Well-aligned chitosan-based ultrafine fibers committed teno-lineage differentiation of human induced pluripotent stem cells for Achilles tendon regeneration

Effect of scaffold morphology and cell co-culture on tenogenic differentiation of HADMSC on centrifugal melt electrospun poly (L‑lactic acid) fibrous meshes

Engineered tendon grafts offer a promising alternative for grafting during the reconstruction of complex tendon tears. The tissue-engineered tendon substitutes have the advantage of increased biosafety and the option to customize their biochemical and biophysical properties to promote tendon regeneration. In this study, we developed a novel centrifugal melt electrospinning (CME) technique, with the goal of optimizing the fabrication parameters to generate fibrous scaffolds for tendon tissue engineering. The effects of CME processing parameters, including rotational speed, voltage, and temperature, on fiber properties (i.e. orientation, mean diameter, and productivity) were systematically investigated. By using this solvent-free and environmentally friendly method, we fabricated both random and aligned poly (L-lactic acid) (PLLA) fibrous scaffolds with controllable mesh thickness. We also investigated and compared their morphology, surface hydrophilicity, and mechanical properties. We seeded human adipose derived mesenchymal stem cells (HADMSC) on various PLLA fibrous scaffolds and conditioned the constructs in tenogenic differentiation medium for up to 21 days, to investigate the effects of fiber alignment and scaffold thickness on cell behavior. Aligned fibrous scaffolds induced cell elongation and orientation through a contact guidance phenomenon and promoted HADMSC proliferation and differentiation towards tenocytes. At the early stage, thinner scaffolds were beneficial for HADMSC proliferation, but the scaffold thickness had no significant effects on cell proliferation for longer-term cell culture. We further co-seeded HADMSC and human umbilical vein endothelial cells (HUVEC) on aligned PLLA fibrous mats and determined how the vascularization affected HADMSC tenogenesis. We found that co-cultured HADMSC-HUVEC expressed more tendon-related markers on the aligned fibrous scaffold. The co-culture systems promoted in vitro HADMSC differentiation towards tenocytes. These aligned fibrous scaffolds fabricated by CME technique could potentially be utilized to repair and regenerate tendon defects and injuries with cell co-culture and controlled vascularization.

Electrospun Fibers for Recruitment and Differentiation of Stem Cells in Regenerative Medicine

Electrospinning is a popular technique used to mimic the natural sub-micron features of the native tissue. The ultra-fine fibers provide a favorable extracellular matrix-like environment for regulation of cellular functions. This article summarizes and reviews the current advances in electrospun fiber application and focuses on the novel strategies applied for tissue regeneration and repair. It explores the different factors affecting the attachment and proliferation of mesenchymal stem cells (MSCs) on the electrospun substrates. The influence of different features of electrospun fibers in the differentiation of MSCs into specific lineages (bone, cartilage, tendon/ligament, and nerves) has been elaborated. In addition, the different techniques to mimic the hierarchical features of tissues and its effect on cellular functions are reviewed. Additionally, the new developments like three-dimensional (3D) electrospinning, 3D spheroid double strategy and the comparative analysis of dynamic and static culture on electrospun scaffolds are discussed. With the intricate understanding of the interaction between the cells and the electrospun fiber matrix we can aim to combine the newer strategies to overcome the existing challenges and improve the potential application of electrospun fibers in the field of tissue regeneration and repair.

Mechanical Actuation Systems for the Phenotype Commitment of Stem Cell-Based Tendon and Ligament Tissue Substitutes

High tensile forces transmitted by tendons and ligaments make them susceptible to tearing or complete rupture. The present standard reparative technique is the surgical implantation of auto- or allografts, which often undergo failure.Currently, different cell types and biomaterials are used to design tissue engineered substitutes. Mechanical stimulation driven by dedicated devices can precondition these constructs to a remarkable degree, mimicking the local in vivo environment. A large number of dynamic culture instruments have been developed and many appealing results collected. Of the cells that have been used, tendon stem cells are the most promising for a reliable stretch-induced tenogenesis, but their reduced availability represents a serious limitation to upscaled production. Biomaterials used for scaffold fabrication include both biological molecules and synthetic polymers, the latter being improved by nanotechnologies which reproduce the architecture of native tendons. In addition to cell type and scaffold material, other variables which must be defined in mechanostimulation protocols are the amplitude, frequency, duration and direction of the applied strain. The ideal conditions seem to be those producing intermittent tension rather than continuous loading. In any case, all physical parameters must be adapted to the specific response of the cells used and the tensile properties of the scaffold. Tendon/ligament grafts in animals usually have the advantage of mechanical preconditioning, especially when uniaxial cyclic forces are applied to cells engineered into natural or decellularized scaffolds. However, due to the scarcity of in vivo research, standard protocols still need to be defined for clinical applications.

