Three Dimensional Collagen Scaffold Promotes Intrinsic Vascularisation for Tissue Engineering Applications

Porous collagen scaffolds enable endothelial lumen formation in vitro under both static and dynamic growth conditions

Despite recent advances in the field of tissue engineering, the development of complex tissue-like structures in vitro is compromised by the lack of integration of a functioning vasculature. In this study, we propose a mesoscale three-dimensional (3D) in vitro vascularized connective tissue model and demonstrate its feasibility to prompt the self-assembly of endothelial cells into vessel-like structures. Moreover, we investigate the effect of perfusion on the organization of the cells. For this purpose, primary endothelial cells (HUVECs) and a cell line of human foreskin fibroblasts are cultivated in ECM-like matrices made up of freeze-dried collagen scaffolds permeated with collagen type I hydrogel. A tailored bioreactor is designed to investigate the effect of perfusion on self-organization of HUVECs. Immunofluorescent staining, two-photon microscopy, second-harmonic generation imaging, and scanning electron microscopy are applied to visualize the spatial arrangement of the cells. The analyses reveal the formation of hollow, vessel-like structures of HUVECs in hydrogel-permeated collagen scaffolds under both static and dynamic conditions. In conclusion, we demonstrate the feasibility of a 3D porous collagen scaffolding system that enables and maintains the self-organization of HUVECs into vessel-like structures independent of a dynamic flow.

Three-dimensional printing and decellularized-extracellular-matrix based methods for advances in artificial blood vessel fabrication: A review

Blood vessels are the tubes through which blood flows and are divided into three types: millimeter-scale arteries, veins, and capillaries as well as micrometer-scale capillaries. Arteries and veins are the conduits that carry blood, while capillaries are where blood exchanges substances with tissues. Blood vessels are mainly composed of collagen fibers, elastic fibers, glycosaminoglycans and other macromolecular substances. There are about 19 feet of blood vessels per square inch of skin in the human body, which shows how important blood vessels are to the human body. Because cardiovascular disease and vascular trauma are common in the population, a great number of researches have been carried out in recent years by simulating the structures and functions of the person's own blood vessels to create different levels of tissue-engineered blood vessels that can replace damaged blood vessels in the human body. However, due to the lack of effective oxygen and nutrient delivery mechanisms, these tissue-engineered vessels have not been used clinically. Therefore, in order to achieve better vascularization of engineered vascular tissue, researchers have widely explored the design methods of vascular systems of various sizes. In the near future, these carefully designed and constructed tissue engineered blood vessels are expected to have practical clinical applications. Exploring how to form multi-scale vascular networks and improve their compatibility with the host vascular system will be very beneficial in achieving this goal. Among them, 3D printing has the advantages of high precision and design flexibility, and the decellularized matrix retains active ingredients such as collagen, elastin, and glycosaminoglycan, while removing the immunogenic substance DNA. In this review, technologies and advances in 3D printing and decellularization-based artificial blood vessel manufacturing methods are systematically discussed. Recent examples of vascular systems designed are introduced in details, the main problems and challenges in the clinical application of vascular tissue restriction are discussed and pointed out, and the future development trends in the field of tissue engineered blood vessels are also prospected.

Technology for the formation of engineered microvascular network models and their biomedical applications

Tissue engineering and regenerative medicine have made great progress in recent decades, as the fields of bioengineering, materials science, and stem cell biology have converged, allowing tissue engineers to replicate the structure and function of various levels of the vascular tree. Nonetheless, the lack of a fully functional vascular system to efficiently supply oxygen and nutrients has hindered the clinical application of bioengineered tissues for transplantation. To investigate vascular biology, drug transport, disease progression, and vascularization of engineered tissues for regenerative medicine, we have analyzed different approaches for designing microvascular networks to create models. This review discusses recent advances in the field of microvascular tissue engineering, explores potential future challenges, and offers methodological recommendations.

Use of collagen and auricular cartilage in bioengineering: scaffolds for tissue regeneration

The aim of this study was the development of collagen and collagen/auricular cartilage scaffolds for application in dermal regeneration. Collagen was obtained from bovine tendon by a 72 h-long treatment, while bovine auricular cartilage was treated for 24 h and divided into two parts, external (perichondrium, E) and internal (elastic cartilage, I). The scaffolds were prepared by mixing collagen (C) with the internal part (CI) or the external part (CE) in a 3:1 ratio. Differential scanning calorimetry, scanning electron microscopy (SEM) analysis, microcomputed tomography imaging (micro-CT) and swelling degree were used to characterize the scaffolds. Cytotoxicity, cell adhesion, and cell proliferation assays were performed using the cell line NIH/3T3. All samples presented a similar denaturation temperature (Td) around 48 °C, while CE presented a second Td at 51.2 °C. SEM micrographs showed superficial pores in all scaffolds and micro-CT exhibited interconnected pore spaces with porosity above 60% (sizes between 47 and 149 µm). The order of swelling was CE < CI < C and the scaffolds did not present cytotoxicity, showing attachment rates above 75%-all samples showed a similar pattern of proliferation until 168 h, whereas CI tended to decrease after this time. The scaffolds were easily obtained, biocompatible and had adequate morphology for cell growth. All samples showed high adhesion, whereas collagen-only and collagen/external part scaffolds presented a better cell proliferation rate and would be indicated for possible use in dermal regeneration.

3D-Printed Collagen-Nanocellulose Hybrid Bioscaffolds with Tailored Properties for Tissue Engineering Applications

Hybrid collagen (Coll) bioscaffolds have emerged as a promising solution for tissue engineering (TE) and regenerative medicine. These innovative bioscaffolds combine the beneficial properties of Coll, an important structural protein of the extracellular matrix, with various other biomaterials to create platforms for long-term cell growth and tissue formation. The integration or cross-linking of Coll with other biomaterials increases mechanical strength and stability and introduces tailored biochemical and physical factors that mimic the natural tissue microenvironment. This work reports on the fabrication of chemically cross-linked hybrid bioscaffolds with enhanced properties from the combination of Coll, nanofibrillated cellulose (NFC), carboxymethylcellulose (CMC), and citric acid (CA). The bioscaffolds were prepared by 3D printing ink containing Coll-NFC-CMC-CA followed by freeze-drying, dehydrothermal treatment, and neutralization. Cross-linking through the formation of ester bonds between the polymers and CA in the bioscaffolds was achieved by exposing the bioscaffolds to elevated temperatures in the dry state. The morphology, pores/porosity, chemical composition, structure, thermal behavior, swelling, degradation, and mechanical properties of the bioscaffolds in the dry and wet states were investigated as a function of Coll concentration. The bioscaffolds showed no cytotoxicity to MG-63 human bone osteosarcoma cells as tested by different assays measuring different end points. Overall, the presented hybrid Coll bioscaffolds offer a unique combination of biocompatibility, stability, and structural support, making them valuable tools for TE.

