage to gel often occurs due to the squeezing pressure from injection devices and the mechanical forces from limb movement, and leads to the rapid degradation of gel matrix and the leakage of the load material. The self-healing injectable hydrogels can overcome these drawbacks via automatically repairing gel structural defects and restoring gel function. The polysaccharide hydrogels constructed through the Schiff base reaction own advantages including simple fabrication, injectability, and self-healing under physiological conditions, and therefore have drawn extensive attention and investigation recently. In this short review, the preparation and self-healing properties of the polysaccharide hydrogels that is established on the Schiff base reaction are focused on and their biological applications in drug delivery and cell therapy are discussed.

Design and characterization of hydrogel nanoparticles with tunable network characteristics for sustained release of a VEGF-mimetic peptide

Peptides that mimic the bioactivity of growth factors are rapidly emerging as therapeutics for a variety of drug delivery applications including therapeutic neovascularization. Neovascularization requires controlled and sustained delivery of proangiogenic factors to stimulate reperfusion of ischemic tissues. To this end, hydrogel nanoparticles were designed to provide sustained and tunable diffusion-based release of a pro-angiogenic peptide, QK. Inverse phase mini-emulsion polymerization (IPMP) was used to generate crosslinked poly(ethylene) glycol (PEG) hydrogel nanoparticles entrapped with the QK peptide. Peptide release kinetics were tuned through adjustments in nanoparticle crosslink density. This was achieved by altering the mole fraction of the crosslinking agent which allowed for the synthesis of low crosslink density (0.754 mmol cm^-3) and high crosslink density (0.810 mmol cm^-3) nanoparticles. Nanoparticle tracking analysis revealed narrow particle size distributions and similar particle sizes regardless of crosslink density (225 ± 75 nm and 233 ± 73 nm, for low and high crosslink density nanoparticles, respectively). The zeta potential was found to be -26 mV for blank nanoparticles and +4 mV in the case of QK-loaded nanoparticles. The resulting nanoparticle crosslink density impacted both peptide loading as well as release kinetics. In terms of cumulative fractional release and weight of peptide released per mass of nanoparticle, higher crosslink density nanoparticles resulted in slower peptide release kinetics. The IPMP process preserved the QK secondary structure and its bioactivity as confirmed using circular dichroism spectroscopy and a Matrigel tubulogenesis assay, respectively, with released peptide. The presented nanoparticles hold great potential for use as drug delivery carriers for stimulation of therapeutic neovascularization of ischemic tissues.

Soft tissue fillers for adipose tissue regeneration: From hydrogel development toward clinical applications

There is a clear and urgent clinical need to develop soft tissue fillers that outperform the materials currently used for adipose tissue reconstruction. Recently, extensive research has been performed within this field of adipose tissue engineering as the commercially available products and the currently existing techniques are concomitant with several disadvantages. Commercial products are highly expensive and associated with an imposing need for repeated injections. Lipofilling or free fat transfer has an unpredictable outcome with respect to cell survival and potential resorption of the fat grafts. Therefore, researchers are predominantly investigating two challenging adipose tissue engineering strategies: in situ injectable materials and porous 3D printed scaffolds. The present work provides an overview of current research encompassing synthetic, biopolymer-based and extracellular matrix-derived materials with a clear focus on emerging fabrication technologies and developments realized throughout the last decade. Moreover, clinical relevance of the most promising materials will be discussed, together with potential concerns associated with their application in the clinic.

Lipoprotein Receptor-related Protein 6 Signaling is Necessary for Vasculogenic Differentiation of Human Dental Pulp Stem Cells

The aim of this study was to evaluate the effects of Wnt signaling through lipoprotein receptor-related protein 6 (LRP6) and Frizzled6 on the endothelial differentiation of dental pulp stem cells (DPSCs). DPSCs were stably transduced with enhanced green fluorescent protein (EGFP)-tagged lentiviral vectors (short hairpin RNA-LRP6, short hairpin RNA-Frizzled6, or empty vector controls). We evaluated the effects of LRP6 and Frizzled6 on expression of endothelial markers and on capillary tube formation mediated by DPSCs induced with recombinant human Wnt1 (rhWnt1) and/or recombinant human vascular endothelial growth factor165 (rhVEGF165). In vivo, tooth slices/scaffolds were seeded with LRP6-silenced, Frizzled6-silenced, or vector control DPSC cells and transplanted into immunodeficient mice. The density of blood vessels generated by DPSCs differentiated into vascular endothelial cells was analyzed by immunohistochemistry for EGFP. The rhWnt1 and rhVEGF165 induced expression of active β-catenin in control DPSCs and in Frizzled6-silenced DPSCs, but not in LRP6-silenced DPSCs. Furthermore, VEGF and interleukin-8 were downregulated in LRP6-silenced DPSCs, but not in control DPSCs or in Frizzled6-silenced DPSCs (P < .05). Likewise, rhWnt1 and rhVEGF165 induced expression of the endothelial marker VEGF receptor-2 in control DPSCs and in Frizzled6-silenced DPSCs, but not in LRP6-silenced DPSCs. These data correlated with a trend for lower density of capillary sprouts generated by LRP6-silenced DPSCs when compared with control DPSCs in Matrigel. In vivo, tooth slice/scaffolds seeded with DPSC-short hairpinRNA-LRP6 cells showed lower density of human blood vessels (ie, EGFP-positive blood vessels), when compared with tooth slice/scaffolds seeded with vector control cells (P < .05). Collectively, these data demonstrated that LRP6 signaling is necessary for the vasculogenic differentiation of human DPSCs.