[Effect of silk fibroin/poly ( L-lactic acid-co-e-caprolactone) nanofibrous scaffold on tendon-bone healing of rabbits]

Objective

To explore the effect of silk fibroin/poly( L-lactic acid-co-e-caprolactone) [SF/P(LLA-CL)] nanofibrous scaffold on tendon-bone healing of rabbits.

Methods

SF/P(LLA-CL) nanofibrous scaffold was fabricated by electrospinning methods. The morphology of the scaffold was observed by scanning electron microscope (SEM). Pre-osteoblasts MC3T3-E1 cells were seeded on the scaffold and cultured for 1, 3, and 5 days. Cell adhesion and proliferation were also observed by SEM. Meanwhile, twenty-four New Zealand white rabbits were randomly divided into the autogenous tendon group (control group) and the autogenous tendon wrapped with SF/P(LLA-CL) scaffold group (experimental group), with twelve rabbits in each group. An extra-articular model was established, the effect was evaluated by histological examination and mechanical testing.

Results

The morphology of SF/P(LLA-CL) nanofibrous scaffold was random, with a diameter of (219.4±66.5) nm. SEM showed that the MC3T3-E1 cells seeded on the scaffold were in the normal shape, growing well, and proliferating with time course. The results of histological examination showed that inflammatory cells infltrated into the graft-host bone interface at 6 weeks after operation in both groups. Besides, the width of interface showed no significant difference between groups. At 12 weeks after operation, protruding new bone tissue could be observed at the interface in the experimental group, while scar tissue but no new bone tissue could be seen at the interface in the control group. Mechanical testing showed that there was no significant difference in the failure load and the stiffness between groups at 6 weeks after operation ( P>0.05). The failure load and the stiffness in the experimental group were significantly higher than those in the control group at 12 weeks after operation ( P<0.05).

Conclusion

The SF/P(LLA-CL) nanofibrous scaffold has good cell biocompatibility and can effectively promote tendon-bone healing, thus providing new method for modifying graft for ACL reconstruction in the clinical practice.

3D Mimicry of Native-Tissue-Fiber Architecture Guides Tendon-Derived Cells and Adipose Stem Cells into Artificial Tendon Constructs

Tendon and ligament (T/L) function is intrinsically related with their unique hierarchically and anisotropically organized extracellular matrix. Their natural healing capacity is, however, limited. Here, continuous and aligned electrospun nanofiber threads (CANT) based on synthetic/natural polymer blends mechanically reinforced with cellulose nanocrystals are produced to replicate the nanoscale collagen fibrils grouped into microscale collagen fibers that compose the native T/L. CANT are then incrementally assembled into 3D hierarchical scaffolds, resulting in woven constructions, which simultaneously mimic T/L nano-to-macro architecture, nanotopography, and nonlinear biomechanical behavior. Biological performance is assessed using human-tendon-derived cells (hTDCs) and human adipose stem cells (hASCs). Scaffolds nanotopography and microstructure induce a high cytoskeleton elongation and anisotropic organization typical of tendon tissues. Moreover, the expression of tendon-related markers (Collagen types I and III, Tenascin-C, and Scleraxis) by both cell types, and the similarities observed on their expression patterns over time suggest that the developed scaffolds not only prevent the phenotypic drift of hTDCs, but also trigger tenogenic differentiation of hASCs. Overall, these results demonstrate a feasible approach for the scalable production of 3D hierarchical scaffolds that exhibit key structural and biomechanical properties, which can be advantageously explored in acellular and cellular T/L TE strategies.

