Cell based advanced therapeutic medicinal products for bone repair: Keep it simple?

A comprehensive review on using injectable chitosan microgels for osteochondral tissue repair

Restoring cartilage to healthy state is challenging due to low cell density and hence low regenerative capacity. The current platforms are not compatible with clinical translation and require dedicated handling of trained personnel. However, by engineering and implanting cell microaggregates in higher concentrations, efficient formation of new cartilage can be achieved, even in the absence of exogenous growth factors. Therefore, one-step surgeries are preferable for novel treatments and we need cell laden microgels allowing the formation of microaggregaets in vivo. Injectability is a key parameter for in situ forming the shape and minimally invasive clinical applications. Hydrogels as bioinks can restore damaged tissues to their primary shape. Chitosan is a polysaccharide derived from chitin with abundant usage in tissue engineering. This review highlights the use of chitosan as an injectable hydrogel for osteochondral defects. Several studies focused on encapsulating mesenchymal stem cells within chitosan hydrogels have been categorized and incorporating microfluidic devices has been identified in the forefront to form microgels. Additionally, the printability is another convenience of chitosan for using in 3D printing for cartilage tissue engineering which is described in this review.

Site-specific periosteal cells with distinct osteogenic and angiogenic characteristics

OBJECTIVES

This study aimed to investigate the site-specific characteristics of rat mandible periosteal cells (MPCs) and tibia periosteal cells (TPCs) to assess the potential application of periosteal cells (PCs) in bone tissue engineering (BTE).

MATERIALS AND METHODS

MPCs and TPCs were isolated and characterized. The potential of proliferation, migration, osteogenesis and adipogenesis of MPCs and TPCs were evaluated by CCK-8, scratch assay, Transwell assay, alkaline phosphatase staining and activity, Alizarin Red S staining, RT‒qPCR, and Western blot (WB) assays, respectively. Then, these cells were cocultured with human umbilical vein endothelial cells (HUVECs) to investigate their angiogenic capacity, which was assessed by scratch assay, Transwell assay, Matrigel tube formation assay, RT‒qPCR, and WB assays.

RESULTS

MPCs exhibited higher osteogenic potential, higher alkaline phosphatase activity, and more mineralized nodule formation, while TPCs showed a greater capability for proliferation, migration, and adipogenesis. MPCs showed higher expression of angiogenic factors, and the conditioned medium of MPCs accelerated the migration of HUVECs, while MPC- conditioned medium induced the formation of more tubular structure in HUVECs in vitro. These data suggest that compared to TPCs, MPCs exert more consequential proangiogenic effects on HUVECs.

CONCLUSIONS

PCs possess skeletal site-specific differences in biological characteristics. MPCs exhibit more eminent osteogenic and angiogenic potentials, which highlights the potential application of MPCs for BTE.

CLINICAL RELEVANCE

Autologous bone grafting as the main modality for maxillofacial bone defect repair has many limitations. Constituting an important cell type in bone repair and regeneration, MPCs show greater potential for application in BTE, which provides a promising treatment option for maxillofacial bone defect repair.

The Evolving Landscape of Potency Assays

There is a "goldilocks" aspect to potency assays. On the one hand, a comprehensive evaluation of the cell product with detailed quantitative measurement of the critical quality attribute/s of the desired biological activity is required. On the other hand, the potency assay benefits from simplification and lean approaches that avoid unnecessary complication and enhance robustness, to provide a reproducible and scalable product. There is a need to balance insightful knowledge of complex biological healing processes with straightforward manufacture of an advanced therapeutic medicinal product (ATMP) that can be administered in a trustworthy cost-effective manner. While earlier chapters within this book have highlighted numerous challenges facing the potency assay conundrum, this chapter offers a forward-looking perspective regarding the many recent advances concerning acellular products, cryopreservation, induced MSC, cell priming, nanotechnology, 3D culture, regulatory guidelines and evolving institutional roles, that are likely to facilitate potency assay development in the future.

