The use of three-dimensional nanostructures to instruct cells to produce extracellular matrix for regenerative medicine strategies

Hierarchically patterned triple-layered gelatin-based electrospun membrane functionalized by cell-specific extracellular matrix for periodontal regeneration

OBJECTIVES

Regenerating the periodontium poses a critical challenge in oral medicine. To repair various periodontal defects, it is necessary to adopt a bio-scaffold that provides both the architecture and bioactive cues for local stem cells to migrate, reside, proliferate, and differentiate. The objective of this study is to combine a cell-specific decellularized extracellular matrix (ECM) and a biomimetic electrospinning scaffold to regenerate severely destructed periodontium.

METHODS

SEM, water contact angle (WCA), live/dead staining, swelling ratio, tensile test and immune-fluorescent staining were used to define the suitable topography for certain dental stem cells seeding and culturing. Transwell assay, CCK-8, Alizarin Red staining and PCR immune-fluorescent staining were used to determine ideal cell-specific ECM for PDLSCs/BMSCs migration, viability, and oriented differentiation. A biodegradable triple-layered electrospun scaffold (TLS) was fabricated by electrospinning with aligned fibers on both surfaces and a polyporous structure in the middle. The morphology and inter-porous structure of the TLS were characterized by SEM and mercury intrusion porosimetry (MIP). The surface of the TLS was functionalized with cell-specific ECM (Bi-ECM-TLS) through decellularization of the cell sheets cultured on the scaffold. The regenerative outcome of Bi-ECM-TLS was assessed by an in-situ rat periodontal defect model. Micro-CT, HE-staining, Masson's trichome staining, Sirius Red staining and Immunofluorescent staining were used for histological analysis.

RESULTS

Aligned Gelatin/PCL fibrous membrane (GPA) was most effective for both PDLSCs and BMSCs in culture with WCA around 50 degrees and better mechanical strength than the rest. MSCs favored the same type of ECM (cell-specific ECM), and their regenerative properties were effectively induced with better chemotaxis, proliferative and differentiating behaviors. TLS characterization showed that TLS possessed aligned-random-aligned structure and inter-porous structure. In a rat model of periodontal defects, the TLS functionalized by BMSC-specific ECM for bone regeneration and PDLSC-specific ECM demonstrated highest BV/TV ratio, best bone structure and ligament fiber orientation and blood vessel formation, suggesting optimal performance in regenerating both alveolar bone and periodontal ligaments over TLS, single-ECM loaded TLS and r-Bi-ECM-TLS.

SIGNIFICANCE

This study highlights the importance of combining a cell-specific decellularized ECM and a biomimetic electrospinning scaffold for targeted periodontal tissue regeneration, with potential implications for periodontal tissue engineering and improved patient outcomes.

Effect of In Vitro Culture Length on the Bone-Forming Capacity of Osteoblast-Derived Decellularized Extracellular Matrix Melt Electrowritten Scaffolds

Recent advancements in decellularization have seen the development of extracellular matrix (ECM)-decorated scaffolds for bone regeneration; however, little is understood of the impact of in vitro culture prior to decellularization on the performances of these constructs. Therefore, this study investigated the effect of in vitro culture on ECM-decorated melt electrowritten polycaprolactone scaffold bioactivity. The scaffolds were seeded with osteoblasts and cultured for 1, 2, or 4 weeks to facilitate bone-specific ECM deposition and subsequently decellularized to form an acellular ECM-decorated scaffold. The utilization of mild chemicals and DNase was highly efficient in removing DNA while preserving ECM structure and composition. ECM decoration of the melt electrowritten fibers was observed within the first week of culture, with increased ECM at 2 and 4 week culture periods. Infiltration of re-seeded cells as well as overall bone regeneration in a rodent calvarial model was impeded by a longer culture period. Thus, it was demonstrated that the length of culture has a key influence on the osteogenic properties of decellularized ECM-decorated scaffolds, with long-term culture (2+ weeks) causing pore obstruction and creating a physical barrier which interfered with bone formation. These findings have important implications for the development of effective ECM-decorated scaffolds for bone regeneration.