Biomimetic tendon extracellular matrix composite gradient scaffold enhances ligament-to-bone junction reconstruction

Management of ligament/tendon-to-bone-junction healing remains a formidable challenge in the field of orthopedic medicine to date, due to deficient vascularity and multi-tissue transitional structure of the junction. Numerous strategies have been employed to improve ligament-bone junction healing, including delivery of stem cells, bioactive factors, and synthetic materials, but these methods are often inadequate at recapitulating the complex structure-function relationships at native tissue interfaces. Here, we developed an easily-fabricated and effective biomimetic composite to promote the regeneration of ligament-bone junction by physically modifying the tendon extracellular matrix (ECM) into a Random-Aligned-Random composite using ultrasound treatment. The differentiation potential of rabbit bone marrow stromal cells on the modified ECM were examined in vitro. The results demonstrated that the modified ECM enhanced expression of chondrogenesis and osteogenesis-associated epigenetic genes (Jmjd1c, Kdm6b), transcription factor genes (Sox9, Runx2) and extracellular matrix genes (Col2a1, Ocn), resulting in higher osteoinductivity than the untreated tendon ECM in vitro. In the rabbit anterior cruciate ligament (ACL) reconstruction model in vivo, micro-computed tomography (Micro-CT) and histological analysis showed that the modified Random-Aligned-Random composite scaffold enhanced bone and fibrocartilage formation at the interface, more efficaciously than the unmodified tendon ECM. Therefore, these results demonstrated that the biomimetic Random-Aligned-Random composite could be a promising scaffold for ligament/tendon-bone junction repair.

STATEMENT OF SIGNIFICANCE

The native transitional region consists of several distinct yet contiguous tissue regions, composed of soft tissue, non-calcified fibrocartilage, calcified fibrocartilage, and bone. A stratified graft whose phases are interconnected with each other is essential for supporting the formation of functionally continuous multi-tissue regions. Various techniques have been attempted to improve adherence of the ligament/tendon graft to bone, including utilization of stem cells, growth factors and biomaterials, but these methods are often inadequate at recapitulating the complex structure-function relationships at native tissue interfaces. Here, we developed an easily-fabricated and effective biomimetic composite to promote the regeneration of ligament-bone junction by physically modifying the tendon extracellular matrix (ECM) into a Random-Aligned-Random composite using ultrasound treatment. The modified ECM enhanced expression of chondrogenesis and osteogenesis-associated epigenetic genes expression in vitro. In the rabbit anterior crucial ligament reconstruction model in vivo, results showed that the modified Random-Aligned-Random composite enhances the bone and fibrocartilage formation in the interface, proving to be more efficient than the unmodified tendon ECM. Therefore, these results demonstrated that the biomimetic Random-Aligned-Random composite could be a promising scaffold for ligament/tendon-bone junction repair.

Tissue-specific bioactivity of soluble tendon-derived and cartilage-derived extracellular matrices on adult mesenchymal stem cells

BACKGROUND

Biological scaffolds composed of tissue-derived extracellular matrix (ECM) can promote homologous (i.e., tissue-specific) cell differentiation through preservation of biophysical and biochemical motifs found in native tissues. Solubilized ECMs derived from decellularized tendon and cartilage have recently been promoted as tissue-specific biomaterials, but whether tissue-specific bioactivity is preserved following solubilization is unknown. This study explored the tissue-specific bioactivity of soluble decellularized tendon and cartilage ECMs on human bone marrow-derived mesenchymal stem cells (MSCs) presented across different culture microenvironments, including two-dimensional (2D) tissue culture plastic, aligned electrospun nanofibers, cell pellets, and cell-seeded photocrosslinkable hydrogels.

METHODS

Tendon and cartilage ECMs were decellularized using established methods and solubilized either via pepsin digestion or urea extraction. The effect of soluble ECMs on cell proliferation and differentiation was initially explored by supplementing basal medium of human MSCs cultured on 2D tissue culture plastic. In subsequent experiments, MSCs were cultured on aligned electrospun nanofibers, ascell pellets, or encapsulated within photocrosslinkable methacrylated gelatin (GelMA) hydrogels. Urea-extracted tendon and cartilage ECMs were added as supplements.

RESULTS

Pepsin-digested ECMs did not promote homologous differentiation in human MSCs, whether provided as a medium supplement or three-dimensional (3D) hydrogels. In contrast, urea-extracted ECMs tended to promote tissue-specific differentiation of MSCs cultured in 2D and 3D microenvironments. The application of the small molecule TGF-β signaling inhibitor SB-431542 largely negated the tissue-specific gene expression patterns mediated by tendon and cartilage ECMs. This suggests that the action of endogenous TGF-β was required, but was not sufficient, to impart tissue-specific bioactivity of urea-extracted ECMs. When urea-extracted cartilage ECM was incorporated within a photocurable GelMA hydrogel it independently enhanced chondrogenesis in encapsulated MSCs, and showed additive prochondrogenesis upon TGF-β supplementation in the medium.