Omentin‑1 induces osteoblast viability and differentiation via the TGF‑β/Smad signaling pathway in osteoporosis

Osteoporosis is a bone-related disease that results from impaired bone formation and excessive bone resorption. The potential value of adipokines has been investigated previously, due to their influence on osteogenesis. However, the osteogenic effects induced by omentin-1 remain unclear. The aim of the present study was to determine the regulatory effects of omentin-1 on osteoblast viability and differentiation, as well as to explore the underlying molecular mechanism. The present study investigated the effects of omentin-1 on the viability and differentiation of mouse pre-osteoblast cells (MC3T3-E1) using quantitative and qualitative measures. A Cell Counting Kit-8 assay was used to assess the viability of MC3T3-E1 cells following treatment with different doses of omentin-1. Omentin-1 and bone morphogenetic protein (BMP) inhibitor were added to osteogenic induction mediums in different ways to assess their effect. The alkaline phosphatase (ALP) activity and Alizarin Red S (ARS) staining of MC3T3-E1 cells treated with omentin-1 and/or BMP inhibitor were used to examine the effects of omentin-1 on differentiation and mineralization. Western blotting was used to further explore its potential mechanism, and to study the role of omentin-1 on the viability and differentiation of osteoblasts. The results showed that omentin-1 altered the viability of MC3T3-E1 cells in a dose-dependent manner. Omentin-1 treatment significantly increased the expression of members of the TGF-β/Smad signaling pathway. In the omentin-1 group, the ALP activity of the MC3T3-E1 cells was increased, and the ARS staining area was also increased. The mRNA and protein expression levels of BMP2, Runt-related transcription factor 2, collagen1, osteopontin, osteocalcin and osterix in the omentin-1 group were also significantly upregulated. All these effects were reversed following treatment with SIS3 HCl. These results demonstrated that omentin-1 can significantly promote osteoblast viability and differentiation via the TGF-β/Smad signaling pathway, thereby promoting bone formation and preventing osteoporosis.

Periosteal Tissue Engineering: Current Developments and Perspectives

Periosteum, a highly vascularized bilayer connective tissue membrane plays an indispensable role in the repair and regeneration of bone defects. It is involved in blood supply and delivery of progenitor cells and bioactive molecules in the defect area. However, sources of natural periosteum are limited, therefore, there is a need to develop tissue-engineered periosteum (TEP) mimicking the composition, structure, and function of natural periosteum. This review explores TEP construction strategies from the following perspectives: i) different materials for constructing TEP scaffolds; ii) mechanical properties and surface topography in TEP; iii) cell-based strategies for TEP construction; and iv) TEP combined with growth factors. In addition, current challenges and future perspectives for development of TEP are discussed.

The Bone-Forming Properties of Periosteum-Derived Cells Differ Between Harvest Sites

The development of alternatives for autologous bone grafts is a major focus of bone tissue engineering. To produce living bone-forming implants, skeletal stem and progenitor cells (SSPCs) are envisioned as key ingredients. SSPCs can be obtained from different tissues including bone marrow, adipose tissue, dental pulp, and periosteum. Human periosteum-derived cells (hPDCs) exhibit progenitor cell characteristics and have well-documented in vivo bone formation potency. Here, we have characterized and compared hPDCs derived from tibia with craniofacial hPDCs, from maxilla and mandible, respectively, each representing a potential source for cell-based tissue engineered implants for craniofacial applications. Maxilla and mandible-derived hPDCs display similar growth curves as tibial hPDCs, with equal trilineage differentiation potential toward chondrogenic, osteogenic, and adipogenic cells. These craniofacial hPDCs are positive for SSPC-markers CD73, CD164, and Podoplanin (PDPN), and negative for CD146, hematopoietic and endothelial lineage markers. Bulk RNA-sequencing identified genes that are differentially expressed between the three sources of hPDC. In particular, differential expression was found for genes of the HOX and DLX family, for SOX9 and genes involved in skeletal system development. The in vivo bone formation, 8 weeks after ectopic implantation in nude mice, was observed in constructs seeded with tibial and mandibular hPDCs. Taken together, we provide evidence that hPDCs show different profiles and properties according to their anatomical origin, and that craniofacial hPDCs are potential sources for cell-based bone tissue engineering strategies. The mandible-derived hPDCs display - both in vitro and in vivo - chondrogenic and osteogenic differentiation potential, which supports their future testing for use in craniofacial bone regeneration applications.

