Engineered decellularized matrices to instruct bone regeneration processes

Advances in reparative materials for infectious bone defects and their applications in maxillofacial regions

Infectious bone defects are characterized by the partial loss or destruction of bone tissue resulting from bacterial contaminations subsequent to diseases or external injuries. Traditional bone transplantation and clinical methods are insufficient in meeting the treatment demands for such diseases. As a result, researchers have increasingly focused on the development of more sophisticated biomaterials for improved therapeutic outcomes in recent years. This review endeavors to investigate specific reparative materials utilized for the treatment of infectious bone defects, particularly those present in the maxillofacial region, with a focus on biomaterials capable of releasing therapeutic substances, functional contact biomaterials, and novel physical therapy materials. These biomaterials operate via heightened antibacterial or osteogenic properties in order to eliminate bacteria and/or stimulate bone cells regeneration in the defect, ultimately fostering the reconstitution of maxillofacial bone tissue. Based upon some successful applications of new concept materials in bone repair of other parts, we also explore their future prospects and potential uses in maxillofacial bone repair later in this review. We highlight that the exploration of advanced biomaterials holds promise in establishing a solid foundation for the development of more biocompatible, effective, and personalized treatments for reconstructing infectious maxillofacial defects.

Zizyphus mauritiana seed extract: Paving the way for next-generation bone constructs with nano-fluorohydroxyapatite/carboxymethyl chitosan nanocomposite scaffold

This is the first study that explored the potential use of Zizyphus mauritiana seed extract (ZSE) to synthesize nano-fluorohydroxyapatite/carboxymethyl chitosan nanocomposite scaffolds at different concentrations (CFZ1, CFZ2 and CFZ3) using co-precipitation method. The proposed scaffolds showed presence of intermolecular H bonding interactions between the constituents, according to the FTIR. The mechanical studies revealed shore hardness of 72 ± 4.6 and optimal compressive modulus in case of CFZ3 [1654.48 ± 1.6 MPa], that was comparable with that of human cortical bone. The SEM, TEM and platelet adhesion images corroborated uniformly distributed needle like particles in case of CFZ3 with an average size ranging from 22 to 26 nm, linked rough morphology, and appropriate hemocompatibility. The markedly up regulation in the ALP activity and protein adsorption upon increasing ZSE concentration demonstrated that CFZ nanocomposite scaffolds were compatible with osteoblastic cells relative to CF nanocomposite. The cytotoxicity study indicated that CFZ nanocomposite do not induce toxicity over MG-63 and did not aggravate LDH leakage in contrast to CF. The histopathological investigations on albino rats confirmed significantly improved regeneration of bone in the repair of a critical-size [8 mm] calvarium defect. Therefore, CFZ3 nanocomposite scaffold represents a simple, off-the-shelf solution to the combined challenges associated with bone defects.

Influence of Extracellular Matrix Components on the Differentiation of Periodontal Ligament Stem Cells in Collagen I Hydrogel

Regeneration of periodontal tissues requires an integrated approach to the restoration of the periodontal ligament, cementum, and alveolar bone surrounding the teeth. Current strategies in endogenous regenerative dentistry widely use biomaterials, in particular the decellularized extracellular matrix (dECM), to facilitate the recruitment of populations of resident cells into damaged tissues and stimulate their proliferation and differentiation. The purpose of our study was to evaluate the effect of the exogenous components of the extracellular matrix (hyaluronic acid, laminin, fibronectin) on the differentiation of periodontal ligament stem cells (PDLSCs) cultured with dECM (combinations of decellularized tooth matrices and periodontal ligament) in a 3D collagen I hydrogel. The immunohistochemical expression of various markers in PDLSCs was assessed quantitatively and semi-quantitatively on paraffin sections. The results showed that PDLSCs cultured under these conditions for 14 days exhibited phenotypic characteristics consistent with osteoblast-like and odontoblast-like cells. This potential has been demonstrated by the expression of osteogenic differentiation markers (OC, OPN, ALP) and odontogenic markers (DSPP). This phenomenon corresponds to the in vivo state of the periodontal ligament, in which cells at the interface between bone and cementum tend to differentiate into osteoblasts or cementoblasts. The addition of fibronectin to the dECM most effectively induces the differentiation of PDLSCs into osteoblast-like and odontoblast-like cells under 3D culture conditions. Therefore, this bioengineered construct has a high potential for future use in periodontal tissue regeneration.

Endogenous Regeneration of Alveolar Bone by Decellularized Tooth Matrix

The efficiency of bone tissue regeneration by decellularized tooth matrix, demineralized tooth matrix, and commercial xenograft Bio-Oss Spongiosa was compared on the model of a critical-size circular defect in the alveolar bone of the upper jaw of adult Wistar rats. The defect healing dynamics was assessed using histological, histomorphometrical, and immunohistochemical methods on days 30 and 60. In contrast to demineralized matrix and commercial xenograft, decellularized matrix induces the formation of the new bone tissue by day 60. Decellularized matrix can be considered as a biomaterial for cell-free tissue engineering for alveolar bone restoration in dentistry and maxillofacial surgery.

Decellularized Matrix Induced Spontaneous Odontogenic and Osteogenic Differentiation in Periodontal Cells

The regeneration of periodontal tissues is a decisive factor in the treatment of periodontitis. Currently, to achieve complete periodontal regeneration, many studies have evaluated the effectiveness of decellularized tissue-engineered constructs on periodontal regeneration. We studied the possibilities of osteogenic and odontogenic differentiation of periodontal progenitor and stem cells (SCs) of the periosteum and periodontal ligament, in decellularized tooth matrix (dTM) and periodontal ligament (dPDL), in 2D and 3D culture. The cell culture of periodontal cells without decellularized matrices was used as control. On the 14th day of cultivation of PDLSCs, PSCs, and PDLSCs + PSCs on dTM and/or dPDL scaffolds in 2D conditions, in all scaffold variants, a dense monolayer of spindle-shaped cells was intensely stained for markers of osteogenic differentiation, such as osteopontin and osteocalcin. Periodontal cells in the collagen I hydrogel (3D-dimensional culture) were more diverse in shape and, in combination of dTM and dPDL, in addition to osteogenic expression, expressed dentin sialophosphoprotein, an odontogenic differentiation marker. Thus, collagen I hydrogel contributed to the formation of conditions similar to those in vivo, and the combination of dTM with dPDL apparently formed a microenvironment that promoted osteogenic and odontogenic differentiation of periodontal cells.