CONCLUSIONS

Urea-extracted ECM fractions of decellularized tendon and cartilage are soluble supplements capable of enhancing tissue-specific differentiation of adult stem cells.

Braided and Stacked Electrospun Nanofibrous Scaffolds for Tendon and Ligament Tissue Engineering

Tendon and ligament injuries are a persistent orthopedic challenge given their poor innate healing capacity. Nonwoven electrospun nanofibrous scaffolds composed of polyesters have been used to mimic the mechanics and topographical cues of native tendons and ligaments. However, nonwoven nanofibers have several limitations that prevent broader clinical application, including poor cell infiltration, as well as tensile and suture-retention strengths that are inferior to native tissues. In this study, multilayered scaffolds of aligned electrospun nanofibers of two designs-stacked or braided-were fabricated. Mechanical properties, including structural and mechanical properties and suture-retention strength, were determined using acellular scaffolds. Human bone marrow-derived mesenchymal stem cells (MSCs) were seeded on scaffolds for up to 28 days, and assays for tenogenic differentiation, histology, and biochemical composition were performed. Braided scaffolds exhibited improved tensile and suture-retention strengths, but reduced moduli. Both scaffold designs supported expression of tenogenic markers, although the effect was greater on braided scaffolds. Conversely, cell infiltration was superior in stacked constructs, resulting in enhanced cell number, total collagen content, and total sulfated glycosaminoglycan content. However, when normalized against cell number, both designs modulated extracellular matrix protein deposition to a similar degree. Taken together, this study demonstrates that multilayered scaffolds of aligned electrospun nanofibers supported tenogenic differentiation of seeded MSCs, but the macroarchitecture is an important consideration for applications of tendon and ligament tissue engineering.

Current concepts on tenogenic differentiation and clinical applications

Tendon is a tissue that transmits force from muscle to bone. Chronic or acute tendon injuries are very common, and are always accompanied by pain and a limited range of motion in patients. In clinical settings, management of tendon injuries still remains a big challenge. Cell therapies, such as the application of stem cells for tenogenic differentiation, were suggested to be an ideal strategy for clinical translation. However, there is still a lack of specific methods for tenogenic differentiation due to the limited understanding of tendon biology currently. This review focuses on the summary of current published strategies for tenogenic differentiation, such as the application of growth factors, mechanical stimulation, biomaterials, coculture, or induced pluripotent stem cells. Current clinical applications of stem cells for treatment of tendon injuries and their limitations have also been discussed in this review.

Osteogenic and tenogenic induction of hBMSCs by an integrated nanofibrous scaffold with chemical and structural mimicry of the bone–ligament connection

A novel electrospinning nanofiber collecting device was designed and utilized to fabricate an integrated PCL nanofibrous scaffold with a “random–aligned–random” structure.

Crimped Nanofibrous Biomaterials Mimic Microstructure and Mechanics of Native Tissue and Alter Strain Transfer to Cells

To fully recapitulate tissue microstructure and mechanics, fiber crimping must exist within biomaterials used for tendon/ligament engineering. Existing crimped nanofibrous scaffolds produced via electrospinning are dense materials that prevent cellular infiltration into the scaffold interior. In this study, we used a sacrificial fiber population to increase the scaffold porosity and evaluated the effect on fiber crimping. We found that increasing scaffold porosity increased fiber crimping and ensured that the fibers properly uncrimped as the scaffolds were stretched by minimizing fiber-fiber interactions. Constitutive modeling demonstrated that the fiber uncrimping produced a nonlinear mechanical behavior similar to that of native tendon and ligament. Interestingly, fiber crimping altered strain transmission to the nuclei of cells seeded on the scaffolds, which may account for previously observed changes in gene expression. These crimped biomaterials are useful for developing functional fiber-reinforced tissues and for studying the effects of altered fiber crimping due to damage or degeneration.