Engineering large and geometrically controlled vascularized nerve tissue in collagen hydrogels to restore large-sized volumetric muscle loss

Developing scalable vascularized and innervated tissue is a critical challenge for the successful clinical application of tissue-engineered constructs. Collagen hydrogels are extensively utilized in cell-mediated vascular network formation because of their naturally excellent biological properties. However, the substantial increase in hydrogel contraction induced by populated cells limits their long-term use. Previous studies attempted to mitigate this issue by concentrating collagen pre-polymer solutions or synthesizing covalently crosslinked collagen hydrogels. However, these methods only partially reduce hydrogel contraction while hindering blood vessel formation within the hydrogels. To address this challenge, we introduced additional support in the form of a supportive spacer to counteract the contraction forces of populated cells and prevent hydrogel contraction. This approach was found to promote cell spreading, resist hydrogel contraction, control hydrogel/tissue geometry, and even facilitate the engineering of functional blood vessels and host nerve growth in just one week. Subsequently, implanting these engineered tissues into muscle defect sites resulted in timely anastomosis with the host vasculature, leading to enhanced myogenesis, increased muscle innervation, and the restoration of injured muscle functionality. Overall, this innovative strategy expands the applicability of collagen hydrogels in fabricating large vascularized nerve tissue constructs for repairing volumetric muscle loss (∼63 %) and restoring muscle function.

Nanomaterials-incorporated hydrogels for 3D bioprinting technology

In the field of tissue engineering and regenerative medicine, various hydrogels derived from the extracellular matrix have been utilized for creating engineered tissues and implantable scaffolds. While these hydrogels hold immense promise in the healthcare landscape, conventional bioinks based on ECM hydrogels face several challenges, particularly in terms of lacking the necessary mechanical properties required for 3D bioprinting process. To address these limitations, researchers are actively exploring novel nanomaterial-reinforced ECM hydrogels for both mechanical and functional aspects. In this review, we focused on discussing recent advancements in the fabrication of engineered tissues and monitoring systems using nanobioinks and nanomaterials via 3D bioprinting technology. We highlighted the synergistic benefits of combining numerous nanomaterials into ECM hydrogels and imposing geometrical effects by 3D bioprinting technology. Furthermore, we also elaborated on critical issues remaining at the moment, such as the inhomogeneous dispersion of nanomaterials and consequent technical and practical issues, in the fabrication of complex 3D structures with nanobioinks and nanomaterials. Finally, we elaborated on plausible outlooks for facilitating the use of nanomaterials in biofabrication and advancing the function of engineered tissues.

Novel stereological method for estimation of cell counts in 3D collagen scaffolds

Current methods for assessing cell proliferation in 3D scaffolds rely on changes in metabolic activity or total DNA, however, direct quantification of cell number in 3D scaffolds remains a challenge. To address this issue, we developed an unbiased stereology approach that uses systematic-random sampling and thin focal-plane optical sectioning of the scaffolds followed by estimation of total cell number (StereoCount). This approach was validated against an indirect method for measuring the total DNA (DNA content); and the Bürker counting chamber, the current reference method for quantifying cell number. We assessed the total cell number for cell seeding density (cells per unit volume) across four values and compared the methods in terms of accuracy, ease-of-use and time demands. The accuracy of StereoCount markedly outperformed the DNA content for cases with ~ 10,000 and ~ 125,000 cells/scaffold. For cases with ~ 250,000 and ~ 375,000 cells/scaffold both StereoCount and DNA content showed lower accuracy than the Bürker but did not differ from each other. In terms of ease-of-use, there was a strong advantage for the StereoCount due to output in terms of absolute cell numbers along with the possibility for an overview of cell distribution and future use of automation for high throughput analysis. Taking together, the StereoCount method is an efficient approach for direct cell quantification in 3D collagen scaffolds. Its major benefit is that automated StereoCount could accelerate research using 3D scaffolds focused on drug discovery for a wide variety of human diseases.

Antibacterial Activity and Cytocompatibility of Electrospun PLGA Scaffolds Surface-Modified by Pulsed DC Magnetron Co-Sputtering of Copper and Titanium

Biocompatible poly(lactide-co-glycolide) scaffolds fabricated via electrospinning are having promising properties as implants for the regeneration of fast-growing tissues, which are able to degrade in the body. The hereby-presented research work investigates the surface modification of these scaffolds in order to improve antibacterial properties of this type of scaffolds, as it can increase their application possibilities in medicine. Therefore, the scaffolds were surface-modified by means of pulsed direct current magnetron co-sputtering of copper and titanium targets in an inert atmosphere of argon. In order to obtain different amounts of copper and titanium in the resulting coatings, three different surface-modified scaffold samples were produced by changing the magnetron sputtering process parameters. The success of the antibacterial properties’ improvement was tested with the methicillin-resistant bacterium Staphylococcus aureus. In addition, the resulting cell toxicity of the surface modification by copper and titanium was examined using mouse embryonic and human gingival fibroblasts. As a result, the scaffold samples surface-modified with the highest copper to titanium ratio show the best antibacterial properties and no toxicity against mouse fibroblasts, but have a toxic effect to human gingival fibroblasts. The scaffold samples with the lowest copper to titanium ratio display no antibacterial effect and toxicity. The optimal poly(lactide-co-glycolide) scaffold sample is surface-modified with a medium ratio of copper and titanium that has antibacterial properties and is non-toxic to both cell cultures.

Development of fibroblast/endothelial cell-seeded collagen scaffolds for in vitro prevascularization

The development of vascularized scaffolds remains one of the major challenges in tissue engineering, and co-culturing with endothelial cells is known as one of the possible approaches for this purpose. In this approach, optimization of cell culture conditions, scaffolds, and fabrication techniques is needed to develop tissue equivalents that will enable in vitro formation of a capillary network. Prevascularized equivalents will be more physiologically comparable to the native tissues and potentially prevent insufficient vascularization after implantation. This study aimed to culture human umbilical vein endothelial cells (HUVECs), alone or in co-culture with fibroblasts, on collagen scaffolds prepared by simple fabrication approaches for in vitro prevascularization. Different concentrations and ratios of HUVECs and fibroblasts seeded on collagen gel and sponge scaffolds under several culture conditions were examined. Cell viability, scaffolds morphology, and structure were analyzed. Collagen gel scaffolds showed good cell proliferation and viability, with higher proliferation rates for cells cultured in a 2:1 (fibroblasts: HUVECs) ratio and kept in endothelial cell growth medium. However, these matrices were unable to support endothelial cell sprouting. Collagen sponges were highly porous and showed good cell viability. However, they became fragile over time in culture, and they still lack signs of vascularization. Collagen scaffolds were a good platform for cell growth and viability. However, under the experimental conditions of this study, the HUVEC/fibroblast-seeded scaffolds were not suitable platforms to generate in vitro prevascularized equivalents. Our findings will be a valuable starting point to optimize culture microenvironments and scaffolds during fabrication of prevascularized scaffolds.

