Engineering the Microcirculation

Tissue-Engineered Microvessels: A Review of Current Engineering Strategies and Applications

Microvessels, including arterioles, capillaries, and venules, play an important role in regulating blood flow, enabling nutrient and waste exchange, and facilitating immune surveillance. Due to their important roles in maintaining normal function in human tissues, a substantial effort has been devoted to developing tissue-engineered models to study endothelium-related biology and pathology. Various engineering strategies have been developed to recapitulate the structural, cellular, and molecular hallmarks of native human microvessels in vitro. In this review, recent progress in engineering approaches, key components, and culture platforms for tissue-engineered human microvessel models is summarized. Then, tissue-specific models, and the major applications of tissue-engineered microvessels in development, disease modeling, drug screening and delivery, and vascularization in tissue engineering, are reviewed. Finally, future research directions for the field are discussed.

In Vivo Vascularization Chamber for the Implantation of Embryonic Kidneys

A major obstacle to the implantation of ex vivo engineered tissues is the incorporation of functional vascular supply to support the growth of new tissue and to minimize ischemic injury. Existing prevascularization systems, such as arteriovenous (AV) loop-based systems, require microsurgery, limiting their use to larger animals. We aimed to develop an implantable device that can be prevascularized to enable vascularization of tissues in small rodents, and test its application on the vascularization of embryonic kidneys. Implanting the chamber between the abdominal aorta and the inferior vena cava, we detected endothelial cells and vascular networks after 48 h of implantation. Loading the chamber with collagen I (C), Matrigel (M), or Matrigel + vascular endothelial growth factor) (MV) had a strong influence on vascularization speed: Chambers loaded with C took 7 days to vascularize, 4 days for chambers with M, and 2 days for chambers with MV. Implantation of E12.5 mouse embryonic kidneys into prevascularized chambers (C, MV) was followed with significant growth and ureteric branching over 22 days. In contrast, the growth of kidneys in non-prevascularized chambers was stunted. We concluded that our prevascularized chamber is a valuable tool for vascularizing implanted tissues and tissue-engineered constructs. Further optimization will be necessary to control the directional growth of vascular endothelial cells within the chamber and the vascularization grade. Impact Statement Vascularization of engineered tissue, or organoids, constructs is a major hurdle in tissue engineering. Failure of vascularization is associated with prolonged ischemia time and potential tissue damage due to hypoxic effects. The method presented, demonstrates the use of a novel chamber that allows rapid vascularization of native and engineered tissues. We hope that this technology helps to stimulate research in the field of tissue vascularization and enables researchers to generate larger engineered vascularized tissues.

Accelerating Patterned Vascularization Using Granular Hydrogel Scaffolds and Surgical Micropuncture

Bulk hydrogel scaffolds are common in reconstructive surgery. They allow for the staged repair of soft tissue loss by providing a base for revascularization. Unfortunately, they are limited by both slow and random vascularization, which may manifest as treatment failure or suboptimal repair. Rapidly inducing patterned vascularization within biomaterials has profound translational implications for current clinical treatment paradigms and the scaleup of regenerative engineering platforms. To address this long-standing challenge, a novel microsurgical approach and granular hydrogel scaffold (GHS) technology are co-developed to hasten and pattern microvascular network formation. In surgical micropuncture (MP), targeted recipient blood vessels are perforated using a microneedle to accelerate cell extravasation and angiogenic outgrowth. By combining MP with an adjacent GHS with precisely tailored void space architecture, microvascular pattern formation as assessed by density, diameter, length, and intercapillary distance is rapidly guided. This work opens new translational opportunities for microvascular engineering, advancing reconstructive surgery, and regenerative medicine.

Stimuli-responsive hydrogels: smart state of-the-art platforms for cardiac tissue engineering

Biomedicine and tissue regeneration have made significant advancements recently, positively affecting the whole healthcare spectrum. This opened the way for them to develop their applications for revitalizing damaged tissues. Thus, their functionality will be restored. Cardiac tissue engineering (CTE) using curative procedures that combine biomolecules, biomimetic scaffolds, and cells plays a critical part in this path. Stimuli-responsive hydrogels (SRHs) are excellent three-dimensional (3D) biomaterials for tissue engineering (TE) and various biomedical applications. They can mimic the intrinsic tissues' physicochemical, mechanical, and biological characteristics in a variety of ways. They also provide for 3D setup, adequate aqueous conditions, and the mechanical consistency required for cell development. Furthermore, they function as competent delivery platforms for various biomolecules. Many natural and synthetic polymers were used to fabricate these intelligent platforms with innovative enhanced features and specialized capabilities that are appropriate for CTE applications. In the present review, different strategies employed for CTE were outlined. The light was shed on the limitations of the use of conventional hydrogels in CTE. Moreover, diverse types of SRHs, their characteristics, assembly and exploitation for CTE were discussed. To summarize, recent development in the construction of SRHs increases their potential to operate as intelligent, sophisticated systems in the reconstruction of degenerated cardiac tissues.

A tissue engineered 3D printed calcium alkali phosphate bioceramic bone graft enables vascularization and regeneration of critical-size discontinuity bony defects in vivo

Introduction: Recently, efforts towards the development of patient-specific 3D printed scaffolds for bone tissue engineering from bioactive ceramics have continuously intensified. For reconstruction of segmental defects after subtotal mandibulectomy a suitable tissue engineered bioceramic bone graft needs to be endowed with homogenously distributed osteoblasts in order to mimic the advantageous features of vascularized autologous fibula grafts, which represent the standard of care, contain osteogenic cells and are transplanted with the respective blood vessel. Consequently, inducing vascularization early on is pivotal for bone tissue engineering. The current study explored an advanced bone tissue engineering approach combining an advanced 3D printing technique for bioactive resorbable ceramic scaffolds with a perfusion cell culture technique for pre-colonization with mesenchymal stem cells, and with an intrinsic angiogenesis technique for regenerating critical size, segmental discontinuity defects in vivo applying a rat model. To this end, the effect of differing Si-CAOP (silica containing calcium alkali orthophosphate) scaffold microarchitecture arising from 3D powder bed printing (RP) or the Schwarzwalder Somers (SSM) replica fabrication technique on vascularization and bone regeneration was analyzed in vivo. In 80 rats 6-mm segmental discontinuity defects were created in the left femur. Methods: Embryonic mesenchymal stem cells were cultured on RP and SSM scaffolds for 7d under perfusion to create Si-CAOP grafts with terminally differentiated osteoblasts and mineralizing bone matrix. These scaffolds were implanted into the segmental defects in combination with an arteriovenous bundle (AVB). Native scaffolds without cells or AVB served as controls. After 3 and 6 months, femurs were processed for angio-µCT or hard tissue histology, histomorphometric and immunohistochemical analysis of angiogenic and osteogenic marker expression. Results: At 3 and 6 months, defects reconstructed with RP scaffolds, cells and AVB displayed a statistically significant higher bone area fraction, blood vessel volume%, blood vessel surface/volume, blood vessel thickness, density and linear density than defects treated with the other scaffold configurations. Discussion: Taken together, this study demonstrated that the AVB technique is well suited for inducing adequate vascularization of the tissue engineered scaffold graft in segmental defects after 3 and 6 months, and that our tissue engineering approach employing 3D powder bed printed scaffolds facilitated segmental defect repair.