Modulation of human multipotent and pluripotent stem cells using surface nanotopographies and surface-immobilised bioactive signals: A review

The ability to control the interactions of stem cells with synthetic surfaces is proving to be effective and essential for the quality of passaged stem cells and ultimately the success of regenerative medicine. The stem cell niche is crucial for stem cell self-renewal and differentiation. Thus, mimicking the stem cell niche, and here in particular the extracellular matrix (ECM), in vitro is an important goal for the expansion of stem cells and their applications. Here, surface nanotopographies and surface-immobilised biosignals have been identified as major factors that control stem cell responses. The development of tailored surfaces having an optimum nanotopography and displaying suitable biosignals is proposed to be essential for future stem cell culture, cell therapy and regenerative medicine applications. While early research in the field has been restricted by the limited availability of micro- and nanofabrication techniques, new approaches involving the use of advanced fabrication and surface immobilisation methods are starting to emerge. In addition, new cell types such as induced pluripotent stem cells (iPSCs) have become available in the last decade, but have not been fully understood. This review summarises significant advances in the area and focuses on the approaches that are aimed at controlling the behavior of human stem cells including maintenance of their self-renewal ability and improvement of their lineage commitment using nanotopographies and biosignals. More specifically, we discuss developments in biointerface science that are an important driving force for new biomedical materials and advances in bioengineering aiming at improving stem cell culture protocols and 3D scaffolds for clinical applications. Cellular responses revolve around the interplay between the surface properties of the cell culture substrate and the biomolecular composition of the cell culture medium. Determination of the precise role played by each factor, as well as the synergistic effects amongst the factors, all of which influence stem cell responses is essential for future developments. This review provides an overview of the current state-of-the-art in the design of complex material surfaces aimed at being the next generation of tools tailored for applications in cell culture and regenerative medicine.

STATEMENT OF SIGNIFICANCE

This review focuses on the effect of surface nanotopographies and surface-bound biosignals on human stem cells. Recently, stem cell research attracts much attention especially the induced pluripotent stem cells (iPSCs) and direct lineage reprogramming. The fast advance of stem cell research benefits disease treatment and cell therapy. On the other hand, surface property of cell adhered materials has been demonstrated very important for in vitro cell culture and regenerative medicine. Modulation of cell behavior using surfaces is costeffective and more defined. Thus, we summarise the recent progress of modulation of human stem cells using surface science. We believe that this review will capture a broad audience interested in topographical and chemical patterning aimed at understanding complex cellular responses to biomaterials.

Electrostatic flocking of chitosan fibres leads to highly porous, elastic and fully biodegradable anisotropic scaffolds

Electrostatic flocking - a common textile technology which has been applied in industry for decades - is based on the deposition of short polymer fibres in a parallel aligned fashion on flat or curved substrates, covered with a layer of a suitable adhesive. Due to their highly anisotropic properties the resulting velvet-like structures can be utilised as scaffolds for tissue engineering applications in which the space between the fibres can be defined as pores. In the present study we have developed a fully resorbable compression elastic flock scaffold from a single material system based on chitosan. The fibres and the resulting scaffolds were analysed concerning their structural and mechanical properties and the biocompatibility was tested in vitro. The tensile strength and Young's modulus of the chitosan fibres were analysed as a function of the applied sterilisation technique (ethanol, supercritical carbon dioxide, γ-irradiation and autoclaving). All sterilisation methods decreased the Young's modulus (from 14GPa to 6-12GPa). The tensile strength was decreased after all treatments - except after the autoclaving of chitosan fibres submerged in water. Compressive strength of the highly porous flock scaffolds was 18±6kPa with a elastic modulus in the range of 50-100kPa. The flocked scaffolds did not show any cytotoxic effect during indirect or direct culture of human mesenchymal stem cells or the sarcoma osteogenic cell line Saos-2. Furthermore cell adhesion and proliferation of both cell types could be observed. This is the first demonstration of a fully biodegradable scaffold manufactured by electrostatic flocking.

STATEMENT OF SIGNIFICANCE

Most tissues possess anisotropic fibrous structures. In contrast, most of the commonly used scaffolds have an isotropic morphology. By utilising the textile technology of electrostatic flocking, highly porous and clearly anisotropic scaffolds can be manufactured. Flocking leads to parallel aligned short fibres, glued on the surface of a substrate. Such structures are characterised by a high and adjustable porosity, accompanied by distinct stiffness in fibre direction. The present article describes for the first time a fully biodegradable flock scaffold, solely made of chitosan. Utilisation of only one material for manufacturing of flock substrate, adhesive and fibres allow a uniform degradation of the whole construct. Such a new type of scaffold can be of great interest for a variety of biomedical applications.

