The effect of hypoxia on in vitro prevascularization of a thick soft tissue

Photobiomodulation and Vascularization in Conduit-Based Peripheral Nerve Repair: A Narrative Review

Background: Peripheral nerve injuries pose a significant clinical issue for patients, especially in the most severe cases wherein complete transection (neurotmesis) results in total loss of sensory/motor function. Nerve guidance conduits (NGCs) are a common treatment option that protects and guides regenerating axons during recovery. However, treatment outcomes remain limited and often fail to achieve full reinnervation, especially in critically sized defects (>3 cm) where a lack of vascularization leads to neural necrosis. Conclusions: A multitreatment approach is, therefore, necessary to improve the efficacy of NGCs. Stimulating angiogenesis within NGCs can help alleviate oxygen deficiency through rapid inosculation with the host vasculature, whereas photobiomodulation therapy (PBMT) has demonstrated beneficial therapeutic effects on regenerating nerve cells and neovascularization. In this review, we discuss the current trends of NGCs, vascularization, and PBMT as treatments for peripheral nerve neurotmesis and highlight the need for a combinatorial approach to improve functional and clinical outcomes.

Organotypic stromal cells impact endothelial cell transcriptome in 3D microvessel networks

Endothelial cells line all major blood vessels and serve as integral regulators of many functions including vessel diameter, cellular trafficking, and transport of soluble mediators. Despite similar functions, the phenotype of endothelial cells is highly organ-specific, yet our understanding of the mechanisms leading to organ-level differentiation is incomplete. We generated 3D microvessel networks by combining a common naïve endothelial cell with six different stromal cells derived from the lung, skin, heart, bone marrow, pancreas, and pancreatic cancer. Single cell RNA-Seq analysis of the microvessel networks reveals five distinct endothelial cell populations, for which the relative proportion depends on the stromal cell population. Morphologic features of the organotypic vessel networks inversely correlate with a cluster of endothelial cells associated with protein synthesis. The organotypic stromal cells were each characterized by a unique subpopulation of cells dedicated to extracellular matrix organization and assembly. Finally, compared to cells in 2D monolayer, the endothelial cell transcriptome from the 3D in vitro heart, skin, lung, and pancreas microvessel networks are more similar to the in vivo endothelial cells from the respective organs. We conclude that stromal cells contribute to endothelial cell and microvessel network organ tropism, and create an endothelial cell phenotype that more closely resembles that present in vivo.

Toward Hierarchical Assembly of Aligned Cell Sheets into a Conical Cardiac Ventricle Using Microfabricated Elastomers

Despite current efforts in organ-on-chip engineering to construct miniature cardiac models, they often lack some physiological aspects of the heart, including fiber orientation. This motivates the development of bioartificial left ventricle models that mimic the myofiber orientation of the native ventricle. Herein, an approach relying on microfabricated elastomers that enables hierarchical assembly of 2D aligned cell sheets into a functional conical cardiac ventricle is described. Soft lithography and injection molding techniques are used to fabricate micro-grooves on an elastomeric polymer scaffold with three different orientations ranging from -60° to +60°, each on a separate trapezoidal construct. The width of the micro-grooves is optimized to direct the majority of cells along the groove direction and while periodic breaks are used to promote cell-cell contact. The scaffold is wrapped around a central mandrel to obtain a conical-shaped left ventricle model inspired by the size of a human left ventricle 19 weeks post-gestation. Rectangular micro-scale holes are incorporated to alleviate oxygen diffusional limitations within the 3D scaffold. Cardiomyocytes within the 3D left ventricle constructs showed high viability in all layers after 7 days of cultivation. The hierarchically assembled left ventricle also provided functional readouts such as calcium transients and ejection fraction.

Repair of peripheral nerve injuries using a prevascularized cell-based tissue-engineered nerve conduit

One of the major challenges in the development of a larger and longer nerve conduit for peripheral nerve repair is the limitation in oxygen and nutrient diffusion within the tissue after transplantation preventing Schwann cell and axonal migration. This restriction is due to the slow neovascularization process of the graft starting from both nerve endings. To overcome this limitation, we propose the design of a living tissue-engineered nerve conduit made of an internal tube with a three-dimensional structure supporting axonal migration, which is inserted inside a hollow external tube that plays the role of an epineurium and is strong enough to be stitched to the severed nerve stumps. The internal tube is made of a rolled living fibroblast sheet and can be seeded with endothelial cells to promote the formation of a network containing capillary-like structures which allow rapid inosculation with the host nerve microvasculature after grafting. Human nerve conduits were grafted in immunodeficient rats to bridge a 15 mm sciatic nerve gap. Human capillaries within the pre-vascularized nerve conduit successfully connected to the host circulation 2 weeks after grafting. Twenty-two weeks after surgery, rats transplanted with the nerve conduits had a similar motor function recovery compared to the autograft group. By promoting rapid vascularization of the internal nerve tube from both ends of the nerve stumps, this endothelialized nerve conduit model displays a favorable environment to enhance axonal migration in both larger caliber and longer nerve grafts.