Three-dimensional-printed polycaprolactone scaffolds with interconnected hollow-pipe structures for enhanced bone regeneration

Three-dimensional (3D)-printed scaffolds are widely used in tissue engineering to help regenerate critical-sized bone defects. However, conventional scaffolds possess relatively simple porous structures that limit the delivery of oxygen and nutrients to cells, leading to insufficient bone regeneration. Accordingly, in the present study, perfusable and permeable polycaprolactone scaffolds with highly interconnected hollow-pipe structures that mimic natural micro-vascular networks are prepared by an indirect one-pot 3D-printing method. In vitro experiments demonstrate that hollow-pipe-structured (HPS) scaffolds promote cell attachment, proliferation, osteogenesis and angiogenesis compared to the normal non-hollow-pipe-structured scaffolds. Furthermore, in vivo studies reveal that HPS scaffolds enhance bone regeneration and vascularization in rabbit bone defects, as observed at 8 and 12 weeks, respectively. Thus, the fabricated HPS scaffolds are promising candidates for the repair of critical-sized bone defects.

Design Challenges in Polymeric Scaffolds for Tissue Engineering

Numerous surgical procedures are daily performed worldwide to replace and repair damaged tissue. Tissue engineering is the field devoted to the regeneration of damaged tissue through the incorporation of cells in biocompatible and biodegradable porous constructs, known as scaffolds. The scaffolds act as host biomaterials of the incubating cells, guiding their attachment, growth, differentiation, proliferation, phenotype, and migration for the development of new tissue. Furthermore, cellular behavior and fate are bound to the biodegradation of the scaffold during tissue generation. This article provides a critical appraisal of how key biomaterial scaffold parameters, such as structure architecture, biochemistry, mechanical behavior, and biodegradability, impart the needed morphological, structural, and biochemical cues for eliciting cell behavior in various tissue engineering applications. Particular emphasis is given on specific scaffold attributes pertaining to skin and brain tissue generation, where further progress is needed (skin) or the research is at a relatively primitive stage (brain), and the enumeration of some of the most important challenges regarding scaffold constructs for tissue engineering.

Multicenter phase III trial of regenerative treatment for chronic tympanic membrane perforation

OBJECTIVE

To evaluate the efficacy and safety of regenerative treatment for tympanic membrane perforation (TMP) using gelatin sponge, basic fibroblast growth factor (bFGF), and fibrin glue.

METHODS

This was a multicenter, non-randomized, single-arm study conducted at tertiary referral centers. Twenty patients with chronic TMP (age 23-78 years, 6 males, 14 females) were registered from three institutions. All treated patients were included in the safety analysis population. The edges of the TMP were disrupted mechanically by myringotomy and several pieces of gelatin sponge immersed in bFGF were placed and fixed with fibrin glue to cover the perforation. The TMP was examined 4 ± 1 weeks later. The protocol was repeated up to four times until closure was complete. The main outcome measures were closure or a decrease in size of the TMP, hearing improvement, and air-bone gap evaluated 16 weeks after the final regenerative procedure (FRP). Adverse events (AEs) were monitored throughout the study.

RESULTS

Total closure of the TMP at 16 weeks was achieved in 15 out of 20 patients (75.0%, 95% confidence interval [CI]: 50.9%-91.3%) and the mean decrease in size was 92.2% (95%CI: 82.9%-100.0%). The ratio of hearing improvement and the air-bone gap at 16 weeks after FRP were 100% (20/20; 95%CI: 83.2%-100%) and 5.3 ± 4.2 dB (p <0.0001), respectively. Thirteen out of 20 patients (65.0%) experienced at least one AE, but no serious AEs occurred.

CONCLUSION

The results indicate that the current regenerative treatment for TMP using gelatin sponge, bFGF, and fibrin glue is safe and effective.

Melt Electrospinning of Polymers: Blends, Nanocomposites, Additives and Applications

Melt electrospinning has been developed in the last decade as an eco-friendly and solvent-free process to fill the gap between the advantages of solution electrospinning and the need of a cost-effective technique for industrial applications. Although the benefits of using melt electrospinning compared to solution electrospinning are impressive, there are still challenges that should be solved. These mainly concern to the improvement of polymer melt processability with reduction of polymer degradation and enhancement of fiber stability; and the achievement of a good control over the fiber size and especially for the production of large scale ultrafine fibers. This review is focused in the last research works discussing the different melt processing techniques, the most significant melt processing parameters, the incorporation of different additives (e.g., viscosity and conductivity modifiers), the development of polymer blends and nanocomposites, the new potential applications and the use of drug-loaded melt electrospun scaffolds for biomedical applications.

Cell-cell interaction in a coculture system consisting of CRISPR/Cas9 mediated GFP knock-in HUVECs and MG-63 cells in alginate-GelMA based nanocomposites hydrogel as a 3D scaffold

The interaction between osteogenic and angiogenic cells through a coculturing system in biocompatible materials has been considered for successfully engineering vascularized bone tissue equivalents. In this study, we developed a hydrogel-blended scaffold consisted of gelatin methacryloyl (GelMA) and alginate enriched with hydroxyapatite nanoparticles (HAP) to model an in vitro prevascularized bone construct. The hydrogel-based scaffold revealed a higher mechanical stiffness than those of pure (GelMA), alginate, and (GelMA+ HAP) hydrogels. In the present study, we generated a green fluorescent protein (GFP) knock-in umbilical vein endothelial cells (HUVECs) cell line using the CRISPR/Cas9 technology. The GFP was inserted into the human-like ROSA locus of HUVECs genome. HUVECs expressing GFP were cocultured with OB-like cells (MG-63) within three-dimensionally (3D) fabricated hydrogel to investigate the response of cocultured osteoblasts and endothelial cells in a 3D structure. Cell viability under the 3D cocultured gel was higher than the 3D monocultured. Compared to the 3D monocultured condition, the cells were aligned and developed into the vessel-like structures. During 14 days of culture periods, the cells displayed actin protrusions by the formation of spike-like filopodia in the 3D cocultured model. Angiogenic and osteogenic-related genes such as CD31, vWF, and osteocalcin showed higher expression in the cocultured versus the monocultured. These results have collectively indicated that the 3D cocultured hydrogel facilitates interaction among cells, thereby having a greater effect on angiogenic and osteogenic properties in the absence of induction media.

Microchannel Molding Combined with Layer-by-Layer Approach for the Formation of Three-Dimensional Tube-like Structures by Endothelial Cells

The development of a functional in vitro model for microcirculation is an unresolved challenge, with major impact for the creation and regeneration of organs in the tissue engineering. The absence of prevascularized engineered tissues limits enormously their efficacy and integration. Therefore, in this study, the in vitro formation of tubular-like structures with human umbilical vein endothelial cells (HUVECs) is investigated thanks to three-dimensional polycarbonate (PC) microchannel (μCh) scaffolds, surface biofunctionalized with hyaluronic acid/chitosan (HA/CHI) layer-by-layer (LbL) films grafted with adhesive (RGD) and angiogenic (SVV and QK) peptides, alone and in combination. The importance of this work lies in the formation of capillaries in the order of tens of μm, developing spontaneous microvessels, without the complexity of microfluidic approaches, and in a short time-scale. Ellipsometry, confocal laser scanning microscopy, and fluorospectrometry are used to characterize the biofunctionalized microchannels. PC-μCh scaffolds functionalized with (HA/CHI)_12.5 film (PC-LbL) and further grafted with RGD and QK peptides (PC-RGD+QK) or with RGD and SVV peptides (PC-RGD+SVV) are then tested for in vitro blood vessel formation. These assays evidence a rapid formation of tubular-like structures after 2 h of incubation. Moreover, a coculture system involving HUVECs and human pericytes derived from placenta (hPCs-PL) stabilizes the tubes for a longer time.

