Tissue engineering of replacement skin: the crossroads of biomaterials, wound healing, embryonic development, stem cells and regeneration

Advanced therapies combating acute and chronic skin wounds are likely to be brought about using our knowledge of regenerative medicine coupled with appropriately tissue-engineered skin substitutes. At the present time, there are no models of an artificial skin that completely replicate normal uninjured skin. Natural biopolymers such as collagen and fibronectin have been investigated as potential sources of biomaterial to which cells can attach. The first generation of degradable polymers used in tissue engineering were adapted from other surgical uses and have drawbacks in terms of mechanical and degradation properties. This has led to the development of synthetic degradable gels primarily as a way to deliver cells and/or molecules in situ, the so-called smart matrix technology. Tissue or organ repair is usually accompanied by fibrotic reactions that result in the production of a scar. Certain mammalian tissues, however, have a capacity for complete regeneration without scarring; good examples include embryonic or foetal skin and the ear of the MRL/MpJ mouse. Investigations of these model systems reveal that in order to achieve such complete regeneration, the inflammatory response is altered such that the extent of fibrosis and scarring is diminished. From studies on the limited examples of mammalian regeneration, it may also be possible to exploit such models to further clarify the regenerative process. The challenge is to identify the factors and cytokines expressed during regeneration and incorporate them to create a smart matrix for use in a skin equivalent. Recent advances in the use of DNA microarray and proteomic technology are likely to aid the identification of such molecules. This, coupled with recent advances in non-viral gene delivery and stem cell technologies, may also contribute to novel approaches that would generate a skin replacement whose materials technology was based not only upon intelligent design, but also upon the molecules involved in the process of regeneration.     

Bone regeneration using coculture of mesenchymal stem cells and angiogenic cells

Cellular strategies remain a crucial component in bone tissue engineering （BTE）. So far, the outcome of cell-based strategies from initial clinical trials is far behind compared to animal studies, which is suggested to be related to insufficient nutrient and oxygen supply inside the Ussue-engineered constructs. Cocultures, by introducing angiogenic cells into osteogenic cell cultures, might provide a solution for improving vascularization and hence increasing bone formation for cell-based constructs. So far, pre-clinical studies demonstrated that cocultures enhance vascularization and bone formation compared to monocultures. However, there has been no report on the application of cocultures in clinics. Therefore, this mini-review aims to provide an overview regarding （i） critical parameters in cocultures and the outcomes of cocultures compared to monocultures in the currently available pre-clinical studies using human mesenchymal stem cells implanted in orthotopic animal models; and （ii） the usage of monocultures in clinical application in BTE.     

Nitric-Oxide-Releasing Biomaterial Regulation of the Stem Cell Microenvironment in Regenerative Medicine

Stem cell therapy has proven to be an attractive solution for the treatment of degenerative diseases or injury. However, poor cell engraftment and survival within injured tissues limits the successful use of stem cell therapy within the clinical setting. Nitric oxide (NO) is an important signaling molecule involved in various physiological processes. Emerging evidence supports NO's diverse roles in modulating stem cell behavior, including survival, migration, differentiation, and paracrine secretion of proregenerative factors. Thus, there has been a shift in research focus to concentrate efforts on the delivery of therapeutic concentration ranges of NO to the target tissue sites. Combinatory therapies utilizing biomaterials that control NO generation and support stem cell delivery can be holistic and synergistic approaches to significantly improve tissue regeneration. Here, the focus is on recent developments of various therapeutic platforms, engineered to both transport NO and to enhance stem-cell-mediated regeneration of damaged tissues. New and emerging revelations of how the stem cell microenvironment can be regulated by NO-releasing biomaterials are also highlighted.     

Transdifferentiation of autologous bone marrow cells on a collagen-poly(ε-caprolactone) scaffold for tissue engineering in complete lack of native urothelium

Urological reconstructive surgery is sometimes hampered by a lack of tissue. In some cases, autologous urothelial cells (UCs) are not available for cell expansion and ordinary tissue engineering. In these cases, we wanted to explore whether autologous mesenchymal stem cells (MSCs) from bone marrow could be used to create urological transplants. MSCs from human bone marrow were cultured in vitro with medium conditioned by normal human UCs or by indirect co-culturing in culture well inserts. Changes in gene expression, protein expression and cell morphology were studied after two weeks using western blot, RT-PCR and immune staining. Cells cultured in standard epithelial growth medium served as controls. Bone marrow MSCs changed their phenotype with respect to growth characteristics and cell morphology, as well as gene and protein expression, to a UC lineage in both culture methods, but not in controls. Urothelial differentiation was also accomplished in human bone marrow MSCs seeded on a three-dimensional poly(ε-caprolactone) (PCL)–collagen construct. Human MSCs could easily be harvested by bone marrow aspiration and expanded and differentiated into urothelium. Differentiation could take place on a three-dimensional hybrid PCL-reinforced collagen-based scaffold for creation of a tissue-engineered autologous transplant for urological reconstructive surgery.     