Hybrid Polycaprolactone/Alginate Scaffolds Functionalized with VEGF to Promote de Novo Vessel Formation for the Transplantation of Islets of Langerhans

Although regarded as a promising treatment for type 1 diabetes, clinical islet transplantation in the portal vein is still hindered by a low transplantation outcome. Alternative transplantation sites have been proposed, but the survival of extra-hepatically transplanted islets of Langerhans critically depends on quick revascularization after engraftment. This study aims at developing a new 3D scaffold platform that can actively boost vascularization and may find an application for extra-hepatic islet transplantation. The construct consists of a 3D ring-shaped polycaprolactone (PCL) scaffold with heparinized surface to electrostatically bind vascular endothelial growth factor (VEGF), surrounding a hydrogel core for islets encapsulation. Heparin immobilization improves the amount of VEGF retained by the construct, up to 3.6 fold, compared to untreated PCL scaffolds. In a chicken chorioallanthoic membrane model, VEGF immobilized on the construct enhances angiogenesis in close proximity and on the surface of the scaffolds. After 7 days, islets encapsulated in the alginate core show functional response to glucose stimuli comparable to free-floating islets. Thus, the developed platform has the potential to support rapid vascularization and islet endocrine function.

Enhancing Therapeutic Efficacy and Reducing Cell Dosage in Stem Cell Transplantation Therapy for Ischemic Limb Diseases by Modifying the Cell Injection Site

In conventional stem cell transplantation therapies for ischemic limb diseases, stem cells are generally transplanted into the ischemic region (IR), and most of the transplanted cells undergo hypoxia-mediated cell death. Due to massive cell death, the therapeutic efficacy is reduced and a high dose of stem cells is necessitated for the therapies. In this study, we investigated whether the therapeutic efficacy can be improved and the cell dosage can be reduced in the therapy for limb ischemia simply by modifying the stem cell injection site to a site where cell engraftment is improved and blood vessel sprouting is efficiently stimulated. Human mesenchymal stem cells (hMSCs) cultured under hypoxic condition, which simulates cells transplanted to IR, underwent extensive cell death in vitro. Importantly, cell death was significantly attenuated when hMSCs adhered first under normoxic condition for 24 h and then were exposed to hypoxic condition, which simulates cells transplanted to the border zone (BZ) in the upper thigh and migrated to IR. hMSCs, at doses of 2 × 10(5) or 2 × 10(6) cells, were injected into the IR or BZ of 5-week-old female athymic mice after ischemic hindlimb induction. Compared with human mesenchymal stem cell (hMSC) transplantation to the IR of mouse ischemic limbs, transplantation to the BZ significantly enhanced cell engraftment and paracrine factor secretion, which effectively stimulated vessel sprouting, enhanced blood perfusion in IR, and enabled the cell dosage reduction. Therefore, modification of the stem cell transplantation site would improve the current stem cell therapies for ischemic limb diseases in terms of cell dosage reduction and therapeutic efficacy enhancement.

Biomaterials with persistent growth factor gradients in vivo accelerate vascularized tissue formation

Gradients of soluble factors play an important role in many biological processes, including blood vessel assembly. Gradients can be studied in detail in vitro, but methods that enable the study of spatially distributed soluble factors and multi-cellular processes in vivo are limited. Here, we report on a method for the generation of persistent in vivo gradients of growth factors in a three-dimensional (3D) biomaterial system. Fibrin loaded porous poly (ethylene glycol) (PEG) scaffolds were generated using a particulate leaching method. Platelet derived growth factor BB (PDGF-BB) was encapsulated into poly (lactic-co-glycolic acid) (PLGA) microspheres which were placed distal to the tissue-material interface. PLGA provides sustained release of PDGF-BB and its diffusion through the porous structure results in gradient formation. Gradients within the scaffold were confirmed in vivo using near-infrared fluorescence imaging and gradients were present for more than 3 weeks. The diffusion of PDGF-BB was modeled and verified with in vivo imaging findings. The depth of tissue invasion and density of blood vessels formed in response to the biomaterial increased with magnitude of the gradient. This biomaterial system allows for generation of sustained growth factor gradients for the study of tissue response to gradients in vivo.

Effects of bioactive cements incorporating zinc-bioglass nanoparticles on odontogenic and angiogenic potential of human dental pulp cells

Background

The objective of this study was to investigate the effects of bioactive calcium phosphate cements (CPC, α-tricalcium phosphate-based) incorporating zinc-bioglass (ZnBG) on the odontogenic differentiation and angiogenesis of human dental pulp cells (HDPCs).

Methods

BGs with varying concentrations of Zn (0, 2.5 and 5%) were produced via a sol-gel process. The proliferation of HDPCs on CPC/BGs was determined by MTS assay. Alizarin red staining, RT-PCR, and ALP activity were used to assess odontogenic differentiation, and western blot analysis was used to asses signaling pathways. In vitro angiogenesis was examined via mRNA expression of angiogenic genes and tubule formation.

Results

All cement formulations showed no cytotoxicity. The CPCs with ZnBG showed increased ALP activity, enhanced formation of mineralized nodules, and upregulated mRNA expression of DMP-1, DSPP, Runx2, and osterix in a time- and dose-dependent manner, relative to CPCs without Zn. ZnBG upregulated integrins α1, α2, β1, and β3 and activated integrin downstream signal pathways, such as p-FAK, p-Akt, p-paxillin, RhoA, MAPK, and NF-κB, as well as canonical and non-canonical Wnt signaling. In addition, ZnBG upregulated VEGF mRNA in HDPCs and increased the tubular structure in endothelial cells.

Conclusions

Our results demonstrate that ZnBG incorporated within CPCs activates odontogenic differentiation and promotes angiogenesis in vitro through integrin, Wnt, MAPK, and NF-κB pathways. Thus, CPCs incorporating ZnBG are promising matrices in tissue engineering to stimulate endodontic regeneration.