Preliminary Study on the Antigen-Removal from Extracellular Matrix via Different Decellularization

Due to the abundance of bioactive components, surficial decoration with cell-derived extracellular matrix (ECM) is a promising strategy to improve the biological functionality of the tissue engineering scaffolds. However, decellularization is necessary to remove antigenic components in the ECM that may trigger adverse immune response. Freeze-thaw (FT) cycles and treatment with Triton X-100/ammonium hydroxide (TN) are two commonly used decellularization methods for ECM, but their effects on both growth factor retention and antigen removal are still controversial. The objectives of this study are to compare the preservation of ECM texture and beneficial ingredients and the removal of cellular antigens by these two methods. First, the constructs combined bone marrow mesenchymal stem cell-derived ECM and poly(lactic-co-glycolic acid) (PLGA) membrane are prepared and decellularized using FT and TN treatments. Moreover, the effects of decellularization on the ultrastructure and the composition of ECM-decorated PLGA membrane are compared by scanning electron microscope observation and protein quantification. Furthermore, the ECM deposited on PLGA is stripped off and then implanted subcutaneously in rats, and the host macrophage and local lymphocyte responses were investigated. Finally, ECM-decorated porous PLGA scaffolds are implanted into rat calvarial defects, and the new bone formation is evaluated. Our results showed that both methods effectively removed DNA. TN treatment partially retained collagen, glycosaminoglycan, bone morphogenetic protein-2, and vascular endothelial growth factor, and better preserved structural integrity than FT treatment. ECM implants decellularized by both methods induced a mild host response after subcutaneous implantation. Although the total content of residual DNA in the two ECMs digested by the DNA enzyme seemed to be similar and very low, the interfaces between implanted materials and natural tissues in the TN group recruited lower numbers of CD68⁺ macrophages, CD68⁺CD86⁺ (M1) macrophages, and CD4⁺ T lymphocytes than that in FT group, implying that there exist other ECM antigens to influence immune response besides DNA. Furthermore, ECM-decorated scaffolds decellularized by TN treatment induced greater bone formation than that of bare scaffolds in vivo, demonstrating the effective retention of ECM bioactive components after decellularization. This study showed that TN treatment was a more effective and safer decellularization method than FT cycles. Impact statement Decellularization is a prerequisite for extracellular matrix (ECM) application, but there is still no standard for its selection. This study demonstrated that detergent treatment was more effective than freeze-thaw (FT) cycles in removing ECM antigens besides DNA, and the prepared ECM elicited a milder allogenic immune response, which ensured the safety of ECM. Moreover, detergent better preserved the ECM integrity than FT cycles, and effectively retained growth factors, and the decellularized ECM-decorated scaffolds significantly promoted bone repair, which ensured the effectiveness of ECM. This study provides the theoretical and experimental bases for the decellularization strategy of ECM-modified tissue engineering scaffolds.

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.

Osteogenic Capacity of the Prefabricated Periosteofascial Flap using Vascular Induction with Skeletonized Pedicle Transfer in Rabbit Calvarium

The study investigated the osteogenic capacity of a prefabricated periosteal flap created using only skeletonized pedicle transfer without fascia or muscle for vascular induction in rabbit calvarium. A critical-sized bone defect was made in the parietal bone centered on the sagittal suture, and the demineralized bone matrix was implanted. The periosteofascia over the defect was used as a form of prefabricated periosteofascial flap (PPF group, N=10), conventional periosteofascial flap (CPF group, N=10), and nonvascularized free periosteofascial graft (FPG group, N=6). The prefabricated flap was designed via vascular induction by transferring the central artery and vein of the right auricle onto the periosteofascia for 4 weeks prior to flap elevation. A quantitative comparison of volume restoration and radiodensity in the bone defect and a histological study were performed after 6 weeks of covering the bone defect with periosteofascia. The volume restoration of the bone defect covered with the PPF (43.4%) was not different from that of the CPF (46.2%), but significantly increased compared with that of the FPG (24.6%). The radiodensity of the bone defect covered with the PPF (-186.3 HU) was not different from that of the CPF (-153.6 HU), but significantly increased compared with that of the FPG (-329.8 HU). The results were based on adequate vascular development of the periosteum and were closely related to the osteogenic changes in the implanted demineralized bone matrix (DBM). In conclusion, even in the PPF created by transferring only skeletonized vascular pedicles, the osteogenic capacity of the periosteofascial flap is well maintained.

The optimization of pepsin processing time to balance the mechanical and biological properties of xenogeneic bone

Xenogeneic bone showed great prospects to treat large bone defects due to its bionic composition and structure, but the immunogenicity limited its wide applications. Previously, we developed a pepsin treating method to eliminate the immunogenicity of xenogeneic bone. In this study, we further investigated the effect of pepsin processing time on the biological and mechanical properties. The results indicated that increased pepsin treating time impaired the mechanical properties of xenogeneic bone. And MC3T3-E1 cells showed enhanced adhesion ability, as well as increased production of alkaline phosphatase and calcium nodulus production on the xenogeneic bone processed by pepsin for 24 hr (P24), as compared with xenogeneic bone processed by pepsin for 30 hr (P30) and 36 hr (P36). In addition, we found no significant inflammatory responses after implanting different xenogeneic bone into the intermuscular site of rats. These results suggested that xenogeneic bone processed by pepsin for 24 hr may be a preferable choice when using the xenogeneic bone as biomaterials for further researches.

Hydrogel composite scaffolds with an attenuated immunogenicity component for bone tissue engineering applications

Xenogeneic bones are potential templates for bone regeneration. In this study, decellularized porcine bone powder with attenuated immunogenicity was incorporated into a photocurable hydrogel, gelatin methacryloyl (GelMA), to obtain scaffolds with good mechanical properties for bone tissue engineering. The decellularized bone powder (DCB)-GelMA hybrid scaffolds had higher compressive strength and stiffness values when the DCB content was increased. In vitro evaluations revealed the biocompatibility of these scaffolds. The scaffolds could induce human bone marrow mesenchymal stem cells (hMSCs) to undergo osteogenic differentiation even in the absence of an induction medium. The efficiency of the scaffolds for bone regeneration applications was further evaluated using an in vivo cranial bone defect model in rats. Micro-CT images showed that the hybrid scaffolds with 20% DCB content had the best effect in promoting new bone regeneration. Thus, it was concluded that the DCB-GelMA hybrid scaffolds have high potential in bone tissue engineering applications.

Decellularized matrix could affect the proliferation and differentiation of periodontal ligament stem cells in vitro

OBJECTIVE AND BACKGROUND

Recently, decellularized matrix (DCM) is considered as a new biomaterial for tissue regeneration. To explore the possible application of DCM in periodontal regeneration, the effect of DCM from three different cells on the proliferation and differentiation of human periodontal ligament stem cells (PDLSCs) was investigated.