In vivo bone regeneration with injectable chitosan/hydroxyapatite/collagen composites and mesenchymal stem cells

For reconstruction of irregular bone defects, injectable biomaterials are more appropriate than the preformed biomaterials. We herein develop a biomimetic in situ-forming composite consisting of chitosan (CS) and mineralized collagen fibrils (nHAC), which has a complex hierarchical structure similar to natural bone. The CS/nHAC composites with or without mesenchymal stem cells (MSCs) are injected into cancellous bone defects at the distal end of rabbit femurs. Defects are assessed by radiographic, histological diagnosis and Raman microscopy until 12 weeks. The results show that MSCs improve the biocompatibility of CS/nHAC composites and enhance new bone formation in vivo at 12 weeks. It can be concluded that the injectable CS/nHAC composites combined with MSCs may be a novel method for reconstruction of irregular bone defects.     

Genomic expression of mesenchymal stem cells to altered nanoscale topographies.

The understanding of cellular response to the shape of their environment would be of benefit in the development of artificial extracellular environments for potential use in the production of biomimetic surfaces. Specifically, the understanding of how cues from the extracellular environment can be used to understand stem cell differentiation would be of special interest in regenerative medicine.In this paper, the genetic profile of mesenchymal stem cells cultured on two osteogenic nanoscale topographies (pitted surface versus raised islands) are compared with cells treated with dexamethasone, a corticosteroid routinely used to stimulate bone formation in culture from mesenchymal stem cells, using 19k gene microarrays as well as 101 gene arrays specific for osteoblast and endothelial biology.The current studies show that by altering the shape of the matrix a cell response (genomic profile) similar to that achieved with chemical stimulation can be elicited. Here, we show that bone formation can be achieved with efficiency similar to that of dexamethasone with the added benefit that endothelial cell development is not inhibited. We further show that the mechanism of action of the topographies and dexamethasone differs. This could have an implication for tissue engineering in which a simultaneous, targeted, development of a tissue, such as bone, without the suppression of angiogenesis to supply nutrients to the new tissue is required. The results further demonstrate that perhaps the shape of the extracellular matrix is critical to tissue development.     

Evaluation of synovium-derived mesenchymal stem cells and 3D printed nanocomposite scaffolds for tissue engineering

Stem cells and scaffolds play a very important role in tissue engineering. Here, we isolated synovium-derived mesenchymal stem cells (SMSCs) from synovial membrane tissue and characterized stem-cell properties. Gelatin nanoparticles (NP) were prepared using a two-step desolvation method and then pre-mixed into different host matrix (silk fibroin (SF), gelatin (Gel), or SF–Gel mixture) to generate various 3D printed nanocomposite scaffolds (NP/SF, NP/SF–Gel, NP/Gel-1, and NP/Gel-2). The microstructure was examined by scanning electron microscopy. Biocompatibility assessment was performed through CCK-8 assay by coculturing with SMSCs at 1, 3, 7 and 14 days. According to the results, SMSCs are similar to other MSCs in their surface epitope expression, which are negative for CD45 and positive for CD44, CD90, and CD105. After incubation in lineage-specific medium, SMSCs could differentiate into chondrocytes, osteocytes and adipocytes. 3D printed nanocomposite scaffolds exhibited a good biocompatibility in the process of coculturing with SMSCs and had no negative effect on cell behavior. The study provides a strategy to obtain SMSCs and fabricate 3D printed nanocomposite scaffolds, the combination of which could be used for practical applications in tissue engineering.     