A tyrosine-rich amelogenin peptide promotes neovasculogenesis in vitro and ex vivo

The formation of new blood vessels has been shown to be fundamental in the repair of many damaged tissues, and we have recently shown that the adult human periodontal ligament contains multipotent stem/progenitor cells that are capable of undergoing vasculogenic and angiogenic differentiation in vitro and ex vivo. Enamel matrix protein (EMP) is a heterogeneous mixture of mainly amelogenin-derived proteins produced during tooth development and has been reported to be sometimes effective in stimulating these processes, including in clinical regeneration of the periodontal ligament. However, the identity of the specific bioactive component of EMP remains unclear. In the present study we show that, while the high-molecular-weight Fraction A of enamel matrix derivative (a heat-treated form of EMP) is unable to stimulate the vasculogenic differentiation of human periodontal ligament cells (HPC) in vitro, the low-molecular-weight Fraction C significantly up-regulates the expression of the endothelial markers VEGFR2, Tie-1, Tie-2, VE-cadherin and vWF and markedly increases the internalization of low-density lipoprotein. Furthermore, we also demonstrate, for the first time, that the synthetic homolog of the 45-amino acid tyrosine-rich amelogenin peptide (TRAP) present in Fraction C is likely to be responsible for its vasculogenesis-inducing activity. Moreover, the chemically synthesized TRAP peptide is also shown here to be capable of up-regulating the angiogenic differentiation of the HPC, based on its marked stimulation of in vitro cell migration and tubule formation and of blood vessel formation assay in a chick embryo chorioallantoic membrane model ex vivo. This novel peptide, and modified derivatives, might thereby represent a new class of regenerative drug that has the ability to elicit new blood vessel formation and promote wound healing in vivo.

FGF-1 delivery from multilayer alginate microbeads stimulates a rapid and persistent increase in vascular density

In recent years, great advances have been made in the use of islet transplantation as a treatment for type I diabetes. Indeed, it is possible that stimulation of local neovascularization upon transplantation could improve functional graft outcomes. In the present study, we investigate the use of multilayered alginate microbeads to provide a sustained delivery of FGF-1, and whether this results in increased neovascularization in vivo. Multilayered alginate microbeads, loaded with either 150ng or 600ng of FGF-1 in the outer layer, were surgically implanted into rats using an omentum pouch model and compared to empty microbead implants. Rats were sacrificed at 4days, 1week, and 6weeks. Staining for CD31 showed that both conditions of FGF-1 loaded microbeads resulted in a significantly higher vessel density at all time points studied. Moreover, at 6weeks, alginate microbeads containing 600ng FGF-1 provided a greater vascular density compared to both the control group and the microbeads loaded with 150ng FGF-1. Omenta analyzed via staining for smooth muscle alpha actin showed no variation in mural cell density at either 4days or 1week. At 6weeks, however, omenta exposed to microbeads loaded with 600ng FGF-1 showed an increase in mural cell staining compared to controls. These results suggest that the sustained delivery of FGF-1 from multilayered alginate microbeads results in a rapid and persistent vascular response. An increase in the local blood supply could reduce the number of islets required for transplantation in order to achieve clinical efficacy.

Effects of enamel matrix proteins on multi-lineage differentiation of periodontal ligament cells in vitro

The adult periodontal ligament (PDL) is considered to contain progenitor cells that are involved in the healing of periodontal wounds. Treatment with enamel matrix derivative (EMD), a heat-treated preparation derived from enamel matrix proteins (EMPs), has been shown to be of some clinical benefit in eliciting periodontal regeneration in vivo. Although there is extensive information available about the effects of EMD on periodontal regeneration, the precise influence of this material on alveolar bone and the formation of blood vessels and proprioceptive sensory nerves, prominent features of functionally active periodontal tissue, remain unclear. The aim of the present study was therefore to examine the effects of EMD on the ability of human periodontal ligament cells (HPCs) to undergo multi-lineage differentiation in vitro. Our results showed that HPCs treated with EMD under non-selective growth conditions did not show any evidence of osteogenic, adipogenic, chondrogenic, neovasculogenic, neurogenic and gliogenic "terminal" differentiation. In contrast, under selective lineage-specific culture conditions, EMD up-regulated osteogenic, chondrogenic and neovasculogenic genes and "terminal" differentiation, but suppressed adipogenesis, neurogenesis and gliogenesis. These findings thus demonstrate for the first time that EMD can differentially modulate the multi-lineage differentiation of HPCs in vitro.

Controlled proteolytic cleavage site presentation in biomimetic PEGDA hydrogels enhances neovascularization in vitro

The volume of tissue that can be engineered is limited by the extent to which vascularization can be stimulated within the scaffold. The ability of a scaffold to induce vascularization is highly dependent on its rate of degradation. We present a novel approach for engineering poly (ethylene glycol) diacrylate (PEGDA) hydrogels with controlled protease-mediated degradation independent of alterations in hydrogel mechanical and physical properties. Matrix metalloproteinase (MMP)-sensitive peptides containing one (SSite) or three (TriSite) proteolytic cleavage sites were engineered and conjugated to PEGDA macromers followed by photopolymerization to form PEGDA hydrogels with tethered cell adhesion ligands of YRGDS and with either single or multiple MMP-sensitive peptide domains between cross links. These hydrogels were investigated as provisional matrices for inducing neovascularization, while maintaining the structural integrity of the hydrogel network. We show that hydrogels made from SSite and TriSite peptide-containing PEGDA macromers polymerized under the same conditions do not result in alterations in hydrogel swelling, mesh size, or compressive modulus, but result in statistically different hydrogel degradation times with TriSite gels degrading in 1-3 h compared to 2-4 days in SSite gels. In both polymer types, increases in the PEGDA concentration result in decreases in hydrogel swelling and mesh size, and increases in the compressive modulus and degradation time. Furthermore, TriSite gels support vessel invasion over a 0.3-3.6 kPa range of compressive modulus, while SSite gels do not support invasion in hydrogels above compressive modulus values of 0.4 kPa. In vitro data demonstrate that TriSite gels result in enhanced vessel invasion areas by sevenfold and depth of invasion by twofold compared to SSite gels by 3 weeks. This approach allows for controlled, localized, and cell-mediated matrix remodeling and can be tailored to tissues that may require more rapid regeneration and neovascularization.

Advances in growth factor delivery for therapeutic angiogenesis

Therapeutic angiogenesis is a new revascularization strategy involving the administration of growth factors to induce new vessel formation. The biology and delivery of angiogenic growth factors involved in vessel formation have been extensively studied but success in translating the angiogenic capacity of growth factors into benefits for vascular disease patients is still limited. This could be attributed to issues related to patient selection, growth factor delivery methods or lack of vessel maturation. Comprehensive understanding of the cellular and molecular cross-talk during the different stages of vascular development is needed for the design of efficient therapeutic strategies. The presentation of angiogenic factors either in series or in parallel using a strategy that mimics physiological events, such as concentration and spatio-temporal profiles, is an immediate requirement for functional blood vessel formation. This review provides an overview of the recent delivery strategies of angiogenic factors and discusses targeting neovascular maturation as a promising approach to induce stable and functional vessels for therapeutic angiogenesis.