Decellularization in Tissue Engineering and Regenerative Medicine: Evaluation, Modification, and Application Methods

Reproduction of different tissues using scaffolds and materials is a major element in regenerative medicine. The regeneration of whole organs with decellularized extracellular matrix (dECM) has remained a goal despite the use of these materials for different purposes. Recently, decellularization techniques have been widely used in producing scaffolds that are appropriate for regenerating damaged organs and may be able to overcome the shortage of donor organs. Decellularized ECM offers several advantages over synthetic compounds, including the preserved natural microenvironment features. Different decellularization methods have been developed, each of which is appropriate for removing cells from specific tissues under certain conditions. A variety of methods have been advanced for evaluating the decellularization process in terms of cell removal efficiency, tissue ultrastructure preservation, toxicity, biocompatibility, biodegradability, and mechanical resistance in order to enhance the efficacy of decellularization methods. Modification techniques improve the characteristics of decellularized scaffolds, making them available for the regeneration of damaged tissues. Moreover, modification of scaffolds makes them appropriate options for drug delivery, disease modeling, and improving stem cells growth and proliferation. However, considering different challenges in the way of decellularization methods and application of decellularized scaffolds, this field is constantly developing and progressively moving forward. This review has outlined recent decellularization and sterilization strategies, evaluation tests for efficient decellularization, materials processing, application, and challenges and future outlooks of decellularization in regenerative medicine and tissue engineering.

The Effect of Ozone Treatment on the Physicochemical Properties and Biocompatibility of Electrospun Poly(ε)caprolactone Scaffolds

Ozonation has been proved as a viable surface modification technique providing certain properties to the scaffolds that are essential in tissue engineering. However, the ozone (O3) treatment of PCL scaffolds in aqueous environments has not yet been presented. O3 treatment performed in aqueous environments is more effective compared with traditional, executed in ambient air treatment due to more abundant production of hydroxyl radicals (•OH) within the O3 reaction with water molecules. During interaction with •OH, the scaffold acquires functional groups which improve wettability properties and encapsulate growth factors. In this study, a poly(ε)caprolactone (PCL) scaffold was fabricated using solution electrospinning and was subsequently ozonated in a water reactor. The O3 treatment resulted in the expected occurrence of oxygen-containing functional groups, which improved scaffold wettability by almost 27% and enhanced cell proliferation for up to 14 days. The PCL scaffold was able to withhold 120 min of O3 treatment, maintaining fibrous morphology and mechanical properties.

Cell-Derived Extracellular Matrix for Tissue Engineering and Regenerative Medicine

Cell-derived extracellular matrices (CD-ECMs) captured increasing attention since the first studies in the 1980s. The biological resemblance of CD-ECMs to their in vivo counterparts and natural complexity provide them with a prevailing bioactivity. CD-ECMs offer the opportunity to produce microenvironments with costumizable biological and biophysical properties in a controlled setting. As a result, CD-ECMs can improve cellular functions such as stemness or be employed as a platform to study cellular niches in health and disease. Either on their own or integrated with other materials, CD-ECMs can also be utilized as biomaterials to engineer tissues de novo or facilitate endogenous healing and regeneration. This review provides a brief overview over the methodologies used to facilitate CD-ECM deposition and manufacturing. It explores the versatile uses of CD-ECM in fundamental research and therapeutic approaches, while highlighting innovative strategies. Furthermore, current challenges are identified and it is accentuated that advancements in methodologies, as well as innovative interdisciplinary approaches are needed to take CD-ECM-based research to the next level.

Eclectic characterisation of chemically modified cell-derived matrices obtained by metabolic glycoengineering and re-assessment of commonly used methods

Azide-bearing cell-derived extracellular matrices ("clickECMs") have emerged as a highly exciting new class of biomaterials. They conserve substantial characteristics of the natural extracellular matrix (ECM) and offer simultaneously small abiotic functional groups that enable bioorthogonal bioconjugation reactions. Despite their attractiveness, investigation of their biomolecular composition is very challenging due to the insoluble and highly complex nature of cell-derived matrices (CDMs). Yet, thorough qualitative and quantitative analysis of the overall material composition, organisation, localisation, and distribution of typical ECM-specific biomolecules is essential for consistent advancement of CDMs and the understanding of the prospective functions of the developed biomaterial. In this study, we evaluated frequently used methods for the analysis of complex CDMs. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and (immune)histochemical staining methods in combination with several microscopic techniques were found to be highly eligible. Commercially available colorimetric protein assays turned out to deliver inaccurate information on CDMs. In contrast, we determined the nitrogen content of CDMs by elementary analysis and converted it into total protein content using conversion factors which were calculated from matching amino acid compositions. The amount of insoluble collagens was assessed based on the hydroxyproline content. The Sircol™ assay was identified as a suitable method to quantify soluble collagens while the Blyscan™ assay was found to be well-suited for the quantification of sulphated glycosaminoglycans (sGAGs). Eventually, we propose a series of suitable methods to reliably characterise the biomolecular composition of fibroblast-derived clickECM.

