Surface Topography Regulates Osteogenic Differentiation of MSCs via Crosstalk between FAK/MAPK and ILK/β-Catenin Pathways in a Hierarchically Porous Environment

A 3D In-vitro model of the human dentine interface shows long-range osteoinduction from the dentine surface

Emerging regenerative cell therapies for alveolar bone loss have begun to explore the use of cell laden hydrogels for minimally invasive surgery to treat small and spatially complex maxilla-oral defects. However, the oral cavity presents a unique and challenging environment for in vivo bone tissue engineering, exhibiting both hard and soft periodontal tissue as well as acting as key biocenosis for many distinct microbial communities that interact with both the external environment and internal body systems, which will impact on cell fate and subsequent treatment efficacy. Herein, we design and bioprint a facile 3D in vitro model of a human dentine interface to probe the effect of the dentine surface on human mesenchymal stem cells (hMSCs) encapsulated in a microporous hydrogel bioink. We demonstrate that the dentine substrate induces osteogenic differentiation of encapsulated hMSCs, and that both dentine and β-tricalcium phosphate substrates stimulate extracellular matrix production and maturation at the gel-media interface, which is distal to the gel-substrate interface. Our findings demonstrate the potential for long-range effects on stem cells by mineralized surfaces during bone tissue engineering and provide a framework for the rapid development of 3D dentine-bone interface models.

Bio-integrated scaffold facilitates large bone regeneration dominated by endochondral ossification

Repair of large bone defects caused by severe trauma, non-union fractures, or tumor resection remains challenging because of limited regenerative ability. Typically, these defects heal through mixed routines, including intramembranous ossification (IMO) and endochondral ossification (ECO), with ECO considered more efficient. Current strategies to promote large bone healing via ECO are unstable and require high-dose growth factors or complex cell therapy that cause side effects and raise expense while providing only limited benefit. Herein, we report a bio-integrated scaffold capable of initiating an early hypoxia microenvironment with controllable release of low-dose recombinant bone morphogenetic protein-2 (rhBMP-2), aiming to induce ECO-dominated repair. Specifically, we apply a mesoporous structure to accelerate iron chelation, this promoting early chondrogenesis via deferoxamine (DFO)-induced hypoxia-inducible factor-1α (HIF-1α). Through the delicate segmentation of click-crosslinked PEGylated Poly (glycerol sebacate) (PEGS) layers, we achieve programmed release of low-dose rhBMP-2, which can facilitate cartilage-to-bone transformation while reducing side effect risks. We demonstrate this system can strengthen the ECO healing and convert mixed or mixed or IMO-guided routes to ECO-dominated approach in large-size models with clinical relevance. Collectively, these findings demonstrate a biomaterial-based strategy for driving ECO-dominated healing, paving a promising pave towards its clinical use in addressing large bone defects.

Engineering mesoporous bioactive glasses for emerging stimuli-responsive drug delivery and theranostic applications

Mesoporous bioactive glasses (MBGs), which belong to the category of modern porous nanomaterials, have garnered significant attention due to their impressive biological activities, appealing physicochemical properties, and desirable morphological features. They hold immense potential for utilization in diverse fields, including adsorption, separation, catalysis, bioengineering, and medicine. Despite possessing interior porous structures, excellent morphological characteristics, and superior biocompatibility, primitive MBGs face challenges related to weak encapsulation efficiency, drug loading, and mechanical strength when applied in biomedical fields. It is important to note that the advantageous attributes of MBGs can be effectively preserved by incorporating supramolecular assemblies, miscellaneous metal species, and their conjugates into the material surfaces or intrinsic mesoporous networks. The innovative advancements in these modified colloidal inorganic nanocarriers inspire researchers to explore novel applications, such as stimuli-responsive drug delivery, with exceptional in-vivo performances. In view of the above, we outline the fabrication process of calcium-silicon-phosphorus based MBGs, followed by discussions on their significant progress in various engineered strategies involving surface functionalization, nanostructures, and network modification. Furthermore, we emphasize the recent advancements in the textural and physicochemical properties of MBGs, along with their theranostic potentials in multiple cancerous and non-cancerous diseases. Lastly, we recapitulate compelling viewpoints, with specific considerations given from bench to bedside.

Mechanical factors influence β-catenin localization and barrier properties

Mechanical forces are of major importance in regulating vascular homeostasis by influencing endothelial cell behavior and functions. Adherens junctions are critical sites for mechanotransduction in endothelial cells. β-catenin, a component of adherens junctions and the canonical Wnt signaling pathway, plays a role in mechanoactivation. Evidence suggests that β-catenin is involved in flow sensing and responds to tensional forces, impacting junction dynamics. The mechanoregulation of β-catenin signaling is context-dependent, influenced by the type and duration of mechanical loads. In endothelial cells, β-catenin's nuclear translocation and signaling are influenced by shear stress and strain, affecting endothelial permeability. The study investigates how shear stress, strain, and surface topography impact adherens junction dynamics, regulate β-catenin localization, and influence endothelial barrier properties. Insight box Mechanical loads are potent regulators of endothelial functions through not completely elucidated mechanisms. Surface topography, wall shear stress and cyclic wall deformation contribute overlapping mechanical stimuli to which endothelial monolayer respond to adapt and maintain barrier functions. The use of custom developed flow chamber and bioreactor allows quantifying the response of mature human endothelial to well-defined wall shear stress and gradients of strain. Here, the mechanoregulation of β-catenin by substrate topography, wall shear stress, and cyclic stretch is analyzed and linked to the monolayer control of endothelial permeability.

Inhibition of Integrin-Associated Kinases FAK and ILK on the In Vitro Model of Skin Wound Healing

Dermal fibroblasts are essential cells of skin tissue responsible for its normal functioning. In skin wounds, the differentiation of resident fibroblasts into myofibroblasts occurs, which is accompanied by the rearrangement of actin cytoskeleton with the expression of alpha-smooth muscle actin. This transformation is a prerequisite for a successful wound healing. At the same time, different studies indicate that extracellular matrix may be involved in regulation of this process. Since the connection between cells and matrix is provided by transmembrane integrin receptors, this work was aimed at studying the dynamics of signaling pathways associated with integrins on an in vitro model of wound healing using human skin fibroblasts. It was shown that the healing of simulated wound was accompanied by a change in the level of integrins as well as integrin-associated kinases ILK (integrin-linked kinase) and FAK (focal adhesion kinase). Pharmacological inhibition of ILK and FAK caused the suppression of p38 and Akt which proteins are involved in regulation of the actin cytoskeleton. Moreover, it resulted in an inefficient wound closure in vitro. The results of this study support the involvement of integrin-associated kinases in regulation of fibroblast-to-myofibroblast transition during wound healing.