Fibrin-loaded porous poly(ethylene glycol) hydrogels as scaffold materials for vascularized tissue formation

Vascular network formation within biomaterial scaffolds is essential for the generation of properly functioning engineered tissues. In this study, a method is described for generating composite hydrogels in which porous poly(ethylene glycol) (PEG) hydrogels serve as scaffolds for mechanical and structural support, and fibrin is loaded within the pores to induce vascularized tissue formation. Porous PEG hydrogels were generated by a salt leaching technique with 100-150-μm pore size and thrombin (Tb) preloaded within the scaffold. Fibrinogen (Fg) was loaded into pores with varying concentrations and polymerized into fibrin due to the presence of Tb, with loading efficiencies ranging from 79.9% to 82.4%. Fibrin was distributed throughout the entire porous hydrogels, lasted for greater than 20 days, and increased hydrogel mechanical stiffness. A rodent subcutaneous implant model was used to evaluate the influence of fibrin loading on in vivo response. At weeks 1, 2, and 3, all hydrogels had significant tissue invasion, but no difference in the depth of invasion was found with the Fg concentration. Hydrogels with fibrin loading induced more vascularization, with a significantly higher vascular density at 20 mg/mL (week 1) and 40 mg/mL (weeks 2 and 3) Fg concentration compared to hydrogels without fibrin. In conclusion, we have developed a composite hydrogel that supports rapid vascularized tissue ingrowth, and thus holds great potential for tissue engineering applications.

Assessing the permeability of engineered capillary networks in a 3D culture

Many pathologies are characterized by poor blood vessel growth and reduced nutrient delivery to the surrounding tissue, introducing a need for tissue engineered blood vessels. Our lab has developed a 3D co-culture method to grow interconnected networks of pericyte-invested capillaries, which can anastamose with host vasculature following implantation to restore blood flow to ischemic tissues. However, if the engineered vessels contain endothelial cells (ECs) that are misaligned or contain wide junctional gaps, they may function improperly and behave more like the pathologic vessels that nourish tumors. The purpose of this study was to test the resistance to permeability of these networks in vitro, grown with different stromal cell types, as a metric of vessel functionality. A fluorescent dextran tracer was used to visualize transport across the endothelium and the pixel intensity was quantified using a customized MATLAB algorithm. In fibroblast-EC co-cultures, the dextran tracer easily penetrated through the vessel wall and permeability was high through the first 5 days of culture, indicative of vessel immaturity. Beyond day 5, dextran accumulated at the periphery of the vessel, with very little transported across the endothelium. Quantitatively, permeability dropped from initial levels of 61% to 39% after 7 days, and to 7% after 2 weeks. When ECs were co-cultured with bone marrow-derived mesenchymal stem cells (MSCs) or adipose-derived stem cells (AdSCs), much tighter control of permeability was achieved. Relative to the EC-fibroblast co-cultures, permeabilities were reduced 41% for the EC-MSC co-cultures and 50% for the EC-AdSC co-cultures after 3 days of culture. By day 14, these permeabilities decreased by 68% and 77% over the EC-fibroblast cultures. Co-cultures containing stem cells exhibit elevated VE-cadherin levels and more prominent EC-EC junctional complexes when compared to cultures containing fibroblasts. These data suggest the stromal cell identity influences the functionality and physiologic relevance of engineered capillary networks.

Tooth slice/scaffold model of dental pulp tissue engineering

Multipotency is a defining characteristic of post-natal stem cells. The human dental pulp contains a small subpopulation of stem cells that exhibit multipotency, as demonstrated by their ability to differentiate into odontoblasts, neural cells, and vascular endothelial cells. These discoveries highlight the fundamental role of stem cells in the biology of the dental pulp and suggest that these cells are uniquely suited for dental pulp tissue-engineering purposes. The availability of experimental approaches specifically designed for studies of the differentiation potential of dental pulp stem cells has played an important role in these discoveries. The objective of this review is to describe the development and characterization of the Tooth Slice/Scaffold Model of Dental Pulp Tissue Engineering. In addition, we discuss the multipotency of dental pulp stem cells, focusing on the differentiation of these cells into functional odontoblasts and into vascular endothelial cells.

A review of available prosthetics for ventral hernia repair

OBJECTIVE

To review mesh products currently available for ventral hernia repair and to evaluate their efficacy in complex repair, including contaminated and reoperative fields.

BACKGROUND

Although commonly referenced, the concept of the ideal prosthetic has never been fully realized. With the development of newer prosthetics and approaches to the ventral hernia repair, many surgeons do not fully understand the properties of the available prosthetics or the circumstances that warrant the use of a specific mesh.

METHODS

A systematic review of published literature from 1951 to June of 2009 was conducted to identify articles relating to ventral hernia repairs and the use of prosthetics in herniorrhaphy.

RESULTS

Important differences exist between the synthetics, composites, and biologic prosthetics used for ventral hernia repair in terms of mechanics, cost, and the ideal situation in which each should be used.

CONCLUSIONS

The use of synthetic mesh remains an appropriate solution for most ventral hernia repairs. Laparoscopic ventral hernia repair has created a niche for both expanded polytetrafluoroethylene and composite mesh, as they are suited to intraperitoneal placement. Preliminary studies have demonstrated that the newer biologic prosthetics are reasonable options for hernia repair in contaminated fields and for large abdominal wall defects; however, more studies need to be done before advocating the use of these biologics in other settings.