3D-Printing of Hierarchically Designed and Osteoconductive Bone Tissue Engineering Scaffolds

In Bone Tissue Engineering (BTE), autologous bone-regenerative cells are combined with a scaffold for large bone defect treatment (LBDT). Microporous, polylactic acid (PLA) scaffolds showed good healing results in small animals. However, transfer to large animal models is not easily achieved simply by upscaling the design. Increasing diffusion distances have a negative impact on cell survival and nutrition supply, leading to cell death and ultimately implant failure. Here, a novel scaffold architecture was designed to meet all requirements for an advanced bone substitute. Biofunctional, porous subunits in a load-bearing, compression-resistant frame structure characterize this approach. An open, macro- and microporous internal architecture (100 µm-2 mm pores) optimizes conditions for oxygen and nutrient supply to the implant's inner areas by diffusion. A prototype was 3D-printed applying Fused Filament Fabrication using PLA. After incubation with Saos-2 (Sarcoma osteogenic) cells for 14 days, cell morphology, cell distribution, cell survival (fluorescence microscopy and LDH-based cytotoxicity assay), metabolic activity (MTT test), and osteogenic gene expression were determined. The adherent cells showed colonization properties, proliferation potential, and osteogenic differentiation. The innovative design, with its porous structure, is a promising matrix for cell settlement and proliferation. The modular design allows easy upscaling and offers a solution for LBDT.

Biomimetic Biomolecules in Next Generation Xeno-Hybrid Bone Graft Material Show Enhanced In Vitro Bone Cells Response

Bone defects resulting from trauma, disease, surgery or congenital malformations are a significant health problem worldwide. Consequently, bone is the second most transplanted tissue just after blood. Although bone grafts (BGs) have been used for decades to improve bone repairs, none of the currently available BGs possesses all the desirable characteristics. One way to overcome such limitations is to introduce the feature of controlled release of active bone-promoting biomolecules: however, the administration of, e.g., recombinant Bone morphogenetic proteins (BMPs) have been used in concentrations overshooting physiologically occurring concentrations and has thus raised concerns as documented side effects were recorded. Secondly, most such biomolecules are very sensitive to organic solvents and this hinders their use. Here, we present a novel xeno-hybrid bone graft, SmartBonePep^®, with a new type of biomolecule (i.e., intrinsically disordered proteins, IDPs) that is both resistant to processing with organic solvent and both triggers bone cells proliferation and differentiation. SmartBonePep^® is an advanced and improved modification of SmartBone^®, which is a bone substitute produced by combining naturally-derived mineral bone structures with resorbable polymers and collagen fragments. Not only have we demonstrated that Intrinsically Disordered Proteins (IDPs) can be successfully and safely loaded onto a SmartBonePep^®, withstanding the hefty manufacturing processes, but also made them bioavailable in a tuneable manner and proved that these biomolecules are a robust and resilient biomolecule family, being a better candidate with respect to other biomolecules for effectively producing the next generation bone grafts. Most other biomolecules which enhances bone formation, e.g., BMP, would not have tolerated the organic solvent used to produce SmartBonePep^®.

Outer-inner dual reinforced micro/nano hierarchical scaffolds for promoting osteogenesis

Biomimetic scaffolds have been extensively studied for guiding osteogenesis through structural cues. Inspired by the natural bone growth process, we have employed a hierarchical outer-inner dual reinforcing strategy, which relies on the interfacial ionic bond interaction between amine/calcium and carboxyl groups, to build a nanofiber/particle dual strengthened hierarchical silk fibroin scaffold. This scaffold can provide an applicable form of osteogenic structural cue and mimic the natural bone forming process. Owing to the active interaction between compositions located in the outer pore space and the inner pore wall, the scaffold has over 4 times improvement in the mechanical properties, followed by a significant alteration of the cell-scaffold interaction pattern, demonstrated by over 2 times elevation in the spreading area and enhanced osteogenic activity potentially involving the activities of integrin, vinculin and Yes-associated protein (YAP). The in vivo performance of the scaffold identified the inherent osteogenic effect of the structural cue, which promotes rapid and uniform regeneration. Overall, the hierarchical scaffold is promising in promoting uniform bone regeneration through its specific structural cue endowed by its micro-nano construction.