A Clinical, Biological, and Biomaterials Perspective into Tendon Injuries and Regeneration

Tendon injury is common and debilitating, and it is associated with long-term pain and ineffective healing. It is estimated to afflict 25% of the adult population and is often a career-ending disease in athletes and racehorses. Tendon injury is associated with high morbidity, pain, and long-term suffering for the patient. Due to the low cellularity and vascularity of tendon tissue, once damage has occurred, the repair process is slow and inefficient, resulting in mechanically, structurally, and functionally inferior tissue. Current treatment options focus on pain management, often being palliative and temporary and ending in reduced function. Most treatments available do not address the underlying cause of the disease and, as such, are often ineffective with variable results. The need for an advanced therapeutic that addresses the underlying pathology is evident. Tissue engineering and regenerative medicine is an emerging field that is aimed at stimulating the body's own repair system to produce de novo tissue through the use of factors such as cells, proteins, and genes that are delivered by a biomaterial scaffold. Successful tissue engineering strategies for tendon regeneration should be built on a foundation of understanding of the molecular and cellular composition of healthy compared with damaged tendon, and the inherent differences seen in the tissue after disease. This article presents a comprehensive clinical, biological, and biomaterials insight into tendon tissue engineering and regeneration toward more advanced therapeutics.

Highly aligned core-shell structured nanofibers for promoting phenotypic expression of vSMCs for vascular regeneration

This study was designed to assess the efficacy of hyaluronan (HA) functionalized well-aligned nanofibers of poly-l-lactic acid (PLLA) in modulating the phenotypic expression of vascular smooth muscle cells (vSMCs) for blood vessel regeneration. Highly aligned HA/PLLA nanofibers in core-shell structure were prepared using a novel stable jet electrospinning approach. Formation of a thin HA-coating layer atop each PLLA nanofiber surface endowed the uni-directionally oriented fibrous mats with increased anisotropic wettability and mechanical compliance. The HA/PLLA nanofibers significantly promoted vSMC to elongation, orientation, and proliferation, and also up-regulated the expression of contractile genes/proteins (e.g., α-SMA, SM-MHC) as well as the synthesis of elastin. Six weeks of in vivo scaffold replacement of rabbit carotid arteries showed that vascular conduits made of circumferentially aligned HA/PLLA nanofibers could maintain patency and promoted oriented vSMC regeneration, lumen endothelialization, and capillary formation. This study demonstrated the synergistic effects of nanotopographical and biochemical cues in one biomimetic scaffold design for efficacious vascular regeneration.

A newly identified mechanism involved in regulation of human mesenchymal stem cells by fibrous substrate stiffness

Stiffness of biomaterial substrates plays a critical role in regulation of cell behavior. Although the effect of substrate stiffness on cell behavior has been extensively studied, molecular mechanisms of regulation rather than those involving cytoskeletal activities still remain elusive. In this study, we fabricated aligned ultrafine fibers and treated the fiber with different annealing temperatures to produce fibrous substrates with different stiffness. Human mesenchymal stem cells (hMSCs) were then cultured on these fibrous substrates. Our results showed that annealing treatment did not change the diameter of electrospun fibers but increased their polymer crystallinity and mechanical properties. The mRNA expression of RUNX2 was upregulated while the mRNA expression of scleraxis was downregulated in response to an increase in substrate stiffness, suggesting that increased stiffness favorably drives hMSCs into the osteogenic lineage. With subsequent induction of osteogenic differentiation, osteogenesis of hMSCs on stiffer substrates was increased compared to that of the cells on control substrates. Cells on stiffer substrates increasingly activated AKT and YAP and upregulated transcript expression of YAP target genes compared to those on control substrates, and inhibition of AKT led to decreased expression of YAP and RUNX2. Furthermore, macrophage migration inhibitory factor (MIF) was increasingly produced by the cell on stiffer substrates, and knocking down MIF by siRNA resulted in decreased AKT phosphorylation. Taken together, we hereby demonstrate that simply using the annealing approach can manipulate stiffness of an aligned fibrous substrate without altering the material chemistry, and substrate stiffness dictates hMSC differentiation through the MIF-mediated AKT/YAP/RUNX2 pathway.