Immobilization and bioactivity evaluation of FGF-1 and FGF-2 on powdered silicon-doped hydroxyapatite and their scaffolds for bone tissue engineering

Fibroblast growth factors (FGFs) are polypeptides that control the proliferation and differentiation of various cell types including osteoblasts. FGFs are also strong inducers of angiogenesis, necessary to obtain oxygen and nutrients during tissue repair. With the aim to incorporate these desirable FGF biological properties into bioceramics for bone repair, silicon substituted hydroxyapatites (Si-HA) were used as materials to immobilize bioactive FGF-1 and FGF-2. Thus, the binding of these growth factors to powdered Si-HA and Si-HA scaffolds was carried out efficiently in the present study and both FGFs maintained its biological activity on osteoblasts after its immobilization. The improvement of cell adhesion and proliferation onto Si-HA scaffolds suggests the potential utility of these FGF/scaffolds for bone tissue engineering.

Dental pulp tissue engineering

Dental pulp is a highly specialized mesenchymal tissue that has a limited regeneration capacity due to anatomical arrangement and post-mitotic nature of odontoblastic cells. Entire pulp amputation followed by pulp space disinfection and filling with an artificial material cause loss of a significant amount of dentin leaving as life-lasting sequelae a non-vital and weakened tooth. However, regenerative endodontics is an emerging field of modern tissue engineering that has demonstrated promising results using stem cells associated with scaffolds and responsive molecules. Thereby, this article reviews the most recent endeavors to regenerate pulp tissue based on tissue engineering principles and provides insightful information to readers about the different aspects involved in tissue engineering. Here, we speculate that the search for the ideal combination of cells, scaffolds, and morphogenic factors for dental pulp tissue engineering may be extended over future years and result in significant advances in other areas of dental and craniofacial research. The findings collected in this literature review show that we are now at a stage in which engineering a complex tissue, such as the dental pulp, is no longer an unachievable goal and the next decade will certainly be an exciting time for dental and craniofacial research.

Strategies for vascularization of polymer scaffolds

Biocompatible, degradable polymer scaffolds combined with cells or biological signals are being investigated as alternatives to traditional options for tissue reconstruction and transplantation. These approaches are already in clinical use as engineered tissues that enhance wound healing and skin regeneration. The continued enhancement of these material strategies is highly dependent on the ability to promote rapid and stable neovascularization (new blood vessel formation) within the scaffold. Whereas neovascularization therapies have shown some promise for the treatment of ischemic tissues, vascularization of polymer scaffolds in tissue engineering strategies provides a unique challenge owing to the volume and the complexity of the tissues targeted. In this article, we examine recent advances in research focused on promoting neovascularization in polymer scaffolds for tissue engineering applications. These approaches include the use of growth factors, cells, and novel surgical approaches to both enhance and control the nature of the vascular networks formed. The continued development of these approaches may lead to new tissue engineering strategies for the generation of skin and other tissues or organs.

Vascular differentiation of bone marrow stem cells is directed by a tunable three-dimensional matrix

Microenvironmental cues are critical in regulating cell behavior and fate. The roles that matrix mechanical signals play in regulating cell behavior have recently been elucidated. An artificial matrix that can maintain the appropriate characteristics for transplanted stem cells is therefore needed to achieve a desired cell phenotype. The objective of this study was to develop a three-dimensional (3-D) matrix with tunable physical and mechanical properties and investigate their effects on mesenchymal stem cell (MSC) differentiation towards vascular cell types. In this study we developed an extracellular microenvironment by modifying fibrinogen with various polyethylene glycol (PEG) derivatives. We hypothesized that adjusting the type of PEG derivative to modify the resultant physical and mechanical characteristics of fibrin would allow us to create a tunable system for use in culture or in vivo in conjunction with a regenerative medicine strategy. Human MSC (hMSC) were entrapped into PEGylated fibrin matrices at a density of 50,000 cells ml(-1). Cell phenotypes were confirmed by immunofluorescent staining as well as the use of oligonucleotide arrays. Vascular phenotypes were correlated with measured mechanical properties and fiber diameters of the PEGylated fibrin matrices. Blocking studies were performed to identify mechanistic factors controlling MSC differentiation through selected blocking of matrix degradation or cell contraction. Cell-matrix interactions were also examined in vivo. Our results demonstrate that transdifferentiation of MSC towards an endothelial cell phenotype is profoundly affected by the 3-D matrix microenvironment. Our work provides a predictive road map for the creation of fibrin-based matrices that support robust endothelial cell gene expression and tubulogenesis.

Formation of human capillaries in vitro: the engineering of prevascularized matrices

Initial take, development, and function of transplanted engineered tissue substitutes are crucially dependent on rapid and adequate blood perfusion. Therefore, the development of rapidly and efficiently vascularized tissue grafts is vital for tissue engineering and regenerative medicine. Here we report on the construction of a network of highly organotypic capillaries in engineered tissue substitutes. We employed a three-dimensional culture system consisting of human microvascular endothelial cells. These were reproducibly expanded at high purity and subsequently seeded into biodegradable, fibrin-based hydrogels. The process of capillary formation in vitro followed the principles of both angiogenesis and postnatal vasculogenesis and a distinct sequence of other developmental steps that closely resemble embryonic neovascularization. Capillary lumen formation in vitro was initiated by the deposition of a basement membrane and intensive pinocytosis, followed by the generation of intracellular vacuoles, successive fusion of these vacuoles, and finally the formation of a long, continuous lumen. After transplantation the vascular structures were stabilized by mural cells of the recipient animal. Our findings suggest that the in vitro engineering of prevascularized matrices is within reach.

Materials for engineering vascularized adipose tissue

Loss of adipose tissue can occur due to congenital and acquired lipoatrophies, trauma, tumor resection, and chronic disease. Clinically, it is difficult to regenerate or reconstruct adipose tissue. The extensive microvsacular network present in adipose, and the sensitivity of adipocytes to hypoxia, hinder the success of typical tissue transfer procedures. Materials that promote the formation of vascularized adipose tissue may offer alternatives to current clinical treatment options. A number of synthetic and natural biomaterials common in tissue engineering have been investigated as scaffolds for adipose regeneration. While these materials have shown some promise they do not account for the unique extracellular microenvironment of adipose. Adipose derived hydrogels more closely approximate the physical and chemical microenvironment of adipose tissue, promote preadipocyte differentiation and vessel assembly in vitro, and stimulate vascularized adipose formation in vivo. The combination of these materials with techniques that promote rapid and stable vascularization could lead to new techniques for engineering stable, vascularized adipose tissue for clinical application. In this review we discuss materials used for adipose tissue engineering and strategies for vascularization of these scaffolds.