Customizable 3D printed perfusion bioreactor for the engineering of stem cell microenvironments

Faithful modeling of tissues and organs requires the development of systems reflecting their dynamic 3D cellular architecture and organization. Current technologies suffer from a lack of design flexibility and complex prototyping, preventing their broad adoption by the scientific community. To make 3D cell culture more available and adaptable we here describe the use of the fused deposition modeling (FDM) technology to rapid-prototype 3D printed perfusion bioreactors. Our 3D printed bioreactors are made of polylactic acid resulting in reusable systems customizable in size and shape. Following design confirmation, our bioreactors were biologically validated for the culture of human mesenchymal stromal cells under perfusion for up to 2 weeks on collagen scaffolds. Microenvironments of various size/volume (6-12 mm in diameter) could be engineered, by modulating the 3D printed bioreactor design. Metabolic assay and confocal microscopy confirmed the homogenous mesenchymal cell distribution throughout the material pores. The resulting human microenvironments were further exploited for the maintenance of human hematopoietic stem cells. Following 1 week of stromal coculture, we report the recapitulation of 3D interactions between the mesenchymal and hematopoietic fractions, associated with a phenotypic expansion of the blood stem cell populations.Our data confirm that perfusion bioreactors fit for cell culture can be generated using a 3D printing technology and exploited for the 3D modeling of complex stem cell systems. Our approach opens the gates for a more faithful investigation of cellular processes in relation to a dynamic 3D microenvironment.

Bacterial collagen-templated synthesis and assembly of inorganic particles

Collagen has been used as a common template for mineralization and assembly of inorganic particles, because of the special arrangement of its fibrils and the presence of charged residues. Streptococcal bacterial collagen, which is inherently secreted on the surface ofStreptococcus pyogenes, has been progressively used as an alternative for type I animal collagen. Bacterial collagen is rich in charged amino acids, which can act as a substrate for the nucleation and growth of inorganic particles. Here, we show that bacterial collagen can be used to nucleate three different inorganic materials: hydroxyapatite crystals, silver nanoparticles, and silica nanoparticles. Collagen/mineral composites show an even distribution of inorganic particles along the collagen fibers, and the particles have a more homogenous size compared with minerals that are formed in the absence of the collagen scaffold. Furthermore, the gelation of silica occurring during mineralization represents a means to produce processable self-standing collagen composites, which is challenging to achieve with bacterial collagen alone. Overall, we highlight the advantage of simply combining bacterial collagen with minerals to expand their applications in the fields of biomaterials and tissue engineering, especially for bone regenerative scaffolds.

Chemical Bonding of Biomolecules to the Surface of Nano-Hydroxyapatite to Enhance Its Bioactivity

Hydroxyapatite (HA) is a significant constituent of bones or teeth and is widely used as an artificial bone graft. It is often used to replace the lost bones or in reconstructing alveolar bones before dental implantation. HA with biological functions finds its importance in orthopedic surgery and dentistry to increase the local concentration of calcium ions, which activate the growth and differentiation of mesenchymal stem cells (MSC). To make relevant use of HA in bone transplantation, the surfaces of orthopedic and dental implants are frequently coated with nanosized hydroxyapatite (nHA), but its low dispersibility and tendency to form aggregates, the purpose of the surface modification of bone implants is defeated. To overcome these drawbacks and to improve the histocompatibility of bone implants or to use nHA in therapeutic applications of implants in the treatment of bone diseases, various studies suggested the attachment of biomolecules (growth factors) or drugs through chemical bonding at the surface of nHA. The growth factors or drugs bonded physically at the surface of nHA are mostly unstable and burst released immediately. Therefore, reported studies suggested that the surface of nHA needs to be modified through the chemical bonding of biologically active molecules at the surface of bone implants such as proteins, peptides, or naturally occurring polysaccharides to prevent the aggregation of nHA and to get homogenous dispersion of nHA in solution. The role of irradiation in producing bioactive and antibacterial nHA through morphological variations in surfaces of nHA is also summarized by considering internal structures and the formation of reactive oxygen species on irradiation. This mini-review aims to highlight the importance of small molecules such as proteins, peptides, drugs, and photocatalysts in surface property modification of nHA to achieve stable, bioactive, and antibacterial nHA to act as artificial bone implants (scaffolds) in combination with biodegradable polymers.

Barrier Membrane in Regenerative Therapy: A Narrative Review

Guided bone and tissue regeneration remains an integral treatment modality to regenerate bone surrounding teeth and dental implants. Barrier membranes have been developed and produced commercially to allow space for bone regeneration and prevent the migration of unwanted cells. Ideal membrane properties, including biocompatibility, sufficient structural integrity and suitable shelf life with easy clinical application, are important to ensure good clinical regenerative outcomes. Membranes have various types, and their clinical application depends on the origin, material, structure and properties. This narrative review aims to describe the currently available barrier membranes in terms of history, main features, types, indication and clinical application and classify them into various groups. Various membranes, including those which are resorbable and non-resorbable, synthetic, added with growth factors and composed of modern materials, such as high-grade polymer (Polyetheretherketone), are explored in this review.

Cell-Seeded Biomaterial Scaffolds: The Urgent Need for Unanswered Accelerated Angiogenesis

One of the most arduous challenges in tissue engineering is neovascularization, without which there is a lack of nutrients delivered to a target tissue. Angiogenesis should be completed at an optimal density and within an appropriate period of time to prevent cell necrosis. Failure to meet this challenge brings about poor functionality for the tissue in comparison with the native tissue, extensively reducing cell viability. Prior studies devoted to angiogenesis have provided researchers with some biomaterial scaffolds and cell choices for angiogenesis. For example, while most current angiogenesis approaches require a variety of stimulatory factors ranging from biomechanical to biomolecular to cellular, some other promising stimulatory factors have been underdeveloped (such as electrical, topographical, and magnetic). When it comes to choosing biomaterial scaffolds in tissue engineering for angiogenesis, key traits rush to mind including biocompatibility, appropriate physical and mechanical properties (adhesion strength, shear stress, and malleability), as well as identifying the appropriate biomaterial in terms of stability and degradation profile, all of which may leave essential trace materials behind adversely influencing angiogenesis. Nevertheless, the selection of the best biomaterial and cells still remains an area of hot dispute as such previous studies have not sufficiently classified, integrated, or compared approaches. To address the aforementioned need, this review article summarizes a variety of natural and synthetic scaffolds including hydrogels that support angiogenesis. Furthermore, we review a variety of cell sources utilized for cell seeding and influential factors used for angiogenesis with a concentrated focus on biomechanical factors, with unique stimulatory factors. Lastly, we provide a bottom-to-up overview of angiogenic biomaterials and cell selection, highlighting parameters that need to be addressed in future studies.