Enhanced Osteogenesis of Bone Marrow-Derived Mesenchymal Stem Cells by a Functionalized Silk Fibroin Hydrogel for Bone Defect Repair

Silk fibroin (SF) from Bombyx mori is a promising natural material for the synthesis of biocompatible and biodegradable hydrogels for use in biomedical applications from tissue engineering to drug delivery. However, weak gelation performance and the lack of biochemical cues to trigger cell proliferation and differentiation currently significantly limit its application in these areas. Herein, a biofunctional hydrogel containing SF (2.0%) and a small peptide gelator (e.g., NapFFRGD = 1.0 wt%) is generated via cooperative molecular self-assembly. The introduction of NapFFRGD to SF is shown to significantly improve its gelation properties by lowering both its threshold gelation concentration to 2.0% and gelation time to 20 min under physiological conditions (pH = 7.4, 37 °C), as well as functionalizing the SF hydrogel with cell-adhesive motifs (e.g., RGD). Besides mediating cell adhesion, the RGD ligands incorporated within the SF-RGD gel promote the osteogenic differentiation of bone marrow-derived mesenchymal stem cells encapsulated within the gel matrix, leading to bone regeneration in a mouse calvarial defect model, compared with a blank SF gel (2.0%, pH = 7.4). This work suggests that SF could be easily tailored with bioactive peptide gelators to afford bioactive hydrogels with favorable microenvironments for tissue regeneration applications.

Circulating healing (CH) cells expressing BST2 are functionally activated by the injury-regulated systemic factor HGFA

Background

Restoration of damaged tissues through the activation of endogenous progenitors is an attractive therapeutic option. A deep evaluation of the intrinsic stem/progenitor cell properties as well as the reciprocal interactions with injured environments is of critical importance.

Methods

Here, we show that bone marrow stromal cell antigen 2 (BST2) allows the isolation of a population of circulating progenitors, the circulating healing (CH) cells, characterized by a distinctive core signature. The bone marrow (BM) origin of BST2^pos CH cells has been strengthened by the co-expression of leptin receptor, the hallmark of a subpopulation of BM-skeletal stem cells.

Results

BST2^pos CH cells retained the capacity to (i) respond to injury signals generated by a bone fracture, (ii) modify the expression of cell motility genes following damage, and (iii) react to hepatocyte growth factor-activator (HGFA), an injury-related stimulus sufficient to induce their transition into G_ALERT, a state in which cells are functionally activated and participate in tissue repair.

Conclusions

Taken together, these results could pave the way for the identification of new strategies to enhance and potentiate endogenous regenerative mechanisms for future therapies.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1056-1) contains supplementary material, which is available to authorized users.

Deciphering the combined effect of bone morphogenetic protein 6 and calcium phosphate on bone formation capacity of periosteum derived cells-based tissue engineering constructs

Cell based combination products with growth factors on optimal carriers represent a promising tissue engineering strategy to treat large bone defects. In this concept, bone morphogenetic protein (BMP) and calcium phosphate (CaP)-based scaffolds can act as potent components of the constructs to steer stem cell specification, differentiation and initiate subsequent in vivo bone formation. However, limited insight into BMP dosage and the cross-talk between BMP and CaP materials, hampers the optimization of in vivo bone formation and subsequent clinical translation. Herein, we combined human periosteum derived progenitor cells with different doses of BMP6 and with three types of clinical grade CaP-scaffolds (ChronOs®, ReproBone™, & CopiOs®). Comprehensive cellular and molecular analysis was performed based on in vitro cell metabolic activity and signaling pathway activation, as well as in vivo ectopic bone forming capacity after 2 weeks and 5 weeks in nude mice. Our data showed that cells seeded on CaP scaffolds with an intermediate Ca²⁺ release rate combined with low or medium dosage of BMP6 demonstrated a robust new bone formation after 5 weeks, which was contributed by both donor and host cells. This phenomenon might be due to the delicate balance between Ca²⁺ and BMP pathways, allowing an appropriate activation of the canonical BMP signaling pathway that is required for in vivo bone formation. For high BMP6 dosage, we found that the BMP6 dosage overrides the effect of the Ca²⁺ release rate and this appeared to be a dominant factor for ectopic bone formation. Taken together, this study illustrates the importance of matching BMP dosage and CaP properties to allow an appropriate activation of canonical BMP signaling that is crucial for in vivo bone formation. It also provides insightful knowledge with regard to clinical translation of cell-based constructs for bone regeneration. STATEMENT OF SIGNIFICANCE: The combination of bone morphogenetic proteins (BMP) and calcium phosphate (CaP)-based biomaterials with mesenchymal stromal cells represents a promising therapeutic strategy to treat large bone defects, an unmet medical need. However, there is limited insight into the optimization of these combination products, which hampers subsequent successful clinical translation. Our data reveal a delicate balance between Ca²⁺ and BMP pathways, allowing an appropriate activation of canonical BMP signaling required for in vivo bone formation. Our findings illustrate the importance of matching BMP dosage and CaP properties in the development of cell-based constructs for bone regeneration.