METHODS

DCM derived from human periodontal ligament cells (PDLCs), dental pulp cells (DPCs), and gingival fibroblasts (GFs) were fabricated using Triton X-100/NH₄ OH combined with DNase I. Allogeneic PDLSCs were cultured on PDLC-DCM, DPC-DCM, and GF-DCM, respectively. The proliferative capacity of PDLSCs was evaluated by PicoGreen assay kit. The expression of alkaline phosphatase (ALP), runt-related transcription factor-2 (RUNX2), osteocalcin (OCN), collagen I (COL1), periostin (POSTN), and cementum protein 1 (CEMP1) were detected by qRT-PCR and western blotting.

RESULTS

PDLC-DCM, DPC-DCM, and GF-DCM had similar and integrated networks of extracellular matrix, as well as significantly decreased DNA content. Compared with control group in which PDLSCs were directly seeded in culture plates, PDLC-DCM, DPC-DCM, and GF-DCM promoted the proliferation of re-seeded PDLSCs. Additionally, PDLSCs on DCM exhibited higher mRNA and protein expression levels of ALP, RUNX2, OCN, and COL1. The expression of POSTN in PDLC-DCM group was significantly higher than control group at both mRNA and protein levels.

CONCLUSIONS

PDLC-DCM, DPC-DCM, and GF-DCM could enhance the proliferation of PDLSCs. PDLC-DCM facilitated osteogenic differentiation and periodontal ligament differentiation of PDLSCs, while DPC-DCM and GF-DCM promoted osteogenic differentiation.

The Therapeutic Potential of Hematopoietic Stem Cells in Bone Regeneration

The repair process of bone fractures is a complex biological mechanism requiring the recruitment and in situ functionality of stem/stromal cells from the bone marrow (BM). BM mesenchymal stem/stromal cells have been widely explored in multiple bone tissue engineering applications, whereas the use of hematopoietic stem cells (HSCs) has been poorly investigated in this context. A reasonable explanation is the fact that the role of HSCs and their combined effect with other elements of the hematopoietic niches in the bone-healing process is still elusive. Therefore, in this review we intend to highlight the influence of HSCs in the bone repair process, mainly through the promotion of osteogenesis and angiogenesis at the bone injury site. For that, we briefly describe the main biological characteristics of HSCs, as well as their hematopoietic niches, while reviewing the biomimetic engineered BM niche models. Moreover, we also highlighted the role of HSCs in translational in vivo transplantation or implantation as promoters of bone tissue repair.

Bone Matrix Non-Collagenous Proteins in Tissue Engineering: Creating New Bone by Mimicking the Extracellular Matrix

Engineering biomaterials that mimic the extracellular matrix (ECM) of bone is of significant importance since most of the outstanding properties of the bone are due to matrix constitution. Bone ECM is composed of a mineral part comprising hydroxyapatite and of an organic part of primarily collagen with the rest consisting on non-collagenous proteins. Collagen has already been described as critical for bone tissue regeneration; however, little is known about the potential effect of non-collagenous proteins on osteogenic differentiation, even though these proteins were identified some decades ago. Aiming to engineer new bone tissue, peptide-incorporated biomimetic materials have been developed, presenting improved biomaterial performance. These promising results led to ongoing research focused on incorporating non-collagenous proteins from bone matrix to enhance the properties of the scaffolds namely in what concerns cell migration, proliferation, and differentiation, with the ultimate goal of designing novel strategies that mimic the native bone ECM for bone tissue engineering applications. Overall, this review will provide an overview of the several non-collagenous proteins present in bone ECM, their functionality and their recent applications in the bone tissue (including dental) engineering field.

Fabrication of Decellularized Engineered Extracellular Matrix through Bioreactor-Based Environment for Bone Tissue Engineering

Extracellular matrix (ECM)-contained grafts can be achieved by decellularization of native bones or synthetic scaffolds. Limitations associated with harvesting the native bone has raised interest in preparing in vitro ECM bioscaffold for bone tissue engineering. Here, we intend to develop an ECM-contained construct via decellularizing an engineered gelatin-coated β-tricalcium phosphate (gTCP) scaffold. In order to find an optimal protocol for decellularization of cell-loaded gTCP scaffolds, they were seeded with buccal fat pad-derived stem cells. Then, four decellularization protocols including sodium dodecyl sulfate, trypsin, Triton X-100, and combined solution methods were compared for the amounts of residual cells and remnant collagen and alteration of scaffold structure. Then, the efficacy of the selected protocol in removing cells from gTCP scaffolds incubated in a rotating and perfusion bioreactor for 24 days was evaluated and compared with static condition using histological analysis. Finally, decellularized scaffolds, reloaded with cells, and their cytotoxicity and osteoinductive capability were evaluated. Complete removal of cells from gTCP scaffolds was achieved from all protocols. However, treatment with Triton X-100 showed significantly higher amount of remnant ECM. Bioreactor-incubated scaffolds possessed greater magnitude of ECM proteins including collagen and glycosaminoglycans. Reseeding the decellularized scaffolds also represented higher osteoinductivity of bioreactor-based scaffolds. Application of Triton X-100 as decellularization protocol and usage of bioreactors are suggested as a suitable technique for designing ECM-contained grafts for bone tissue engineering.

Orthotopic Bone Formation by Streamlined Engineering and Devitalization of Human Hypertrophic Cartilage

Most bones of the human body form and heal through endochondral ossification, whereby hypertrophic cartilage (HyC) is formed and subsequently remodeled into bone. We previously demonstrated that HyC can be engineered from human mesenchymal stromal cells (hMSC), and subsequently devitalized by apoptosis induction. The resulting extracellular matrix (ECM) tissue retained osteoinductive properties, leading to ectopic bone formation. In this study, we aimed at engineering and devitalizing upscaled quantities of HyC ECM within a perfusion bioreactor, followed by in vivo assessment in an orthotopic bone repair model. We hypothesized that the devitalized HyC ECM would outperform a clinical product currently used for bone reconstructive surgery. Human MSC were genetically engineered with a gene cassette enabling apoptosis induction upon addition of an adjuvant. Engineered hMSC were seeded, differentiated, and devitalized within a perfusion bioreactor. The resulting HyC ECM was subsequently implanted in a 10-mm rabbit calvarial defect model, with processed human bone (Maxgraft^®) as control. Human MSC cultured in the perfusion bioreactor generated a homogenous HyC ECM and were efficiently induced towards apoptosis. Following six weeks of in vivo implantation, microcomputed tomography and histological analyses of the defects revealed an increased bone formation in the defects filled with HyC ECM as compared to Maxgraft^®. This work demonstrates the suitability of engineered devitalized HyC ECM as a bone substitute material, with a performance superior to a state-of-the-art commercial graft. Streamlined generation of the devitalized tissue transplant within a perfusion bioreactor is relevant towards standardized and automated manufacturing of a clinical product.