CLINICAL RELEVANCE

Materials that promote formation of vascularized adipose tissue have the potential to serve as alternatives or supplements to existing treatment options, for adipose defects or deficiencies resulting from chronic disease, lipoatrophies, trauma, and tumor resection.

The stimulation of angiogenesis and collagen deposition by copper

Copper is known to trigger endothelial cells towards angiogenesis. Different approaches have been investigated to develop vascularisation in biomaterials. The angiogenic and healing potential of copper ions in combination with two major angiogenic factors was examined. A 3D culture system in which, under stimulation by FGF-2 and to a lesser degree with VEGF, endothelial cells assembled into structures resembling to an angiogenic process was used. The combination of CuSO(4) with increasing doses of VEGF or FGF-2 enhanced the complexity of angiogenic networks in a significant manner. In vivo studies were also conducted by incorporating FGF-2 with CuSO(4) in a cylindrical collagen-based scaffold. CuSO(4) enhanced significantly the invasion of microvessel compared to control implants and to 20ng FGF-2+/-CuSO(4). Vascular infiltration was also significantly improved by combination of CuSO(4) with FGF-2, compared to FGF-2 alone (0.2 and 1microg). Nevertheless, in comparison with CuSO(4) alone, there was a significant increase only with 1microg of FGF-2 combined with CuSO(4). Significantly, collagen fiber deposition was enhanced following the combinatory loading in comparison to that with FGF-2 alone but not with CuSO(4) only. Thus, copper associated with growth factors may have synergistic effects which are highly attractive in the fields of tissue engineering (e.g., bone) and biomaterials.

Reclaiming a natural beauty: whole-organ engineering with natural extracellular materials

The ability to engineer whole organs as replacements for allografts and xenografts is an ongoing pursuit in regenerative medicine. While challenges remain, including systemic tissue integration with angiogenesis, lymphatiogenesis and neurogenesis, ongoing efforts are working to develop novel technologies to produce implantable engineered scaffolds and potentially engineered whole organs. Natural extracellular matrix materials, commonly utilized in vitro, are now being used as effective, natural, acellular allografts, and are being integrated into nanoscale scaffolds and matrices with programmable responsiveness. Based on the significant use of natural scaffolds for tissue regeneration and bioengineering strategies, this review focuses on recent and ongoing efforts to engineer whole organs, such as the tooth, featuring natural extracellular matrix molecules.

Phthalimide neovascular factor 1 (PNF1) modulates MT1-MMP activity in human microvascular endothelial cells

We are creating synthetic pharmaceuticals with angiogenic activity and potential to promote vascular invasion. We previously demonstrated that one of these molecules, phthalimide neovascular factor 1 (PNF1), significantly expands microvascular networks in vivo following sustained release from poly(lactic-co-glycolic acid) (PLAGA) films. In addition, to probe PNF1 mode of action, we recently applied a novel pathway-based compendium analysis to a multi-timepoint, controlled microarray data set of PNF1-treated (vs. control) human microvascular endothelial cells (HMVECs), and we identified induction of tumor necrosis factor-alpha (TNF-alpha) and, subsequently, transforming growth factor-beta (TGF-beta) signaling networks by PNF1. Here we validate this microarray data set with quantitative real-time polymerase chain reaction (RT-PCR) analysis. Subsequently, we probe this data set and identify three specific TGF-beta-induced genes with regulation by PNF1 conserved over multiple timepoints-amyloid beta (A4) precursor protein (APP), early growth response 1 (EGR-1), and matrix metalloproteinase 14 (MMP14 or MT1-MMP)-that are also implicated in angiogenesis. We further focus on MMP14 given its unique role in angiogenesis, and we validate MT1-MMP modulation by PNF1 with an in vitro fluorescence assay that demonstrates the direct effects that PNF1 exerts on functional metalloproteinase activity. We also utilize endothelial cord formation in collagen gels to show that PNF1-induced stimulation of endothelial cord network formation in vitro is in some way MT1-MMP-dependent. Ultimately, this new network analysis of our transcriptional footprint characterizing PNF1 activity 1-48 h post-supplementation in HMVECs coupled with corresponding validating experiments suggests a key set of a few specific targets that are involved in PNF1 mode of action and important for successful promotion of the neovascularization that we have observed by the drug in vivo.

Mathematical modelling of tissue-engineered angiogenesis

We present a mathematical model for the vascularisation of a porous scaffold following implantation in vivo. The model is given as a set of coupled non-linear ordinary differential equations (ODEs) which describe the evolution in time of the amounts of the different tissue constituents inside the scaffold. Bifurcation analyses reveal how the extent of scaffold vascularisation changes as a function of the parameter values. For example, it is shown how the loss of seeded cells arising from slow infiltration of vascular tissue can be overcome using a prevascularisation strategy consisting of seeding the scaffold with vascular cells. Using certain assumptions it is shown how the system can be simplified to one which is partially tractable and for which some analysis is given. Limited comparison is also given of the model solutions with experimental data from the chick chorioallantoic membrane (CAM) assay.

Complexity in biomaterials for tissue engineering

The molecular and physical information coded within the extracellular milieu is informing the development of a new generation of biomaterials for tissue engineering. Several powerful extracellular influences have already found their way into cell-instructive scaffolds, while others remain largely unexplored. Yet for commercial success tissue engineering products must be not only efficacious but also cost-effective, introducing a potential dichotomy between the need for sophistication and ease of production. This is spurring interest in recreating extracellular influences in simplified forms, from the reduction of biopolymers into short functional domains, to the use of basic chemistries to manipulate cell fate. In the future these exciting developments are likely to help reconcile the clinical and commercial pressures on tissue engineering.