Clot-entrapped blood cells in synergy with human mesenchymal stem cells create a pro-angiogenic healing response

Blood clots stop bleeding and provide cell-instructive microenvironments. Still, in vitro models used to study implant performance typically neglect any possible interactions of recruited cells with surface-adhering blood clots. Here we study the interaction and synergies of bone marrow derived human mesenchymal stem cells (hMSCs) with surface-induced blood clots in an in vitro model by fluorescence microscopy, scanning and correlative light and electron microscopy, ELISA assays and zymography. The clinically used alkali-treated rough titanium (Ti) surfaces investigated here are known to enhance blood clotting compared to native Ti and to improve the healing response, but the underlying mechanisms remain elusive. Here we show that the presence of blood clots synergistically increased hMSC proliferation, extracellular matrix (ECM) remodelling and the release of matrix fragments and angiogenic VEGF, but did not increase the osteogenic differentiation of hMSCs. While many biomaterials are nowadays engineered to release pro-angiogenic factors, we show here that clot-entrapped blood cells on conventional materials in synergy with hMSCs are potent producers of pro-angiogenic factors. Our data might thus not only explain why alkali-treatment is beneficial for Ti implant integration, but they suggest that the physiological importance of blood clots to create pro-angiogenic environments on implants has been greatly underestimated. The importance of blood clots might have been missed because the pro-angiogenic functions get activated only upon stimulation by synergistic interactions with the invading cells.

A novel cytotherapy device for rapid screening, enriching and combining mesenchymal stem cells into a biomaterial for promoting bone regeneration

Bone defects are a common challenge in clinic, usually warranting bone grafts. However, current strategies to obtain effective graft materials have many drawbacks. Mesenchymal stem cell (MSC)-based therapy is a promising alternative. We designed an innovative appliance named the stem cell screen-enrich-combine(-biomaterials) circulating system (SECCS). In this study, 42 patients who required bone graft underwent SECCS-based treatment. Their bone marrow samples and beta-tricalcium phosphate (β-TCP) granules were processed in the SECCS for 10-15 minutes, to produce MSC/β-TCP composites. These composites were grafted back into bone defect sites. The results showed 85.53% ± 7.95% autologous MSCs were successfully screened, enriched, and seeded on the β-TCP scaffolds synchronously. The cell viability remained unchanged after SECCS processing. Clinically, all patients obtained satisfactory bone healing. Thus, without in vitro culture, the SECCS can produce bioactive MSC/β-TCP composites for bone regeneration during surgery. The SECCS represents a convenient, rapid, low-cost, and safe method for bone regeneration.

Bionic Design, Materials and Performance of Bone Tissue Scaffolds

Design, materials, and performance are important factors in the research of bone tissue scaffolds. This work briefly describes the bone scaffolds and their anatomic structure, as well as their biological and mechanical characteristics. Furthermore, we reviewed the characteristics of metal materials, inorganic materials, organic polymer materials, and composite materials. The importance of the bionic design in preoperative diagnosis models and customized bone scaffolds was also discussed, addressing both the bionic structure design (macro and micro structure) and the bionic performance design (mechanical performance and biological performance). Materials and performance are the two main problems in the development of customized bone scaffolds. Bionic design is an effective way to solve these problems, which could improve the clinical application of bone scaffolds, by creating a balance between mechanical performance and biological performance.

Sphingosine 1-phosphate (S1P) signalling: Role in bone biology and potential therapeutic target for bone repair

The lipid mediator sphingosine 1-phosphate (S1P) affects cellular functions in most systems. Interest in its therapeutic potential has increased following the discovery of its G protein-coupled receptors and the recent availability of agents that can be safely administered in humans. Although the role of S1P in bone biology has been the focus of much less research than its role in the nervous, cardiovascular and immune systems, it is becoming clear that this lipid influences many of the functions, pathways and cell types that play a key role in bone maintenance and repair. Indeed, S1P is implicated in many osteogenesis-related processes including stem cell recruitment and subsequent differentiation, differentiation and survival of osteoblasts, and coupling of the latter cell type with osteoclasts. In addition, S1P's role in promoting angiogenesis is well-established. The pleiotropic effects of S1P on bone and blood vessels have significant potential therapeutic implications, as current therapeutic approaches for critical bone defects show significant limitations. Because of the complex effects of S1P on bone, the pharmacology of S1P-like agents and their physico-chemical properties, it is likely that therapeutic delivery of S1P agents will offer significant advantages compared to larger molecular weight factors. Hence, it is important to explore novel methods of utilizing S1P agents therapeutically, and improve our understanding of how S1P and its receptors modulate bone physiology and repair.