Decellularised extracellular matrix decorated PCL PolyHIPE scaffolds for enhanced cellular activity, integration and angiogenesis

Wound healing involves a complex series of events where cell–cell and cell-extracellular matrix (ECM) interactions play a key role. Wounding can be simple, such as the loss of the epithelial integrity, or deeper and more complex, reaching to subcutaneous tissues, including blood vessels, muscles and nerves. Rapid neovascularisation of the wounded area is crucial for wound healing as it has a key role in supplying oxygen and nutrients during the highly demanding proliferative phase and transmigration of inflammatory cells to the wound area. One approach to circumvent delayed neovascularisation is the exogenous use of pro-angiogenic factors, which is expensive, highly dose-dependent, and the delivery of them requires a very well-controlled system to avoid leaky, highly permeable and haemorrhagic blood vessel formation. In this study, we decorated polycaprolactone (PCL)-based polymerised high internal phase emulsion (PolyHIPE) scaffolds with fibroblast-derived ECM to assess fibroblast, endothelial cell and keratinocyte activity in vitro and angiogenesis in ex ovo chick chorioallantoic membrane (CAM) assays. Our results showed that the inclusion of ECM in the scaffolds increased the metabolic activity of three types of cells that play a key role in wound healing and stimulated angiogenesis in ex ovo CAM assays over 7 days. Herein, we demonstrated that fibroblast-ECM functionalised PCL PolyHIPE scaffolds appear to have great potential to be used as an active wound dressing to promote angiogenesis and wound healing.

Decellularisation of in vitro generated extracellular matrix (ECM) provides an effective way to stimulate angiogenesis and wound healing.

The Role of Brain Microvascular Endothelial Cell and Blood-Brain Barrier Dysfunction in Schizophrenia

BACKGROUND

Despite decades of research, little clarity exists regarding pathogenic mechanisms related to schizophrenia. Investigations on the disease biology of schizophrenia have primarily focused on neuronal alterations. However, there is substantial evidence pointing to a significant role for the brain's microvasculature in mediating neuroinflammation in schizophrenia.

SUMMARY

Brain microvascular endothelial cells (BMEC) are a central element of the microvasculature that forms the blood-brain barrier (BBB) and shields the brain against toxins and immune cells via paracellular, transcellular, transporter, and extracellular matrix proteins. While evidence for BBB dysfunction exists in brain disorders, including schizophrenia, it is not known if BMEC themselves are functionally compromised and lead to BBB dysfunction.

KEY MESSAGES

Genome-wide association studies, postmortem investigations, and gene expression analyses have provided some insights into the role of the BBB in schizophrenia pathophysiology. However, there is a significant gap in our understanding of the role that BMEC play in BBB dysfunction. Recent advances differentiating human BMEC from induced pluripotent stem cells (iPSC) provide new avenues to examine the role of BMEC in BBB dysfunction in schizophrenia.

Pre-Seeding of Simple Electrospun Scaffolds with a Combination of Endothelial Cells and Fibroblasts Strongly Promotes Angiogenesis

BACKGROUND

Introduction of pro-angiogenic cells into tissue-engineered (TE) constructs (prevascularisation) is a promising approach to overcome delayed neovascularisation of such constructs post-implantation. Accordingly, in this study, we examined the contribution of human dermal microvascular endothelial cells (HDMECs) and human dermal fibroblasts (HDFs) alone and in combination on the formation of new blood vessels in ex-ovo chick chorioallantoic membrane (CAM) assay.

METHODS

Poly-3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) and polycaprolactone (PCL) were first examined in terms of their physical, mechanical, and biological performances. The effect of gelatin coating and co-culture conditions on enhancing endothelial cell viability and growth was then investigated. Finally, the angiogenic potential of HDMECs and HDFs were assessed macroscopically and histologically after seeding on simple electrospun PHBV scaffolds either in isolation or in indirect co-culture using an ex-ovo CAM assay.

RESULTS

The results demonstrated that PHBV was slightly more favourable than PCL for HDMECs in terms of cell metabolic activity. The gelatin coating of PHBV scaffolds and co-culture of HDMECs with HDFs both showed a positive impact on HDMECs viability and growth. Both cell types induced angiogenesis over 7 days in the CAM assay either in isolation or in co-culture. The introduction of HDMECs to the scaffolds resulted in the production of more blood vessels in the area of implantation than the introduction of HDFs, but the co-culture of HDMECs and HDFs gave the most significant angiogenic activity.

CONCLUSION

Our findings showed that the in vitro prevascularisation of TE constructs with HDMECs and HDFs alone or in co-culture promotes angiogenesis in implantable TE constructs.