Extracellular matrix-based biomaterials for cardiac regeneration and repair

The spectrum of ischemic heart diseases, encompassing acute myocardial infarction to heart failure, represents the leading cause of death worldwide. Although extensive progress in cardiovascular diagnoses and therapy has been made, the prevalence of the disease continues to increase. Cardiac regeneration has a promising perspective for the therapy of heart failure. Recently, extracellular matrix (ECM) has been shown to play an important role in cardiac regeneration and repair after cardiac injury. There is also evidence that the ECM could be directly used as a drug to promote cardiomyocyte proliferation and cardiac regeneration. Increasing evidence supports that applying ECM biomaterials to maintain heart function recovery is an important approach to apply the concept of cardiac regenerative medicine to clinical practice in the future. Here, we will introduce the essential role of cardiac ECM in cardiac regeneration and summarize the approaches of delivering ECM biomaterials to promote cardiac repair in this review.

Rejuvenating Strategies for Stem Cell-Based Therapies in Aging

Recent advances in our understanding of tissue regeneration and the development of efficient approaches to induce and differentiate pluripotent stem cells for cell replacement therapies promise exciting avenues for treating degenerative age-related diseases. However, clinical studies and insights from model organisms have identified major roadblocks that normal aging processes impose on tissue regeneration. These new insights suggest that specific targeting of environmental niche components, including growth factors, ECM, and immune cells, and intrinsic stem cell properties that are affected by aging will be critical for the development of new strategies to improve stem cell function and optimize tissue repair processes.

clickECM: Development of a cell-derived extracellular matrix with azide functionalities

In vitro cultured cells produce a complex extracellular matrix (ECM) that remains intact after decellularization. The biological complexity derived from the variety of distinct ECM molecules makes these matrices ideal candidates for biomaterials. Biomaterials with the ability to guide cell function are a topic of high interest in biomaterial development. However, these matrices lack specific addressable functional groups, which are often required for their use as a biomaterial. Due to the biological complexity of the cell-derived ECM, it is a challenge to incorporate such functional groups without affecting the integrity of the biomolecules within the ECM. The azide-alkyne cycloaddition (click reaction, Huisgen-reaction) is an efficient and specific ligation reaction that is known to be biocompatible when strained alkynes are used to avoid the use of copper (I) as a catalyst. In our work, the ubiquitous modification of a fibroblast cell-derived ECM with azides was achieved through metabolic oligosaccharide engineering by adding the azide-modified monosaccharide Ac₄GalNAz (1,3,4,6-tetra-O-acetyl-N-azidoacetylgalactosamine) to the cell culture medium. The resulting azide-modified network remained intact after removing the cells by lysis and the molecular structure of the ECM proteins was unimpaired after a gentle homogenization process. The biological composition was characterized in order to show that the functionalization does not impair the complexity and integrity of the ECM. The azides within this "clickECM" could be accessed by small molecules (such as an alkyne-modified fluorophore) or by surface-bound cyclooctynes to achieve a covalent coating with clickECM.

STATEMENT OF SIGNIFICANCE

The clickECM was produced by the incorporation of azide-functionalized sugar analogues into the extracellular glycans of fibroblast cell cultures by metabolic oligosaccharide engineering. By introducing these azide groups into the glycan structures, we enabled this cell-derived ECM for bioorthogonal click reactions. Click chemistry provides extremely specific reactions with high efficiency, high selectivity, and high reaction yields. We could show that the azide functionalities within the clickECM are chemically accessible. Based on our here described clickECM technique it will be possible to create and investigate new clickECM materials with tunable bioactive properties and additional functionalities, which offers a promising approach for basic and applied research in the field of biomaterial science, biomedical applications, and tissue engineering.