Exploring the role of polymers to overcome ongoing challenges in the field of extracellular vesicles

Extracellular vesicles (EVs) are naturally occurring nanoparticles released from all eucaryotic and procaryotic cells. While their role was formerly largely underestimated, EVs are now clearly established as key mediators of intercellular communication. Therefore, these vesicles constitute an attractive topic of study for both basic and applied research with great potential, for example, as a new class of biomarkers, as cell-free therapeutics or as drug delivery systems. However, the complexity and biological origin of EVs sometimes complicate their identification and therapeutic use. Thus, this rapidly expanding research field requires new methods and tools for the production, enrichment, detection, and therapeutic application of EVs. In this review, we have sought to explain how polymer materials actively contributed to overcome some of the limitations associated to EVs. Indeed, thanks to their infinite diversity of composition and properties, polymers can act through a variety of strategies and at different stages of EVs development. Overall, we would like to emphasize the importance of multidisciplinary research involving polymers to address persistent limitations in the field of EVs.

Synergistic effect of hierarchical topographic structure on 3D-printed Titanium scaffold for enhanced coupling of osteogenesis and angiogenesis

The significance of the osteogenesis-angiogenesis relationship in the healing process of bone defects has been increasingly emphasized in recent academic research. Surface topography plays a crucial role in guiding cellular behaviors. Metal-organic framework (MOF) is an innovative biomaterial with nanoscale structural and topological features, enabling the modulation of scaffold physicochemical properties. This study involved the loading of varying quantities of UiO-66 nanocrystals onto alkali-heat treated 3D-printed titanium scaffolds, resulting in the formation of hierarchical micro/nano topography named UiO-66/AHTs. The physicochemical properties of these scaffolds were subsequently characterized. Furthermore, the impact of these scaffolds on the osteogenic potential of BMSCs, the angiogenic potential of HUVECs, and their intercellular communication were investigated. The findings of this study indicated that 1/2UiO-66/AHT outperformed other groups in terms of osteogenic and angiogenic induction, as well as in promoting intercellular crosstalk by enhancing paracrine effects. These results suggest a promising biomimetic hierarchical topography design that facilitates the coupling of osteogenesis and angiogenesis.

Amino-Functionalized Zirconium-Based Metal-Organic Frameworks as Bifunctional Nanomaterials to Treat Bone Tumors and Promote Osteogenesis

Bone tumor patients often encounter challenges associated with cancer cell residues and bone defects postoperation. To address this, there is an urgent need to develop a material that can enable tumor treatment and promote bone repair. Metal-organic frameworks (MOFs) have attracted the interest of many researchers due to their special porous structure, which has great potential in regenerative medicine and drug delivery. However, few studies explore MOFs with dual antitumor and bone regeneration properties. In this study, we investigated amino-functionalized zirconium-based MOF nanoparticles (UiO-66-NH2 NPs) as bifunctional nanomaterials for bone tumor treatment and osteogenesis promotion. UiO-66-NH2 NPs loading with doxorubicin (DOX) (DOX@UiO-66-NH2 NPs) showed good antitumor efficacy both in vitro and in vivo. Additionally, DOX@UiO-66-NH2 NPs significantly reduced lung injury compared to free DOX in vivo. Interestingly, the internalized UiO-66-NH2 NPs notably promoted the osteogenic differentiation of preosteoblasts. RNA-sequencing data revealed that PI3K-Akt signaling pathways or MAPK signaling pathways might be involved in this enhanced osteogenesis. Overall, UiO-66-NH2 NPs exhibit dual functionality in tumor treatment and bone repair, making them highly promising as a bifunctional material with broad application prospects.

Focal adhesion kinase (FAK): its structure, characteristics, and signaling in skeletal system

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase and distributes important regulatory functions in skeletal system. Mesenchymal stem cell (MSC) possesses significant migration and differentiation capacity, is an important source of distinctive bone cells production and a prominent bone development pathway. MSC has a wide range of applications in tissue bioengineering and regenerative medicine, and is frequently employed for hematopoietic support, immunological regulation, and defect repair, although current research is insufficient. FAK has been identified to cross-link with many other keys signaling pathways in bone biology and is considered as a fundamental "crossroad" on the signal transduction pathway and a "node" in the signal network to mediate MSC lineage development in skeletal system. In this review, we summarized the structure, characteristics, cellular signaling, and the interactions of FAK with other signaling pathways in the skeletal system. The discovery of FAK and its mediated molecules will lead to a new knowledge of bone development and bone construction as well as considerable potential for therapeutic use in the treatment of bone-related disorders such as osteoporosis, osteoarthritis, and osteosarcoma.

Periplaneta americana extract promotes osteoblast differentiation of human alveolar bone marrow mesenchymal stem cells

OBJECTIVES

This study aims to investigate the effects of Traditional Chinese medicine, Periplaneta americana extract (PAE), on osteoblast differentiation of human alveolar bone marrow-derived mesenchymal stem cells (hABMMSCs).

MATERIALS AND METHODS

Human alveolar bone marrow-derived mesenchymal stem cells were treated with different concentrations of PAE. Cell Counting Kit-8 (CCK-8) assay and transwell migration assay were conducted to evaluate cell proliferation and migration, respectively. Alkaline phosphatase (ALP) staining, ALP activity assay, and Alizarin red S staining were performed to detect osteogenesis in hABMMSCs. In addition, real-time quantitative polymerase chain reaction (RT-qPCR) and western blot (WB) assay were performed to evaluate expression levels of osteogenic markers. Finally, RNA sequencing analysis and WB were carried out to elucidate the underlying mechanism.

RESULTS

A total of 0.1 mg/ml PAE promoted cell proliferation and migration. PAE also increased ALP activity and mineralized nodule formation of hABMMSCs. In addition, PAE upregulated the expression of osteogenesis-related genes (RUNX2, COL1A1, and BGLAP). RNA-sequencing analysis revealed that PAE activated the focal adhesion signaling pathway. Treatment with Defactinib, an inhibitor of FAK, attenuated the effects induced by PAE.

CONCLUSIONS

PAE could enhance osteoblast differentiation of hABMMSCs through focal adhesion signaling pathway, suggesting a therapeutic potential for the alveolar bone defect.

Replication of natural surface topographies to generate advanced cell culture substrates

Surface topographies of cell culture substrates can be used to generate in vitro cell culture environments similar to the in vivo cell niches. In vivo, the physical properties of the extracellular matrix (ECM), such as its topography, provide physical cues that play an important role in modulating cell function. Mimicking these properties remains a challenge to provide in vitro realistic environments for cells. Artificially generated substrates' topographies were used extensively to explore this important surface cue. More recently, the replication of natural surface topographies has been enabling to exploration of characteristics such as hierarchy and size scales relevant for cells as advanced biomimetic substrates. These substrates offer more realistic and mimetic environments regarding the topographies found in vivo. This review will highlight the use of natural surface topographies as a template to generate substrates for in-vitro cell culture. This review starts with an analysis of the main cell functions that can be regulated by the substrate's surface topography through cell-substrate interactions. Then, we will discuss research works wherein substrates for cell biology decorated with natural surface topographies were used and investigated regarding their influence on cellular performance. At the end of this review, we will highlight the advantages and challenges of the use of natural surface topographies as a template for the generation of advanced substrates for cell culture.