Biomimetic Trachea Engineering via a Modular Ring Strategy Based on Bone-Marrow Stem Cells and Atelocollagen for Use in Extensive Tracheal Reconstruction

The fabrication of biomimetic tracheas with a architecture of cartilaginous rings alternately interspersed between vascularized fibrous tissue (CRVFT) has the potential to perfectly recapitulate the normal tracheal structure and function. Herein, the development of a customized chondroitin-sulfate-incorporating type-II atelocollagen (COL II/CS) scaffold with excellent chondrogenic capacity and a type-I atelocollagen (COL I) scaffold to facilitate the formation of vascularized fibrous tissue is described. An efficient modular ring strategy is then adopted to develop a CRVFT-based biomimetic trachea. The in vitro engineering of cartilaginous rings is achieved via the recellularization of ring-shaped COL II/CS scaffolds using bone marrow stem cells as a mimetic for native cartilaginous ring tissue. A CRVFT-based trachea with biomimetic mechanical properties, composed of bionic biochemical components, is additionally successfully generated in vivo via the alternating stacking of cartilaginous rings and ring-shaped COL I scaffolds on a silicone pipe. The resultant biomimetic trachea with pedicled muscular flaps is used for extensive tracheal reconstruction and exhibits satisfactory therapeutic outcomes with structural and functional properties similar to those of native trachea. This is the first study to utilize stem cells for long-segmental tracheal cartilaginous regeneration and this represents a promising method for extensive tracheal reconstruction.

Combination of Polysaccharide Nanofibers Derived from Cellulose and Chitin Promotes the Adhesion, Migration and Proliferation of Mouse Fibroblast Cells

Extracellular matrix (ECM) as a structural and biochemical scaffold to surrounding cells plays significant roles in cell adhesion, migration, proliferation and differentiation. Herein, we show the novel combination of TEMPO-oxidized cellulose nanofiber (TOCNF) and surface-N-deacetylated chitin nanofiber (SDCtNF), respectively, having carboxylate and amine groups on each crystalline surface, for mouse fibroblast cell culture. The TOCNF/SDCtNF composite scaffolds demonstrated characteristic cellular behavior, strongly depending on the molar ratios of carboxylates and amines of polysaccharide NFs. Pure TOCNF substrate exhibited good cell attachment, although intact carboxylate-free CNF made no contribution to cell adhesion. By contrast, pure SDCtNF induced crucial cell aggregation to form spheroids; nevertheless, the combination of TOCNF and SDCtNF enhanced cell attachment and subsequent proliferation. Molecular blend of carboxymethylcellulose and acid-soluble chitosan made nearly no contribution to cell culture behavior. The wound healing assay revealed that the polysaccharide combination markedly promoted skin repair for wound healing. Both of TOCNF and SDCtNF possessed rigid nanofiber nanoarchitectures with native crystalline forms and regularly-repeated functional groups, of which such structural characteristics would provide a potential for developing cell culture scaffolds having ECM functions, possibly promoting good cellular adhesion, migration and growth in the designated cellular microenvironments.

Collagen Nanofilm-Coated Partially Deproteinized Bone Combined With Bone Mesenchymal Stem Cells for Rat Femoral Defect Repair by Bone Tissue Engineering

BACKGROUND

The advantages of good biocompatibility, low degradation and low antigenicity of collagen, and the osteogenic differentiation characteristics of bone mesenchymal stem cells (BMSCs) were used to promote the recovery of bone defects using partially deproteinized bone (PDPB) by bone tissue engineering (BTE).

METHODS

The BMSCs were identified by examining their potential for osteogenic, lipogenic, and chondrogenic differentiation. The prepared pure PDPB was ground into bone blocks 4 × 2 × 2 mm in size, which were divided into the following groups: PDPB group, PDPB + collagen group, PDPB + collagen + BMSC group, PDPB with a composite collagen nanofilm, and BMSCs injected into the tail vein. At 2, 4, 6, and 8 weeks after surgery, the effects of the implants in the different groups on bone defect repair were continuously and dynamically observed through x-ray examination, gross specimen observation, histological evaluation, and microvascularization detection.

RESULTS

Postoperative x-ray examination and gross specimen observation revealed that, after 4 to 8 weeks, the external contour of the graft was gradually weakened, and the transverse comparison showed that the absorption of the graft and fusion of the defect were more obvious in PDPB + collagen + BMSC group than in PDPB group and PDPB + collagen group, and the healing was better (P < 0.05). Hematoxylin and eosin staining of histological sections showed very active proliferation of trabecular hematopoietic cells in groups PDPB + collagen + BMSC and PDPB + collagen. Masson's trichrome staining for evaluation of bone defect repair showed that the mean percent area of collagen fibers was greater in PDPB + collagen + BMSC group than in the PDPB group, with degradation of the scaffold material and the completion of repair. Immunofluorescence staining showed significantly enhanced expression of the vascular marker CD31 in group C (P < 0.05).

CONCLUSIONS

The proposed hybrid structure of the collagen matrix and PDPB provides an ideal 3-dimensional microenvironment for patient-specific BTE and cell therapy applications. The results showed that collagen appeared to regulate MSC-mediated osteogenesis and increase the migration and invasion of BMSCs. The combination of collagen nanofilm and biological bone transplantation with BMSC transplantation enhanced the proliferation and potential of the BMSCs for bone regeneration, successfully promoting bone repair after implantation at the defect site. This method may provide a new idea for treating clinical bone defects through BTE.

Avoiding artefacts in MicroCT imaging of collagen scaffolds: Effect of phosphotungstic acid (PTA)-staining and crosslink density

X-ray micro-computed tomography (μ-CT) can be used to provide both qualitative and quantitative information on the structure of three-dimensional (3D) bioactive scaffolds. When performed in a dry state, μ-CT accurately reflects the structure of collagen-based scaffolds, but imaging in a wet state offers challenges with radiolucency. Here we have used phosphotungstic acid (PTA) as a contrast agent to visualise fully hydrated collagen scaffolds in a physiologically relevant environment. A systematic investigation was performed to understand the effects of PTA on the results of μ-CT imaging by varying sample processing variables such as crosslinking density, hydration medium and staining duration.