ECM and ECM-like materials - Biomaterials for applications in regenerative medicine and cancer therapy

Regenerative strategies such as stem cell-based therapies and tissue engineering applications are being developed with the aim to replace, remodel, regenerate or support damaged tissues and organs. In addition to careful cell type selection, the design of appropriate three-dimensional (3D) scaffolds is essential for the generation of bio-inspired replacement tissues. Such scaffolds are usually made of degradable or non-degradable biomaterials and can serve as cell or drug carriers. The development of more effective and efficient drug carrier systems is also highly relevant for novel cancer treatment strategies. In this review, we provide a summary of current approaches that employ ECM and ECM-like materials, or ECM-synthetic polymer hybrids, as biomaterials in the field of regenerative medicine. We further discuss the utilization of such materials for cell and drug delivery, and highlight strategies for their use as vehicles for cancer therapy.

Microstructure-dependent mechanical properties of electrospun core-shell scaffolds at multi-scale levels

Mechanical factors among many physiochemical properties of scaffolds for stem cell-based tissue engineering significantly affect tissue morphogenesis by controlling stem cell behaviors including proliferation and phenotype-specific differentiation. Core-shell electrospinning provides a unique opportunity to control mechanical properties of scaffolds independent of surface chemistry, rendering a greater freedom to tailor design for specific applications. In this study, we synthesized electrospun core-shell scaffolds having different core composition and/or core-to-shell dimensional ratios. Two independent biocompatible polymer systems, polyetherketoneketone (PEKK) and gelatin as the core materials while maintaining the shell polymer with polycaprolactone (PCL), were utilized. The mechanics of such scaffolds was analyzed at the microscale and macroscales to determine the potential implications it may hold for cell-material and tissue-material interactions. The mechanical properties of individual core-shell fibers were controlled by core-shell composition and structure. The individual fiber modulus correlated with the increase in percent core size ranging from 0.55±0.10GPa to 1.74±0.22GPa and 0.48±0.12GPa to 1.53±0.12GPa for the PEKK-PCL and gelatin-PCL fibers, respectively. More importantly, it was demonstrated that mechanical properties of the scaffolds at the macroscale were dominantly determined by porosity under compression. The increase of scaffold porosity from 70.2%±1.0% to 93.2%±0.5% by increasing the core size in the PEKK-PCL scaffold resulted in the decrease of the compressive elastic modulus from 227.67±20.39kPa to 14.55±1.43kPa while a greater changes in the porosity of gelatin-PCL scaffold from 54.5%±4.2% to 89.6%±0.4% resulted in the compressive elastic modulus change from 484.01±30.18kPa to 17.57±1.40kPa. On the other hand, the biphasic behaviors under tensile mechanical loading result in a range from a minimum of 5.42±1.05MPa to a maximum of 12.00±1.96MPa for the PEKK-PCL scaffolds, and 10.19±4.49MPa to 22.60±2.44MPa for the gelatin-PCL scaffolds. These results suggest a feasible approach for precisely controlling the local and global mechanical characteristics, in addition to independent control over surface chemistry, to achieve a desired tissue morphogenesis using the core-shell electrospinning.

Acellular matrix in urethral reconstruction

The treatment of severe urethral stenosis has always been a challenge even for skilled urologists. Classic urethroplasty, skin flaps and buccal mucosa grafting may not be used for long and complex strictures. In the quest for an ideal urethral substitute, acellular scaffolds have demonstrated the ability to induce tissue regeneration layer by layer. After several experimental studies, the use of acellular matrices for urethral reconstruction has become a clinical reality over the last decade. In this review we analyze advantages and limitations of both biological and polymeric scaffolds that have been reported in experimental and human studies. Important aspects such as graft extension, surgical technique and cell-seeding versus cell-free grafts will be discussed.