Mineral Distribution Spatially Patterns Bone Marrow Stromal Cell Behavior on Monolithic Bone Scaffolds

Interfaces between soft tissue and bone are characterized by transitional gradients in composition and structure that mediate substantial changes in mechanical properties. For interfacial tissue engineering, scaffolds with mineral gradients have shown promise in controlling osteogenic behavior of seeded bone marrow stromal cells (bMSCs). Previously, we have demonstrated a 'top-down' method for creating monolithic bone-derived scaffolds with patterned mineral distributions similar to native tissue. In the present work, we evaluated the ability of these scaffolds to pattern osteogenic behavior in bMSCs in basic, osteogenic, and chondrogenic biochemical environments. Immunohistochemical (IHC) and histological stains were used to characterize cellular behavior as a function of local mineral content. Alkaline phosphatase, an early marker of osteogenesis, and osteocalcin, a late marker of osteogenesis, were positively correlated with mineral content in basic, osteogenic, and chondrogenic media. The difference in bMSC behavior between the mineralized and demineralized regions was most pronounced in an basic biochemical environment. In the mineralized regions of the scaffold, osteogenic markers were clearly present as early as 4 days in culture. In osteogenic media, osteogenic behavior was observed across the entire scaffold, whereas in chondrogenic media, there was an overall reduction in osteogenic biomarkers. Overall, these results indicate local mineral content of the scaffold plays a key role in spatially patterning bMSC behavior. Our results can be utilized for the development of interfacial tissue engineered scaffolds and understanding the role of local environment in determining bMSC behavior. STATEMENT OF SIGNIFICANCE: Soft tissue-to-bone interfaces, such as tendon-bone, ligament-bone, and cartilage-bone, are ubiquitous in mammalian musculoskeletal systems. These interfacial tissues have distinct, hierarchically-structured gradients of cellular, biochemical, and materials components. Given the complexity of the biological structures, interfacial tissues present unique challenges for tissue engineering. Here, we demonstrate that material-derived cues can spatially pattern osteogenic behavior in bone marrow stromal cells (bMSCs). Specifically, we observed that when the bMSCs are cultured on bone-derived scaffolds with mineral gradients, cells in contact with higher mineral content display osteogenic behavior at earlier times than those on the unmineralized substrate. The ability to pattern the cellular complexity found in native interfaces while maintaining biologically relevant structures is a key step towards creating engineered tissue interfaces.

Mesenchymal stem cells modifications for enhanced bone targeting and bone regeneration

In pathological bone conditions (e.g., osteoporotic fractures or critical size bone defects), increasing the pool of osteoblast progenitor cells is a promising therapeutic approach to facilitate bone healing. Since mesenchymal stem cells (MSCs) give rise to the osteogenic lineage, a number of clinical trials investigated the potential of MSCs transplantation for bone regeneration. However, the engraftment of transplanted cells is often hindered by insufficient oxygen and nutrients supply and the tendency of MSCs to home to different sites of the body. In this review, we discuss various approaches of MSCs transplantation for bone regeneration including scaffold and hydrogel constructs, genetic modifications and surface engineering of the cell membrane aimed to improve homing and increase cell viability, proliferation and differentiation.

Development of a decellularized porcine bone matrix for potential applications in bone tissue regeneration

Aim: The objectives of this study were to develop a new decellularized bone matrix (DBM) and to investigate its effect on the in vitro cell behavior of human bone marrow-derived mesenchymal stem cells (hMSCs), compared with porous β-tricalcium phosphate (β-TCP) scaffolds. Materials & methods: Triton X-100 and deoxycholate sodium solution, combining DNase I and RNase, were used to decellularize porcine bones. The DBM were then characterized by DNA contents and matrix components. hMSCs were then seeded on the DBM and β-TCP scaffolds to study cell behavior. Results: Results showed that most porcine cells were removed and the matrix components of the DBM were maintained. Cell culture results showed that DBM promoted cell attachment and proliferation of hMSCs but did not significantly promote the gene expression of osteogenic genes, compared with β-TCP scaffolds. Conclusion: DBM has similar function on cell behavior to β-TCP scaffolds that have promising potential in bone tissue regeneration.

Adult Stem Cells for Bone Regeneration and Repair

The regeneration of bone fractures, resulting from trauma, osteoporosis or tumors, is a major problem in our super-aging society. Bone regeneration is one of the main topics of concern in regenerative medicine. In recent years, stem cells have been employed in regenerative medicine with interesting results due to their self-renewal and differentiation capacity. Moreover, stem cells are able to secrete bioactive molecules and regulate the behavior of other cells in different host tissues. Bone regeneration process may improve effectively and rapidly when stem cells are used. To this purpose, stem cells are often employed with biomaterials/scaffolds and growth factors to accelerate bone healing at the fracture site. Briefly, this review will describe bone structure and the osteogenic differentiation of stem cells. In addition, the role of mesenchymal stem cells for bone repair/regrowth in the tissue engineering field and their recent progress in clinical applications will be discussed.

Extracellular Matrices to Modulate the Innate Immune Response and Enhance Bone Healing

Extracellular matrices (ECMs) have emerged as promising off-the-shelf products to induce bone regeneration, with the capacity not only to activate osteoprogenitors, but also to influence the immune response. ECMs generated starting from living cells such as mesenchymal stromal cells (MSCs) have the potential to combine advantages of native tissue-derived ECMs (e.g., physiological presentation of multiple regulatory factors) with those of synthetic ECMs (e.g., customization and reproducibility of composition). MSC-derived ECMs could be tailored by enrichment not only in osteogenic cytokines, but also in immunomodulatory factors, to skew the innate immune response toward regenerative processes. After reviewing the different immunoregulatory properties of ECM components, here we propose different approaches to engineer ECMs enriched in factors capable to regulate macrophage polarization, recruit host immune and mesenchymal cells, and stimulate the synthesis of other immunoinstructive cytokines. Finally, we offer a perspective on the possible evolution of the paradigm based on biological and chemico-physical design considerations, and the use of gene editing approaches.