Immersing samples in 0.3% PTA solution overnight completely stained the samples and the treatment provided a successful route for μ-CT analysis of crosslinked samples. However, significant structural artefacts were observed for samples which were either non-crosslinked or had low levels of crosslinking, which had a heterogeneous interior architecture with collapsed pores at the scaffold periphery. This work highlights the importance of optimising the choice of processing and staining conditions to ensure accurate visualisation for hydrated 3D collagen scaffolds in an aqueous medium.

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Important new insights on artefact prevention in scaffold imaging.

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First time μ-CT imaging of a hydrated collagen scaffold is reported.

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Optimisation of the use of phosphotungstic acid in μ-CT imaging.

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Correlation between crosslinking degree and artefact creation is established.

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A solution to processing and staining issues is highlighted.

Comparison of Vestibular Depth Relapse and Wound Healing After Reconstructive Preprosthetic Surgery Using Cryopreserved Amniotic Membrane and Acellular Dermal Matrix - A Comparative Study

Introduction:

The significance of membranes as wound dressing in oral surgeries has been reported by previous studies. The aim of the present split-mouth randomized clinical study was to assess and compare the wound dressing properties of acellular dermal matrix (ADM) and cryopreserved human amniotic membrane (AM) after reconstructive preprosthetic oral surgery.

Materials and Methods:

Twenty-eight patients with complete mandibular edentulism and resorbed alveolar bone were included. After taking mandibular impression, a clear acrylic splint with increased labial flange height was created. In each participant, labial vestibular depth was elevated using the Clark's technique. Subsequently, half of the exposed periosteum was covered with ADM while the other half was covered with cryopreserved human AM. Vestibule depth and relapse in the two sides were measured immediately after vestibuloplasty and at the end of the 1^st week, 2^nd week, 1^st month, and 3^rd months with graduations of 0.1 mm. Furthermore, after 3 and 7 days, samples were collected from graft material, and the macrophage population was analyzed by flow cytometry.

Results:

There was no significant difference in the relapse of vestibule depth between the two grafts at different time intervals. However, the frequency of wound-infiltrating macrophages (CD68⁺ cells) was significantly higher in areas covered by ADM after 3 and 7 days.

Discussion:

ADM is as effective as cryopreserved AM in terms of maintaining the postoperative vestibular depth. On the other hand, our results suggested that the onset of healing phase in ADM-covered areas occurs faster compared to the periosteum covered with cryopreserved human AM. This clinical trial showed significantly faster postoperative healing onset when ADM was used than when cryopreserved human AM was applied on the periosteum.

Piezoelectric core-shell PHBV/PDX blend scaffolds for reduced superficial wound contraction and scarless tissue regeneration

The use of non-invasive scaffold materials which can mimic the innate piezoelectric properties of biological tissues is a promising strategy to promote native tissue regeneration. Piezoelectric and cell instructive electrospun core-shell PDX/PHBV mats have been engineered to promote native tissue and skin regeneration. In depth physicochemical characterisation, in vitro and in vivo studies of a rat model showed that the 20/80 PDX/PHBV composition possessed the right balance of physicochemical and piezoelectric properties leading to enhanced fibroblast stimulation, proliferation and migration, reduced fibroblast-mediated contraction and macrophage-induced inflammation, improved keratinocyte proliferation, proper balance between endothelial cell phenotypes, decreased in vivo fibrosis and accelerated in vivo scarless wound regeneration. Overall, this study highlights the importance of exploiting cell-material interactions to match tissue biological needs to sustain the wound healing cascade.

Facile fabrication of Luffa cylindrica-assisted 3D hydroxyapatite scaffolds

The incidence of bone-related disorders is abruptly increasing worldwide, and the current therapies available are not sufficient to fulfill the growing demands of patients. Porous three-dimensional (3D) structures cast in combination with ceramics and polymers, with an intention to mimic native bone tissues, are gaining importance because of their better physicochemical and biological activities. The purpose of this study is to prepare a porous scaffold using Luffa cylindrica (LC) as a template coated with hydroxyapatite and gelatin. Guar gum (GG) was used as a binder, and hydroxyapatite powder was added to slurry of 10% gelatin and 1% GG in which pieces of LC were dipped followed by sintering at 900°C. The fabricated scaffolds (LC-GG) were analyzed by using different characterization techniques along with evaluation of porosity and water retention ability. The results revealed that the as-formed scaffolds have 70% porosity with more than 90% water retention ability. The degree of spreading of lymphocytes over the scaffold surface was less in comparison with that of the control, which showed the immunocompatibility of the fabricated scaffold. Based on the aforementioned findings, it is assumed that the synthesized porous structures can suitably be used for biomedical applications.

Vascularization strategies in tissue engineering approaches for soft tissue repair

Insufficient vascularization during tissue repair is often associated with poor clinical outcomes. This is a concern especially when patients have critical-sized injuries, where the size of the defect restricts vascularity, or even in small defects that have to be treated under special conditions, such as after radiation therapy (relevant to tumor resection) that hinders vascularity. In fact, poor vascularization is one of the major obstacles for clinical application of tissue engineering methods in soft tissue repair. As a key issue, lack of graft integration, caused by inadequate vascularization after implantation, can lead to graft failure. Moreover, poor vascularization compromises the viability of cells seeded in deep portions of scaffolds/graft materials, due to hypoxia and insufficient nutrient supply. In this article we aim to review vascularization strategies employed in tissue engineering techniques to repair soft tissues. For this purpose, we start by providing a brief overview of the main events during the physiological wound healing process in soft tissues. Then, we discuss how tissue repair can be achieved through tissue engineering, and considerations with regards to the choice of scaffold materials, culture conditions, and vascularization techniques. Next, we highlight the importance of vascularization, along with strategies and methods of prevascularization of soft tissue equivalents, particularly cell-based prevascularization. Lastly, we present a summary of commonly used in vitro methods during the vascularization of tissue-engineered soft tissue constructs.