Optimization of a concentrated growth factor/mesoporous bioactive glass composite scaffold and its application in rabbit mandible defect regeneration

Maxillofacial bone defect repair and regeneration remains a tremendous challenge in the field of stomatology. However, the limited osteoinductivity of artificial materials and the high cost of bioactive agents restrain their clinical translation. This study aimed to construct an economical and efficient concentrated growth factor/mesoporous bioactive glass (CGF/MBG) composite scaffold for bone regeneration. The biochemical composition and biological effects of different forms of CGFs were systematically compared, and the results showed that CGF-conditioned medium effectively promoted proliferation, migration and osteogenesis of allogenic BMSCs. Gel phase CGF (gpCGF) exhibited superior bioactivity and osteoinductivity to liquid phase CGF (lpCGF) and liquid/gel mixed phase CGF (lgpCGF), and was further applied to construct CGF/MBG scaffolds. In vitro studies demonstrated that co-culture with gpCGF-conditioned medium further enhanced the biocompatibility of MBG, increasing cell adhesion and proliferation on the scaffold. On this basis, two compositing approaches to construct the scaffold by fibrin gel formation (CGF/FG/MBG) and freeze-drying (fdCGF/MBG) were applied, and the biological efficacy of CGFs was compared in vivo. In a rabbit mandibular defect model, higher osteogenic efficiency in in situ bone regeneration of CGF/FG/MBG composite scaffolds was proved, compared with fdCGF/MBG. Taken together, the CGF/FG/MBG composite scaffold is expected to be an efficient bone repairing therapy for clinical translation, and the CGF-composited scaffold using gpCGF and the fibrin gel formation method is a promising way to enhance the bioactivity and osteoinductivity of current clinical bone repairing materials, providing new thoughts on the development of future orthopedic biomaterials.

In situ self-assembled organoid for osteochondral tissue regeneration with dual functional units

The regeneration of hierarchical osteochondral units is challenging due to difficulties in inducing spatial, directional and controllable differentiation of mesenchymal stem cells (MSCs) into cartilage and bone compartments. Emerging organoid technology offers new opportunities for osteochondral regeneration. In this study, we developed gelatin-based microcryogels customized using hyaluronic acid (HA) and hydroxyapatite (HYP), respectively for inducing cartilage and bone regeneration (denoted as CH-Microcryogels and OS-Microcryogels) through in vivo self-assembly into osteochondral organoids. The customized microcryogels showed good cytocompatibility and induced chondrogenic and osteogenic differentiation of MSCs, while also demonstrating the ability to self-assemble into osteochondral organoids with no delamination in the biphasic cartilage-bone structure. Analysis by mRNA-seq showed that CH-Microcryogels promoted chondrogenic differentiation and inhibited inflammation, while OS-Microcryogels facilitated osteogenic differentiation and suppressed the immune response, by regulating specific signaling pathways. Finally, the in vivo engraftment of pre-differentiated customized microcryogels into canine osteochondral defects resulted in the spontaneous assembly of an osteochondral unit, inducing simultaneous regeneration of both articular cartilage and subchondral bone. In conclusion, this novel approach for generating self-assembling osteochondral organoids utilizing tailor-made microcryogels presents a highly promising avenue for advancing the field of tissue engineering.

Biophysical cues to improve the immunomodulatory capacity of mesenchymal stem cells: The progress and mechanisms

Mesenchymal stem cells (MSCs) can maintain immune homeostasis and many preclinical trials with MSCs have been carried out around the world. In vitro culture of MSCs has been found to result in the decline of immunomodulatory capacity, migration and proliferation. To address these problems, simulating the extracellular environment for preconditioning of MSCs is a promising and inexpensive method. Biophysical cues in the external environment that MSCs are exposed to have been shown to affect MSC migration, residency, differentiation, secretion, etc. We review the main ways in which MSCs exert their immunomodulatory ability, and summarize recent advances in mechanical preconditioning of MSCs to enhance immunomodulatory capacity and related mechanical signal sensing and transduction mechanisms.

3D printing of conch-like scaffolds for guiding cell migration and directional bone growth

Regeneration of severe bone defects remains an enormous challenge in clinic. Developing regenerative scaffolds to directionally guide bone growth is a potential strategy to overcome this hurdle. Conch, an interesting creature widely spreading in ocean, has tough spiral shell that can continuously grow along the spiral direction. Herein, inspired by the physiological features of conches, a conch-like (CL) scaffold based on β-TCP bioceramic material was successfully prepared for guiding directional bone growth via digital light processing (DLP)-based 3D printing. Benefiting from the spiral structure, the CL scaffolds significantly improved cell adhesion, proliferation and osteogenic differentiation in vitro compared to the conventional 3D scaffolds. Particularly, the spiral structure in the scaffolds could efficiently induce cells to migrate from the bottom to the top of the scaffolds, which was like “cells climbing stairs”. Furthermore, the capability of guiding directional bone growth for the CL scaffolds was demonstrated by a special half-embedded femoral defects model in rabbits. The new bone tissue could consecutively grow into the protruded part of the scaffolds along the spiral cavities. This work provides a promising strategy to construct biomimetic biomaterials for guiding directional bone tissue growth, which offers a new treatment concept for severe bone defects, and even limb regeneration.

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A conch-like scaffold was firstly developed for guiding directional bone growth.

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The CL scaffolds efficiently induced cells “climbing stairs”- like-migrating.

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The CL scaffolds showed improved bioactivities benefited from the spiral structure.

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This work provided a new treatment concept for severe bone defects.

Response of mesenchymal stem cells to surface topography of scaffolds and the underlying mechanisms

Mesenchymal stem/stromal cells (MSCs) serve as essential components of regenerative medicine. Their destiny is influenced by the interaction of the cells with the external environment. In addition to the biochemical cues in a microenvironment, physical cues of the topography of the surrounding materials such as the extracellular matrix emerge as a crucial regulator of stem cell destiny and function. With recent advances in technologies of materials production and surface modification, surfaces with micro/nanotopographical characteristics can be fabricated to mimic the micro/nanoscale mechanical stimuli of the extracellular matrix environment and regulate the biological behavior of cells. Understanding the interaction of cells with the topography of a surface is conducive to the control of stem cell fate for application in regenerative medicine. However, the mechanisms by which topography affects the biological behavior of stem cells have not been fully elucidated. This review will present the effects of surface topography at the nano/micrometer scale on stem cell adhesion, morphology, proliferation, migration, and differentiation. It also focuses on discussing current theories about the sensing and recognition of surface topology cues, the transduction of the extracellular cues into plasma, and the final activation of related signaling pathways and downstream gene expression in MSCs. These insights will provide a theoretical basis for the future design of biomaterial scaffolds for application in regenerative medicine and tissue engineering.