STATEMENT OF SIGNIFICANCE

Stiffness of biomaterial substrates plays a critical role in regulation of cell behavior. Although the effect of substrate stiffness on cell behavior has been extensively studied, molecular mechanisms of regulation rather than those involving cytoskeletal activities still remain elusive. In this manuscript, we report our new findings that simply using the annealing approach can manipulate stiffness of an aligned fibrous substrate without altering the material chemistry, and substrate stiffness dictates human mesenchymal stem cell (hMSC) differentiation through the macrophage migration inhibitory factor-mediated AKT/YAP/RUNX2 pathway. The findings are novel and interesting because we have identified a new mechanism rather than those involving cytoskeleton activity, by which substrate stiffness regulates hMSC behavior.

Stem cell regenerative potential for plastic and reconstructive surgery

Stem cells represent heterogeneous population of undifferentiated cells with unique characteristics of long term self renewal and plasticity. Moreover, they are capable of active migration to diseased tissues, secretion of different bioactive molecules, and they have immunosuppressive potential as well. They occur in all tissues through life and are involved in process of embryogenesis and regeneration. During last decades stem cells attracted significant attention in each field of medicine, including plastic and reconstructive surgery. The main goal of the present review article is to present and discuss the potential of stem cells and to provide information about their safe utilization in chronic wounds and fistulae healing, scar management, breast reconstruction, as well as in bone, tendon and peripheral nerve regeneration.

Aligned nanofibers direct human dermal fibroblasts to tenogenic phenotype in vitro and enhance tendon regeneration in vivo

Aim: To explore the effect of aligned nanofibers on inducing tenogenic phenotype of human dermal fibroblasts (hDFs) in vitro and on inducing de novo tendon regeneration in vivo. Materials & methods: Random and aligned nanofibers were electrospun, seeded with hDFs and cultured in vitro, and in vivo implanted without cell seeding to bridge segmental defect of rat Achilles tendon. Results: In vitro, the well-aligned nanofibers could elongate hDFs, induce a tenogenic phenotype and form better organized neotendon respectively compared with random nanofibers. In vivo, the bridged nanofibers of aligned group could better recruit host cells and regenerate Achilles tendon de novo with enhanced tenogenic gene expression. Conclusion: Aligned nanofibers could induce tenogenic phenotype in vitro and regenerate tendon in vivo.

Comparison of the Biological Characteristics of Mesenchymal Stem Cells Derived from Bone Marrow and Skin

Mesenchymal stem cells (MSCs) exhibit high proliferation and self-renewal capabilities and are critical for tissue repair and regeneration during ontogenesis. They also play a role in immunomodulation. MSCs can be isolated from a variety of tissues and have many potential applications in the clinical setting. However, MSCs of different origins may possess different biological characteristics. In this study, we performed a comprehensive comparison of MSCs isolated from bone marrow and skin (BMMSCs and SMSCs, resp.), including analysis of the skin sampling area, separation method, culture conditions, primary and passage culture times, cell surface markers, multipotency, cytokine secretion, gene expression, and fibroblast-like features. The results showed that the MSCs from both sources had similar cell morphologies, surface markers, and differentiation capacities. However, the two cell types exhibited major differences in growth characteristics; the primary culture time of BMMSCs was significantly shorter than that of SMSCs, whereas the growth rate of BMMSCs was lower than that of SMSCs after passaging. Moreover, differences in gene expression and cytokine secretion profiles were observed. For example, secretion of proliferative cytokines was significantly higher for SMSCs than for BMMSCs. Our findings provide insights into the different biological functions of both cell types.

Osteogenic differentiation and bone regeneration of iPSC-MSCs supported by a biomimetic nanofibrous scaffold