Bone marrow mesenchymal stem cells: Aging and tissue engineering applications to enhance bone healing

Bone has well documented natural healing capacity that normally is sufficient to repair fractures and other common injuries. However, the properties of bone change throughout life, and aging is accompanied by increased incidence of bone diseases and compromised fracture healing capacity, which necessitate effective therapies capable of enhancing bone regeneration. The therapeutic potential of adult mesenchymal stem cells (MSCs) for bone repair has been long proposed and examined. Actions of MSCs may include direct differentiation to become bone cells, attraction and recruitment of other cells, or creation of a regenerative environment via production of trophic growth factors. With systemic aging, MSCs also undergo functional decline, which has been well investigated in a number of recent studies. In this review, we first describe the changes in MSCs during aging and discuss how these alterations can affect bone regeneration. We next review current research findings on bone tissue engineering, which is considered a promising and viable therapeutic solution for structural and functional restoration of bone. In particular, the importance of MSCs and bioscaffolds is highlighted. Finally, potential approaches for the prevention of MSC aging and the rejuvenation of aged MSC are discussed.

Reconstructing Bone with Natural Bone Graft: A Review of In Vivo Studies in Bone Defect Animal Model

Bone defects caused by fracture, disease or congenital defect remains a medically important problem to be solved. Bone tissue engineering (BTE) is a promising approach by providing scaffolds to guide and support the treatment of bone defects. However, the autologous bone graft has many defects such as limited sources and long surgical procedures. Therefore, xenograft bone graft is considered as one of the best substitutions and has been effectively used in clinical practice. Due to better preserved natural bone structure, suitable mechanical properties, low immunogenicity, good osteoinductivity and osteoconductivity in natural bone graft, decellularized and demineralized bone matrix (DBM) scaffolds were selected and discussed in the present review. In vivo animal models provide a complex physiological environment for understanding and evaluating material properties and provide important reference data for clinical trials. The purpose of this review is to outline the in vivo bone regeneration and remodeling capabilities of decellularized and DBM scaffolds in bone defect models to better evaluate the potential of these two types of scaffolds in BTE. Taking into account the limitations of the state-of-the-art technology, the results of the animal bone defect model also provide important information for future design of natural bone composite scaffolds.

3D Bone Biomimetic Scaffolds for Basic and Translational Studies with Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) are recognized as an attractive tool owing to their self-renewal and differentiation capacity, and their ability to secrete bioactive molecules and to regulate the behavior of neighboring cells within different tissues. Accumulating evidence demonstrates that cells prefer three-dimensional (3D) to 2D culture conditions, at least because the former are closer to their natural environment. Thus, for in vitro studies and in vivo utilization, great effort is being dedicated to the optimization of MSC 3D culture systems in view of achieving the intended performance. This implies understanding cell–biomaterial interactions and manipulating the physicochemical characteristics of biomimetic scaffolds to elicit a specific cell behavior. In the bone field, biomimetic scaffolds can be used as 3D structures, where MSCs can be seeded, expanded, and then implanted in vivo for bone repair or bioactive molecules release. Actually, the union of MSCs and biomaterial has been greatly improving the field of tissue regeneration. Here, we will provide some examples of recent advances in basic as well as translational research about MSC-seeded scaffold systems. Overall, the proliferation of tools for a range of applications witnesses a fruitful collaboration among different branches of the scientific community.

Injectable cartilaginous template transformed BMSCs into vascularized bone

Regeneration of alveolar bone for dental implant remains a major issue, partifcularly for patients suffering from severe bone adsorption and irregular socket trauma. Recapitulating embryological development is becoming an attractive approach for engineer organ or three-dimensional tissues from stem cells. In this study, we aimed to develop an injectable “cartilaginous” graft with adequate mechanical resistance and ideal bone remodelling potential. The cartilaginous graft was composed of a particulate decellularised cartilage matrix (PDCM), chondrogenically primed bone mesenchymal stem cell (BMSC) bricks (CB), and enriched platelet-rich plasma (P) gel. In immunodeficient mice, we found that angiogenesis occurred quickly inside PDCM-CB-P constructs after implantation, thereby improving tissue survival and bone formation. In rabbit tibia bone defects around implants, we confirmed that CBs not only transformed into bone tissue rapidly, but also significantly promoted bone remodelling and replacement of PDCM, thus realising osseointegration of dental implants within 3 months. In conclusion, CBs exhibited the potential for endochondral ossification in vivo, and application of a cartilaginous template composed of PDCM, CB, and P provided a minimally-invasive, “free material residual” approach to regenerate alveolar bone tissues in vivo. This method could have applications in peri-implant bone regeneration.

The effect of decellularized tissue engineered constructs on periodontal regeneration

AIM

To evaluate the effect of decellularized tissue engineered constructs on cell differentiation in vitro and periodontal regeneration in vivo.

MATERIALS AND METHODS

Periodontal ligament cell (PDLC) sheets were loaded on polycaprolactone (PCL) scaffolds and then decellularized. Constructs were assessed for their effect on allogenic PDLC and mesenchymal stem cell (MSC) differentiation in vitro, as evaluated by gene expression of bone and periodontal ligament tissue markers post-seeding. Expression of MSC marker STRO-1 was assessed by immunostaining. Decellularized constructs were evaluated in a rat periodontal defect model to assess their biocompatibility and tissue integration. Microcomputed topography (μCT) and histological assessment were performed to assess the regenerative potential of the constructs at 2 and 4 weeks postoperatively.

RESULTS

There was upregulation of bone marker gene expression by PDLCs especially on the 14th day. MSCs lacked bone markers expression, but showed increased collagen I marker expression on day 14. STRO-1 expression by the MSCs decreased over the three timepoints when seeded on decellularized sheets. Histological assessment demonstrated the biocompatibility of the decellularized constructs in vivo. More new attachment formation was observed on the decellularized constructs compared to scaffold only controls.

CONCLUSION

Decellularized tissue engineered constructs are capable of inducing cell differentiation in vitro and have the potential to facilitate periodontal regeneration in vivo.

Decellularized materials derived from TSP2-KO mice promote enhanced neovascularization and integration in diabetic wounds

Decellularized biologic scaffolds are gaining popularity over synthetic biomaterials as naturally derived materials capable of promoting improved healing. Nevertheless, the most widely used biologic material - acellular dermal matrix (ADM) - exhibits slow repopulation and remodeling, which prevents integration. Additionally, engineering control of these materials is limited because they require a natural source for their production. In the current report, we demonstrate the feasibility of using genetically engineered animals to create decellularized biologic scaffolds with favorable extracellular matrix (ECM) properties. Specifically, we utilized skin from thrombospondin (TSP)-2 KO mice to derive various decellularized products. Scanning electron microscopy and mechanical testing showed that TSP-2 KO ADM exhibited an altered structure and a reduction in elastic modulus and ultimate tensile strength, respectively. When a powdered form of KO ADM was implanted subcutaneously, it was able to promote enhanced vascularization over WT. Additionally, when implanted subcutaneously, intact slabs of KO ADM were populated by higher number of host cells when compared to WT. In vitro studies confirmed the promigratory properties of KO ADM. Specifically, degradation products released by pepsin digestion of KO ADM induced greater cell migration than WT. Moreover, cell-derived ECM from TSP-2 null fibroblasts was more permissive to fibroblast migration. Finally, ADMs were implanted in a diabetic wound model to examine their ability to accelerate wound healing. KO ADM exhibited enhanced remodeling and vascular maturation, indicative of efficient integration. Overall, we demonstrate that genetic manipulation enables engineered ECM-based materials with increased regenerative potential.