Promoting angiogenesis via manipulation of VEGF responsiveness with notch signaling

Promoting angiogenesis via delivery of vascular endothelial growth factor (VEGF) and other angiogenic factors is both a potential therapy for cardiovascular diseases and a critical aspect for tissue regeneration. The recent demonstration that VEGF signaling is modulated by the Notch signaling pathway, however, suggests that inhibiting Notch signaling may enhance regional neovascularization, by altering the responsiveness of local endothelial cells to angiogenic stimuli. We tested this possibility with in vitro assays using human endothelial cells, as well as in a rodent hindlimb ischemia model. Treatment of cultured human endothelial cells with DAPT, a gamma secretase inhibitor, increased cell migration and sprout formation in response to VEGF stimulation with a biphasic dependence on DAPT concentration. Further, delivery of an appropriate combination of DAPT and VEGF from an injectable alginate hydrogel system into ischemic hindlimbs led to a faster recovery of blood flow than VEGF or DAPT alone; perfusion levels reached 80% of the normal level by week 4 with combined DAPT and VEGF delivery. Direct intramuscular or intraperitoneal injection of DAPT did not result in the same level of improvement, suggesting that appropriate presentation of DAPT (gel delivery) is important for its activity. DAPT delivery from the hydrogels also did not lead to any adverse side effects, in contrast to systemic introduction of DAPT. Altogether, these results suggest a new approach to promote angiogenesis by controlling Notch signaling, and may provide new options to treat patients with diseases that diminish angiogenic responsiveness.

Biocompatibility of Poly-epsilon-caprolactone-hydroxyapatite composite on mouse bone marrow-derived osteoblasts and endothelial cells

Background

Tissue-engineered bone may be developed by seeding the cells capable of both osteogenesis and vascularization on biocompatible composite scaffolds. The current study investigated the performance of mice bone marrow-derived osteogenic cells and endothelial cells as seeded on hydroxyapatite (HA) and poly-ε-caprolactone (PCL) composite scaffolds.

Methods

Mononuclear cells were induced to osteoblasts and endothelial cells respectively, which were defined by the expression of osteocalcin, alkaline phosphatase (ALP), and deposits of calcium-containing crystal for osteoblasts, or by the expression of vascular endothelial growth factor receptor-2 (VEGFR-2) and von Willebrand factor (vWF), and the formation of a capillary network in Matrigel™ for endothelial cells. Both types of cell were seeded respectively on PCL-HA scaffolds at HA to PCL weight ratio of 1:1, 1:4, or 0:1 and were evaluated using scanning electron microscopy, ALP activity (of osteoblasts) and nitric oxide production (of endothelial cells) plus the assessment of cell viability.

Results

The results indicated that HA led to a positive stimulation of osteoblasts viability and ALP activity, while HA showed less influence on endothelial cells viability. An elevated nitric oxide production of endothelial cells was observed in HA-containing group.

Conclusion

Supplement of HA into PCL improved biocompatible for bone marrow-derived osteoblasts and endothelial cells. The PCL-HA composite integrating with two types of cells may provide a useful system for tissue-engineered bone grafts with vascularization.

Insulin enhances proliferation and viability of human umbilical vein endothelial cells

This investigation is a follow-up to our previous in vivo studies revealing that rapid stretch increases tissue insulin in murine skin flaps, coincident with the up-regulation of key angiogenic effectors and enhanced vascularization. In the present study, we used human umbilical vein endothelial cells (HUVECs) as an in vitro model system to determine the role of insulin in the chemical signals regulating the processes of proliferation and viability (survival). MTT-based colorimetric methods demonstrated that insulin enhances proliferation and survival of HUVECs. Western blot analysis revealed that protein kinase B (pAkt [Thr(308)]) and vascular endothelial growth factor (VEGF) were the insulin-responsive intermediates in proliferating endothelial cells (ECs). In insulin-enhanced survival, both pAkt (Thr(308)) and pAkt (Ser(473)) were activated in HUVECs. However, no change in VEGF expression accompanied pAkt activation. The beneficial effects of insulin were abrogated by insulin receptor (IR)/insulin-like growth factor receptor (IGFR) or phosphoinositide-3 kinase (PI3-K) blockade, suggesting that insulin-induced EC proliferation and viability are mediated through pIR/pIGFR and PI3-K effectors. These data provide new insights into the beneficial effects of insulin on vascularization and tissue viability, providing a mechanistic link to the enhancement of healing in acutely stretched skin.

Expansion of microvascular networks in vivo by phthalimide neovascular factor 1 (PNF1)

Phthalimide neovascular factor (PNF1, formerly SC-3-149) is a potent stimulator of proangiogenic signaling pathways in endothelial cells. In this study, we evaluated the in vivo effects of sustained PNF1 release to promote ingrowth and expansion of microvascular networks surrounding biomaterial implants. The dorsal skinfold window chamber was used to evaluate the structural remodeling response of the local microvasculature. PNF1 was released from poly(lactic-co-glycolic acid) (PLAGA) films, and a transport model was utilized to predict PNF1 penetration into the surrounding tissue. PNF1 significantly expanded microvascular networks within a 2mm radius from implants after 3 and 7 days by increasing microvessel length density and lumenal diameter of local arterioles and venules. Staining of histological sections with CD11b showed enhanced recruitment of circulating white blood cells, including monocytes, which are critical for the process of vessel enlargement through arteriogenesis. As PNF1 has been shown to modulate MT1-MMP, a facilitator of CCL2 dependent leukocyte transmigration, aspects of window chamber experiments were repeated in CCR2(-/-) (CCL2 receptor) mouse chimeras to more fully explore the critical nature of monocyte recruitment on the therapeutic benefits of PNF1 function in vivo.

Novel pathway compendium analysis elucidates mechanism of pro-angiogenic synthetic small molecule

MOTIVATION

Computational techniques have been applied to experimental datasets to identify drug mode-of-action. A shortcoming of existing approaches is the requirement of large reference databases of compound expression profiles. Here, we developed a new pathway-based compendium analysis that couples multi-timepoint, controlled microarray data for a single compound with systems-based network analysis to elucidate drug mechanism more efficiently.