Cauda equina-derived extracellular matrix for fabrication of nanostructured hybrid scaffolds applied to neural tissue engineering

Extracellular matrix (ECM) components have become important candidate materials for use as neural scaffolds for neural tissue engineering. In the current study, we prepared cauda equina-derived ECM materials for the production of scaffolds. Natural porcine cauda equina was decellularized using Triton X-100 and sodium deoxycholate, shattered physically, and made into a suspension by differential centrifugation. The decellularization procedure resulted in the removal of >94% of the nuclear material and preserved the extracellular collagen and sulfated glycosaminoglycan. Immunofluorescent staining confirmed the presence of collagen type I, laminin, and fibronectin in the ECM. The cauda equine-derived ECM was blended with poly(l-lactide-co-glycolide) (PLGA) to fabricate nanostructured scaffolds using electrospinning. The incorporation of the ECM increased the hydrophilicity of the scaffolds. Fourier transform infrared spectroscopy and multiphoton-induced autofluorescence images showed the presence of the ECM in the scaffolds. ECM/PLGA scaffolds were beneficial for the survival of Schwann cells compared with scaffolds consisting of PLGA alone, and the aligned fibers could regulate cell morphologic features by modulating cellular orientation. Axons in the dorsal root ganglia explants extended to a greater extent along ECM/PLGA compared with PLGA-alone fibers. The cauda equina ECM might be a promising material for forming scaffolds for use in neural tissue engineering.

Effective cell collection method using collagenase and ultrasonic vibration

This study proposes a novel cell collection method based on collagenase treatment and ultrasonic vibration. The method collects calf chondrocytes from a reusable metal cell culture substrate. To develop our concept, we calculated the natural vibration modes of the cell culture substrate by a finite element method, and conducted eigenvalue and piezoelectric-structural analyses. Selecting the first out-of-plane vibration mode of the substrate, which has a single nodal circle, we designed and fabricated the cell collection device. The excited vibration mode properly realized our intentions. We then evaluated the cell collection ratio and the growth response, and observed the morphology of the collected cells. The collagenase and ultrasonic vibration treatment collected comparable numbers of cells to conventional trypsin and pipetting treatment, but improved the proliferating cell statistics. Morphological observations revealed that the membranes of cells collected by the proposed method remain intact; consequently, the cells are larger and rougher than cells collected by the conventional method. Therefore, we present a promising cell collection method for adhesive cell culturing process.

Electrospun poly(L-lactide)/poly(ε-caprolactone) blend nanofibrous scaffold: characterization and biocompatibility with human adipose-derived stem cells

The essence of tissue engineering is the fabrication of autologous cells or induced stem cells in naturally derived or synthetic scaffolds to form specific tissues. Polymer is thought as an appealing source of cell-seeded scaffold owing to the diversity of its physicochemical property and can be electrospun into nano-size to mimic natural structure. Poly (L-lactic acid) (PLLA) and poly (ε-caprolactone) (PCL) are both excellent aliphatic polyester with almost “opposite” characteristics. The controlling combination of PLLA and PCL provides varying properties and makes diverse applications. Compared with the copolymers of the same components, PLLA/PCL blend demonstrates its potential in regenerative medicine as a simple, efficient and scalable alternative. In this study, we electrospun PLLA/PCL blends of different weight ratios into nanofibrous scaffolds (NFS) and their properties were detected including morphology, porosity, degradation, ATR-FTIR analysis, stress-stain assay, and inflammatory reaction. To explore the biocompatibility of the NFS we synthesized, human adipose-derived stem cells (hASCs) were used to evaluate proliferation, attachment, viability and multi-lineage differentiation. In conclusion, the electrospun PLLA/PCL blend nanofibrous scaffold with the indicated weight ratios all supported hASCs well. However, the NFS of 1/1 weight ratio showed better properties and cellular responses in all assessments, implying it a biocompatible scaffold for tissue engineering.

Taking a deep look: modern microscopy technologies to optimize the design and functionality of biocompatible scaffolds for tissue engineering in regenerative medicine

This review focuses on modern nonlinear optical microscopy (NLOM) methods that are increasingly being used in the field of tissue engineering (TE) to image tissue non-invasively and without labelling in depths unreached by conventional microscopy techniques. With NLOM techniques, biomaterial matrices, cultured cells and their produced extracellular matrix may be visualized with high resolution. After introducing classical imaging methodologies such as µCT, MRI, optical coherence tomography, electron microscopy and conventional microscopy two-photon fluorescence (2-PF) and second harmonic generation (SHG) imaging are described in detail (principle, power, limitations) together with their most widely used TE applications. Besides our own cell encapsulation, cell printing and collagen scaffolding systems and their NLOM imaging the most current research articles will be reviewed. These cover imaging of autofluorescence and fluorescence-labelled tissue and biomaterial structures, SHG-based quantitative morphometry of collagen I and other proteins, imaging of vascularization and online monitoring techniques in TE. Finally, some insight is given into state-of-the-art three-photon-based imaging methods (e.g. coherent anti-Stokes Raman scattering, third harmonic generation). This review provides an overview of the powerful and constantly evolving field of multiphoton microscopy, which is a powerful and indispensable tool for the development of artificial tissues in regenerative medicine and which is likely to gain importance also as a means for general diagnostic medical imaging.