Porous Heat-Treated Polyacrylonitrile Scaffolds for Bone Tissue Engineering

Heat-treated polyacrylonitrile (HT-PAN), also referred to as black orlon (BO), is a promising carbon-based material used for applications in tissue engineering and regenerative medicine. To the best of our knowledge, no such complex bone morphology-mimicking three-dimensional (3D) BO structure has been reported to date. We report that BO can be easily made into 3D cryogel scaffolds with porous structures, using succinonitrile as a porogen. The cryogels possess a porous morphology, similar to bone tissue. The prepared scaffolds showed strong osteoconductive activity, providing excellent support for the adhesion, proliferation, and mitochondrial activity of human bone-derived cells. This effect was more apparent in scaffolds prepared from a matrix with a higher content of PAN (i.e., 10% rather than 5%). The scaffolds with 10% of PAN also showed enhanced mechanical properties, as revealed by higher compressive modulus and higher compressive strength. Therefore, these scaffolds have a robust potential for use in bone tissue engineering.

Directing osteogenic differentiation of BMSCs by cell-secreted decellularized extracellular matrixes from different cell types

Cell-secreted decellularized extracellular matrixes (D-ECM) are promising for conferring bioactivity and directing cell fate to facilitate tissue regeneration.

Prefabrication of a functional bone graft with a pedicled periosteal flap as an in vivo bioreactor

The in vivo bioreactor principle, which focuses on using the body as a living bioreactor to cultivate stem cells, bioscaffolds, and growth factors and leveraging the body’s self-regenerative capacity to regenerate new tissue, has been considered a potential approach for bone defect reconstruction. The histological characteristics of the periosteum allow it to possess a remarkable capacity to induce bone growth and remodeling, making it suitable as an in vivo bioreactor strategy for bone graft prefabrication. The present study was designed to prefabricate vascularized bone grafts using pedicled periosteal flaps and decellularized bone matrix (DBM) scaffolds in a rabbit model. The muscular pouches created in the femoral muscle were acted as a control. Our histological results revealed that both the periosteal flap group and muscular pouch group induced bone tissue formation on the DBM surface at both 8 and 16 weeks postoperatively. However, micro-computed tomography (microCT) scanning, biomechanical, and histomorphometric findings indicated that bone grafts from the periosteal flap group showed larger bone mass, faster bone formation rates, higher vascular density, and stronger biomechanical properties than in the muscular pouch group. We suggest that using the pedicled periosteal flap as an in vivo bioreactor is a promising approach for functional bone graft prefabrication.

"Tissue Papers" from Organ-Specific Decellularized Extracellular Matrices

Using an innovative, tissue-independent approach to decellularized tissue processing and biomaterial fabrication, the development of a series of "tissue papers" derived from native porcine tissues/organs (heart, kidney, liver, muscle), native bovine tissue/organ (ovary and uterus), and purified bovine Achilles tendon collagen as a control from decellularized extracellular matrix particle ink suspensions cast into molds is described. Each tissue paper type has distinct microstructural characteristics as well as physical and mechanical properties, is capable of absorbing up to 300% of its own weight in liquid, and remains mechanically robust (E = 1-18 MPa) when hydrated; permitting it to be cut, rolled, folded, and sutured, as needed. In vitro characterization with human mesenchymal stem cells reveals that all tissue paper types support cell adhesion, viability, and proliferation over four weeks. Ovarian tissue papers support mouse ovarian follicle adhesion, viability, and health in vitro, as well as support, and maintain the viability and hormonal function of nonhuman primate and human follicle-containing, live ovarian cortical tissues ex vivo for eight weeks postmortem. "Tissue papers" can be further augmented with additional synthetic and natural biomaterials, as well as integrated with recently developed, advanced 3D-printable biomaterials, providing a versatile platform for future multi-biomaterial construct manufacturing.

A model system for developing a tissue engineered meniscal enthesis

The meniscus acts as a stabilizer, lubricator, and load distributer in the knee joint. The mechanical stability of the meniscus depends on its connection to the underlying bone by a fibrocartilage to bone transition zone called the meniscal enthesis. Tissue engineered menisci hold great promise as a treatment alternative however lack a means of integrated fixation to the underlying bone needed in order for a tissue engineered meniscal replacement to be successful. Tissue engineering the meniscal enthesis is a difficult task given the complex gradients of cell type, mineral, and extracellular matrix molecules. Therefore, there is a need for a simplified and high throughput enthesis model to test experimental parameters. The goal of this study was to develop a simplified enthesis model to test collagen integration with decellularized bone. We found that injection molding collagen into tubing loaded with decellularized bone plugs resulted in a scaffold with three regions: bone, bone-collagen, and collagen. Furthermore, collagen formation was directed in the axial direction by using mechanical fixation at the bony ends. The results of this study showed that this technique can be used to mimic the native enthesis morphology and serves as ideal test platform to generate a model tissue engineered enthesis.

STATEMENT OF SIGNIFICANCE

The meniscal enthesis is a complex structure that is essential to mechanical stability of the meniscus and the knee joint. Several studies document the development of anatomically shaped tissue engineered meniscus constructs, but none have focused on how to integrate such tissues with underlying bone. This study establishes a simplified construct to model the meniscal enthesis composed of a collagen gel seeded with meniscal fibrochondrocytes integrated with decellularized cancellous bone. Mechanical fixation at the bony ends induced tissue integration of fibers into the bony tissue, which is critical for mechanical performance and has yet to be shown in enthesis literature. Our test platform is amenable to targeted experiments investigating mineralization gradients, collagen fiber alignment, cell population phenotype, and media conditioning with experimental impact on enthesis studies for meniscus, tendon, and ligament.