Evaluation of synovium-derived mesenchymal stem cells and 3D printed nanocomposite scaffolds for tissue engineering

Abstract

Stem cells and scaffolds play a very important role in tissue engineering. Here, we isolated synovium-derived mesenchymal stem cells (SMSCs) from synovial membrane tissue and characterized stem-cell properties. Gelatin nanoparticles (NP) were prepared using a two-step desolvation method and then pre-mixed into different host matrix (silk fibroin (SF), gelatin (Gel), or SF–Gel mixture) to generate various 3D printed nanocomposite scaffolds (NP/SF, NP/SF–Gel, NP/Gel-1, and NP/Gel-2). The microstructure was examined by scanning electron microscopy. Biocompatibility assessment was performed through CCK-8 assay by coculturing with SMSCs at 1, 3, 7 and 14 days. According to the results, SMSCs are similar to other MSCs in their surface epitope expression, which are negative for CD45 and positive for CD44, CD90, and CD105. After incubation in lineage-specific medium, SMSCs could differentiate into chondrocytes, osteocytes and adipocytes. 3D printed nanocomposite scaffolds exhibited a good biocompatibility in the process of coculturing with SMSCs and had no negative effect on cell behavior. The study provides a strategy to obtain SMSCs and fabricate 3D printed nanocomposite scaffolds, the combination of which could be used for practical applications in tissue engineering.     

矿化柞蚕丝胶膜表面粗糙度的调控及其对骨髓间充质干细胞生长行为的影响

生物材料表面的粗糙度是影响细胞行为的重要因素之一。为了调控丝蛋白生物材料表面的粗糙度,并评价材料表面粗糙度对细胞生长行为的影响,首先,通过湿化学共沉淀法,以柞蚕丝胶(AS)溶液为模板,诱导了羟基磷灰石(HAp)晶体成核,进而调控了AS膜表面的粗糙度。然后,采用SEM、粗糙仪、FTIR及EDX等对HAp/AS复合膜表面形貌、粗糙度及成分进行了表征。最后,通过SEM和CellTiter 96?AQueous单溶液细胞增殖检测试剂盒(MTS)检测了骨髓间充质干细胞(BMSCs)在HAp/AS复合膜表面的形貌及增殖率。结果表明:纯AS膜的表面粗糙度为0.15μm,矿化1、8及24h后,表面粗糙度分别为0.38、0.46和1.20μm;矿化24h后,在HAp/AS复合膜表面可观察到直径为30~80nm的球状复合物,生成的矿化物为HAp;HAp/AS复合膜具有良好的细胞相容性,表面粗糙度为1.20μm的复合膜能够显著促进BMSCs的增殖,粗糙度对BMSCs在HAp/AS复合膜表面的粘附和形貌有着重要的影响。因此,可通过矿化的方法在生物大分子表面诱导HAp晶体的成核与生长,从而调控材料的表面粗糙度,研究材料界面上的细胞行为。     

Collagen gel three-dimensional matrices combined with adhesive proteins stimulate neuronal differentiation of mesenchymal stem cells

Three-dimensional gel matrices provide specialized microenvironments that mimic native tissues and enable stem cells to grow and differentiate into specific cell types. Here, we show that collagen three-dimensional gel matrices prepared in combination with adhesive proteins, such as fibronectin (FN) and laminin (LN), provide significant cues to the differentiation into neuronal lineage of mesenchymal stem cells (MSCs) derived from rat bone marrow. When cultured within either a three-dimensional collagen gel alone or one containing either FN or LN, and free of nerve growth factor (NGF), the MSCs showed the development of numerous neurite outgrowths. These were, however, not readily observed in two-dimensional culture without the use of NGF. Immunofluorescence staining, western blot and fluorescence-activated cell sorting analyses demonstrated that a large population of cells was positive for NeuN and glial fibrillary acidic protein, which are specific to neuronal cells, when cultured in the three-dimensional collagen gel. The dependence of the neuronal differentiation of MSCs on the adhesive proteins containing three-dimensional gel matrices is considered to be closely related to focal adhesion kinase (FAK) activation through integrin receptor binding, as revealed by an experiment showing no neuronal outgrowth in the FAK-knockdown cells and stimulation of integrin β1 gene. The results provided herein suggest the potential role of three-dimensional collagen-based gel matrices combined with adhesive proteins in the neuronal differentiation of MSCs, even without the use of chemical differentiation factors. Furthermore, these findings suggest that three-dimensional gel matrices might be useful as nerve-regenerative scaffolds.     

Adhesion formation of primary human osteoblasts and the functional response of mesenchymal stem cells to 330nm deep microgrooves.