Recapitulating Cardiac Structure and Function In Vitro from Simple to Complex Engineering

Cardiac tissue engineering aims to generate in vivo-like functional tissue for the study of cardiac development, homeostasis, and regeneration. Since the heart is composed of various types of cells and extracellular matrix with a specific microenvironment, the fabrication of cardiac tissue in vitro requires integrating technologies of cardiac cells, biomaterials, fabrication, and computational modeling to model the complexity of heart tissue. Here, we review the recent progress of engineering techniques from simple to complex for fabricating matured cardiac tissue in vitro. Advancements in cardiomyocytes, extracellular matrix, geometry, and computational modeling will be discussed based on a technology perspective and their use for preparation of functional cardiac tissue. Since the heart is a very complex system at multiscale levels, an understanding of each technique and their interactions would be highly beneficial to the development of a fully functional heart in cardiac tissue engineering.

Free Gingival Graft and Collagen Matrix Revascularization in an Enoral Open Wound Situation

PURPOSE

Vestibuloplasty with free gingival grafting is a frequently performed surgical procedure to generate sufficient keratinized mucosa (KM) around dental implants. Avascular porcine collagen matrices (CM) have been proclaimed to be sufficient substitutes as alternatives to free gingival grafts (FGGs). However, the process of graft integration and vascularization is still incompletely understood.

METHODS

In 18 patients a vestibuloplasty in the lower edentulous jaw situation was performed during implant exposure, either with FGGs from the palate or a porcine CM (mucoderm). Tissue perfusion of the soft tissue grafts was measured using laser-doppler-spectrophotometer intraoperatively and on postoperative days 2, 5, 10, 30 and between days 60 and 90. With graft perfusion expressed by oxygen saturation [SO2%], the relative amount of hemoglobin [rHb], blood flow, and velocity [AU] was detected and compared between groups and the surrounding mucosa.

RESULTS

Healing was uneventful in both groups, with mature KM around dental implants after healing. Blood flow and velocity significantly increased until postoperative day 10, comparable to perfusion values of the surrounded mucosa. Intergroup comparisons revelated no significant differences concerning the flow between CM and FGGs. Oxygen saturation also significantly increased within the first 5 postoperative days in both groups. Hemoglobin content did not show any differences during the investigated period.

CONCLUSIONS

The perfusion mainly progresses within the first postoperative week with only minimal further detectable alterations until the final investigation, comparable in both groups. Although integration of FGGs (revascularized) and the CM (new tissue formation) is biologically different, both transplants show comparable perfusion patterns, leading to sufficient KM.

Advances in generation of three-dimensional skin equivalents: pre-clinical studies to clinical therapies

The inability of two-dimensional cell culture systems to adequately map the structure and function of complex organs like skin necessitates the development of three-dimensional (3D) skin models. A diverse range of 3D skin equivalents have been developed over the last few decades for studying complex properties of skin as well as for drug discovery and clinical applications for skin regeneration in chronic wounds, such as diabetic foot ulcers, where the normal mechanism of wound healing is compromised. These 3D skin substitutes also serve as a suitable alternative to animal models in industrial applications and fundamental research. With the emergence of tissue engineering, new scaffolds and matrices have been integrated into 3D cell culture systems, along with gene therapy approaches, to increase the efficacy of transplanted cells in skin regeneration. This review summarizes recent approaches to the development of skin equivalents as well as different models for studying skin diseases and properties and current therapeutic applications of skin substitutes.

Comprehensive Survey on Nanobiomaterials for Bone Tissue Engineering Applications

One of the most important ideas ever produced by the application of materials science to the medical field is the notion of biomaterials. The nanostructured biomaterials play a crucial role in the development of new treatment strategies including not only the replacement of tissues and organs, but also repair and regeneration. They are designed to interact with damaged or injured tissues to induce regeneration, or as a forest for the production of laboratory tissues, so they must be micro-environmentally sensitive. The existing materials have many limitations, including impaired cell attachment, proliferation, and toxicity. Nanotechnology may open new avenues to bone tissue engineering by forming new assemblies similar in size and shape to the existing hierarchical bone structure. Organic and inorganic nanobiomaterials are increasingly used for bone tissue engineering applications because they may allow to overcome some of the current restrictions entailed by bone regeneration methods. This review covers the applications of different organic and inorganic nanobiomaterials in the field of hard tissue engineering.

Collagen Membrane for Guided Bone Regeneration in Dental and Orthopedic Applications

Treatment of cortical bone defects is a clinical challenge. Guided bone regeneration (GBR), commonly used in oral and maxillofacial dental surgery, may show promise for orthopedic applications in repair of cortical bone defects. However, a limitation in the use of GBR for cortical bone defects is the lack of an ideal scaffold that provides sufficient mechanical support to bridge the cortical bone with minimal interference in the repair process. We have developed a new collagen membrane, CelGro™, for use in GBR. We report the material characterization of CelGro and evaluate the performance of CelGro in translational preclinical and clinical studies. The results show CelGro has a bilayer structure of different fiber alignment and is composed almost exclusively of type I collagen. CelGro was found to be completely acellular and free from xenoantigen, α-gal (galactose-alpha-1,3-galactose). In the preclinical study of a rabbit cortical bone defect model, CelGro demonstrated enhanced bone-remodeling activity and cortical bone healing. Microcomputed tomography evaluation showed early bony bridging over the defect area 30 days postoperatively, and nearly complete restoration of mature cortical bone at the bone defect site 60 days postoperatively. Histological analysis 60 days after surgery further confirmed that CelGro enables bridging of the cortical bone defect by induction of newly formed cortical bone. Compared to a commercially available collagen membrane, Bio-Gide^®, CelGro showed much better cortical alignment and reduced porosity at the defect interface. As selection of orthopedic patients with cortical bone defects is complex, we conducted a clinical study evaluating the performance of CelGro in guided bone regeneration around dental implants. CelGro was used in GBR procedures in a total of 16 implants placed in 10 participants. Cone-beam computed tomography images show significantly increased bone formation both horizontally and vertically, which provides sufficient support to stabilize implants within 4 months. Together, the findings of our study demonstrate that CelGro is an ideal membrane for GBR not only in oral and maxillofacial reconstructive surgery but also in orthopedic applications (Clinical Trial ID ACTRN12615000027516).

An in-silico study of cancer cell survival and spatial distribution within a 3D microenvironment

3D cell cultures are in-vitro models representing a significant improvement with respect to traditional monolayers. Their diffusion and applicability, however, are hampered by the complexity of 3D systems, that add new physical variables for experimental analyses. In order to account for these additional features and improve the study of 3D cultures, we here present SALSA (ScAffoLd SimulAtor), a general purpose computational tool that can simulate the behavior of a population of cells cultured in a 3D scaffold. This software allows for the complete customization of both the polymeric template structure and the cell population behavior and characteristics. In the following the technical description of SALSA will be presented, together with its validation and an example of how it could be used to optimize the experimental analysis of two breast cancer cell lines cultured in collagen scaffolds. This work contributes to the growing field of integrated in-silico/in-vitro analysis of biological systems, which have great potential for the study of complex cell population behaviours and could lead to improve and facilitate the effectiveness and diffusion of 3D cell culture models.