Design and development of nanocomposite scaffolds for auricular reconstruction

Auricular reconstruction using sculpted autologous costal cartilage is effective, but complex and time consuming and may incur donor site sequelae and morbidity. Conventional synthetic alternatives are associated with infection and extrusion in up to about 15% of cases. We present a novel POSS-PCU nanocomposite auricular scaffold, which aims to reduce extrusion rates by mimicking the elastic modulus of human ears and by encouraging desirable cellular interactions. The fabrication, physicochemical properties (including nanoscale topography) and cellular interactions of these scaffolds were compared to Medpor®, the current synthetic standard. Our scaffold had a more similar elastic modulus (5.73 ± 0.17MPa) to ear cartilage (5.02 ± 0.17MPa) compared with Medpor®, which was much stiffer (140.9 ± 0.04MPa). POSS-PCU supported fibroblast ingrowth and proliferation; significantly higher collagen production was also produced by cells on the POSS-PCU than those on Medpor®. This porous POSS-PCU nanocomposite scaffold is therefore a promising alternative biomaterial for auricular surgical reconstruction.

FROM THE CLINICAL EDITOR

In this paper, a novel POSS-PCU nanocomposite auricular scaffold is described to reduce extrusion rates by having a much closer elastic modulus of human ears than the currently available synthetic standard. Enabling desirable cellular interactions may lead to the successful clinical application of these novel scaffolds.

Absence of immune responses with xenogeneic collagen and elastin

Novel tissue-engineering approaches for cardiovascular matrices based on xenogeneic extracellular matrix protein (ECMp) constituents require a detailed evaluation of their interaction with essential immune cell subsets playing a role in innate or adaptive immunity. Therefore, in this study, the effects of xenogeneic (porcine, bovine) collagen type I and elastin as the two main components of the heart valve ECM were analyzed in comparison to their human equivalents. First, their potential to induce maturation and cytokine secretion of human dendritic cells (DC) was tested by flow cytometry. Second, the influence on proliferation and cytokine release of purified human B and T cells was measured. We could demonstrate that xenogeneic collagen type I and elastin are not able to trigger the maturation of DC as verified by the lack of CD83 induction accompanied by a low tumor necrosis factor-α release. Moreover, both ECMp showed no effect on the proliferation and the interleukin-6 release of either unstimulated or prestimulated B cells. Additionally, anti-CD3-induced purified T cell proliferation and secretion of cytokines was not affected. All in vitro data verify the low immunogenicity of porcine and bovine collagen type I and elastin and favor their suitability for tissue-engineered scaffolds.

Hyaluronan-Based Three-Dimensional Microenvironment Potently Induces Cardiovascular Progenitor Cell Populations

The relationship between stem cell niches in vivo and their surrounding microenvironment is still relatively unknown. Recent advances have indicated that extrinsic factors within the cardiovascular progenitor cell niche influence maintenance of a multipotent state as well as drive cell-fate decisions. We have previously shown the direct effects of extracellular matrix (ECM) proteins and have now investigated the effects of dimension on the induction of a cardiovascular progenitor cell (CPC) population. We have shown here that the three-dimensionality of a hyaluronan-based hydrogel greatly induces a CPC population, as marked by Flk-1. We have compared the effects of a 3D microenvironment to those of conventional 2D cell culture practices and have found that the 3D microenvironment potently induces a progenitor cell state.

Electrospun synthetic human elastin:collagen composite scaffolds for dermal tissue engineering

We present an electrospun synthetic human elastin:collagen composite scaffold aimed at dermal tissue engineering. The panel of electrospun human tropoelastin and ovine type I collagen blends comprised 80% tropoelastin+20% collagen, 60% tropoelastin+40% collagen and 50% tropoelastin+50% collagen. Electrospinning efficiency decreased with increasing collagen content under the conditions used. Physical and mechanical characterization encompassed fiber morphology, porosity, pore size and modulus, which were prioritized to identify the optimal candidate for dermal tissue regeneration. Scaffolds containing 80% tropoelastin and 20% collagen (80T20C) were selected on this basis for further cell interaction and animal implantation studies. 80T20C enhanced proliferation and migration rates of dermal fibroblasts in vitro and were well tolerated in a mouse subcutaneous implantation study where they persisted over 6 weeks. The 80T20C scaffolds supported fibroblast infiltration, de novo collagen deposition and new capillary formation.