Effect of Chemically Induced Hypoxia on Osteogenic and Angiogenic Differentiation of Bone Marrow Mesenchymal Stem Cells and Human Umbilical Vein Endothelial Cells in Direct Coculture

Bone is an active tissue where bone mineralization and resorption occur simultaneously. In the case of fracture, there are numerous factors required to facilitate bone healing including precursor cells and blood vessels. To evaluate the interaction between bone marrow-derived mesenchymal stem cells (BMSC)—the precursor cells able to differentiate into bone-forming cells and human umbilical vein endothelial cells (HUVEC)—a cell source widely used for the study of blood vessels. We performed direct coculture of BMSC and HUVEC in normoxia and chemically induced hypoxia using Cobalt(II) chloride and Dimethyloxaloylglycine and in the condition where oxygen level was maintained at 1% as well. Cell proliferation was analyzed by crystal violet staining. Osteogenesis was examined by Alizarin Red and Collagen type I staining. Expression of angiogenic factor-vascular endothelial growth factor (VEGF) and endothelial marker-von Willebrand factor (VWF) were demonstrated by immunohistochemistry and enzyme-linked immunosorbent assay. The quantitative polymerase chain reaction was also used to evaluate gene expression. The results showed that coculture in normoxia could retain both osteogenic differentiation and endothelial markers while hypoxic condition limits cell proliferation and osteogenesis but favors the angiogenic function even after 1 of day treatment.

Assessing the Minimum Time-Period of Normoxic Preincubation for Stable Adipose Stromal Cell-Derived Vascular Networks

Introduction

Pre-vascularization of tissue engineered grafts is a promising strategy to facilitate their improved viability following in vivo implantation. In this process, endothelial cells (ECs) form capillary-like networks that can anastomose with host vasculature. Adipose-derived stromal cells (ASCs) are a commonly used cell population for tissue engineering and contain a subpopulation of ECs capable of assembling into robust vascular networks and anastomosing with the host. However, their initial vascular assembly is significantly impaired in hypoxic conditions (2% O₂). In this study, we explored the minimum period of normoxic (20% O₂) pre-treatment required to enable the formation of stable vascular networks.

Methods

ASC-derived vascular structures were allowed to preassemble in fibrin hydrogels in normoxia for 0, 2, 4, or 6 days and then transplanted into hypoxic environments for 6 days. Total vascular length, pericyte coverage, cell proliferation, apoptosis rates, and ECM production was assessed.

Results

Vascular assembly increased with time over the 6 days of culture. We found that 4 days was the minimum period of time required for stable vascular assembly. We compared the major differences in cell behavior and network structure at Days 2 and 4. Neither proliferation nor apoptosis differed, however, the Day 4 time-point was associated with a significant increase in pericyte coverage (46.1 ± 2.6%) compared to Day 2 (24.3 ± 5.3%).

Conclusions

These data suggest oxygen tension may be a mediator of EC-pericyte interactions during vascular assembly. Pre-vascularization strategies should incorporate a normoxic period of to enable successful vascular formation and development.

Prevascularization of natural nanofibrous extracellular matrix for engineering completely biological three-dimensional prevascularized tissues for diverse applications

Self-sustainability after implantation is one of the critical obstacles facing large engineered tissues. A preformed functional vascular network provides an effective solution for solving the mass transportation problem. With the support of mural cells, endothelial cells (ECs) can form microvessels within engineered tissues. As an important mural cell, human mesenchymal stem cells (hMSCs) not only stabilize the engineered microvessel network, but also preserve their multi-potency when grown under optimal culture conditions. A prevascularized hMSC/extracellular matrix (ECM) sheet fabricated by the combination of hMSCs, ECs and a naturally derived nanofibrous ECM scaffold offers great opportunity for engineering mechanically strong and completely biological three-dimensional prevascularized tissues. The objective of this study was to create a prevascularized hMSC/ECM sheet by co-culturing ECs and hMSCs on a nanofibrous ECM scaffold. Physiologically low oxygen (2% O2 ) was introduced during the 7 day hMSC culture to preserve the stemness of hMSCs and thereby their capability to secrete angiogenic factors. The ECs were then included to form microvessels under normal oxygen (20% O2 ) for up to 7 days. The results showed that a branched and mature vascular network was formed in the co-culture condition. Angiogenic factors vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF) and angiopoietin-1 (Ang-1) were significantly increased by low-oxygen culture of hMSCs, which further stabilized and supported the maturation of microvessels. A differentiation assay of the prevascularized ECM scaffold demonstrated a retained hMSC multi-potency in the hypoxia cultured samples. The prevascularized hMSC/ECM sheet holds great promise for engineering three-dimensional prevascularized tissues for diverse applications.

The balance between proliferation and transcription of angiogenic factors of mesenchymal stem cells in hypoxia

Bridging large bone defects with mesenchymal stromal cells-seeded scaffolds remains a big challenge in orthopedic surgery, due to the lack of vascularization. Within such a cell-scaffold construct, cells are exposed to ischemic conditions. When human mesenchymal stem cells (hMSCs) encounter hypoxic conditions, they show higher cell proliferation than at ambient oxygen levels. However, when hMSCs are exposed to prolonged ischemia, cell proliferation ceases completely. Exposure of hMSCs to hypoxic conditions is known to result in the transcription of angiogenic factors (AGF), which can promote the development of new blood vessels. In this study, we investigated at which oxygen level hMSC proliferation and the transcription of AGF were optimal. Human bone marrow-derived hMSCs were cultured at 0.1, 1, 2, 3, 4, 5, and 21% oxygen. Cell proliferation over 14 days was assayed using a DNA quantification method. hMSC metabolic activity over 14 days was measured using a MTT test. Quantitative RT-PCR was used to assess mRNA levels of angiogenic factors at the tested oxygen percentages. hMSCs showed the highest cell proliferation rate at 1% oxygen. The highest corrected cell metabolic rate was found at 21% oxygen, followed by 2% oxygen. HIF1α transcription did not increase under hypoxic conditions compared to 21% oxygen conditions. However, transcription of VEGF and ANG-1 was significantly higher at 2% oxygen than at 21% O₂. The optimum oxygen range at which hMSCs proliferated rapidly and angiogenic factors ANG-1 and VEGF simultaneously came to expression was from 1 to 2% oxygen.