The plasticity of biocompatibility

Biocompatibility concerns the phenomena that occur within the interactions between biomaterials and human patients, which ultimately control the performance of many facets of medical technology. It involves aspects of materials science, many different forms of engineering and nanotechnology, chemistry, biophysics, molecular and cellular biology, immunology, pathology and a myriad of clinical applications. It is not surprising that an overarching framework of mechanisms of biocompatibility has been difficult to elucidate and validate. This essay discusses one fundamental reason for this; we have tended to consider biocompatibility pathways as essentially linear sequences of events which follow well-understood processes of materials science and biology. The reality, however, is that the pathways may involve a great deal of plasticity, in which many additional idiosyncratic factors, including those of genetic, epigenetic and viral origin, exert influence, as do complex mechanical, physical and pharmacological variables. Plasticity is an inherent core feature of the performance of synthetic materials; here we follow the more recent biological applications of plasticity concepts into the sphere of biocompatibility pathways. A straightforward linear pathway may result in successful outcomes for many patients; we may describe this in terms of classic biocompatibility pathways. In other situations, which usually command much more attention because of their unsuccessful outcomes, these plasticity-driven processes follow alternative biocompatibility pathways; often, the variability in outcomes with identical technologies is due to biological plasticity rather than material or device deficiency.

Testing the Role of Focal Adhesion Kinase (FAK) in Topography-Mediated Stem Cell Differentiation by Inhibiting FAK Phosphorylation

Stem cell differentiation can be modulated by the substrate topographies. Focal adhesion kinase (FAK) has been identified as a key regulator in topography-induced stem cell mechanotransduction. This chapter will describe a protocol to study the effect of FAK phosphorylation inhibition on topography-mediated stem cell differentiation. The FAK phosphorylation was inhibited using a FAK inhibitor and the effects on stem cell differentiation were examined using western blot and immunofluorescence staining.

Construction of a Hierarchical Micro-/Submicro-/Nanostructured 3D-Printed Ti6Al4V Surface Feature to Promote Osteogenesis: Involvement of Sema7A through the ITGB1/FAK/ERK Signaling Pathway

Constructing hierarchical hybrid structures is considered a facile method to improve the osseointegration of implants. Herein, a hierarchical micro-/submicro-/nanostructured surface feature of Ti6Al4V implants (3DAT group) was successfully constructed by combining the inherently formed three-dimensional (3D)-printed microscale topography, acid-etched sub-micropits, and anodized nanotubes. Compared with the classical SLA surface, the microscale topography and sub-micropits increased the three-dimensional space for the cell growth and mechanical stability of implants, while the modification of nanotubes dramatically improved the surface hydrophilicity, protein adsorption, and biomineralization. Most importantly, the 3DAT surface feature possessed excellent osteogenic performance in vitro and in vivo, with the involvement of semaphorin 7A (Sema7A) as revealed by RNA-seq through the ITGB1/FAK/ERK signaling pathway. The present study suggested that the hierarchically structured surface design strategy could accelerate the osseointegration rate of 3D-printed Ti6Al4V implants, promising personalized reconstruction of bone defects.

The Bumpy Road to Stem Cell Therapies: Rational Design of Surface Topographies to Dictate Stem Cell Mechanotransduction and Fate

Cells sense and respond to a variety of physical cues from their surrounding microenvironment, and these are interpreted through mechanotransductive processes to inform their behavior. These mechanisms have particular relevance to stem cells, where control of stem cell proliferation, potency, and differentiation is key to their successful application in regenerative medicine. It is increasingly recognized that surface micro- and nanotopographies influence stem cell behavior and may represent a powerful tool with which to direct the morphology and fate of stem cells. Current progress toward this goal has been driven by combined advances in fabrication technologies and cell biology. Here, the capacity to generate precisely defined micro- and nanoscale topographies has facilitated the studies that provide knowledge of the mechanotransducive processes that govern the cellular response as well as knowledge of the specific features that can drive cells toward a defined differentiation outcome. However, the path forward is not fully defined, and the "bumpy road" that lays ahead must be crossed before the full potential of these approaches can be fully exploited. This review focuses on the challenges and opportunities in applying micro- and nanotopographies to dictate stem cell fate for regenerative medicine. Here, key techniques used to produce topographic features are reviewed, such as photolithography, block copolymer lithography, electron beam lithography, nanoimprint lithography, soft lithography, scanning probe lithography, colloidal lithography, electrospinning, and surface roughening, alongside their advantages and disadvantages. The biological impacts of surface topographies are then discussed, including the current understanding of the mechanotransductive mechanisms by which these cues are interpreted by the cells, as well as the specific effects of surface topographies on cell differentiation and fate. Finally, considerations in translating these technologies and their future prospects are evaluated.

Biocompatibility pathways and mechanisms for bioactive materials: The bioactivity zone

This essay analyzes the scientific evidence that forms the basis of bioactive materials, covering the fundamental understanding of bioactivity phenomena and correlation with the mechanisms of biocompatibility of biomaterials. This is a detailed assessment of performance in areas such as bone-induction, cell adhesion, immunomodulation, thrombogenicity and antimicrobial behavior. Bioactivity is the modulation of biological activity by characteristics of the interfacial region that incorporates the material surface and the immediate local host tissue. Although the term ‘bioactive material’ is widely used and has a well understood general meaning, it would be useful now to concentrate on this interfacial region, considered as ‘the bioactivity zone’. Bioactivity phenomena are either due to topographical/micromechanical characteristics, or to biologically active species that are presented in the bioactivity zone. Examples of topographical/micromechanical effects are the modulation of the osteoblast – osteoclast balance, nanotopographical regulation of cell adhesion, and bactericidal nanostructures. Regulation of bioactivity by biologically active species include their influence, especially of metal ions, on signaling pathways in bone formation, the role of cell adhesion molecules and bioactive peptides in cell attachment, macrophage polarization by immunoregulatory molecules and antimicrobial peptides. While much experimental data exists to demonstrate the potential of such phenomena, there are considerable barriers to their effective clinical translation. This essay shows that there is solid scientific evidence of the existence of bioactivity mechanisms that are associated with some types of biomaterials, especially when the material is modified in a manner designed to specifically induce that activity.

Biophysical Stimuli as the Fourth Pillar of Bone Tissue Engineering

The repair of critical bone defects remains challenging worldwide. Three canonical pillars (biomaterial scaffolds, bioactive molecules, and stem cells) of bone tissue engineering have been widely used for bone regeneration in separate or combined strategies, but the delivery of bioactive molecules has several obvious drawbacks. Biophysical stimuli have great potential to become the fourth pillar of bone tissue engineering, which can be categorized into three groups depending on their physical properties: internal structural stimuli, external mechanical stimuli, and electromagnetic stimuli. In this review, distinctive biophysical stimuli coupled with their osteoinductive windows or parameters are initially presented to induce the osteogenesis of mesenchymal stem cells (MSCs). Then, osteoinductive mechanisms of biophysical transduction (a combination of mechanotransduction and electrocoupling) are reviewed to direct the osteogenic differentiation of MSCs. These mechanisms include biophysical sensing, transmission, and regulation. Furthermore, distinctive application strategies of biophysical stimuli are presented for bone tissue engineering, including predesigned biomaterials, tissue-engineered bone grafts, and postoperative biophysical stimuli loading strategies. Finally, ongoing challenges and future perspectives are discussed.