Modeling neural differentiation on micropatterned substrates coated with neural matrix components

Topographical and biochemical characteristics of the substrate are critical for neuronal differentiation including axonal outgrowth and regeneration of neural circuits in vivo. Contact stimuli and signaling molecules allow neurons to develop and stabilize synaptic contacts. Here we present the development, characterization and functional validation of a new polymeric support able to induce neuronal differentiation in both PC12 cell line and adult primary skin-derived precursor cells (SKPs) in vitro. By combining a photolithographic technique with use of neural extracellular matrix (ECM) as a substrate, a biocompatible and efficient microenvironment for neuronal differentiation was developed.

Stem cell microenvironments--unveiling the secret of how stem cell fate is defined

Stem cells are defined as unspecialized cells that are capable of long term self-renewal and differentiation into specialized cell types. These unique properties make them an attractive cell source for regenerative medicine applications. Although the functions of various stem cells have been extensively studied in the development of organisms and in diseases, the specific factors and conditions that control stem cell fate, specifically the conditions that allow them to remain unspecialized, are not well studied. It has been suggested that adult stem cell survival and maintenance, as well as proliferation and differentiation, are controlled by the three-dimensional (3D) microenvironment, the so-called niche. Major functional niche components include supporting niche cells, growth-modulating soluble factors stored within the niches, and the extracellular matrix (ECM). In this article, we review work highlighting the growing complexity of stem cell-ECM interactions and their impact on the fields of biomaterials research and regenerative medicine.

Electrospun cellular microenvironments: Understanding controlled release and scaffold structure

Electrospinning is a versatile technique in tissue engineering for the production of scaffolds. To guide tissue development, scaffolds must provide specific biochemical, structural and mechanical cues to cells and deliver them in a controlled fashion over time. Electrospun scaffold design thus includes aspects of both controlled release and structural cues. Controlled multicomponent and multiphasic drug delivery can be achieved by the careful application and combination of novel electrospinning techniques, i.e., emulsion and co-axial electrospinning. Drug distribution and polymer properties influence the resulting release kinetics. Pore size is far more relevant as a structural parameter than previously recognized. It enables cell proliferation and ingrowth, whereas fiber diameter predominantly influences cell fate. Both parameters can be exploited by combining multiple fiber types in the form of multifiber and multilayer scaffolds. Such scaffolds are required to reproduce more complex tissue structures.

Recapitulation of the embryonic cardiovascular progenitor cell niche

Stem or progenitor cell populations are often established in unique niche microenvironments that regulate cell fate decisions. Although niches have been shown to be critical for the normal development of several tissues, their role in the cardiovascular system is poorly understood. In this study, we characterized the cardiovascular progenitor cell (CPC) niche in developing human and mouse hearts, identifying signaling pathways and extracellular matrix (ECM) proteins that are crucial for CPC maintenance and expansion. We demonstrate that collagen IV (ColIV) and β-catenin-dependent signaling are essential for maintaining and expanding undifferentiated CPCs. Since niches are three-dimensional (3D) structures, we investigated the impact of a 3D microenvironment that mimics the in vivo niche ECM. Employing electrospinning technologies, 3D in vitro niche substrates were bioengineered to serve as culture inserts. The three-dimensionality of these structures increased mouse embryonic stem cell differentiation into CPCs when compared to 2D control cultures, which was further enhanced by incorporation of ColIV into the substrates. Inhibiting p300-dependent β-catenin signals with the small molecule IQ1 facilitated further expansion of CPCs. Our study represents an innovative approach to bioengineer cardiac niches that can serve as unique 3D in vitro systems to facilitate CPC expansion and study CPC biology.