Key components of engineering vascularized 3-dimensional bioprinted bone constructs

Vascularization has a pivotal role in engineering successful tissue constructs. However, it remains a major hurdle of bone tissue engineering, especially in clinical applications for the treatment of large bone defects. Development of vascularized and clinically-relevant engineered bone substitutes with sufficient blood supply capable of maintaining implant viability and supporting subsequent host tissue integration remains a major challenge. Since only cells that are 100-200 µm from blood vessels can receive oxygen through diffusion, engineered constructs that are thicker than 400 µm face a challenging oxygenation problem. Following implantation in vivo, spontaneous ingrowth of capillaries in thick engineered constructs is too slow. Thus, it is critical to provide optimal conditions to support vascularization in engineered bone constructs. To achieve this, an in-depth understanding of the mechanisms of angiogenesis and bone development is required. In addition, it is also important to mimic the physiological milieu of native bone to fabricate more successful vascularized bone constructs. Numerous applications of engineered vascularization with cell-and/or microfabrication-based approaches seek to meet these aims. Three-dimensional (3D) printing promises to create patient-specific bone constructs in the future. In this review, we discuss the major components of fabricating vascularized 3D bioprinted bone constructs, analyze their related challenges, and highlight promising future trends.

Vascular Pedicle and Microchannels: Simple Methods Toward Effective In Vivo Vascularization of 3D Scaffolds

Poor vascularization remains a key limiting factor in translating advances in tissue engineering to clinical applications. Vascular pedicles (large arteries and veins) isolated in plastic chambers are known to sprout an extensive capillary network. This study examined the effect vascular pedicles and scaffold architecture have on vascularization and tissue integration of implanted silk scaffolds. Porous silk scaffolds with or without microchannels are manufactured to support implantation of a central vascular pedicle, without a chamber, implanted in the groin of Sprague Dawley rats, and assessed morphologically and morphometrically at 2 and 6 weeks. At both time points, blood vessels, connective tissue, and an inflammatory response infiltrate all scaffold pores externally, and centrally when a vascular pedicle is implanted. At week 2, vascular pedicles significantly increase the degree of scaffold tissue infiltration, and both the pedicle and the scaffold microchannels significantly increase vascular volume and vascular density. Interestingly, microchannels contribute to increased scaffold vascularity without affecting overall tissue infiltration, suggesting a direct effect of biomaterial architecture on vascularization. The inclusion of pedicles and microchannels are simple and effective proangiogenic techniques for engineering thick tissue constructs as both increase the speed of construct vascularization in the early weeks post in vivo implantation.

Application of Regenerative Treatment for Tympanic Membrane Perforation With Cholesteatoma, Tumor, or Severe Calcification

OBJECTIVE

To apply regenerative treatment for tympanic membrane (TM) perforation to patients with severe calcification of the TM, cholesteatomas, or tumors localized to the tympanic cavity.

STUDY DESIGN

Controlled clinical pilot study.

SETTING

General hospitals.

PATIENTS

Forty-five patients (age: 8-85; M = 19, F = 26) were selected from patients with or without TM perforation for the regenerative treatment. Twenty-five patients had cholesteatomas, 3 had tumors, and 17 had severe TM calcification. Patients were classified into three groups based on TM perforation size: less than 1/3 of the TM as Grade I (n = 5), 1/3 to 2/3 as Grade II (n = 19), and over 2/3 as Grade III (n = 21). Twenty patients who underwent standard tympanoplasty type I were selected as historical controls.

MATERIALS AND METHODS

Materials for the TM repair included gelatin sponge with basic fibroblast growth factor and fibrin glue. After lesions were removed through the TM perforation, gelatin sponge immersed in basic fibroblast growth factor was placed over the perforation. Fibrin glue was then dripped onto the sponge. Treatment efficacy was evaluated 6 months posttreatment.

RESULTS

Complete closure of the TM perforation was achieved in 91% (n = 41/45) of the patients in this regenerative treatment. Improvement in average hearing levels and air-bone gap were much better with this treatment than in the historical control group.

CONCLUSION

This new regenerative therapy is useful not only for patients with simple TM perforations but also for those with cholesteatomas, tumors, or severe calcification without requiring conventional surgical procedures. This regenerative therapy is an easy, safe, cost-effective, and minimally-invasive treatment.

Biofabrication offers future hope for tackling various obstacles and challenges in tissue engineering and regenerative medicine: A Perspective

Biofabrication is an emerging multidisciplinary field that makes a revolutionary impact on the researches on life science, biomedical engineering, and both basic and clinical medicine, has progressed tremendously over the past few years. Recently, there has been a big boom in three-dimensional (3D) printing or additive manufacturing (AM) research worldwide, and there is a significant increase not only in the number of researchers turning their attention to AM but also publications demonstrating the potential applications of 3D printing techniques in multiple fields. Biofabrication and bioprinting hold great promise for the innovation of engineering-based organ replacing medicine. In this mini review, various challenges in the field of tissue engineering are focused from the point of view of the biofabrication - strategies to bridge the gap between organ shortage and mission of medical innovation research seek to achieve organ-specific treatments or regenerative therapies. Four major challenges are discussed including (i) challenge of producing organs by AM, (ii) digitalization of tissue engineering and regenerative medicine, (iii) rapid production of organs beyond the biological natural course, and (iv) extracorporeal organ engineering.

3D-porous β-tricalcium phosphate-alginate-gelatin scaffold with DMOG delivery promotes angiogenesis and bone formation in rat calvarial defects

Hypoxia-inducible factor-1α (HIF-1α), a well-studied angiogenesis pathway, plays an essential role in angiogenesis-osteogenesis coupling. Targeting the HIF-1a pathway frequently leads to successful reconstruction of large-sized bone defects through promotion of angiogenesis. Dimethyloxalylglycine (DMOG) small molecule regulates the stability of HIF-1α at normal oxygen tension by mimicking hypoxia, which subsequently accelerates angiogenesis. The current study aims to develop a novel construct by seeding adipose derived mesenchymal stem cells (ADMSCs) onto a scaffold that contains DMOG to induce angiogenesis and regeneration of a critical size calvarial defect in a rat model. The spongy scaffolds have been synthesized in the presence and absence of DMOG and analyzed in terms of morphology, porosity, pore size, mechanical properties and DMOG release profile. The effect of DMOG delivery on cellular behaviors of adhesion, viability, osteogenic differentiation, and angiogenesis were subsequently evaluated under in vitro conditions. Histological analysis of cell-scaffold constructs were also performed following transplantation into the calvarial defect. Physical characteristics of fabricated scaffolds confirmed higher mechanical strength and surface roughness of DMOG-loaded scaffolds. Scanning electron microscopy (SEM) images and MTT assay demonstrated the attachment and viability of ADMSCs in the presence of DMOG, respectively. Osteogenic activity of ADMSCs that included alkaline phosphatase (ALP) activity and calcium deposition significantly increased in the DMOG-loaded scaffold. Computed tomography (CT) imaging combined with histomorphometry and immunohistochemistry analysis showed enhanced bone formation and angiogenesis in the DMOG-loaded scaffolds. Therefore, spongy scaffolds that contained DMOG and had angiogenesis ability could be utilized to enhance bone regeneration of large-sized bone defects.