Transdifferentiation of human MNNG/HOS osteosarcoma cells into vascular endothelial cells in vitro and in vivo

The transdifferentiation of cancer cells into other types of cells in several types of tissues or organs has been studied. However, whether human osteosarcoma MNNG/HOS cells can transdifferentiate into other types of cells has seldom been reported. Meanwhile, the mechanism of tumor angiogenesis is still disputed, and whether MNNG/HOS cells participate in angiogenesis in osteosarcoma remains unknown. In the present study, the investigation was divided into two parts: in vitro and in vivo. In vitro, we cultivated MNNG/HOS cells under hypoxic conditions for 4 days and found that they typically showed a characteristic 'flagstone' appearance as cultured vascular endothelial cells (VECs). MNNG/HOS cells that were cultivated on Matrigel under hypoxic conditions gradually formed tubular-like structures. Furthermore, when cultured under hypoxic conditions for 4 days, MNNG/HOS cells also transcribed and expressed several molecular markers of VECs (CD31, CD34 and vWF). In vivo, MNNG/HOS cells (1x106 cells) were cultivated under hypoxic conditions and subcutaneously injected into nude mice; the mice were sacrificed 49 days after inoculation. Immunohistochemical staining with anti-human CD31 antibody showed evidence of tumor angiogenesis in human osteosarcoma MNNG/HOS cells. The results demonstrated that MNNG/HOS cells can transdifferentiate into vascular endothelial cell-like cells in vitro and in vivo.

Prevascularization of Decellularized Porcine Myocardial Slice for Cardiac Tissue Engineering

Prevacularization strategies have been implemented in tissue engineering to generate microvasculature networks within a scaffold prior to implantation. Prevascularizing scaffolds will shorten the time of functional vascular perfusion with host upon implantation. In this study, we explored key variables affecting the interaction between decellularized porcine myocardium slices (dPMSs) and reseeded stem cells toward the fabrication of prevascularized cardiac tissue. Our results demonstrated that dPMS supports attachment of human mesenchymal stem cells (hMSCs) and rat adipose derived stem cells (rASCs) with high viability. We found that cell seeding efficiency and proliferation are dPMS thickness dependent. Compared to lateral cell seeding, bilateral cell seeding strategy significantly enhanced seeding efficiency, infiltration, and growth in 600 μm dPMS. dPMS induced endothelial differentiation and maturation of hMSCs and rASCs after 1 and 5 days culture, respectively. These results indicate the potential of dPMS as a powerful platform to develop prevascularized scaffolds and fabricate functional cardiac patches.

Identification of HIF-2α-regulated genes that play a role in human microvascular endothelial sprouting during prolonged hypoxia in vitro

During prolonged hypoxic conditions, endothelial cells change their gene expression to adjust to the low oxygen environment. This process is mainly regulated by the hypoxia-inducible factors, HIF-1α and HIF-2α. Although endothelial cells do not form sprouts during prolonged hypoxic culturing, silencing of HIF-2α partially restores sprout formation. The present study identifies novel HIF-2α-target genes that may regulate endothelial sprouting during prolonged hypoxia. The gene expression profile of primary human microvascular endothelial cells (hMVECs) that were cultured at 20 % oxygen was compared to hMVECs that were cultured at 1 % oxygen for 14 days by using genome-wide RNA-sequencing. The differentially regulated genes in hypoxia were compared to the genes that were differentially regulated upon silencing of HIF-2α in hypoxia. Surprisingly, KEGG pathway analysis showed that metabolic pathways were enriched within genes upregulated in response to hypoxia and enriched within genes downregulated upon HIF-2α silencing. Moreover, 51 HIF-2α-regulated genes were screened for their role in endothelial sprouting in hypoxia, of which four genes ARRDC3, MME, PPARG and RALGPS2 directly influenced endothelial sprouting during prolonged hypoxic culturing. The manipulation of specific downstream targets of HIF-2α provides a new, but to be further evaluated, perspective for restoring reduced neovascularization in several pathological conditions, such as diabetic ulcers or other chronic wounds, for improvement of vascularization of implanted tissue-engineered scaffolds.

HIF-2α Expression Regulates Sprout Formation into 3D Fibrin Matrices in Prolonged Hypoxia in Human Microvascular Endothelial Cells

Background

During short-term hypoxia, Hypoxia Inducible Factors (particular their subunits HIF-1α and HIF-2α) regulate the expression of many genes including the potent angiogenesis stimulator VEGF. However, in some pathological conditions chronic hypoxia occurs and is accompanied by reduced angiogenesis.

Objectives

We investigated the effect of prolonged hypoxia on the proliferation and sprouting ability of human microvascular endothelial cells and the involvement of the HIFs and Dll4/Notch signaling.