Induced pluripotent stem cell-derived mesenchymal stem cells (iPSC-MSCs) are a new type of MSCs that come with attractive merits over the iPSCs per se. Aimed for regenerating bone tissues, this study was designed to investigate osteogenic differentiation and bone regeneration capacities of iPSC-MSCs by using biomimetic nanofibers of hydroxyapatite/collagen/chitosan (HAp/Col/CTS). Murine iPSCs were firstly induced to differentiate into iPSC-MSCs and thoroughly characterized. Effects of HAp/Col/CTS nanofibers prepared from electrospinning of Col-doped HAp/CTS nanocomposite, on osteogenic differentiation of the generated iPSC-MSCs were then evaluated in detail, including cell morphology, proliferation, migration, quantified specific osteogenic gene and protein expressions. Compared with different controls (TCP, CTS, and HAp/CTS), the HAp/Col/CTS scaffold was found to have more favorable effects on attachment and proliferation of iPSC-MSCs than others (P<0.01). Expressions of osteogenic genes, Runx2, Ocn, Alp, and Col, were significantly upregulated in iPSC-MSCs cultured on HAp/Col/CTS than CTS (P<0.01). Similarly, there appeared considerably higher secreting activities of osteogenesis protein markers, ALP and Col. Furthermore, mouse cranial defects were created to investigate efficacy of using iPSC-MSCs in combination with HAp/Col/CTS scaffold for regenerative bone repair in vivo. Examinations by computed tomography (CT) imaging, bone mineral density and hematoxylin eosin (HE) staining corroborated that cell-scaffold construct of iPSC-MSCs+HAp/Col/CTS could effectively promote bone regeneration. After 6 weeks of implantation, bone mineral density of the iPSC-MSCs+HAp/Col/CTS group was found to be nearly 2-fold higher than others. Our results demonstrated that biomimetic nanofibers of HAp/Col/CTS promoted the osteogenic differentiation and bone regeneration of iPSC-MSCs. The iPSC-MSCs+HAp/Col/CTS complex could be used as a new 'stem cell-scaffold' system for realizing personalized and efficacious bone regeneration in future.

STATEMENT OF SIGNIFICANCE

In bone tissue engineering, stem cells have become the most important source of seed cells. iPSC-MSCs are a new type of MSCs that come with attractive merits over the iPSCs per se. However, how to obtain befitting iPSC-MSCs and regulate their osteogenic differentiation are the key issues to be addressed. Given the great biomimicking capacity to extracellular matrix, electrospun nanofibers may be explored to modulate osteogenic differentiation of the iPSC-MSCs. This study successfully demonstrated that biomimetic nanofibers of HAp/Col/CTS significantly promoted the osteogenic differentiation and bone regeneration of iPSC-MSCs, which thereby suggests that nanofibrous scaffold supported iPSC-MSCs complex may be a new 'stem cell-scaffold' system for regulating the fate of osteogenic differentiation of iPSC-MSCs towards patient-specific bone regeneration in future.

A novel and facile approach to fabricate a conductive and biomimetic fibrous platform with sub-micron and micron features

A novel and facile method to fabricate a core–shell structure consisting of a conducting fiber core and an electrospun fiber shell is presented.

Stem cell technology for tendon regeneration: current status, challenges, and future research directions

Tendon injuries are a common cause of physical disability. They present a clinical challenge to orthopedic surgeons because injured tendons respond poorly to current treatments without tissue regeneration and the time required for rehabilitation is long. New treatment options are required. Stem cell-based therapies offer great potential to promote tendon regeneration due to their high proliferative, synthetic, and immunomodulatory activities as well as their potential to differentiate to the target cell types and undergo genetic modification. In this review, I first recapped the challenges of tendon repair by reviewing the anatomy of tendon. Next, I discussed the advantages and limitations of using different types of stem cells compared to terminally differentiated cells for tendon tissue engineering. The safety and efficacy of application of stem cells and their modified counterparts for tendon tissue engineering were then summarized after a systematic literature search in PubMed. The challenges and future research directions to enhance, optimize, and standardize stem cell-based therapies for augmenting tendon repair were then discussed.

Scaffolds and tissue regeneration: An overview of the functional properties of selected organic tissues

Tissue engineering plays a significant role both in the re-establishment of functions and regeneration of organic tissues. Success in manufacturing projects for biological scaffolds, for the purpose of tissue regeneration, is conditioned by the selection of parameters such as the biomaterial, the device architecture, and the specificities of the cells making up the organic tissue to create, in vivo, a microenvironment that preserves and further enhances the proliferation of a specific cell phenotype. To support this approach, we have screened scientific publications that show biomedical applications of scaffolds, biomechanical, morphological, biochemical, and hemodynamic characteristics of the target organic tissues, and the possible interactions between different cell matrices and biological scaffolds. This review article provides an overview on the biomedical application of scaffolds and on the characteristics of the (bio)materials commonly used for manufacturing these biological devices used in tissue engineering, taking into consideration the cellular specificity of the target tissue. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1483-1494, 2016.