Accelerated Angiogenic Induction and Vascular Integration in a Novel Synthetic Scaffolding Matrix for Tissue Replacement

Purpose

Reduced or delayed neovascularization is a major obstacle with regard to tissue-engineered constructs. The aim of this study was to evaluate the early microvascular response to a novel degradable ε-caprolactone terpolymer matrix.

Methods

ε-caprolactone terpolymer matrices (Suprathel Plus®; Institute of Textile and Process Engineering, Denkendorf, Germany) were implanted into dorsal skinfold chambers of balb/c mice (n=10). Microcirculatory changes were observed by intravital fluorescence microscopy. Scaffolding matrices from PEGT/PBT copolymer were used as controls (n=10).

Results

The formation of de novo vascular networks within both scaffolding matrices was noted throughout the experiment. A vascular ingrowth of perfused microvessels into the matrices up to 600 μm apart from the edge was noted within 10 days of implantation. The earliest signs of neoangiogenesis were visible in ε-caprolactone terpolymer matrices on day 1. In both scaffolds the new developed vessels extended centripetally from the border of the matrices towards the center and anastomosed to form a perfused microvascular network. There was significantly earlier onset of vascularization, increased vascularized area and higher vessel density in ε-caprolactone terpolymer matrices compared to PEGT/PBT copolymer matrices were observed.

Conclusions

The scaffolding matrix from ε-caprolactone terpolymer allowed for an earlier and more intense induction of angiogenesis and displayed the tendency to vascularize more rapidly within a shorter period of time after transplantation compared to PEGT/PBT copolymer scaffolds, thus indicating its potential application for tissue engineering purposes.

Vascularisation for cardiac tissue engineering: the extracellular matrix

Cardiovascular diseases present a major socio-economic burden. One major problem underlying most cardiovascular and congenital heart diseases is the irreversible loss of contractile heart muscle cells, the cardiomyocytes. To reverse damage incurred by myocardial infarction or by surgical correction of cardiac malformations, the loss of cardiac tissue with a thickness of a few millimetres needs to be compensated. A promising approach to this issue is cardiac tissue engineering. In this review we focus on the problem of in vitro vascularisation as implantation of cardiac patches consisting of more than three layers of cardiomyocytes (> 100 μm thick) already results in necrosis. We explain the need for vascularisation and elaborate on the importance to include non-myocytes in order to generate functional vascularised cardiac tissue. We discuss the potential of extracellular matrix molecules in promoting vascularisation and introduce nephronectin as an example of a new promising candidate. Finally, we discuss current biomaterial- based approaches including micropatterning, electrospinning, 3D micro-manufacturing technology and porogens. Collectively, the current literature supports the notion that cardiac tissue engineering is a realistic option for future treatment of paediatric and adult patients with cardiac disease.

Haemodynamically stimulated and in vivo generated axially vascularized soft-tissue free flaps for closure of complex defects: Evaluation in a small animal model

The arteriovenous (AV) loop model permits the creation of significant volumes of axially vascularized tissue that represents an alternative to conventional free flaps, circumventing their common limitations. However, such AV loop-based flaps have never before been examined in standardized animal models with respect to their suitability for reconstruction of critical bone-exposing defects. In the course of our preliminary studies, we implemented a novel defect model in rats that provides standardized and critical wound conditions and evaluated whether AV loop-generated flaps are suitable for free microsurgical transfer and closure of composite defects. We compared three groups of rodents with similar scapular defects: one received the AV flap, whereas controls were left to heal by secondary intention or with supplementary acellular matrix alone. To create the flaps, AV loops were placed into subcutaneous Teflon chambers filled with acellular matrix and transferred to the thigh region. Flap maturation was evaluated by histological analysis of angiogenesis and cell migration at days 14 and 28 after loop creation. Flap transfer to the scapular region and microsurgical anastomoses were performed after 14 days. Postoperative defect closure and perfusion were continually compared between groups. Within the AV flap chamber, the mean vessel number, cell count and the proportion of proliferating cells increased significantly over time. The novel defect model revealed that stable wound coverage with homogeneous vascular integration was achieved by AV loop-vascularized soft-tissue free flaps compared with controls. In summary, our study indicates for the first time that complex composite defects in rats can successfully be treated with AV loop-based free flaps.

Hypoxic preconditioning of myoblasts implanted in a tissue engineering chamber significantly increases local angiogenesis via upregulation of myoblast vascular endothelial growth factor-A expression and downregulation of miRNA-1, miRNA-206 and angiopoietin-1

Vascularization is a major hurdle for growing three-dimensional tissue engineered constructs. This study investigated the mechanisms involved in hypoxic preconditioning of primary rat myoblasts in vitro and their influence on local angiogenesis postimplantation. Primary rat myoblast cultures were exposed to 90 min hypoxia at <1% oxygen followed by normoxia for 24 h. Real time (RT) polymerase chain reaction evaluation indicated that 90 min hypoxia resulted in significant downregulation of miR-1 and miR-206 (p < 0.05) and angiopoietin-1 (p < 0.05) with upregulation of vascular endothelial growth factor-A (VEGF-A; p < 0.05). The miR-1 and angiopoietin-1 responses remained significantly downregulated after a 24 h rest phase. In addition, direct inhibition of miR-206 in L6 myoblasts caused a significant increase in VEGF-A expression (p < 0.05), further establishing that changes in VEGF-A expression are influenced by miR-206. Of the myogenic genes examined, MyoD was significantly upregulated, only after 24 h rest (p < 0.05). Preconditioned or control myoblasts were implanted with Matrigel™ into isolated bilateral tissue engineering chambers incorporating a flow-through epigastric vascular pedicle in severe combined immunodeficiency mice and the chamber tissue harvested 14 days later. Chambers implanted with preconditioned myoblasts had a significantly increased percentage volume of blood vessels (p = 0.0325) compared with chambers implanted with control myoblasts. Hypoxic preconditioned myoblasts promote vascularization of constructs via VEGF upregulation and downregulation of angiopoietin-1, miR-1 and miR-206. The relatively simple strategy of hypoxic preconditioning of implanted cells - including non-stem cell types - has broad, future applications in tissue engineering of skeletal muscle and other tissues, as a technique to significantly increase implant site angiogenesis.