Cellular Based Strategies for Microvascular Engineering

Vascularization is a major hurdle in complex tissue and organ engineering. Tissues greater than 200 μm in diameter cannot rely on simple diffusion to obtain nutrients and remove waste. Therefore, an integrated vascular network is required for clinical translation of engineered tissues. Microvessels have been described as <150 μm in diameter, but clinically they are defined as <1 mm. With new advances in super microsurgery, vessels less than 1 mm can be anastomosed to the recipient circulation. However, this technical advancement still relies on the creation of a stable engineered microcirculation that is amenable to surgical manipulation and is readily perfusable. Microvascular engineering lays on the crossroads of microfabrication, microfluidics, and tissue engineering strategies that utilize various cellular constituents. Early research focused on vascularization by co-culture and cellular interactions, with the addition of angiogenic growth factors to promote vascular growth. Since then, multiple strategies have been utilized taking advantage of innovations in additive manufacturing, biomaterials, and cell biology. However, the anatomy and dynamics of native blood vessels has not been consistently replicated. Inconsistent results can be partially attributed to cell sourcing which remains an enigma for microvascular engineering. Variations of endothelial cells, endothelial progenitor cells, and stem cells have all been used for microvascular network fabrication along with various mural cells. As each source offers advantages and disadvantages, there continues to be a lack of consensus. Furthermore, discord may be attributed to incomplete understanding about cell isolation and characterization without considering the microvascular architecture of the desired tissue/organ.

Type I Collagen Suspension Induces Neocollagenesis and Myodifferentiation in Fibroblasts In Vitro

The ability of a collagen-based matrix to support cell proliferation, migration, and infiltration has been reported; however, the direct effect of an aqueous collagen suspension on cell cultures has not been studied yet. In this work, the effects of a high-concentration aqueous suspension of a micronized type I equine collagen (EC-I) have been evaluated on a normal mouse fibroblast cell line. Immunofluorescence analysis showed the ability of EC-I to induce a significant increase of type I and III collagen levels, parallel with overexpression of crucial proteins in collagen biosynthesis, maturation, and secretion, prolyl 4-hydroxylase (P4H) and heat shock protein 47 (HSP47), as demonstrated by western blot experiments. The treatment led, also, to an increase of α-smooth muscle actin (α-SMA) expression, evaluated through western blot analysis, and cytoskeletal reorganization, as assessed by phalloidin staining. Moreover, scanning electron microscopy analysis highlighted the appearance of plasma membrane extensions and blebbing of extracellular vesicles. Altogether, these results strongly suggest that an aqueous collagen type I suspension is able to induce fibroblast myodifferentiation. Moreover, our findings also support in vitro models as a useful tool to evaluate the effects of a collagen suspension and understand the molecular signaling pathways possibly involved in the effects observed following collagen treatment in vivo.

Proteins and Peptides as Important Modifiers of the Polymer Scaffolds for Tissue Engineering Applications-A Review

Polymer scaffolds constitute a very interesting strategy for tissue engineering. Even though they are generally non-toxic, in some cases, they may not provide suitable support for cell adhesion, proliferation, and differentiation, which decelerates tissue regeneration. To improve biological properties, scaffolds are frequently enriched with bioactive molecules, inter alia extracellular matrix proteins, adhesive peptides, growth factors, hormones, and cytokines. Although there are many papers describing synthesis and properties of polymer scaffolds enriched with proteins or peptides, few reviews comprehensively summarize these bioactive molecules. Thus, this review presents the current knowledge about the most important proteins and peptides used for modification of polymer scaffolds for tissue engineering. This paper also describes the influence of addition of proteins and peptides on physicochemical, mechanical, and biological properties of polymer scaffolds. Moreover, this article sums up the major applications of some biodegradable natural and synthetic polymer scaffolds modified with proteins and peptides, which have been developed within the past five years.

Natural polypeptides-based electrically conductive biomaterials for tissue engineering

Fabrication of an appropriate scaffold is the key fundamental step required for a successful tissue engineering (TE). The artificial scaffold as extracellular matrix in TE has noticeable role in the fate of cells in terms of their attachment, proliferation, differentiation, orientation and movement. In addition, chemical and electrical stimulations affect various behaviors of cells such as polarity and functionality. Therefore, the fabrication approach and materials used for the preparation of scaffold should be more considered. Various synthetic and natural polymers have been used extensively for the preparation of scaffolds. The electrically conductive polymers (ECPs), moreover, have been used in combination with other polymers to apply electric fields (EF) during TE. In this context, composites of natural polypeptides and ECPs can be taken into account as context for the preparation of suitable scaffolds with superior biological and physicochemical features. In this review, we overviewed the simultaneous usage of natural polypeptides and ECPs for the fabrication of scaffolds in TE.

Vascularization of tissue-engineered skeletal muscle constructs

Skeletal muscle tissue can be created in vitro by tissue engineering approaches, based on differentiation of muscle stem cells. Several approaches exist and generally result in three dimensional constructs composed of multinucleated myofibers to which we refer as myooids. Engineering methods date back to 3 decades ago and meanwhile a wide range of cell types and scaffold types have been evaluated. Nevertheless, in most approaches, myooids remain very small to allow for diffusion-mediated nutrient supply and waste product removal, typically less than 1 mm thick. One of the shortcomings of current in vitro skeletal muscle organoid development is the lack of a functional vascular structure, thus limiting the size of myooids. This is a challenge which is nowadays applicable to almost all organoid systems. Several approaches to obtain a vascular structure within myooids have been proposed. The purpose of this review is to give a concise overview of these approaches.

Hydrogel scaffolds based on blood plasma cryoprecipitate and collagen derived from various sources: Structural, mechanical and biological characteristics

At present there is a growing need for tissue engineering products, including the products of scaffold-technologies. Biopolymer hydrogel scaffolds have a number of advantages and are increasingly being used to provide means of cell transfer for therapeutic treatments and for inducing tissue regeneration. This work presents original hydrogel biopolymer scaffolds based on a blood plasma cryoprecipitate and collagen and formed under conditions of enzymatic hydrolysis. Two differently originated collagens were used for the scaffold formation. During this work the structural and mechanical characteristics of the scaffold were studied. It was found that, depending on the origin of collagen, scaffolds possess differences in their structural and mechanical characteristics. Both types of hydrogel scaffolds have good biocompatibility and provide conditions that maintain the three-dimensional growth of adipose tissue stem cells. Hence, scaffolds based on such a blood plasma cryoprecipitate and collagen have good prospects as cell carriers and can be widely used in regenerative medicine.