Methods and Results

Human microvascular endothelial cells (hMVECs), cultured at 20% oxygen for 14 days and seeded on top of 3D fibrin matrices, formed sprouts when stimulated with VEGF-A/TNFα. In contrast, hMVECs precultured at 1% oxygen for 14 days were viable and proliferative, but did not form sprouts into fibrin upon VEGF-A/TNFα stimulation at 1% oxygen. Silencing of HIF-2α with si-RNA partially restored the inhibition of endothelial sprouting, whereas HIF-1α or HIF-3α by si-RNA had no effect. No involvement of Dll4/Notch pathway in the inhibitory effect on endothelial sprouting by prolonged hypoxia was found. In addition, hypoxia decreased the production of urokinase-type plasminogen activator (uPA), needed for migration and invasion, without a significant effect on its inhibitor PAI-1. This was independent of HIF-2α, as si-HIF-2α did not counteract uPA reduction.

Conclusion

Prolonged culturing of hMVECs at 1% oxygen inhibited endothelial sprouting into fibrin. Two independent mechanisms contribute. Silencing of HIF-2α with si-RNA partially restored the inhibition of endothelial sprouting pointing to a HIF-2α-dependent mechanism. In addition, reduction of uPA contributed to reduced endothelial tube formation in a fibrin matrix during prolonged hypoxia.

Hypoxia Inhibits De Novo Vascular Assembly of Adipose-Derived Stromal/Stem Cell Populations, but Promotes Growth of Preformed Vessels

Vascularization is critical for cell survival within tissue-engineered grafts. Adipose-derived stromal/stem cells (ASCs) are widely used in tissue engineering applications as they are a clinically relevant source of stem cells and endothelial progenitor cells. ASCs have previously been shown to self-assemble into pericyte-stabilized vascular networks in normoxic (20% O2) cultures. This capacity for de novo vascular assembly may accelerate graft vascularization in vivo rather than relying solely on angiogenic ingrowth. However, oxygen depletion within large cell-seeded grafts will be rapid, and it is unclear how this worsening hypoxic environment will impact the vascular assembly of the transplanted cells. The objectives of this study were to determine whether ASC-derived vessels could grow in hypoxia and to assess whether the vessel maturity (i.e., individual cells vs. preformed vessels) influenced this hypoxic response. Utilizing an in vitro vascularization model, ASCs were encapsulated within fibrin gels and cultured in vitro for up to 6 days in either normoxia (20% O2) or hypoxia (0.2% or 2% O2). In a subsequent experiment, vessels were allowed to preform in normoxia for 6 days before an additional 6 days of either normoxia or hypoxia. Viability, vessel growth, pericyte coverage, proliferation, metabolism, and angiogenic factor expression were assessed for each experimental approach. Vessel growth was dramatically inhibited in both moderate and severe hypoxia (47% and 11% total vessel length vs. normoxia, respectively), despite maintaining high cell viability and upregulating endogenous expression of vascular endothelial growth factor in hypoxia. Bromodeoxyuridine labeling indicated significantly reduced proliferation of endothelial cells in hypoxia. In contrast, when vascular networks were allowed to preform for 6 days in normoxia, vessels not only survived but also continued to grow more in hypoxia than those maintained in normoxia. These findings demonstrate that vascular assembly and growth are tightly regulated by oxygen tension and may be differentially affected by hypoxic conditions based on the maturity of the vessels. Understanding this relationship is critical to developing effective approaches to engineer viable tissue-engineered grafts in vivo.

The pre-vascularisation of a collagen-chondroitin sulphate scaffold using human amniotic fluid-derived stem cells to enhance and stabilise endothelial cell-mediated vessel formation

A major problem in tissue engineering (TE) is graft failure in vivo due to core degradation in in vitro engineered constructs designed to regenerate thick tissues such as bone. The integration of constructs post-implantation relies on the rapid formation of functional vasculature. A recent approach to overcome core degradation focuses on the creation of cell-based, pre-engineered vasculature formed within the TE construct in vitro, prior to implantation in vivo. The primary objective of this study was to investigate whether an amniotic fluid-derived stem cell (AFSC)-human umbilical vein endothelial cell (HUVEC) co-culture could be used to engineer in vitro vasculature in a collagen chondroitin sulphate (CCS) scaffold. The secondary objective was to investigate whether hypoxic conditions (2% O2) could enhance microcapillary-like structure formation by this co-culture. The results of this study demonstrate, for the first time, that the AFSC-HUVEC co-culture was capable of pre-vascularising CCS scaffolds within 7 days and that the AFSCs are capable of behaving as pericytes while interacting with HUVECS to form microcapillary-like structures. However, this microcapillary-like structure formation was reduced in hypoxic conditions. qRT-PCR analysis indicated that an upregulation of VEGFR1 and accompanying decrease of VEGFR2 gene expression may be responsible for the poor response of these microcapillary-like structures to hypoxic conditions. Overall, however, these results demonstrate the potential of this newly developed co-culture system for the formation of pre-engineered vasculature within TE constructs.