Cell sheet engineering using the stromal vascular fraction of adipose tissue as a vascularization strategy

Current vascularization strategies for Tissue Engineering constructs, in particular cell sheet-based, are limited by time-consuming and expensive endothelial cell isolation and/or by the complexity of using extrinsic growth factors. Herein, we propose an alternative strategy using angiogenic cell sheets (CS) obtained from the stromal vascular fraction (SVF) of adipose tissue that can be incorporated into more complex constructs. Cells from the SVF were cultured in normoxic and hypoxic conditions for up to 8days in the absence of extrinsic growth factors. Immunocytochemistry against CD31 and CD146 revealed spontaneous organization in capillary-like structures, more complex after hypoxic conditioning. Inhibition of HIF-1α pathway hindered capillary-like structure formation in SVF cells cultured in hypoxia, suggesting a role of HIF-1α. Moreover, hypoxic SVF cells showed a trend for increased secretion of angiogenic factors, which was reflected in increased network formation by endothelial cells cultured on matrigel using that conditioned medium. In vivo implantation of SVF CS in a mouse hind limb ischemia model revealed that hypoxia-conditioned CS led to improved restoration of blood flow. Both in vitro and in vivo data suggest that SVF CS can be used as simple and cost-efficient tools to promote functional vascularization of TE constructs.

STATEMENT OF SIGNIFICANCE

Neovascularization after implantation is a major obstacle for producing clinically viable cell sheet-based tissue engineered constructs. Strategies using endothelial cells and extrinsic angiogenic growth factors are expensive and time consuming and may raise concerns of tumorigenicity. In this manuscript, we describe a simplified approach using angiogenic cell sheets fabricated from the stromal vascular fraction of adipose tissue. The strong angiogenic behavior of these cell sheets, achieved without the use of external growth factors, was further stimulated by low oxygen culture. When implanted in an in vivo model of hind limb ischemia, the angiogenic cell sheets contributed to blood flux recovery. These cell sheets can therefore be used as a straightforward tool to increase the neovascularization of cell sheet-based thick constructs.

Increased stem cells delivered using a silk gel/scaffold complex for enhanced bone regeneration

The low in vivo survival rate of scaffold-seeded cells is still a challenge in stem cell-based bone regeneration. This study seeks to use a silk hydrogel to deliver more stem cells into a bone defect area and prolong the viability of these cells after implantation. Rat bone marrow stem cells were mingled with silk hydrogels at the concentrations of 1.0 × 10⁵/mL, 1.0 × 10⁶/mL and 1.0 × 10⁷/mL before gelation, added dropwise to a silk scaffold and applied to a rat calvarial defect. A cell tracing experiment was included to observe the preservation of cell viability and function. The results show that the hydrogel with 1.0 × 10⁷/mL stem cells exhibited the best osteogenic effect both in vitro and in vivo. The cell-tracing experiment shows that cells in the 1.0 × 10⁷ group still survive and actively participate in new bone formation 8 weeks after implantation. The strategy of pre-mingling stem cells with the hydrogel had the effect of delivering more stem cells for bone engineering while preserving the viability and functions of these cells in vivo.

Three-dimensional tissues using human pluripotent stem cell spheroids as biofabrication building blocks

A recently emerged approach for tissue engineering is to biofabricate tissues using cellular spheroids as building blocks. Human pluripotent stem cells (hPSCs), including human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs), can be cultured to generate large numbers of cells and can presumably be differentiated into all the cell types of the human body in vitro, thus are an ideal cell source for biofabrication. We previously developed a hydrogel-based cell culture system that can economically produce large numbers of hPSC spheroids. With hPSCs and this culture system, there are two potential methods to biofabricate a desired tissue. In Method 1, hPSC spheroids are first utilized to biofabricate an hPSC tissue that is subsequently differentiated into the desired tissue. In Method 2, hPSC spheroids are first converted into tissue spheroids in the hydrogel-based culture system and the tissue spheroids are then utilized to biofabricate the desired tissue. In this paper, we systematically measured the fusion rates of hPSC spheroids without and with differentiation toward cortical and midbrain dopaminergic neurons and found spheroids' fusion rates dropped sharply as differentiation progressed. We found Method 1 was appropriate for biofabricating neural tissues.

Bioprinting for vascular and vascularized tissue biofabrication

Bioprinting is a promising technology to fabricate design-specific tissue constructs due to its ability to create complex, heterocellular structures with anatomical precision. Bioprinting enables the deposition of various biologics including growth factors, cells, genes, neo-tissues and extra-cellular matrix-like hydrogels. Benefits of bioprinting have started to make a mark in the fields of tissue engineering, regenerative medicine and pharmaceutics. Specifically, in the field of tissue engineering, the creation of vascularized tissue constructs has remained a principal challenge till date. However, given the myriad advantages over other biofabrication methods, it becomes organic to expect that bioprinting can provide a viable solution for the vascularization problem, and facilitate the clinical translation of tissue engineered constructs. This article provides a comprehensive account of bioprinting of vascular and vascularized tissue constructs. The review is structured as introducing the scope of bioprinting in tissue engineering applications, key vascular anatomical features and then a thorough coverage of 3D bioprinting using extrusion-, droplet- and laser-based bioprinting for fabrication of vascular tissue constructs. The review then provides the reader with the use of bioprinting for obtaining thick vascularized tissues using sacrificial bioink materials. Current challenges are discussed, a comparative evaluation of different bioprinting modalities is presented and future prospects are provided to the reader.

STATEMENT OF SIGNIFICANCE

Biofabrication of living tissues and organs at the clinically-relevant volumes vitally depends on the integration of vascular network. Despite the great progress in traditional biofabrication approaches, building perfusable hierarchical vascular network is a major challenge. Bioprinting is an emerging technology to fabricate design-specific tissue constructs due to its ability to create complex, heterocellular structures with anatomical precision, which holds a great promise in fabrication of vascular or vascularized tissues for transplantation use. Although a great progress has recently been made on building perfusable tissues and branched vascular network, a comprehensive review on the state-of-the-art in vascular and vascularized tissue bioprinting has not reported so far. This contribution is thus significant because it discusses the use of three major bioprinting modalities in vascular tissue biofabrication for the first time in the literature and compares their strengths and limitations in details. Moreover, the use of scaffold-based and scaffold-free bioprinting is expounded within the domain of vascular tissue fabrication.

Digital biomanufacturing supporting vascularization in 3D bioprinting

Synergies in bioprinting are appearing from individual researchers focusing on divergent aspects of the technology. Many are now evolving from simple mono-dimensional operations to model-controlled multi-material, interpenetrating networks using multi-modal deposition techniques. Bioinks are being designed to address numerous critical process parameters. Both the cellular constructs and architectural design for the necessary vascular component in digitally biomanufactured tissue constructs are being addressed. Advances are occurring from the topology of the circuits to the source of the of the biological microvessel components. Instruments monitoring and control of these activates are becoming interconnected. More and higher quality data are being collected and analysis is becoming richer. Information management and model generation is now describing a "process network." This is promising; more efficient use of both locally and imported raw data supporting accelerated strategic as well as tactical decision making. This allows real time optimization of the immediate bioprinting bioprocess based on such high value criteria as instantaneous progress assessment and comparison to previous activities. Finally, operations up- and down-stream of the deposition are being included in a supervisory enterprise control.

Vascularization of LBL structured nanofibrous matrices with endothelial cells for tissue regeneration

The aligned LBL scaffold promoted host vessel infiltration into the scaffolds and integration with in vitro prefabricated vascular structures.