The surface microtexture of an orthopaedic device can regulate cellular adhesion, a process fundamental in the initiation of osteoinduction and osteogenesis. Advances in fabrication techniques have evolved to include the field of surface modification; in particular, nanotechnology has allowed for the development of experimental nanoscale substrates for investigation into cell nanofeature interactions. Here primary human osteoblasts (HOBs) were cultured on ordered nanoscale groove/ridge arrays fabricated by photolithography. Grooves were 330nm deep and either 10, 25 or 100mum in width. Adhesion subtypes in HOBs were quantified by immunofluorescent microscopy and cell-substrate interactions were investigated via immunocytochemistry with scanning electron microscopy. To further investigate the effects of these substrates on cellular function, 1.7K gene microarray analysis was used to establish gene regulation profiles of mesenchymal stem cells cultured on these nanotopographies. Nanotopographies significantly affected the formation of focal complexes (FXs), focal adhesions (FAs) and supermature adhesions (SMAs). Planar control substrates induced widespread adhesion formation; 100mum wide groove/ridge arrays did not significantly affect adhesion formation yet induced upregulation of genes involved in skeletal development and increased osteospecific function; 25mum wide groove/ridge arrays were associated with a reduction in SMA and an increase in FX formation; and 10mum wide groove/ridge arrays significantly reduced osteoblast adhesion and induced an interplay of up- and downregulation of gene expression. This study indicates that groove/ridge topographies are important modulators of both cellular adhesion and osteospecific function and, critically, that groove/ridge width is important in determining cellular response.     

Development of an early estimation method for predicting later osteogenic differentiation activity of rat mesenchymal stromal cells from their attachment areas

Abstract

Cell morphology has received considerable attention in recent years owing to its possible relationship with cell functions, including proliferation, differentiation, and migration. Recent evidence suggests that extracellular environments can also mediate cell functions, particularly cell adhesion. The aims of this study were to investigate the correlation between osteogenic differentiation activity and the morphology of rat mesenchymal stromal cells (MSCs), and to develop a method of estimating osteogenic differentiation capability of MSCs on biomaterials. We measured the attachment areas of MSCs on substrates with various types of surface after 2 h of seeding, and quantified the amount of osteocalcin secreted from MSCs after 3 weeks of culture under osteogenic differentiation conditions. MSCs with small attachment areas showed a high osteogenic differentiation activity. These findings indicate that cell attachment areas correlate well with the osteogenic differentiation activity of MSCs. They also suggest that the measurement of cell attachment areas is useful for estimating the osteogenic differentiation activity of MSCs and is a practical tool for applications of MSCs in regenerative medicine.     

Development of an early estimation method for predicting later osteogenic differentiation activity of rat mesenchymal stromal cells from their attachment areas

Cell morphology has received considerable attention in recent years owing to its possible relationship with cell functions, including proliferation, differentiation, and migration. Recent evidence suggests that extracellular environments can also mediate cell functions, particularly cell adhesion. The aims of this study were to investigate the correlation between osteogenic differentiation activity and the morphology of rat mesenchymal stromal cells (MSCs), and to develop a method of estimating osteogenic differentiation capability of MSCs on biomaterials. We measured the attachment areas of MSCs on substrates with various types of surface after 2 h of seeding, and quantified the amount of osteocalcin secreted from MSCs after 3 weeks of culture under osteogenic differentiation conditions. MSCs with small attachment areas showed a high osteogenic differentiation activity. These findings indicate that cell attachment areas correlate well with the osteogenic differentiation activity of MSCs. They also suggest that the measurement of cell attachment areas is useful for estimating the osteogenic differentiation activity of MSCs and is a practical tool for applications of MSCs in regenerative medicine.     

The effect of perfusion culture on proliferation and differentiation of human mesenchymal stem cells on biocorrodible bone replacement material

Biocorrodible iron foams were coated with different calcium phosphate phases (CPP) to obtain a bioactive surface and controlled degradation. Further adhesion, proliferation and differentiation of SaOs-2 and human mesenchymal stem cells were investigated under both static and dynamic culture conditions. Hydroxyapatite (HA; [Ca₁₀(PO₄)₆OH₂]) coated foams released 500 μg/g iron per day for Dulbecco's modified eagle medium (DMEM) and 250 μg/g iron per day for McCoys, the unmodified reference 1000 μg/g iron per day for DMEM and 500 μg/g iron per day for McCoys, while no corrosion could be detected on brushite (CaHPO₄) coated foams. Using a perfusion culture system with conditions closer to the in vivo situation, cells proliferated and differentiated on iron foams coated with either brushite or HA while in static cell culture cells could proliferate only on Fe-brushite. We conclude that the degradation behaviour of biocorrodible iron foams can be varied by different calcium phosphate coatings, offering opportunities for design of novel bone implants. Further studies will focus on the influence of different modifications of iron foams on the expression of oxidative stress enzymes. Additional information about in vivo reactions and remodelling behaviour are expected from testing in implantation studies.