STATEMENT OF SIGNIFICANCE

This article describes the development of an amniotic fluid-derived stem cell (AFSC)-human umbilical vein endothelial cell (HUVEC) co-culture for use in engineering in vitro vasculature in a collagen chondroitin sulphate (CCS) scaffold. The article also describes the effect of hypoxic conditions on the networks of microcapillary-like structures formed by this co-culture. The AFSC-HUVEC co-culture was capable of pre-vascularising CCS scaffolds within 7 days. However, microcapillary-like structure formation was reduced in hypoxic conditions. Overall, these results demonstrate the potential of this newly developed co-culture system for the formation of pre-engineered vasculature within TE constructs. The proangiogenic nature of this co-culture has the potential to both enhance bone regeneration while also overcoming the problem of inadequate vascularisation of grafts commonly seen in the field of tissue engineering.

Using bimodal MRI/fluorescence imaging to identify host angiogenic response to implants

Therapies that promote angiogenesis have been successfully applied using various combinations of proangiogenic factors together with a biodegradable delivery vehicle. In this study we used bimodal noninvasive monitoring to show that the host response to a proangiogenic biomaterial can be drastically affected by the mode of implantation and the surface area-to-volume ratio of the implant material. Fluorescence/MRI probes were covalently conjugated to VEGF-bearing biodegradable PEG-fibrinogen hydrogel implants and used to document the in vivo degradation and liberation of bioactive constituents in an s.c. rat implantation model. The hydrogel biodegradation and angiogenic host response with three types of VEGF-bearing implant configurations were compared: preformed cylindrical plugs, preformed injectable microbeads, and hydrogel precursor, injected and polymerized in situ. Although all three were made with identical amounts of precursor constituents, the MRI data revealed that in situ polymerized hydrogels were fully degraded within 2 wk; microbead degradation was more moderate, and plugs degraded significantly more slowly than the other configurations. The presence of hydrogel degradation products containing the fluorescent label in the surrounding tissues revealed a distinct biphasic release profile for each type of implant configuration. The purported in vivo VEGF release profile from the microbeads resulted in highly vascularized s.c. tissue containing up to 16-fold more capillaries in comparison with controls. These findings demonstrate that the configuration of an implant can play an important role not only in the degradation and resorption properties of the materials, but also in consequent host angiogenic response.

Hypoxic signaling during tissue repair and regenerative medicine

In patients with chronic wounds, autologous tissue repair is often not sufficient to heal the wound. These patients might benefit from regenerative medicine or the implantation of a tissue-engineered scaffold. Both wound healing and tissue engineering is dependent on the formation of a microvascular network. This process is highly regulated by hypoxia and the transcription factors hypoxia-inducible factors-1α (HIF-1α) and -2α (HIF-2α). Even though much is known about the function of HIF-1α in wound healing, knowledge about the function of HIF-2α in wound healing is lacking. This review focuses on the function of HIF-1α and HIF-2α in microvascular network formation, wound healing, and therapy strategies.

Engineered microvessels with strong alignment and high lumen density via cell-induced fibrin gel compaction and interstitial flow

The development of engineered microvessels with clinically relevant characteristics is a critical step toward the creation of engineered myocardium. Alignment is one such characteristic that must be achieved, as it both mimics native capillary beds and provides natural inlet and outlet sides for perfusion. A second characteristic that is currently deficient is cross-sectional lumen density, typically under 100 lumens/mm²; the equivalent value for human myocardium is 2000 lumens/mm². Therefore, this study examined the effects of gel compaction and interstitial flow on microvessel alignment and lumen density. Strong microvessel alignment was achieved via mechanically constrained cell-induced fibrin gel compaction following vasculogenesis, and high lumen density (650 lumens/mm²) was achieved by the subsequent application of low levels of interstitial flow. Low interstitial flow also conferred microvessel barrier function.

In vitro models of angiogenesis and vasculogenesis in fibrin gel

In vitro models of endothelial assembly into microvessels are useful for the study of angiogenesis and vasculogenesis. In addition, such models may be used to provide the microvasculature required to sustain engineered tissues. A large range of in vitro models of both angiogenesis and vasculogenesis have utilized fibrin gel as a scaffold. Although fibrin gel is conducive to endothelial assembly, its ultrastructure varies substantially based on the gel formulation and gelation conditions, making it challenging to compare between models. This work reviews existing models of endothelial assembly in fibrin gel and posits that differerences between models are partially caused by microstructural differences in fibrin gel.

Comparison of different strategies for in vivo seeding of prevascularized scaffolds