Comparative proteomic profiling of human osteoblast-derived extracellular matrices identifies proteins involved in mesenchymal stromal cell osteogenic differentiation and mineralization

The extracellular matrix (ECM) is a dynamic component of tissue architecture that physically supports cells and actively influences their behavior. In the context of bone regeneration, cell-secreted ECMs have become of interest as they reproduce tissue-architecture and modulate the promising properties of mesenchymal stem cells (MSCs). We have previously created an in vitro model of human osteoblast-derived devitalized ECM that was osteopromotive for MSCs. The aim of this study was to identify ECM regulatory proteins able to modulate MSC differentiation to broaden the spectrum of MSC clinical applications. To this end, we created two additional models of devitalized ECMs with different mineralization phenotypes. Our results showed that the ECM derived from osteoblast-differentiated MSCs had increased osteogenic potential compared to ECM derived from undifferentiated MSCs and non-ECM cultures. Proteomic analysis revealed that structural ECM proteins and ribosomal proteins were upregulated in the ECM from undifferentiated MSCs. A similar response profile was obtained by treating osteoblast-differentiating MSCs with Activin-A. Extracellular proteins were upregulated in Activin-A ECM, whereas mitochondrial and membrane proteins were downregulated. In summary, this study illustrates that the composition of different MSC-secreted ECMs is important to regulate the osteogenic differentiation of MSCs. These models of devitalized ECMs could be used to modulate MSC properties to regulate bone quality.

Contrast-Enhanced Microtomographic Characterisation of Vessels in Native Bone and Engineered Vascularised Grafts Using Ink-Gelatin Perfusion and Phosphotungstic Acid

Objectives

Bone ischemia and necrosis are challenging to treat, requiring investigation of native and engineered bone revascularisation processes through advanced imaging techniques. This study demonstrates an experimental two-step method for precise bone and vessel analysis in native bones or vascularised bone grafts using X-ray microtomography (μCT), without interfering with further histological processing.

Methods

Distally ligated epigastric arteries or veins of 6 nude rats were inserted in central channels of porous hydroxyapatite cylinders and these pedicled grafts were implanted subcutaneously. One week later, the rats were perfused with ink-gelatin and euthanised and the femurs, tibias, and grafts were explanted. Samples were scanned using μCT, decalcified, incubated with phosphotungstic acid (PTA) for contrast enhancement, rescanned, and processed histologically.

Results

Contrast-enhanced μCT displayed the course and branching of native bone vessels. Histologically, both central (-17%) and epiphyseal vessels (-58%) appeared smaller than in μCT scans. Hydroxyapatite cylinders were thoroughly vascularised but did not display bone formation. Grafts with a central artery had more (+58%) and smaller (-52%) vessel branches compared to grafts with a vein.

Conclusions

We present a relatively inexpensive and easy-to-perform two-step method to analyse bone and vessels by μCT, suitable to assess a variety of bone-regenerative strategies.

Generation of a Bone Organ by Human Adipose-Derived Stromal Cells Through Endochondral Ossification

: Recapitulation of endochondral ossification (ECO) (i.e., generation of marrow-containing ossicles through a cartilage intermediate) has relevance to develop human organotypic models for bone or hematopoietic cells and to engineer grafts for bone regeneration. Unlike bone marrow-derived stromal cells (also known as bone marrow-derived mesenchymal stromal/stem cells), adipose-derived stromal cells (ASC) have so far failed to form a bone organ by ECO. The goal of the present study was to assess whether priming human ASC to a defined stage of chondrogenesis in vitro allows their autonomous ECO upon ectopic implantation. ASC were cultured either as micromass pellets or into collagen sponges in chondrogenic medium containing transforming growth factor-β3 and bone morphogenetic protein-6 for 4 weeks (early hypertrophic templates) or for two additional weeks in medium supplemented with β-glycerophosphate, l-thyroxin, and interleukin1-β to induce hypertrophic maturation (late hypertrophic templates). Constructs were implanted in vivo and analyzed after 8 weeks. In vitro, ASC deposited cartilaginous matrix positive for glycosaminoglycans, type II collagen, and Indian hedgehog. Hypertrophic maturation induced upregulation of type X collagen, bone sialoprotein, and matrix metalloproteinase13 (MMP13). In vivo, both early and late hypertrophic templates underwent cartilage remodeling, as assessed by MMP13- and tartrate-resistant acid phosphatase-positive staining, and developed bone ossicles, including bone marrow elements, although to variable degrees of efficiency. In situ hybridization for human-specific sequences and staining with a human specific anti-CD146 antibody demonstrated the direct contribution of ASC to bone and stromal tissue formation. In conclusion, despite their debated skeletal progenitor nature, human ASC can generate bone organs through ECO when suitably primed in vitro.

SIGNIFICANCE

Recapitulation of endochondral ossification (ECO) (i.e., generation of marrow-containing ossicles through a cartilage intermediate) has relevance to develop human organotypic models for bone or hematopoietic cells and to engineer grafts for bone regeneration. This study demonstrated that expanded, human adult adipose-derived stromal cells can generate ectopic bone through ECO, as previously reported for bone marrow stromal cells. This system can be used as a model in a variety of settings for mimicking ECO during development, physiology, or pathology (e.g., to investigate the role of BMPs, their receptors, and signaling pathways). The findings have also translational relevance in the field of bone regeneration, which, despite several advances in the domains of materials and surgical techniques, still faces various limitations before being introduced in the routine clinical practice.

Fat-Derived Stromal Vascular Fraction Cells Enhance the Bone-Forming Capacity of Devitalized Engineered Hypertrophic Cartilage Matrix

: Engineered and devitalized hypertrophic cartilage (HC) has been proposed as bone substitute material, potentially combining the features of osteoinductivity, resistance to hypoxia, capacity to attract blood vessels, and customization potential for specific indications. However, in comparison with vital tissues, devitalized HC grafts have reduced efficiency of bone formation and longer remodeling times. We tested the hypothesis that freshly harvested stromal vascular fraction (SVF) cells from human adipose tissue-which include mesenchymal, endothelial, and osteoclastic progenitors-enhance devitalized HC remodeling into bone tissue. Human SVF cells isolated from abdominal lipoaspirates were characterized cytofluorimetrically. HC pellets, previously generated by human bone marrow-derived stromal cells and devitalized by freeze/thaw, were embedded in fibrin gel with or without different amounts of SVF cells and implanted either ectopically in nude mice or in 4-mm-diameter calvarial defects in nude rats. In the ectopic model, SVF cells added to devitalized HC directly contributed to endothelial, osteoblastic, and osteoclastic populations. After 12 weeks, the extent of graft vascularization and amount of bone formation increased in a cell-number-dependent fashion (up to, respectively, 2.0-fold and 2.9-fold using 12 million cells per milliliter of gel). Mineralized tissue volume correlated with the number of implanted, SVF-derived endothelial cells (CD31+ CD34+ CD146+). In the calvarial model, SVF activation of HC using 12 million cells per milliliter of gel induced efficient merging among implanted pellets and strongly enhanced (7.3-fold) de novo bone tissue formation within the defects. Our findings outline a bone augmentation strategy based on off-the-shelf devitalized allogeneic HC, intraoperatively activated with autologous SVF cells.