Ion release behavior of vanadium-doped mesoporous bioactive glass particles and the effect of the released ions on osteogenic differentiation of BMSCs via the FAK/MAPK signaling pathway

Vanadium is an important trace element in bone and is involved in bone metabolism, bone formation, and bone growth, but the roles of various vanadium ions, especially of pentavalent vanadium, in bone tissue regenerative repair have been underestimated and even misinterpreted for a long time. The main purposes of this study are to investigate the release profile of Si, Ca, P, and V ions from vanadium doped mesoporous bioactive glass (V-MBG) particles and to explore the effect of pentavalent vanadium ions on proliferation and osteogenic differentiation of BMSCs as well as the corresponding osteogenic signaling pathway. On the basis of preparations of V-MBG particles with different pentavalent vanadium contents, the ion release behavior from V-MBG in distilled water and simulated body fluid was systemically investigated. Furthermore, the cytocompatibility and osteogenic effect of V-MBG extracts were studied in rBMSCs, and the related molecular mechanisms were preliminarily discussed. The results of dissolution experiments showed that the V ionic concentration exhibited a burst increase and then a sustained slow increase in the two media. The resultant V ions from 1.0V-MBG, 4.0V-MBG and 10.0V-MBG at 21 days were about 1.1, 5.8, and 12.5 mg L^-1 in water, respectively, and 1.6, 4.8 and 12.8 mg L^-1 in SBF, respectively. The release behaviors of Si, Ca, P, and V ions were evidently affected by high contents of incorporated vanadium. The cellular results indicated that compared to the control and MBG groups, the V(V) ions in V-MBG extracts at about 19.4 μM markedly promoted the proliferation, the gene and protein expression of BMP-2 and COL-I, and the ALP activity of rBMSCs in non-osteoinductive media, but insignificantly stimulated the OCN protein synthesis. More deeply, V(V) ions at about 19.4 μM significantly upregulated the gene and protein expressions of Itga 2b, FAK, and pERK1/2, demonstrating that V(V) ions could regulate osteogenic differentiation of rBMSCs through the activation of the Itga 2b-FAK-MAPK (pERK1/2) signaling pathway. The in vivo results further confirmed that V-MBG induced and promoted new bone formation in the defect area compared to the PGC and PGC/V-M0 groups. These results would contribute to modify the perception about the biocompatibility and osteogenic promotion of pentavalent vanadium at an appropriate concentration.

3D printed hydrogel/PCL core/shell fiber scaffolds with NIR-triggered drug release for cancer therapy and wound healing

Hydrogel based scaffolds with the ability of on-demand drug delivery gained increasing interests for localized cancer therapy and tissue engineering application. However, most drug-loaded hydrogels are generally not suitable for long-term drug delivery, because of the uncontrolled diffusion of drugs from the swollen hydrogels. Therefore, in this study, a core/shell fiber scaffold was fabricated by coating a homogeneous layer of polycaprolactone (PCL) on the 3D printed alginate-gelatin hydrogel scaffolds. The PCL coatings could reduce the free diffusion of drugs from the core gels. Subsequently, polydopamine (PDA) was coated on the Gel/PCL core/shell scaffolds, endowing the scaffolds with great photothermal effects. Thus, near-infrared (NIR) laser triggered on-demand drug release was realized in this system due to the thermally induced sol-gel transition of the core gels. The released drugs (doxorubicin) and photothermal therapy could effectively prohibit or ablate tumor in vitro and in vivo. Additionally, the Gel/PCL/PDA core/shell scaffold could serve as platform for promoting wound healing. Therefore, the reported Gel/PCL/PDA core/shell scaffolds have the potential for application in localized cancer therapy and tissue regeneration. Especially for those cancer patients suffering surgical resection, the scaffolds could be implanted in the resection site to kill the residual or recurrent cancer cells, as well as to repair the tissue defects caused by surgery. STATEMENT OF SIGNIFICANCE: This paper reported a facile strategy to realize stimuli-triggered on demand drug release in vitro and in vivo. Polycaprolactone (PCL) and polydopamine were sequentially deposited on the surface of 3D printed drug-loaded alginate/gelatin scaffold. PCL encapsulation could effectively reduce the free diffusion of drugs from the core hydrogel, achieving sustained drug release. Polydopamine with good photothermal effects endowed the scaffold with stimuli-triggered drug release in response to near-infrared (NIR) laser irradiation. This scaffold could be applied for localized cancer therapy and tissue regeneration. Especially for those cancer patients suffering surgical resection, the scaffolds could be implanted in the resection site to kill the residual or recurrent cancer cells, as well as to repair the tissue defects caused by surgery.

Strategies to Improve Bone Healing: Innovative Surgical Implants Meet Nano-/Micro-Topography of Bone Scaffolds

Successful fracture healing is dependent on an optimal mechanical and biological environment at the fracture site. Disturbances in fracture healing (non-union) or even critical size bone defects, where void volume is larger than the self-healing capacity of bone tissue, are great challenges for orthopedic surgeons. To address these challenges, new surgical implant concepts have been recently developed to optimize mechanical conditions. First, this review article discusses the mechanical environment on bone and fracture healing. In this context, a new implant concept, variable fixation technology, is introduced. This implant has the unique ability to change its mechanical properties from “rigid” to “dynamic” over the time of fracture healing. This leads to increased callus formation, a more homogeneous callus distribution and thus improved fracture healing. Second, recent advances in the nano- and micro-topography of bone scaffolds for guiding osteoinduction will be reviewed, particularly emphasizing the mimicry of natural bone. We summarize that an optimal scaffold should comprise micropores of 50–150 µm diameter allowing vascularization and migration of stem cells as well as nanotopographical osteoinductive cues, preferably pores of 30 nm diameter. Next to osteoinduction, such nano- and micro-topographical cues may also reduce inflammation and possess an antibacterial activity to further promote bone regeneration.