RESULTS

We applied this approach to a transcriptional regulatory footprint of phthalimide neovascular factor 1 (PNF1)-a novel synthetic small molecule that exhibits significant in vitro endothelial potency-spanning 1-48 h post-supplementation in human micro-vascular endothelial cells (HMVEC) to comprehensively interrogate PNF1 effects. We concluded that PNF1 first induces tumor necrosis factor-alpha (TNF-alpha) signaling pathway function which in turn affects transforming growth factor-beta (TGF-beta) signaling. These results are consistent with our previous observations of PNF1-directed TGF-beta signaling at 24 h, including differential regulation of TGF-beta-induced matrix metalloproteinase 14 (MMP14/MT1-MMP) which is implicated in angiogenesis. Ultimately, we illustrate how our pathway-based compendium analysis more efficiently generates hypotheses for compound mechanism than existing techniques.

Ex vivo method to visualize and quantify vascular networks in native and tissue engineered skin

BACKGROUND AND AIMS

Neovascularization plays a pivotal role in tissue engineering and tissue regeneration. However, reliable technologies to visualize and quantify blood vessel networks in target tissue areas are still pending. In this work, we introduce a new method which allows comparing vascularization levels in normal and tissue-engineered skin.

MATERIALS AND METHODS

Normal skin was isolated, and vascular dermal regeneration was analyzed based on tissue transillumination and computerized digital segmentation. For tissue-engineered skin, a bilateral full skin defect was created in a nude mouse model and then covered with a commercially available scaffold for dermal regeneration. After 3 weeks, the whole skin (including scaffold for dermal regeneration) was harvested, and vascularization levels were analyzed.

RESULTS

The blood vessel network in the skin was better visualized by transillumination than by radio-angiographic studies, the gold standard for angiographies. After visualization, the whole vascular network was digitally segmented showing an excellent overlapping with the original pictures. Quantification over the digitally segmented picture was performed, and an index of vascularization area (VAI) and length (VLI) of the vessel network was obtained in target tissues. VAI/VLI ratio was calculated to obtain the vessel size index.

CONCLUSIONS

We present a new technique which has several advantages compared to others, as animals do not require intravascular perfusions, total areas of interest can be quantitatively analyzed at once, and the same target tissue can be processed for further experimental analysis.

Acute stretch promotes endothelial cell proliferation in wounded healing mouse skin

We have developed a novel in vivo model utilizing acute stretch to investigate endothelial cell proliferation as a marker of vascular growth in healing mouse skin. This study is a follow-up to ones revealing immediate stretch improves blood flow, decreases total tissue necrosis, and induces tissue insulin transcription. Dorsal distally based flaps of skin were stretched for 3 min using linear (skin hook) plus hemispherical load cycling (inflated subcutaneous silicone catheter). Unstretched, wounded skin along the back and sternum served as postoperative controls. Laser Doppler flowmetry demonstrated a threefold increase in flap perfusion at postoperative day 7. A stretch-induced sixfold increase in endothelial cell mitogenesis accompanied enhancements in blood flow and extracorporal wound healing over the sternum. Western blots revealed up-regulation/activation of insulin and mitogenic signaling intermediates in stretched skin. Activated insulin and insulin growth factor receptors (pIR/pIGFR), protein kinase B (Akt, pAkt), vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptor 2 (flk-1) were among the identified stretch-responsive intermediates. These results indicate the benefits of acute stretch are mediated through enhanced vascularity as evidenced by endothelial cell mitogenesis and up-regulation/activation of insulin and key angiogenic effectors in dorsal distally based skin flaps.

Mechanistic exploration of phthalimide neovascular factor 1 using network analysis tools

Neovascularization is essential for the survival and successful integration of most engineering tissues after implantation in vivo. The objective of this study was to elucidate possible mechanisms of phthalimide neovascular factor 1 (PNF1), a new synthetic small molecule proposed for therapeutic induction of angiogenesis. Complementary deoxyribonucleic acid microarray analysis was used to identify 568 transcripts in human microvascular endothelial cells (HMVECs) that were significantly regulated after 24-h stimulation with 30 muM of PNF1, previously known as SC-3-149. Network analysis tools were used to identify genetic networks of the global biological processes involved in PNF1 stimulation and to describe known molecular and cellular functions that the drug regulated most highly. Examination of the most significantly perturbed networks identified gene products associated with transforming growth factor-beta (TGF-beta), which has many known effects on angiogenesis, and related signal transduction pathways. These include molecules integral to the thrombospondin, plasminogen, fibroblast growth factor, epidermal growth factor, ephrin, Rho, and Ras signaling pathways that are essential to endothelial function. Moreover, real-time reverse-transcriptase polymerase chain reaction (RT-PCR) of select genes showed significant increases in TGF-beta-associated receptors endoglin and beta glycan. These experiments provide important insight into the pro-angiogenic mechanism of PNF1, namely, TGF-beta-associated signaling pathways, and may ultimately offer new molecular targets for directed drug discovery.

Spatiotemporal control over growth factor signaling for therapeutic neovascularization

Many of the qualitative roles of growth factors involved in neovascularization have been delineated, but it is unclear yet from an engineering perspective how to use these factors as therapies. We propose that an approach that integrates quantitative spatiotemporal measurements of growth factor signaling using 3-D in vitro and in vivo models, mathematic modeling of factor tissue distribution, and new delivery technologies may provide an opportunity to engineer neovascularization on demand.

In vitro models of angiogenesis

Neovascularization can be categorized into two general processes: vasculogenesis and angiogenesis. Angiogenesis is the formation of new capillaries from pre-existing vessels, requiring growth factor driven recruitment, migration, proliferation, and differentiation of endothelial cells (ECs). Complex cell-cell and cell-extracellular matrix (ECM) interactions contribute to this process, leading finally to a network of tube-like formations of endothelial cells supported by surrounding mural cells. The study of angiogenesis has broad clinical implications in the fields of peripheral and coronary vascular disease, oncology, hematology, wound healing, dermatology, and ophthalmology, among others. As such, novel, clinically relevant models of angiogenesis in vitro are crucial to the understanding of angiogenic processes. We highlight some of the advances made in the development of these models, and discuss the importance of incorporating the three-dimensional cell-matrix and EC-mural cell interactions into these in vitro assays of angiogenesis. This review also discusses our own 3-D angiogenesis assay and some of the in vitro results from our lab as they relate to therapeutic neovascularization and tissue engineering of vascular grafts.