The Arteriovenous (AV) Loop in a Small Animal Model to Study Angiogenesis and Vascularized Tissue Engineering

A functional blood vessel network is a prerequisite for the survival and growth of almost all tissues and organs in the human body. Moreover, in pathological situations such as cancer, vascularization plays a leading role in disease progression. Consequently, there is a strong need for a standardized and well-characterized in vivo model in order to elucidate the mechanisms of neovascularization and develop different vascularization approaches for tissue engineering and regenerative medicine. We describe a microsurgical approach for a small animal model for induction of a vascular axis consisting of a vein and artery that are anastomosed to an arteriovenous (AV) loop. The AV loop is transferred to an enclosed implantation chamber to create an isolated microenvironment in vivo, which is connected to the living organism only by means of the vascular axis. Using 3D imaging (MRI, micro-CT) and immunohistology, the growing vasculature can be visualized over time. By implanting different cells, growth factors and matrices, their function in blood vessel network formation can be analyzed without any disturbing influences from the surroundings in a well controllable environment. In addition to angiogenesis and antiangiogenesis studies, the AV loop model is also perfectly suited for engineering vascularized tissues. After a certain prevascularization time, the generated tissues can be transplanted into the defect site and microsurgically connected to the local vessels, thereby ensuring immediate blood supply and integration of the engineered tissue. By varying the matrices, cells, growth factors and chamber architecture, it is possible to generate various tissues, which can then be tailored to the individual patient's needs.

Human hepatocytes loaded in 3D bioprinting generate mini-liver

BACKGROUND

Because of an increasing discrepancy between the number of potential liver graft recipients and the number of organs available, scientists are trying to create artificial liver to mimic normal liver function and therefore, to support the patient's liver when in dysfunction. 3D printing technique meets this purpose. The present study was to test the feasibility of 3D hydrogel scaffolds for liver engineering.

METHODS

We fabricated 3D hydrogel scaffolds with a bioprinter. The biocompatibility of 3D hydrogel scaffolds was tested. Sixty nude mice were randomly divided into four groups, with 15 mice in each group: control, hydrogel, hydrogel with L02 (cell line HL-7702), and hydrogel with hepatocyte growth factor (HGF). Cells were cultured and deposited in scaffolds which were subsequently engrafted into livers after partial hepatectomy and radiation-induced liver damage (RILD). The engrafted tissues were examined after two weeks. The levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), albumin, total bilirubin, CYP1A2, CYP2C9, glutathione S-transferase (a-GST), and UDP-glucuronosyl transferase (UGT-2) were compared among the groups. Hematoxylin-eosin (HE) staining and immunohistochemistry of cKit and cytokeratin 18 (CK18) of engrafted tissues were evaluated. The survival time of the mice was also compared among the four groups.

RESULTS

3D hydrogel scaffolds did not impact the viability of cells. The levels of ALT, AST, albumin, total bilirubin, CYP1A2, CYP2C9, a-GST and UGT-2 were significantly improved in mice engrafted with 3D scaffold loaded with L02 compared with those in control and scaffold only (P<0.05). HE staining showed clear liver tissue and immunohistochemistry of cKit and CK18 were positive in the engrafted tissue. Mice treated with 3D scaffold+L02 cells had longer survival time compared with those in control and scaffold only (P<0.05).

CONCLUSION

3D scaffold has the potential of recreating liver tissue and partial liver functions and can be used in the reconstruction of liver tissues.

Tissue Engineering by Intrinsic Vascularization in an In Vivo Tissue Engineering Chamber

In reconstructive surgery, there is a clinical need for an alternative to the current methods of autologous reconstruction which are complex, costly and trade one defect for another. Tissue engineering holds the promise to address this increasing demand. However, most tissue engineering strategies fail to generate stable and functional tissue substitutes because of poor vascularization. This paper focuses on an in vivo tissue engineering chamber model of intrinsic vascularization where a perfused artery and a vein either as an arteriovenous loop or a flow-through pedicle configuration is directed inside a protected hollow chamber. In this chamber-based system angiogenic sprouting occurs from the arteriovenous vessels and this system attracts ischemic and inflammatory driven endogenous cell migration which gradually fills the chamber space with fibro-vascular tissue. Exogenous cell/matrix implantation at the time of chamber construction enhances cell survival and determines specificity of the engineered tissues which develop. Our studies have shown that this chamber model can successfully generate different tissues such as fat, cardiac muscle, liver and others. However, modifications and refinements are required to ensure target tissue formation is consistent and reproducible. This article describes a standardized protocol for the fabrication of two different vascularized tissue engineering chamber models in vivo.

Mesenchymal Stem Cells Increase Neo-Angiogenesis and Albumin Production in a Liver Tissue-Engineered Engraftment

The construction of a three-dimensional (3D) liver tissue is limited by many factors; one of them is the lack of vascularization inside the tissue-engineered construct. An engineered liver pocket-scaffold able to increase neo-angiogenesis in vivo could be a solution to overcome these limitations. In this work, a hyaluronan (HA)-based scaffold enriched with human mesenchymal stem cells (hMSCs) and rat hepatocytes was pre-conditioned in a bioreactor system, then implanted into the liver of rats. Angiogenesis and hepatocyte metabolic functions were monitored. The formation of a de novo vascular network within the HA-based scaffold, as well as an improvement in albumin production by the implanted hepatocytes, were detected. The presence of hMSCs in the HA-scaffold increased the concentration of growth factors promoting angiogenesis inside the graft. This event ensured a high blood vessel density, coupled with a support to metabolic functions of hepatocytes. All together, these results highlight the important role played by stem cells in liver tissue-engineered engraftment.

Silk Biomaterials with Vascularization Capacity

Functional vascularization is critical for the clinical regeneration of complex tissues such as kidney, liver or bone. The immobilization or delivery of growth factors has been explored to improve vascularization capacity of tissue engineered constructs, however, the use of growth factors has inherent problems such as the loss of signaling capability and the risk of complications such as immunological responses and cancer. Here, a new method of preparing water-insoluble silk protein scaffolds with vascularization capacity using an all aqueous process is reported. Acid was added temporally to tune the self-assembly of silk in lyophilization process, resulting in water insoluble scaffold formation directly. These biomaterials are mainly noncrystalline, offering improved cell proliferation than previously reported silk materials. These systems also have appropriate softer mechanical property that could provide physical cues to promote cell differentiation into endothelial cells, and enhance neovascularization and tissue ingrowth in vivo without the addition of growth factors. Therefore, silk-based degradable scaffolds represent an exciting biomaterial option, with vascularization capacity for soft tissue engineering and regenerative medicine.

Axially vascularised mandibular constructs: Is it time for a clinical trial?

Applying regenerative therapies in the field of cranio-maxillofacial reconstruction has now become a daily practice. However, regeneration of challenging or irradiated bone defects following head and neck cancer is still far beyond clinical application. As the key factor for sound regeneration is the development of an adequate vascular supply for the construct, the current modalities using extrinsic vascularization are incapable of regenerating such complex defects. Our group has recently introduced the intrinsic axial vascularization technique to regenerate mandibular defects using the arteriovenous loop (AVL). The technique has shown promising results in terms of efficient vascularization and bone regeneration at the preclinical level. In this article, we have conducted a narrative literature review about using the AVL to vascularize tissue-engineering constructs at the preclinical level. We have also conducted a systematic literature review about applying the technique of axial vascularization in the field of craniofacial regeneration. The versatility of the technique and the possible challenges are discussed, and a suggested protocol for the first clinical trial applying the AVL technique for mandibular reconstruction is also presented.