Centering Single Cells in Microgels via Delayed Crosslinking Supports Long-Term 3D Culture by Preventing Cell Escape

Single-cell-laden microgels support physiological 3D culture conditions while enabling straightforward handling and high-resolution readouts of individual cells. However, their widespread adoption for long-term cultures is limited by cell escape. In this work, it is demonstrated that cell escape is predisposed to off-center encapsulated cells. High-speed microscopy reveals that cells are positioned at the microgel precursor droplets' oil/water interface within milliseconds after droplet formation. In conventional microencapsulation strategies, the droplets are typically gelled immediately after emulsification, which traps cells in this off-center position. By delaying crosslinking, driving cells toward the centers of microgels is succeeded. The centering of cells in enzymatically crosslinked microgels prevents their escape during at least 28 d. It thereby uniquely enables the long-term culture of individual cells within <5-µm-thick 3D uniform hydrogel coatings. Single cell analysis of mesenchymal stem cells in enzymatically crosslinked microgels reveals unprecedented high cell viability (>90%), maintained metabolic activity (>70%), and multilineage differentiation capacity (>60%) over a period of 28 d. The facile nature of this microfluidic cell-centering method enables its straightforward integration into many microencapsulation strategies and significantly enhances control, reproducibility, and reliability of 3D single cell cultures.

Biofabrication and Bone Tissue Regeneration: Cell Source, Approaches, and Challenges

The growing occurrence of bone disorders and the increase in aging population have resulted in the need for more effective therapies to meet this request. Bone tissue engineering strategies, by combining biomaterials, cells, and signaling factors, are seen as alternatives to conventional bone grafts for repairing or rebuilding bone defects. Indeed, skeletal tissue engineering has not yet achieved full translation into clinical practice because of several challenges. Bone biofabrication by additive manufacturing techniques may represent a possible solution, with its intrinsic capability for accuracy, reproducibility, and customization of scaffolds as well as cell and signaling molecule delivery. This review examines the existing research in bone biofabrication and the appropriate cells and factors selection for successful bone regeneration as well as limitations affecting these approaches. Challenges that need to be tackled with the highest priority are the obtainment of appropriate vascularized scaffolds with an accurate spatiotemporal biochemical and mechanical stimuli release, in order to improve osseointegration as well as osteogenesis.

Healing of a Large Long-Bone Defect through Serum-Free In Vitro Priming of Human Periosteum-Derived Cells

Clinical translation of cell-based strategies for regenerative medicine demands predictable in vivo performance where the use of sera during in vitro preparation inherently limits the efficacy and reproducibility. Here, we present a bioinspired approach by serum-free pre-conditioning of human periosteum-derived cells, followed by their assembly into microaggregates simultaneously primed with bone morphogenetic protein 2 (BMP-2). Pre-conditioning resulted in a more potent progenitor cell population, while aggregation induced osteochondrogenic differentiation, further enhanced by BMP-2 stimulation. Ectopic implantation displayed a cascade of events that closely resembled the natural endochondral process resulting in bone ossicle formation. Assessment in a critical size long-bone defect in immunodeficient mice demonstrated successful bridging of the defect within 4 weeks, with active contribution of the implanted cells. In short, the presented serum-free process represents a biomimetic strategy, resulting in a cartilage tissue intermediate that, upon implantation, robustly leads to the healing of a large long-bone defect.