Optimized BMSC-derived osteoinductive exosomes immobilized in hierarchical scaffold via lyophilization for bone repair through Bmpr2/Acvr2b competitive receptor-activated Smad pathway

Mesenchymal stem cell-derived exosomes (MSC-exos), with its inherent capacity to modulate cellular behavior, are emerging as a novel cell-free therapy for bone regeneration. Herein, focusing on practical applying problems, the osteoinductivity of MSC-exos produced by different stem cell sources (rBMSCs/rASCs) and culture conditions (osteoinductive/common) were systematically compared to screen out an optimized osteogenic exosome (BMSC-OI-exo). Via bioinformatic analyses by miRNA microarray and in vitro pathway verification by gene silencing and miRNA transfection, we first revealed that the osteoinductivity of BMSC-OI-exo was attributed to multi-component exosomal miRNAs (let-7a-5p, let-7c-5p, miR-328a-5p and miR-31a-5p). These miRNAs targeted Acvr2b/Acvr1 and regulated the competitive balance of Bmpr2/Acvr2b toward Bmpr-elicited Smad1/5/9 phosphorylation. On these bases, lyophilized delivery of BMSC-OI-exo on hierarchical mesoporous bioactive glass (MBG) scaffold was developed to realize bioactivity maintenance and sustained release by entrapment in the surface microporosity of the scaffold. In a rat cranial defect model, the loading of BMSC-OI-exo efficiently enhanced the bone forming capacity of the scaffold and induced rapid initiation of bone regeneration. This paper could provide empirical bases of MSC-exo-based therapy for bone regeneration and theoretical bases of MSC-exo-induced osteogenesis mechanism. The BMSC-OI-exo-loaded MBG scaffold developed here represented a promising bone repairing strategy for future clinical application.

Biomimetic Culture Strategies for the Clinical Expansion of Mesenchymal Stromal Cells

Mesenchymal stromal/stem cells (MSCs) typically require significant ex vivo expansion to achieve the high cell numbers required for research and clinical applications. However, conventional MSC culture on planar (2D) plastic surfaces has been shown to induce MSC senescence and decrease cell functionality over long-term proliferation, and usually, it has a high labor requirement, a high usage of reagents, and therefore, a high cost. In this Review, we describe current MSC-based therapeutic strategies and outline the important factors that need to be considered when developing next-generation cell expansion platforms. To retain the functional value of expanded MSCs, ex vivo culture systems should ideally recapitulate the components of the native stem cell microenvironment, which include soluble cues, resident cells, and the extracellular matrix substrate. We review the interplay between these stem cell niche components and their biological roles in governing MSC phenotype and functionality. We discuss current biomimetic strategies of incorporating biochemical and biophysical cues in MSC culture platforms to grow clinically relevant cell numbers while preserving cell potency and stemness. This Review summarizes the current state of MSC expansion technologies and the challenges that still need to be overcome for MSC clinical applications to be feasible and sustainable.

Designing topographically textured microparticles for induction and modulation of osteogenesis in mesenchymal stem cell engineering

Mesenchymal stem cells are the focus of intense research in bone development and regeneration. The potential of microparticles as modulating moieties of osteogenic response by utilizing their architectural features is demonstrated herein. Topographically textured microparticles of varying microscale features are produced by exploiting phase-separation of a readily soluble sacrificial component from polylactic acid. The influence of varying topographical features on primary human mesenchymal stem cell attachment, proliferation and markers of osteogenesis is investigated. In the absence of osteoinductive supplements, cells cultured on textured microparticles exhibit notably increased expression of osteogenic markers relative to conventional smooth microparticles. They also exhibit varying morphological, attachment and proliferation responses. Significantly altered gene expression and metabolic profiles are observed, with varying histological characteristics in vivo. This study highlights how tailoring topographical design offers cell-instructive 3D microenvironments which allow manipulation of stem cell fate by eliciting the desired downstream response without use of exogenous osteoinductive factors.

Synergistic Effect of Cell-Derived Extracellular Matrices and Topography on Osteogenesis of Mesenchymal Stem Cells

Cell-derived matrices (CDMs) are an interesting alternative to conventional sources of extracellular matrices (ECMs) as CDMs mimic the natural ECM composition better and are therefore attractive as a scaffolding material for regulating the functions of stem cells. Previous research on stem cell differentiation has demonstrated that both surface topography and CDMs have a significant influence. However, not much focus has been devoted to elucidating possible synergistic effects of CDMs and topography on osteogenic differentiation of human bone marrow-derived mesenchymal stem cells (hBM-MSCs). In this study, polydimethylsiloxane (PDMS)-based anisotropic topographies (wrinkles) with various topography dimensions were prepared and subsequently combined with native ECMs produced by human fibroblasts that remained on the surface topography after decellularization. The synergistic effect of CDMs combined with topography on osteogenic differentiation of hBM-MSCs was investigated. The results showed that substrates with specific topography dimensions, coated with aligned CDMs, dramatically enhanced the capacity of osteogenesis as investigated using immunofluorescence staining for identifying osteopontin (OPN) and mineralization. Furthermore, the hBM-MSCs on the substrates decorated with CDMs exhibited a higher percentage of (Yes-associated protein) YAP inside the nucleus, stronger cell contractility, and greater formation of focal adhesions, illustrating that enhanced osteogenesis is partly mediated by cellular tension and mechanotransduction following the YAP pathway. Taken together, our findings highlight the importance of ECMs mediating the osteogenic differentiation of stem cells, and the combination of CDMs and topography will be a powerful approach for material-driven osteogenesis.

Fabrication of Injectable, Porous Hyaluronic Acid Hydrogel Based on an In-Situ Bubble-Forming Hydrogel Entrapment Process

Injectable hydrogels have been widely applied in the field of regenerative medicine. However, current techniques for injectable hydrogels are facing a challenge when trying to generate a biomimetic, porous architecture that is well-acknowledged to facilitate cell behaviors. In this study, an injectable, interconnected, porous hyaluronic acid (HA) hydrogel based on an in-situ bubble self-generation and entrapment process was developed. Through an amide reaction between HA and cystamine dihydrochloride activated by EDC/NHS, CO2 bubbles were generated and were subsequently entrapped inside the substrate due to a rapid gelation-induced retention effect. HA hydrogels with different molecular weights and concentrations were prepared and the effects of the hydrogel precursor solution's concentration and viscosity on the properties of hydrogels were investigated. The results showed that HA10-10 (10 wt.%, MW 100,000 Da) and HA20-2.5 (2.5 wt.%, MW 200,000 Da) exhibited desirable gelation and obvious porous structure. Moreover, HA10-10 represented a high elastic modulus (32 kPa). According to the further in vitro and in vivo studies, all the hydrogels prepared in this study show favorable biocompatibility for desirable cell behaviors and mild host response. Overall, such an in-situ hydrogel with a self-forming bubble and entrapment strategy is believed to provide a robust and versatile platform to engineer injectable hydrogels for a variety of applications in tissue engineering, regenerative medicine, and personalized therapeutics.