SIGNIFICANCE

This study validates an innovative bone substitute material based on allogeneic hypertrophic cartilage that is engineered, devitalized, stored, and clinically used, together with autologous cells, intraoperatively derived from a lipoaspirate. The strategy was tested using human cells in an ectopic model and an orthotopic implantation model, in immunocompromised animals.

Advancing biomaterials of human origin for tissue engineering

Biomaterials have played an increasingly prominent role in the success of biomedical devices and in the development of tissue engineering, which seeks to unlock the regenerative potential innate to human tissues/organs in a state of deterioration and to restore or reestablish normal bodily function. Advances in our understanding of regenerative biomaterials and their roles in new tissue formation can potentially open a new frontier in the fast-growing field of regenerative medicine. Taking inspiration from the role and multi-component construction of native extracellular matrices (ECMs) for cell accommodation, the synthetic biomaterials produced today routinely incorporate biologically active components to define an artificial in vivo milieu with complex and dynamic interactions that foster and regulate stem cells, similar to the events occurring in a natural cellular microenvironment. The range and degree of biomaterial sophistication have also dramatically increased as more knowledge has accumulated through materials science, matrix biology and tissue engineering. However, achieving clinical translation and commercial success requires regenerative biomaterials to be not only efficacious and safe but also cost-effective and convenient for use and production. Utilizing biomaterials of human origin as building blocks for therapeutic purposes has provided a facilitated approach that closely mimics the critical aspects of natural tissue with regard to its physical and chemical properties for the orchestration of wound healing and tissue regeneration. In addition to directly using tissue transfers and transplants for repair, new applications of human-derived biomaterials are now focusing on the use of naturally occurring biomacromolecules, decellularized ECM scaffolds and autologous preparations rich in growth factors/non-expanded stem cells to either target acceleration/magnification of the body's own repair capacity or use nature's paradigms to create new tissues for restoration. In particular, there is increasing interest in separating ECMs into simplified functional domains and/or biopolymeric assemblies so that these components/constituents can be discretely exploited and manipulated for the production of bioscaffolds and new biomimetic biomaterials. Here, following an overview of tissue auto-/allo-transplantation, we discuss the recent trends and advances as well as the challenges and future directions in the evolution and application of human-derived biomaterials for reconstructive surgery and tissue engineering. In particular, we focus on an exploration of the structural, mechanical, biochemical and biological information present in native human tissue for bioengineering applications and to provide inspiration for the design of future biomaterials.

Directing immunomodulation using biomaterials for endogenous regeneration

Stem cell therapy and tissue engineering hold considerable potential for innovative and transformative strategies to repair damaged tissue form and function. Although many approaches are adopting ex vivo expanded cells for transplantation, an alternative is to manipulate the biomaterial-host interactions that recruit the patients' own stem cells endogenously for regeneration. There are several considerations in targeting the biomaterial-host interactions therapeutically, not the least of which is the biomimetic design of extracellular matrix (ECM)-mimicking materials and the administration of navigation cues and small molecules that target specific aspects of the native healing cascades to stimulate homing of endogenous stem cells and, thereafter, their expansion and differentiation. A sequence of coordinated interactions between the local niche cells and implanted biomaterials offers signals and sign posts that may instruct the cells traveling toward the injured tissues. Furthermore, stem cell function is critically influenced by extrinsic signals provided by the niche as well as by the implanted biomaterials. Novel strategies harnessing growth factors and immunological cues to design materials not only can modulate the behavior of stem cells but also can alter innate and adaptive immunity in a controlled manner. We envisage that successful and safe endogenous regeneration will involve at least three aspects, i.e., homing of sufficient stem cells, controlling cell fate determination, and blunting host immune responses to outside biomaterial devices. Improving our understanding of the biological and physicochemical signals of biomimetic biomaterials that govern immunomodulation for in situ tissue regeneration, particularly context-dependent macrophage (Mφ) polarization, will lead to a concurrent improvement in clinical outcomes.

Native extracellular matrix preserves mesenchymal stem cell "stemness" and differentiation potential under serum-free culture conditions

Introduction

Bone marrow-derived mesenchymal stem cells (BM-MSCs) for clinical use should not be grown in media containing fetal bovine serum (FBS), because of serum-related concerns over biosafety and batch-to-batch variability. Previously, we described the preparation and use of a cell-free native extracellular matrix (ECM) made by bone marrow cells (BM-ECM) which preserves stem cell properties and enhances proliferation. Here, we compare colony-forming ability and differentiation of MSCs cultured on BM-ECM with a commercially available matrix (CELLstart™) and tissue culture plastic (TCP) under serum-free conditions.

Methods

Primary MSCs from freshly isolated bone marrow-derived mononuclear cells or passaged MSCs (P1) were grown in serum-containing (SCM) or serum-free (SFM) media on BM-ECM, CELLstart™, or TCP substrates. Proliferation, cell composition (phenotype), colony-forming unit replication, and bone morphogenetic protein-2 (BMP-2) responsiveness were compared among cells maintained on the three substrates.

Results

Proliferation of primary BM-MSCs was significantly higher in SCM than SFM, irrespectively of culture substrate, suggesting that the expansion of these cells requires SCM. In contrast, passaged cells cultured on BM-ECM or CELLstart™ in SFM proliferated to nearly the same extent as cells in SCM. However, morphologically, those on BM-ECM were smaller and more aligned, slender, and long.

Cells grown for 7 days on BM-ECM in SFM were 20–40 % more positive for MSC surface markers than cells cultured on CELLstart™. Cells cultured on TCP contained the smallest number of cells positive for MSC markers. MSC colony-forming ability in SFM, as measured by CFU-fibroblasts, was increased 10-, 9-, and 2-fold when P1 cells were cultured on BM-ECM, CELLstart™, and TCP, respectively. Significantly, CFU-adipocyte and -osteoblast replication of cells grown on BM-ECM was dramatically increased over those on CELLstart™ (2X) and TCP (4-7X). BM-MSCs, cultured in SFM and treated with BMP-2, retained their differentiation capacity better on BM-ECM than on either of the other two substrates.

Conclusions

Our findings indicate that BM-ECM provides a unique microenvironment that supports the colony-forming ability of MSCs in SFM and preserves their stem cell properties. The establishment of a robust culture system, combining native tissue-specific ECM and SFM, provides an avenue for preparing significant numbers of potent MSCs for cell-based therapies in patients.