A First Step in De Novo Synthesis of a Living Pulp Tissue Replacement Using Dental Pulp MSCs and Tissue Growth Factors, Encapsulated within a Bioinspired Alginate Hydrogel

INTRODUCTION

A living, self-supporting pulp tissue replacement in vitro and for transplantation is an attractive yet unmet bioengineering challenge. Our aim is to create 3-dimensional alginate-based microenvironments that replicate the shape of gutta-percha and comprise key elements for the proliferation of progenitor cells and the release of growth factors.

METHODS

An RGD-bearing alginate framework was used to encapsulate dental pulp stem cells and human umbilical vein endothelial cells in a ratio of 1:1. The alginate hydrogel also retained and delivered 2 key growth factors, vascular endothelial growth factor-121 and fibroblast growth factor, in a sufficient amount to induce proliferation. A method was then devised to replicate the shape of gutta-percha using RGD alginate within a custom-made mold of thermoresponsive N-isopropylacrylamide. Plugs of alginate containing different permutations of growth factor-based encapsulates were tested and evaluated for viability, proliferation, and release kinetics between 1 and 14 days.

RESULTS

According to scanning electron microscopic and confocal microscopic observations, the encapsulated human endothelial cells and dental pulp stem cell distribution were frequent and extensive throughout the length of the construct. There were also high levels of viability in all test environments. Furthermore, cell proliferation was higher in the growth factor-based groups. Growth factor release kinetics also showed significant differences between them. Interestingly, the combination of vascular endothelial growth factor and fibroblast growth factor synergize to significantly up-regulate cell proliferation.

CONCLUSIONS

RGD-alginate scaffolds can be fabricated into shapes to fill the pulp space by simple templating. The addition of dual growth factors to cocultures of stem cells within RGD-alginate scaffolds led to the creation of microenvironments that significantly enhance the proliferation of dental pulp stem cell/human umbilical vein endothelial cell combinations.

Isolation of human lymphatic endothelial cells by multi-parameter fluorescence-activated cell sorting

Lymphatic system disorders such as primary lymphedema, lymphatic malformations and lymphatic tumors are rare conditions that cause significant morbidity but little is known about their biology. Isolating highly pure human lymphatic endothelial cells (LECs) from diseased and healthy tissue would facilitate studies of the lymphatic endothelium at genetic, molecular and cellular levels. It is anticipated that these investigations may reveal targets for new therapies that may change the clinical management of these conditions. A protocol describing the isolation of human foreskin LECs and lymphatic malformation lymphatic endothelial cells (LM LECs) is presented. To obtain a single cell suspension tissue was minced and enzymatically treated using dispase II and collagenase II. The resulting single cell suspension was then labelled with antibodies to cluster of differentiation (CD) markers CD34, CD31, Vascular Endothelial Growth Factor-3 (VEGFR-3) and PODOPLANIN. Stained viable cells were sorted on a fluorescently activated cell sorter (FACS) to separate the CD34(Low)CD31(Pos)VEGFR-3(Pos)PODOPLANIN(Pos) LM LEC population from other endothelial and non-endothelial cells. The sorted LM LECs were cultured and expanded on fibronectin-coated flasks for further experimental use.

Acceleration of vascularized bone tissue-engineered constructs in a large animal model combining intrinsic and extrinsic vascularization

During the last decades, a range of excellent and promising strategies in Bone Tissue Engineering have been developed. However, the remaining major problem is the lack of vascularization. In this study, extrinsic and intrinsic vascularization strategies were combined for acceleration of vascularization. For optimal biomechanical stability of the defect site and simplifying future transition into clinical application, a primary stable and approved nanostructured bone substitute in clinically relevant size was used. An arteriovenous (AV) loop was microsurgically created in sheep and implanted, together with the bone substitute, in either perforated titanium chambers (intrinsic/extrinsic) for different time intervals of up to 18 weeks or isolated Teflon(®) chambers (intrinsic) for 18 weeks. Over time, magnetic resonance imaging and micro-computed tomography (CT) analyses illustrate the dense vascularization arising from the AV loop. The bone substitute was completely interspersed with newly formed tissue after 12 weeks of intrinsic/extrinsic vascularization and after 18 weeks of intrinsic/extrinsic and intrinsic vascularization. Successful matrix change from an inorganic to an organic scaffold could be demonstrated in vascularized areas with scanning electron microscopy and energy dispersive X-ray spectroscopy. Using the intrinsic vascularization method only, the degradation of the scaffold and osteoclastic activity was significantly lower after 18 weeks, compared with 12 and 18 weeks in the combined intrinsic-extrinsic model. Immunohistochemical staining revealed an increase in bone tissue formation over time, without a difference between intrinsic/extrinsic and intrinsic vascularization after 18 weeks. This study presents the combination of extrinsic and intrinsic vascularization strategies for the generation of an axially vascularized bone substitute in clinically relevant size using a large animal model. The additional extrinsic vascularization promotes tissue ingrowth and remodeling processes of the bone substitute. Extrinsic vessels contribute to faster vascularization and finally anastomose with intrinsic vasculature, allowing microvascular transplantation of the bone substitute after a shorter prevascularization time than using the intrinsic method only. It can be reasonably assumed that the usage of perforated chambers can significantly reduce the time until transplantation of bone constructs. Finally, this study paves the way for further preclinical testing for proof of the concept as a basis for early clinical applicability.

Novel human-derived extracellular matrix induces in vitro and in vivo vascularization and inhibits fibrosis

The inability to vascularize engineered organs and revascularize areas of infarction has been a major roadblock to delivering successful regenerative medicine therapies to the clinic. These investigations detail an isolated human extracellular matrix derived from the placenta (hPM) that induces vasculogenesis in vitro and angiogenesis in vivo within bioengineered tissues, with significant immune reductive properties. Compositional analysis showed ECM components (fibrinogen, laminin), angiogenic cytokines (angiogenin, FGF), and immune-related cytokines (annexins, DEFA1) in near physiological ratios. Gene expression profiles of endothelial cells seeded onto the matrix displayed upregulation of angiogenic genes (TGFB1, VEGFA), remodeling genes (MMP9, LAMA5) and vascular development genes (HAND2, LECT1). Angiogenic networks displayed a time dependent stability in comparison to current in vitro approaches that degrade rapidly. In vivo, matrix-dosed bioscaffolds showed enhanced angiogenesis and significantly reduced fibrosis in comparison to current angiogenic biomaterials. Implementation of this human placenta derived extracellular matrix provides an alternative to Matrigel and, due to its human derivation, its development may have significant clinical applications leading to advances in therapeutic angiogenesis techniques and tissue engineering.