Scaffolds seeded with multipotent precursor cells were hypothesized to heal critically sized bone defects. However, the success of this concept was limited by low cell survival after transplantation due to a lack of nutrients and oxygen. In vivo prevascularization of scaffolds before cell seeding may improve cell survival, yet the best seeding technique and time point of cell application remain elusive. Thus, the aim of this study was to compare different strategies. Demineralized bone matrix scaffolds were implanted around the saphenous arteriovenous (AV) bundle in nude mice. In vivo seeding was performed 0, 5, or 21 days after implantation using enhanced green fluorescent protein (eGFP)-expressing mesenchymal stem cells (MSCs). Cells were applied either by injection or the repetitive dripping technique. In vitro seeded and subcutaneously implanted scaffolds served as controls. Fourteen days after cell application, the fluorescence intensity of transplanted cells and the extent of newly formed vessels were quantified. We found that the AV flow through model as well as cell application increased vessel formation. In vitro seeding resulted in significantly higher cell numbers than in vivo seeding. With increasing time of prevascularization, the number of cells declined dramatically. In vivo seeding by cell injection was superior to the repetitive dripping protocol. On subcutaneously implanted scaffolds, significantly, more cells were found than on axially perfused scaffolds. We conclude that in vitro seeding is more efficient compared to the two novel in vivo seeding techniques of prevascularized scaffolds. With increasing time of prevascularization, the seeding efficiency for the in vivo methods further decreases, presumably due to the ingrowth of connective tissue. Even though, the presence of MSCs and the longer period of prevascularization enhances vessel formation, this conceivable advantage is limited supposedly by the inferior seeding efficiency.

Directed oxygen gradients initiate a robust early remodeling response in engineered vascular grafts

Whereas functionally different, both organogenesis and wound-healing processes create zones or regions of hypoxia that persist until capillary networks are formed to facilitate oxygen and nutrient delivery. Similarly, regenerative processes within in vitro engineered tissues experience the same hypoxic regions, but without the capacity to form functional capillaries resulting in a major limitation in developing full-thickness organs and tissues. Due to the importance of oxygen in wound healing and tissue regeneration, we hypothesize that directed oxygen gradients can be used to modulate cell function and promote more effective tissue regeneration. The effect of controlled oxygen gradients on human smooth muscle cells (SMCs) was assessed using dual chambered perfusion bioreactors to regulate transport conditions occurring in a model vascular construct. SMCs were seeded onto the ablumenal surface of the scaffold and cultured for 21 days under 3 independent gas environments: (1) 21% oxygen, (2) 11% oxygen, or (3) an ablumen to lumen oxygen gradient from 11% to 21%. When compared to 21% oxygen and 11% oxygen conditions, the directed 11%-21% oxygen gradient resulted in a raised metabolic activity and significantly improved cell migration. After 21 days from seeding, cells were shown to migrate entirely across the scaffold to the vessel lumen (>450 μm). Concomitant with a more uniform cell dispersion, scaffold mechanics were significantly enhanced with increased stiffness and tensile strength. Native oxygen gradients are known to play a pivotal role during organ development; these results show that directed oxygen gradients within in vitro systems can be used to facilitate early remodeling leading to significantly enhanced cell migration and scaffold biomechanics.

Nonsteady state oxygen transport in engineered tissue: implications for design

Engineered tissue constructs are limited in size, and thus clinical relevance, when diffusion is the primary mode of oxygen transport. Understanding the extent of oxygen diffusion and cellular consumption is necessary for the design of engineered tissues, particularly those intended for implantation into hypoxic wound sites. This study presents a combined experimental and computation model to predict design constraints for cellularized fibrin tissues subjected to a step change in the oxygen concentration to simulate transplantation. Nonsteady state analysis of oxygen diffusion and consumption was used to estimate the diffusion coefficient of oxygen (mean±SD, 1.7×10(-9)±8.4×10(-11) m(2)/s) in fibrin hydrogels as well as the Michaelis-Menten parameters, Vmax (1.3×10(-17)±9.2×10(-19) mol·cell(-1)·s(-1)), and Km (8.0×10(-3)±3.5×0(-3) mol/m(3)), of normal human lung fibroblasts. Nondimensionalization of the governing diffusion-reaction equation enabled the creation of a single dimensionless parameter, the Thiele modulus (φ), which encompasses the combined effects of oxygen diffusion, consumption, and tissue dimensions. Tissue thickness is the design parameter with the most pronounced influence on the distribution of oxygen within the system. Additionally, tissues designed such that φ<1 achieve a near spatially uniform and adequate oxygen concentration following the step change. Understanding and optimizing the Thiele modulus will improve the design of engineered tissue implants.

Full range physiological mass transport control in 3D tissue cultures

We report the first demonstration of a microfluidic platform that captures the full physiological range of mass transport in 3-D tissue culture. The basis of our method used long microfluidic channels connected to both sides of a central microtissue chamber at different downstream positions to control the mass transport distribution within the chamber. Precise control of the Péclet number (Pe), defined as the ratio of convective to diffusive transport, over nearly five orders of magnitude (0.0056 to 160) was achieved. The platform was used to systematically investigate the role of physiological mass transport on vasculogenesis. We demonstrate, for the first time, that vasculogenesis can be independently stimulated by interstitial flow (Pe > 10) or hypoxic conditions (Pe < 0.1), and not by the intermediate state (normal living tissue). This simple platform can be applied to physiological and biological studies of 3D living tissue followed by pathological disease studies, such as cancer research and drug screening.