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Serum-free pre-conditioning affects the identity of periosteal progenitor cells

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A reduced CD105⁺, elevated CD34⁺, and upregulated BMP receptor expression was seen

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Priming by aggregation and BMP stimulation induced endochondral bone formation

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Validation in a critical size fracture model confirmed endochondral healing

Concise Review: Organ Engineering: Design, Technology, and Integration

Engineering complex tissues and whole organs has the potential to dramatically impact translational medicine in several avenues. Organ engineering is a discipline that integrates biological knowledge of embryological development, anatomy, physiology, and cellular interactions with enabling technologies including biocompatible biomaterials and biofabrication platforms such as three-dimensional bioprinting. When engineering complex tissues and organs, core design principles must be taken into account, such as the structure-function relationship, biochemical signaling, mechanics, gradients, and spatial constraints. Technological advances in biomaterials, biofabrication, and biomedical imaging allow for in vitro control of these factors to recreate in vivo phenomena. Finally, organ engineering emerges as an integration of biological design and technical rigor. An overall workflow for organ engineering and guiding technology to advance biology as well as a perspective on necessary future iterations in the field is discussed. Stem Cells 2017;35:51-60.

Bioinspired seeding of biomaterials using three dimensional microtissues induces chondrogenic stem cell differentiation and cartilage formation under growth factor free conditions

Cell laden biomaterials are archetypically seeded with individual cells and steered into the desired behavior using exogenous stimuli to control growth and differentiation. In contrast, direct cell-cell contact is instructive and even essential for natural tissue formation. Namely, microaggregation and condensation of mesenchymal progenitor cells triggers chondrogenesis and thereby drives limb formation. Yet a biomimetic strategy translating this approach into a cell laden biomaterial-based therapy has remained largely unexplored. Here, we integrate the microenvironment of cellular condensation into biomaterials by encapsulating microaggregates of a hundred human periosteum-derived stem cells. This resulted in decreased stemness-related markers, up regulation of chondrogenic genes and improved in vivo cartilage tissue formation, as compared to single cell seeded biomaterials. Importantly, even in the absence of exogenous growth factors, the microaggregate laden hydrogels outperformed conventional single cell laden hydrogels containing supraphysiological levels of the chondrogenic growth factor TGFB. Overall, the bioinspired seeding strategy described herein represents an efficient and growth factor-free approach to efficiently steer cell fate and drive tissue formation for biomaterial-based tissue engineering strategies.

Enzymatic Crosslinking of Polymer Conjugates is Superior over Ionic or UV Crosslinking for the On-Chip Production of Cell-Laden Microgels

Cell-laden micrometer-sized hydrogels (microgels) hold great promise for improving high throughput ex-vivo drug screening and engineering biomimetic tissues. Microfluidics is a powerful tool to produce microgels. However, only a limited amount of biomaterials have been reported to be compatible with on-chip microgel formation. Moreover, these biomaterials are often associated with mechanical instability, cytotoxicity, and cellular senescence. To resolve this challenge, dextran-tyramine has been explored as a novel biomaterial for on-chip microgel formation. In particular, dextran-tyramine is compared with two commonly used biomaterials, namely, polyethylene-glycol diacrylate (PEGDA) and alginate, which crosslink through enzymatic reaction, UV polymerization, and ionic interaction, respectively. Human mesenchymal stem cells (hMSCs) encapsulated in dextran-tyramine microgels demonstrate significantly higher (95%) survival as compared to alginate (81%) and PEGDA (69%). Long-term cell cultures demonstrate that hMSCs in PEGDA microgels become senescent after 7 d. Alginate microgels dissolve within 7 d due to Ca²⁺ loss. In contrast, dextran-tyramine based microgels remain stable, sustain hMSCs metabolic activity, and permit for single-cell level analysis for at least 28 d of culture. In conclusion, enzymatically crosslinking dextran-tyramine conjugates represent a novel biomaterial class for the on-chip production of cell-laden microgels, which possesses unique advantages as compared to the commonly used UV and ionic crosslinking biomaterials.

Gelatin-Methacryloyl Hydrogels: Towards Biofabrication-Based Tissue Repair

Research over the past decade on the cell-biomaterial interface has shifted to the third dimension. Besides mimicking the native extracellular environment by 3D cell culture, hydrogels offer the possibility to generate well-defined 3D biofabricated tissue analogs. In this context, gelatin-methacryloyl (gelMA) hydrogels have recently gained increased attention. This interest is sparked by the combination of the inherent bioactivity of gelatin and the physicochemical tailorability of photo-crosslinkable hydrogels. GelMA is a versatile matrix that can be used to engineer tissue analogs ranging from vasculature to cartilage and bone. Convergence of biological and biofabrication approaches is necessary to progress from merely proving cell functionality or construct shape fidelity towards regenerating tissues. GelMA has a critical pioneering role in this process and could be used to accelerate the development of clinically relevant applications.