Vascularisation to improve translational potential of tissue engineering systems for cardiac repair

Cardiac tissue engineering is developing as an alternative approach to heart transplantation for treating heart failure. Shortage of organ donors and complications arising after orthotopic transplant remain major challenges to the modern field of heart transplantation. Engineering functional myocardium de novo requires an abundant source of cardiomyocytes, a biocompatible scaffold material and a functional vasculature to sustain the high metabolism of the construct. Progress has been made on several fronts, with cardiac cell biology, stem cells and biomaterials research particularly promising for cardiac tissue engineering, however currently employed strategies for vascularisation have lagged behind and limit the volume of tissue formed. Over ten years we have developed an in vivo tissue engineering model to construct vascularised tissue from various cell and tissue sources, including cardiac tissue. In this article we review the progress made with this approach and others, together with their potential to support a volume of engineered tissue for cardiac tissue engineering where contractile mass impacts directly on functional outcomes in translation to the clinic. It is clear that a scaled-up cardiac tissue engineering solution required for clinical treatment of heart failure will include a robust vascular supply for successful translation. This article is part of a directed issue entitled: Regenerative Medicine: the challenge of translation.

L-mimosine increases the production of vascular endothelial growth factor in human tooth slice organ culture model

AIM

To assess the pro-angiogenic and pro-inflammatory capacity of the dentine-pulp complex in response to the prolyl hydroxylase inhibitor L-mimosine in a tooth slice organ culture model.

METHODOLOGY

Human teeth were sectioned transversely into 600-μm-thick slices and cultured in medium supplemented with serum and antibiotics. Then, pulps were stimulated for 48 h with L-mimosine. Pulps were subjected to viability measurements based on formazan formation in MTT assays. In addition, histological evaluation of pulps was performed based on haematoxylin and eosin staining. Culture supernatants were subjected to immunoassays for vascular endothelial growth factor (VEGF) to determine the pro-angiogenic capacity and to immunoassays for interleukin (IL)-6 and IL-8 to assess the pro-inflammatory response. Interleukin-1 served as pro-inflammatory control. Echinomycin was used to inhibit hypoxia-inducible factor-1 (HIF-1) alpha activity. Data were analysed using Student's t-test and Mann-Whitney U test.

RESULTS

Pulps within tooth slices remained vital upon L-mimosine stimulation as indicated by formazan formation and histological evaluation. L-mimosine increased VEGF production when normalized to formazan formation in the pulp tissue of the tooth slices (P < 0.05). This effect on VEGF was reduced by echinomycin (P < 0.01). Changes in normalized IL-6 and IL-8 levels upon treatment with L-mimosine did not reach the level of significance (P > 0.05), whilst treatment with IL-1, which served as positive control, increased IL-6 (P < 0.05) and IL-8 levels (P < 0.05).

CONCLUSIONS

The prolyl hydroxylase inhibitor L-mimosine increased VEGF production via HIF-1 alpha in the tooth slice organ culture model whilst inducing no prominent increase in IL-6 and IL-8. Pre-clinical studies will reveal if these in vitro effects translate into dental pulp regeneration.

The adipogenic potential of various extracellular matrices under the influence of an angiogenic growth factor combination in a mouse tissue engineering chamber

The extracellular matrix (ECM) Matrigel™ has frequently and successfully been used to generate new adipose tissue experimentally, but is unsuitable for human application. This study sought to compare the adipogenic potential of a number of alternative, biologically derived or synthetic ECMs with potential for human application, with and without growth factors and a small fat autograft. Eight groups, with six severe combined immunodeficient (SCID) mice per group, were created with bilateral chambers (silicone tubes) implanted around the epigastric vascular pedicle, with one chamber/animal containing a 5mg fat autograft. Two animal groups were created for each of four ECMs (Matrigel™, Myogel, Cymetra® and PuraMatrix™) which filled the bilateral chambers. One group/ECM had no growth factors added to chambers whilst the other group had growth factors (GFs) (vascular endothelial growth factor-A (VEGF-A) plus fibroblast growth factor-2 (FGF-2) plus platelet-derived growth factor-BB (PDGF-BB)) added to both chambers. At 6weeks, chamber tissue was morphometrically assessed for percent and absolute adipose tissue volume. Overall, the triple GF regime significantly increased percent(∗) and absolute(#) adipose tissue volume (p<0.0005(∗#)) compared to chambers without triple GF treatment. The fat autograft also significantly increased percent (p<0.0005) and absolute (p<0.011) adipose tissue volume. Cymetra® (human collagen) constructs yielded the largest total tissue and absolute adipose tissue volume. We found that the pro-angiogenic FGF-2, VEGF-A and PDGF-BB combination in ECMs of synthetic and biological origin produced an overall significantly increased adipose tissue volume at 6weeks and may have clinical application, particularly with Cymetra.

Arrayed Hollow Channels in Silk-based Scaffolds Provide Functional Outcomes for Engineering Critically-sized Tissue Constructs

In the field of regenerative medicine there is a need for scaffolds that support large, critically-sized tissue formation. Major limitations in reaching this goal are the delivery of oxygen and nutrients throughout the bulk of the engineered tissue as well as host tissue integration and vascularization upon implantation. To address these limitations we previously reported the development of a porous scaffold platform made from biodegradable silk protein that contains an array of vascular-like structures that extend through the bulk of the scaffold. Here we report that the hollow channels play a pivotal role in enhancing cell infiltration, delivering oxygen and nutrients to the scaffold bulk, and promoting in vivo host tissue integration and vascularization. The unique features of this protein biomaterial system, including the vascular structures and tunable material properties, render this scaffold a robust and versatile tool for implementation in a variety of tissue engineering, regenerative medicine and disease modeling applications.

Regenerative treatment for soft tissue defects of the external auditory meatus

OBJECTIVE

To establish a regenerative treatment for soft tissue defects of the external auditory meatus (EAM) without conventional surgical therapy.

STUDY DESIGN

Controlled clinical pilot study.

SETTING

General hospitals.

PATIENTS

Sixty-five patients with new or old EAM defects without active inflammation were selected. Ages ranged from 12 to 87 years (average age of 58 yr).

INTERVENTION

Therapeutic nonsurgical treatment of EAM defects. Gelatin sponge, basic fibroblast growth factor (b-FGF), fibrin glue, and water proof transparent dressing were used in the repair procedure. Patients were divided into 2 groups: treatment with (n = 54) and without (n = 11) b-FGF. After mechanically disrupting the edge of the EAM defect, gelatin sponge immersed in b-FGF was placed over the defect and covered with fibrin glue. In cases of extensive EAM defects, the EAM was filled with gelatin sponge/b-FGF, and the auricle was wrapped in water proof dressing. Two or 3 weeks postprocedure, crust over the defect was removed. If complete defect closure was not achieved after 1 treatment course, the treatment was repeated.

MAIN OUTCOME MEASURE

Evaluation of complete closure of EAM defects 3 months posttreatment.

RESULTS

Complete closure of the EAM defect was achieved within 3 treatment courses in 92.6% (50/54) and 18.2% (2/11) of the patients with or without b-FGF, respectively. No inflammation/infection or severe sequelae were observed.

CONCLUSION

This study demonstrated the effectiveness of combining gelatin sponge, b-FGF, and fibrin glue for EAM defect regeneration. This innovative regenerative therapy is an easy, simple, cost-effective and minimally invasive method for treating EAM defects.