Synthesis and Characterization of a Biocomposite Bone Bandage for Controlled Delivery of Bone-Active Drugs in Fracture Nonunions

Fracture nonunions are common in orthopedics and their treatment often involves multiple surgical interventions. The aim of this study was to fabricate and characterize a gelatin-nano-hydroxyapatite membrane (GM)-based bone bandage for controlled delivery of bio-active molecules; recombinant human bone morphogenic protein-2 (rhBMP-2) and zoledronic acid (ZA) to promote osteoinduction and prevent callus resorption, respectively. In vitro cell-material interaction experiments using MC3T3 cells seeded on the GM indicated good biocompatibility. rhBMP-2-functionalized GM promoted osteogenic differentiation of MC3T3 cells and the rhBMP-2 bio-activity thus remained, as indicated by increased levels of alkaline phosphatase compared to only GM. The GM released a small amount (1.1%) of rhBMP-2 in vitro over a period of 5 weeks, demonstrating a strong interaction of rhBMP-2 with the GM. In the first animal study, the GM specimens loaded with rhBMP-2 or with the combination of rhBMP-2 + ZA were placed in the abdominal muscle pouch of rats. In the GM + rhBMP-2 + ZA group, significantly higher bone volume (21.5 ± 5.9 vs 2.7 ± 1.0 mm³) and area (3.3 ± 2.3 vs 1.0 ± 0.4 mm²) of bone were observed compared to GM + rhBMP-2 after 4 weeks, as indicated by micro-computed tomography and histomorphometry, respectively. Finally, a nonunion model in rats was used to evaluate the efficacy of the GM bandage and bio-active molecules in healing of fracture nonunions. The GM functionalized with rhBMP-2 + ZA led to higher bone formation around the fracture (63.9 ± 19.0 vs 31.8 ± 3.7 mm³) and stronger fracture callus (110.8 ± 46.8 vs 45.6 ± 17.8 N) compared to the empty controls. However, the overall union rate was only marginally improved. The GM alone or combined with ZA did not aid in bone healing in this model. Thus, this study shows that controlled delivery of rhBMP-2 + ZA via the developed GM is a promising approach that could aid in earlier full load bearing in patients with nonunion.

The Implication of Spatial Statistics in Human Mesenchymal Stem Cell Response to Nanotubular Architectures

INTRODUCTION

In recent years there has been ample interest in nanoscale modifications of synthetic biomaterials to understand fundamental aspects of cell-surface interactions towards improved biological outcomes. In this study, we aimed at closing in on the effects of nanotubular TiO2 surfaces with variable nanotopography on the response on human mesenchymal stem cells (hMSCs). Although the influence of TiO2 nanotubes on the cellular response, and in particular on hMSC activity, has already been addressed in the past, previous studies overlooked critical morphological, structural and physical aspects that go beyond the simple nanotube diameter, such as spatial statistics.

METHODS

To bridge this gap, we implemented an extensive characterization of nanotubular surfaces generated by anodization of titanium with a focus on spatial structural variables including eccentricity, nearest neighbour distance (NND) and Voronoi entropy, and associated them to the hMSC response. In addition, we assessed the biological potential of a two-tiered honeycomb nanoarchitecture, which allowed the detection of combinatory effects that this hierarchical structure has on stem cells with respect to conventional nanotubular designs. We have combined experimental techniques, ranging from Scanning Electron (SEM) and Atomic Force (AFM) microscopy to Raman spectroscopy, with computational simulations to characterize and model nanotubular surfaces. We evaluated the cell response at 6 hrs, 1 and 2 days by fluorescence microscopy, as well as bone mineral deposition by Raman spectroscopy, demonstrating substrate-induced differential biological cueing at both the short- and long-term.

RESULTS

Our work demonstrates that the nanotube diameter is not sufficient to comprehensively characterize nanotubular surfaces and equally important parameters, such as eccentricity and wall thickness, ought to be included since they all contribute to the overall spatial disorder which, in turn, dictates the overall bioactive potential. We have also demonstrated that nanotubular surfaces affect the quality of bone mineral deposited by differentiated stem cells. Lastly, we closed in on the integrated effects exerted by the superimposition of two dissimilar nanotubular arrays in the honeycomb architecture.

DISCUSSION

This work delineates a novel approach for the characterization of TiO2 nanotubes which supports the incorporation of critical spatial structural aspects that have been overlooked in previous research. This is a crucial aspect to interpret cellular behaviour on nanotubular substrates. Consequently, we anticipate that this strategy will contribute to the unification of studies focused on the use of such powerful nanostructured surfaces not only for biomedical applications but also in other technology fields, such as catalysis.

Bioactive, degradable and multi-functional three-dimensional membranous scaffolds of bioglass and alginate composites for tissue regenerative applications

Multifunctional bioactive hydrogel ECM like membrane for 3D dynamic tissue/disease modelling.

Topography induced stiffness alteration of stem cells influences osteogenic differentiation

Topography-driven alterations to single cell stiffness rather than alterations in cell morphology, is the underlying driver for influencing cell biological processes, particularly stem cell differentiation.

Exosomes: A Novel Therapeutic Agent for Cartilage and Bone Tissue Regeneration

Despite traditionally treating autologous and allogeneic transplantation and emerging tissue engineering (TE)-based therapies, which have commonly performed in clinic for skeletal diseases, as the "gold standard" for care, undesirably low efficacy and other complications remain. Therefore, exploring new strategies with better therapeutic outcomes and lower incidences of unfavorable side effect is imperative. Recently, exosomes, secreted microvesicles of endocytic origin, have caught researcher's eyes in tissue regeneration fields, especially in cartilage and bone-related regeneration. Multiple researchers have demonstrated the crucial roles of exosomes throughout every developing stage of cartilage and bone tissue regeneration, indicating that there may be a potential therapeutic application of exosomes in future clinical use. Herein, we summarize the function of exosomes derived from the primary cells functioning in skeletal diseases and their restoration processes, therapeutic exosomes used to promote cartilage and bone repairing in recent research, and applications of exosomes within the setting of the TE matrix.

Nanotopography on titanium promotes osteogenesis via autophagy-mediated signaling between YAP and β-catenin

Nanostructured titanium implants are recognized for inducing osteogenesis, but the cell signal transductions related to topography are not fully understood. Implant topography is associated with the functionality of osteogenic transcription factors directed by β-catenin in the nucleus, and autophagic flux in the cytoplasm; YAP (Yes-associated protein) is implicated in the destruction of β-catenin in the cytoplasm and is susceptible to autophagic flux. This study investigated whether surface topography of the titanium implant modulates autophagy-lysosome degradation of cytoplasmic YAP. Titanium surfaces were modified with smooth, micro, or nanotopographies. Compared with the smooth and micro surfaces, nanotopography was associated with higher β-catenin nuclear translocation, osteogenic differentiation, and autophagy, and less cytoplasmic YAP. Blockade of the autophagy-lysosome pathway resulted in YAP retention in MC3T3-E1 cells. Cytoplasmic YAP restricted β-catenin nuclear translocation. In the nano surface group, β-catenin accumulation in the nucleus and expression of osteogenesis genes was improved. However, in the absence of cell-cell (confluent) contact, manipulation of YAP and β-catenin localization associated with topography-induced autophagy was lost. In summary, the osteogenesis observed in response to titanium implants with nanotopography involves a signaling link between YAP and β-catenin. STATEMENT OF SIGNIFICANCE: Titanium with rough topographical surfaces is extensively applied in orthopedic and dental clinics. However, the cellular response to topographies that promotes osteogenesis and underlying mechanisms are not fully understood. In this study, we modified titanium surfaces to produce smooth, micro, or nano topographies. Experiments indicated that the nanotopography induced a stronger autophagic response, leading to degraded cytoplasmic YAP. With the lower levels of YAP, β-catenin transported and accumulated in the nucleus to activate TCF/LEF transcription factors, resulting in stronger osteogenesis. Additionally, cell-cell contact was essential in the autophagy-mediated signaling link between YAP and β-catenin. Consequently, our investigation revealed a novel signal transduction in nanotopography-regulated osteogenesis, and supports the modification of biomaterial surfaces to maximize osseointegration.