Overcoming hypoxia to improve tissue-engineering approaches to regenerative medicine

The current clinical successes of tissue engineering are limited primarily to low-metabolism, acellular, pre-vascularized or thin tissues. Mass transport has been identified as the primary culprit, limiting the delivery of nutrients (such as oxygen and glucose) and removal of wastes, from tissues deep within a cellular scaffold. While strategies to develop sufficient vasculature to overcome hypoxia in vitro are promising, inconsistencies between the in vitro and the in vivo environments may still negate the effectiveness of large-volume tissue-engineered scaffolds. While a common theme in tissue engineering is to maximize oxygen supply, studies suggest that moderate oxygenation of cellular scaffolds during in vitro conditioning is preferable to high oxygen levels. Aiming for moderate oxygen values to prevent hypoxia while still promoting angiogenesis may be obtained by tailoring in vitro culture conditions to the oxygen environment the scaffold will experience upon implantation. This review discusses the causes and effects of tissue-engineering hypoxia and the optimization of oxygenation for the minimization of in vivo hypoxia.

Hypoxia impedes vasculogenesis of in vitro engineered bone

To ensure the survival of engineered bone after implantation, we combined human endothelial colony forming cells (ECFCs) and multipotent stromal cells (MSCs) as a proof of concept in a co-culture model to create in vitro prevascularized bone constructs. We hypothesized that a hypoxic stimulus will contribute to prevascularization of engineered bone. Bone marrow-derived MSCs and ECFCs from human adult peripheral blood were allowed to form co-culture pellets containing ECFCs and MSCs (1:4) or MSCs only in controls. After culture under normoxia or hypoxia (5%), pellets were harvested and processed for immunohistochemistry of CD31, α-smooth muscle actin, and osteocalcin. Expression of vascular endothelial growth factor and SDF-1α was analyzed by PCR to elucidate their involvement in hypoxic stimulation of prevascularization. The normoxic condition in co-cultures of MSCs and ECFCs supported the formation and maintenance of prevascular structures, including organized CD31-positive cells embraced by differentiated mural cells. These structures failed to form in hypoxic conditions, thereby rejecting the hypothesis that hypoxia stimulates prevasculogenesis in three-dimensional engineered bone constructs. Further, the formation of prevascular structures was paralleled by increased SDF-1α expression. It is suggested that actual oxygen levels were below 5% in the hypoxic co-cultures, which prevented prevascular structure formation. In conclusion, our normoxic co-culture model containing cells from clinically relevant sources sustained simultaneous endothelial, smooth muscle, and osteogenic differentiation.

PLGA/hydrogel biopapers as a stackable substrate for printing HUVEC networks via BioLP

Two major challenges in tissue engineering are mimicking the native cell-cell arrangements of tissues and maintaining viability of three-dimension (3D) tissues thicker than 300 µm. Cell printing and prevascularization of engineered tissues are promising approaches to meet these challenges. However, the printing technologies used in biofabrication must balance the competing parameters of resolution, speed, and volume, which limit the resolution of thicker 3D structures. We suggest that high-resolution conformal printing techniques can be used to print 2D patterns of vascular cells onto biopaper substrates which can then be stacked to form a thicker tissue construct. Towards this end we created 1 cm × 1 cm × 300 µm biopapers to be used as the transferable, stackable substrate for cell printing. 3.6% w/v poly-lactide-co-glycolide was dissolved in chloroform and poured into molds filled with NaCl crystals. The salt was removed with DI water and the scaffolds were dried and loaded with a Collagen Type I or Matrigel. SEM of the biopapers showed extensive porosity and gel loading throughout. Biological laser printing (BioLP) was used to deposit human umbilical vein endothelial cells (HUVEC) in a simple intersecting pattern to the surface of the biopapers. The cells differentiated and stretched to form networks preserving the printed pattern. In a separate experiment to demonstrate "stackability," individual biopapers were randomly seeded with HUVECs and cultured for 1 day. The mechanically stable and viable biopapers were then stacked and cultured for 4 days. Three-dimensional confocal microscopy showed cell infiltration and survival in the compound multilayer constructs. These results demonstrate the feasibility of stackable "biopapers" as a scaffold to build 3D vascularized tissues with a 2D cell-printing technique.

Hypoxia preconditioning of tissue-engineered mucosa enhances its angiogenic capacity in vitro

Improving vascularization of tissue-engineered oral mucosa (TEM) is a major challenge in the field of plastic surgery. Hypoxia is a stimulator of angiogenesis through a number of mechanisms. Therefore, hypoxia is a critical parameter that can be controlled in an effort to improve angiogenesis. In the present study we studied the secretion of a number of angiogenic factors during hypoxia exposure and evaluated the effect of TEM conditioned medium on endothelial cells. TEM was constructed by seeding human oral mucosa keratinocytes and fibroblasts on acellular human donor skin. TEM was exposed to hypoxia during 6, 12, and 24 h. Cellular hypoxia was assessed by immunolocalization of the hypoxia-inducible factor-1α. Secretion of vascular endothelial growth factor, placental growth factor (PlGF), tissue inhibitors of matrix metalloproteinases-1 and -2, and the activity of matrix metalloproteinase-9 significantly increased during hypoxia exposure. Moreover, conditioned medium from hypoxic TEM strongly enhanced endothelial cell proliferation and migration. In vitro exposure of TEM to hypoxia improves its capacity to support endothelial cell proliferation and migration, which suggests that hypoxia preconditioning of TEM potentially improves angiogenic responses for in vivo implantation.