Investigating the Osteoinductive Potential of a Decellularized Xenograft Bone Substitute

Bone grafting is the second most common tissue transplantation procedure worldwide. One of the alternative methods for bone repair under investigation is a tissue-engineered bone substitute. An ideal property of tissue-engineered bone substitutes is osteoinductivity, defined as the ability to stimulate primitive cells to differentiate into a bone-forming lineage. In the current study, we use a decellularization and oxidation protocol to produce a porcine bone scaffold and examine whether it possesses osteoinductive potential and can be used to create a tissue-engineered bone microenvironment. The decellularization protocol was patented by our lab and consists of chemical decellularization and oxidation steps using combinations of deionized water, trypsin, antimicrobials, peracetic acid, and triton-X100. To test if the bone scaffold was a viable host, preosteoblasts were seeded and analyzed for markers of osteogenic differentiation. The osteoinductive potential was observed in vitro with similar osteogenic markers being expressed in preosteoblasts seeded on the scaffolds and demineralized bone matrix. To assess these properties in vivo, scaffolds with and without preosteoblasts preseeded were subcutaneously implanted in mice for 4 weeks. MicroCT scanning revealed 1.6-fold increased bone volume to total volume ratio and 1.4-fold increase in trabecular thickness in scaffolds after implantation. The histological analysis demonstrates new bone formation and blood vessel formation with pentachrome staining demonstrating osteogenesis and angiogenesis, respectively, within the scaffold. Furthermore, CD31+ staining confirmed the endothelial lining of the blood vessels. These results demonstrate that porcine bone maintains its osteoinductive properties after the application of a patented decellularization and oxidation protocol developed in our laboratory. Future work must be performed to definitively prove osteogenesis of human mesenchymal stem cells, biocompatibility in large animal models, and osteoinduction/osseointegration in a relevant clinical model in vivo. The ability to create a functional bone microenvironment using decellularized xenografts will impact regenerative medicine, orthopedic reconstruction, and could be used in the research of multiple diseases.

Collagen-Based Tissue Engineering Strategies for Vascular Medicine

Cardiovascular diseases (CVDs) account for the 31% of total death per year, making them the first cause of death in the world. Atherosclerosis is at the root of the most life-threatening CVDs. Vascular bypass/replacement surgery is the primary therapy for patients with atherosclerosis. The use of polymeric grafts for this application is still burdened by high-rate failure, mostly caused by thrombosis and neointima hyperplasia at the implantation site. As a solution for these problems, the fast re-establishment of a functional endothelial cell (EC) layer has been proposed, representing a strategy of crucial importance to reduce these adverse outcomes. Implant modifications using molecules and growth factors with the aim of speeding up the re-endothelialization process has been proposed over the last years. Collagen, by virtue of several favorable properties, has been widely studied for its application in vascular graft enrichment, mainly as a coating for vascular graft luminal surface and as a drug delivery system for the release of pro-endothelialization factors. Collagen coatings provide receptor-ligand binding sites for ECs on the graft surface and, at the same time, act as biological sealants, effectively reducing graft porosity. The development of collagen-based drug delivery systems, in which small-molecule and protein-based drugs are immobilized within a collagen scaffold in order to control their release for biomedical applications, has been widely explored. These systems help in protecting the biological activity of the loaded molecules while slowing their diffusion from collagen scaffolds, providing optimal effects on the targeted vascular cells. Moreover, collagen-based vascular tissue engineering substitutes, despite not showing yet optimal mechanical properties for their use in the therapy, have shown a high potential as physiologically relevant models for the study of cardiovascular therapeutic drugs and diseases. In this review, the current state of the art about the use of collagen-based strategies, mainly as a coating material for the functionalization of vascular graft luminal surface, as a drug delivery system for the release of pro-endothelialization factors, and as physiologically relevant in vitro vascular models, and the future trend in this field of research will be presented and discussed.

Generation of a nitric oxide signaling pathway in mesenchymal stem cells promotes endothelial lineage commitment

Enhancing differentiation of mesenchymal stem cells (MSCs) to endothelial cells may improve their ability to vascularize tissue and promote wound healing. This study describes a novel role for nitric oxide (NO) in reprogramming MSCs towards an endothelial lineage and highlights the role of Wnt signaling and epigenetic modification by NO. Rat MSCs were transduced with lentiviral vectors expressing endothelial nitric oxide synthase (pLV-eNOS) and a mutated caveolin gene (pLV-CAV-1^F92A ) to enhance NO generation resulting in increased in vitro capillary tubule formation and endothelial marker gene expression. An exogenous source of NO could also stimulate CD31 expression in MSCs. NO was associated with an arterial-specific endothelial gene expression profile of Notch1, Dll4, and Hey2 and significantly reduced expression of venous markers. Wnt signaling associated with NO was evident through increased gene expression of Wnt3a and β-catenin protein, and expression of the endothelial marker Pecam-1 could be significantly reduced by treatment with the Wnt signaling inhibitor Dkk-1. The role of NO as an epigenetic modifier was evident with reduced gene expression of the methyltransferase, DNMT1, and bisulfite sequencing of the endothelial Flt1 promoter region in NO-producing MSCs showed significant demethylation compared to control cells. Finally, subcutaneous implantation of NO-producing MSCs seeded in a biomaterial scaffold (NovoSorb®) resulted in survival of transplanted cells and the formation of blood vessels. In summary, this study describes, NO as a potent endothelial programming factor which acts as an epigenetic modifier in MSCs and may provide a novel platform for vascular regenerative therapy.

Bioprinting of Vascularized Tissue Scaffolds: Influence of Biopolymer, Cells, Growth Factors, and Gene Delivery

Over the past decades, tissue regeneration with scaffolds has achieved significant progress that would eventually be able to solve the worldwide crisis of tissue and organ regeneration. While the recent advancement in additive manufacturing technique has facilitated the biofabrication of scaffolds mimicking the host tissue, thick tissue regeneration remains challenging to date due to the growing complexity of interconnected, stable, and functional vascular network within the scaffold. Since the biological performance of scaffolds affects the blood vessel regeneration process, perfect selection and manipulation of biological factors (i.e., biopolymers, cells, growth factors, and gene delivery) are required to grow capillary and macro blood vessels. Therefore, in this study, a brief review has been presented regarding the recent progress in vasculature formation using single, dual, or multiple biological factors. Besides, a number of ways have been presented to incorporate these factors into scaffolds. The merits and shortcomings associated with the application of each factor have been highlighted, and future research direction has been suggested.