Three-Dimensional Printed Scaffolds with Controlled Micro-/Nanoporous Surface Topography Direct Chondrogenic and Osteogenic Differentiation of Mesenchymal Stem Cells

The effect of topography in three-dimensional (3D) printed polymer scaffolds on stem cell differentiation is a significantly underexplored area. Compared to two-dimensional (2D) biomaterials on which various well-defined topographies have been incorporated and shown to direct a range of cell behaviors including adhesion, cytoskeleton organization, and differentiation, incorporating topographical features to 3D polymer scaffolds is challenging due to the difficulty of accessing the inside of a porous scaffold. Only the roughened strut surface has been introduced to 3D printed porous scaffolds. Here, a rapid, single-step 3D printing method to fabricate polymeric scaffolds consisting of microstruts (ca. 60 μm) with micro-/nanosurface pores (0.2-2.4 μm) has been developed based on direct ink writing of an agitated viscous polymer solution. The density, size, and alignment of these pores can be controlled by changing the degree of agitation or the speed of printing. Three-dimensional printed scaffolds with micro-/nanoporous struts enhanced chondrogenic and osteogenic differentiation of mesenchymal stem cells (MSCs) without soluble differentiation factors. The topography also selectively affected adhesion, morphology, and differentiation of MSC to chondrogenic and osteogenic lineages depending on the composition of the differentiation medium. This fabrication method can potentially be used for a wide range of polymers where desirable architecture and topography are required.

Multicellularity-interweaved bone regeneration of BMP-2-loaded scaffold with orchestrated kinetics of resorption and osteogenesis

Synchronization of material resorption and new bone formation is vital to achieve harmonious bone regeneration in the treatment of large bone defects. To exposit the resorption/osteogenesis properties in the guided bone repairing, rhBMP-2-loaded trimodal macro/micro/nano-porous bioactive glass scaffolds (TMS-rhBMP-2) were set as substrate model. We penetratingly investigated the particular function of hierarchical structure and incorporated rhBMP-2 in the resorption/osteogenesis, and dissected the cellular interplay throughout the regenerative procedure. The results suggested that rhBMP-2 significantly facilitated osteoclastogenesis-mediated scaffold degradation and strikingly up-regulated mesenchymal stem cells (MSCs)-involved osteogenesis in vitro. Further gene microarray and related proteins expression indicated that in the presence of rhBMP-2, MSCs rather than differentiated MSCs could exert synergistic effects on osteoclastogenesis, osteoclasts maturation and resorptive function; meanwhile, rhBMP-2-induced MSCs osteogenesis was also strengthened by the osteoclasts. In vivo micro-CT, X-ray, kinetic and histological analyses qualitatively and quantitively demonstrated the optimized coupling of bioresorption/osteogenesis and the most rapid regeneration in TMS-rhBMP-2. Consequently, with rhBMP-2 acted as ignitor and MSCs/osteoclasts interaction as booster, a harmonious bone regeneration was obtained. Besides, long-term magnetic resonance imaging (MRI) in virtue of Gd³⁺ suggested that the degradation products mainly distributed in liver and spleen, verifying the accumulation/discharge profiles and safety application of TMS-rhBMP-2 system in vivo. This study will not merely provide guidance for the design of clinical bone repairing materials, but shed substantial light on the multicell-mediated tissue regeneration.

Time-Phase Sequential Utilization of Adipose-Derived Mesenchymal Stem Cells on Mesoporous Bioactive Glass for Restoration of Critical Size Bone Defects

The effective transportation of oxygen, nutrients, and metabolic wastes through new blood vessel networks is key to the survival of engineered constructs in large bone defects. Adipose-derived mesenchymal stem cells (ADSCs), which are regarded as excellent candidates for both bone and blood vessel engineering, are the preferred option for the restoration of massive bone defects. Therefore, we propose to induce ADSCs into osteogenic and endothelial cells differently. A modified hierarchical mesoporous bioactive glass (MBG) scaffold with an enhanced compressive strength was constructed and prevascularized by seeding with endothelial-induced ADSCs (EI-ADSCs). The prevascularized scaffolds were combined with osteogenically induced ADSCs (OI-ADSCs) to repair critical-size bone defects. To validate the angiogenesis of the prevascularized MBG scaffolds in vivo, green fluorescent protein (GFP) was used to label EI-ADSCs. The labeled EI-ADSCs were demonstrated to survive and participate in vascularization at day 7 after subcutaneous implantation in nude mice by double immunofluorescence staining of CD31 and GFP. Regarding the restoration of critical size bone defects, early angiogenesis of rat femur plug defects was evaluated by perfusion of Microfil after 3 weeks. Compared to nonvascularized MBG carrying OI-ADSCs (MBG/OI-ADSCs) and non-cell-seeded MBG scaffolds, the prevascularized MBG carrying OI-ADSCs (Pv-MBG/OI-ADSCs) showed enhanced angiogenesis on the surface and interior. Through dynamic bone formation analysis with sequential fluorescent labeling and Van Gieson's picro-fuchsin staining, we found that the Pv-MBG/OI-ADSCs exhibited the highest mineral deposition rate after surgery, which may be contributed by rapid vascular anastomosis facilitating increased survival of the seeded OI-ADSCs and by the recruitment function for bone mesenchymal stem cells. Therefore, the strategy of time-phase sequential utilization of ADSCs on MBG scaffolds is a practical design for the repair of massive bone defects.

Shaping Cell Fate: Influence of Topographical Substratum Properties on Embryonic Stem Cells

Development of multicellular organisms is a highly orchestrated process, with cells responding to factors and features present in the extracellular milieu. Changes in the surrounding environment help decide the fate of cells at various stages of development. This review highlights recent research that details the effects of mechanical properties of the surrounding environment and extracellular matrix and the underlying molecular mechanisms that regulate the behavior of embryonic stem cells (ESCs). In this study, we review the role of mechanical properties during embryogenesis and discuss the effect of engineered microtopographies on ESC pluripotency.