Collagen-PCL sheath-core bicomponent electrospun scaffolds increase osteogenic differentiation and calcium accretion of human adipose-derived stem cells

Human adipose-derived stem cells (hASCs) are an abundant cell source capable of osteogenic differentiation, and have been investigated as an autologous stem cell source for bone tissue engineering applications. The objective of this study was to determine if the addition of a type-I collagen sheath to the surface of poly(ε-caprolactone) (PCL) nanofibers would enhance viability, proliferation and osteogenesis of hASCs. This is the first study to examine the differentiation behavior of hASCs on collagen-PCL sheath-core bicomponent nanofiber scaffolds developed using a co-axial electrospinning technique. The use of a sheath-core configuration ensured a uniform coating of collagen on the PCL nanofibers. PCL nanofiber scaffolds prepared using a conventional electrospinning technique served as controls. hASCs were seeded at a density of 20 000 cells/cm(2) on 1 cm(2) electrospun nanofiber (pure PCL or collagen-PCL sheath-core) sheets. Confocal microscopy and hASC proliferation data confirmed the presence of viable cells after 2 weeks in culture on all scaffolds. Greater cell spreading occurred on bicomponent collagen-PCL scaffolds at earlier time points. hASCs were osteogenically differentiated by addition of soluble osteogenic inductive factors. Calcium quantification indicated cell-mediated calcium accretion was approx. 5-times higher on bicomponent collagen-PCL sheath-core scaffolds compared to PCL controls, indicating collagen-PCL bicomponent scaffolds promoted greater hASC osteogenesis after two weeks of culture in osteogenic medium. This is the first study to examine the effects of collagen-PCL sheath-core composite nanofibers on hASC viability, proliferation and osteogenesis. The sheath-core composite fibers significantly increased calcium accretion of hASCs, indicating that collagen-PCL sheath-core bicomponent structures have potential for bone tissue engineering applications using hASCs.

Electrospun poly(D/L-lactide-co-L-lactide) hybrid matrix: a novel scaffold material for soft tissue engineering

Electrospinning is a long-known polymer processing technique that has received more interest and attention in recent years due to its versatility and potential use in the field of biomedical research. The fabrication of three-dimensional (3D) electrospun matrices for drug delivery and tissue engineering is of particular interest. In the present study, we identified optimal conditions to generate novel electrospun polymeric scaffolds composed of poly-D/L-lactide and poly-L-lactide in the ratio 50:50. Scanning electron microscopic analyses revealed that the generated poly(D/L-lactide-co-L-lactide) electrospun hybrid microfibers possessed a unique porous high surface area mimicking native extracellular matrix (ECM). To assess cytocompatibility, we isolated dermal fibroblasts from human skin biopsies. After 5 days of in vitro culture, the fibroblasts adhered, migrated and proliferated on the newly created 3D scaffolds. Our data demonstrate the applicability of electrospun poly(D/L-lactide-co-L-lactide) scaffolds to serve as substrates for regenerative medicine applications with special focus on skin tissue engineering.

Elastin-based materials

Elastin is a versatile elastic protein that dominates flexible tissues capable of recoil, and facilitates commensurate cell interactions in these tissues in all higher vertebrates. Elastin's persistence and insolubility hampered early efforts to construct versatile biomaterials. Subsequently the field has progressed substantially through the adapted use of solubilized elastin, elastin-based peptides and the increasing availability of recombinant forms of the natural soluble elastin precursor, tropoelastin. These interactions allow for the formation of a sophisticated range of biomaterial constructs and composites that benefit from elastin's physical properties of innate assembly and elasticity, and cell interactive properties as discussed in this tutorial review.

Stem cell paracrine actions and tissue regeneration

Stem cells have emerged as a key element of regenerative medicine therapies due to their inherent ability to differentiate into a variety of cell phenotypes, thereby providing numerous potential cell therapies to treat an array of degenerative diseases and traumatic injuries. A recent paradigm shift has emerged suggesting that the beneficial effects of stem cells may not be restricted to cell restoration alone, but also due to their transient paracrine actions. Stem cells can secrete potent combinations of trophic factors that modulate the molecular composition of the environment to evoke responses from resident cells. Based on this new insight, current research directions include efforts to elucidate, augment and harness stem cell paracrine mechanisms for tissue regeneration. This article discusses the existing studies on stem/progenitor cell trophic factor production, implications for tissue regeneration and cancer therapies, and development of novel strategies to use stem cell paracrine delivery for regenerative medicine.