Advancing Tissue Engineering: A Tale of Nano-, Micro-, and Macroscale Integration

Tissue engineering has the potential to revolutionize the health care industry. Delivering on this promise requires the generation of efficient, controllable and predictable implants. The integration of nano- and microtechnologies into macroscale regenerative biomaterials plays an essential role in the generation of such implants, by enabling spatiotemporal control of the cellular microenvironment. Here we review the role, function and progress of a wide range of nano- and microtechnologies that are driving the advancements in the field of tissue engineering.

Platelet-Rich Blood Derivatives for Stem Cell-Based Tissue Engineering and Regeneration

Platelet rich blood derivatives have been widely used in different fields of medicine and stem cell based tissue engineering. They represent natural cocktails of autologous growth factor, which could provide an alternative for recombinant protein based approaches. Platelet rich blood derivatives, such as platelet rich plasma, have consistently shown to potentiate stem cell proliferation, migration, and differentiation. Here, we review the spectrum of platelet rich blood derivatives, discuss their current applications in tissue engineering and regenerative medicine, reflect on their effect on stem cells, and highlight current translational challenges.

Biofabrication of bone tissue: approaches, challenges and translation for bone regeneration

The rising incidence of bone disorders has resulted in the need for more effective therapies to meet this demand, exacerbated by an increasing ageing population. Bone tissue engineering is seen as a means of developing alternatives to conventional bone grafts for repairing or reconstructing bone defects by combining biomaterials, cells and signalling factors. However, skeletal tissue engineering has not yet achieved full translation into clinical practice as a consequence of several challenges. The use of additive manufacturing techniques for bone biofabrication is seen as a potential solution, with its inherent capability for reproducibility, accuracy and customisation of scaffolds as well as cell and signalling factor delivery. This review highlights the current research in bone biofabrication, the necessary factors for successful bone biofabrication, in addition to the current limitations affecting biofabrication, some of which are a consequence of the limitations of the additive manufacturing technology itself.

Dental pulp stem cells as a multifaceted tool for bioengineering and the regeneration of craniomaxillofacial tissues

Dental pulp stem cells, or DPSC, are neural crest-derived cells with an outstanding capacity to differentiate along multiple cell lineages of interest for cell therapy. In particular, highly efficient osteo/dentinogenic differentiation of DPSC can be achieved using simple in vitro protocols, making these cells a very attractive and promising tool for the future treatment of dental and periodontal diseases. Among craniomaxillofacial organs, the tooth and salivary gland are two such cases in which complete regeneration by tissue engineering using DPSC appears to be possible, as research over the last decade has made substantial progress in experimental models of partial or total regeneration of both organs, by cell recombination technology. Moreover, DPSC seem to be a particularly good choice for the regeneration of nerve tissues, including injured or transected cranial nerves. In this context, the oral cavity appears to be an excellent testing ground for new regenerative therapies using DPSC. However, many issues and challenges need yet to be addressed before these cells can be employed in clinical therapy. In this review, we point out some important aspects on the biology of DPSC with regard to their use for the reconstruction of different craniomaxillofacial tissues and organs, with special emphasis on cranial bones, nerves, teeth, and salivary glands. We suggest new ideas and strategies to fully exploit the capacities of DPSC for bioengineering of the aforementioned tissues.

Manufacturing of dental pulp cell-based products from human third molars: current strategies and future investigations

In recent years, mesenchymal cell-based products have been developed to improve surgical therapies aimed at repairing human tissues. In this context, the tooth has recently emerged as a valuable source of stem/progenitor cells for regenerating orofacial tissues, with easy access to pulp tissue and high differentiation potential of dental pulp mesenchymal cells. International guidelines now recommend the use of standardized procedures for cell isolation, storage and expansion in culture to ensure optimal reproducibility, efficacy and safety when cells are used for clinical application. However, most dental pulp cell-based medicinal products manufacturing procedures may not be fully satisfactory since they could alter the cells biological properties and the quality of derived products. Cell isolation, enrichment and cryopreservation procedures combined to long-term expansion in culture media containing xeno- and allogeneic components are known to affect cell phenotype, viability, proliferation and differentiation capacities. This article focuses on current manufacturing strategies of dental pulp cell-based medicinal products and proposes a new protocol to improve efficiency, reproducibility and safety of these strategies.