Co-culture systems of osteoblasts and osteoclasts: Simulating in vitro bone remodeling in regenerative approaches

Calcium Phosphates: A Key to Next-Generation In Vitro Bone Modeling

The replication of bone physiology under laboratory conditions is a prime target behind the development of in vitro bone models. The model should be robust enough to elicit an unbiased response when stimulated experimentally, giving reproducible outcomes. In vitro bone tissue generation majorly requires the availability of cellular components, the presence of factors promoting cellular proliferation and differentiation, efficient nutrient supply, and a supporting matrix for the cells to anchor - gaining predefined topology. Calcium phosphates (CaP) are difficult to ignore while considering the above requirements of a bone model. Therefore, the current review focuses on the role of CaP in developing an in vitro bone model addressing the prerequisites of bone tissue generation. Special emphasis is given to the physico-chemical properties of CaP that promote osteogenesis, angiogenesis and provide sufficient mechanical strength for load-bearing applications. Finally, the future course of action is discussed to ensure efficient utilization of CaP in the in vitro bone model development field.

Unveiling the Role of Sik1 in Osteoblast Differentiation: Implications for Osteoarthritis

Osteoarthritis (OA) is a chronic degenerative disease characterized by subchondral osteosclerosis, mainly due to osteoblast activity. This research investigates the function of Sik1, a member of the AMP-activated protein kinase family, in OA. Proteomic analysis was conducted on clinical samples from 30 OA patients, revealing a negative correlation between Sik1 expression and OA. In vitro experiments utilized BMSCs to examine the effect of Sik1 on osteogenic differentiation. BMSCs were cultured and induced toward osteogenesis with specific media. Sik1 overexpression was achieved through lentiviral transfection, followed by analysis of osteogenesis-associated proteins using Western blotting, RT-qPCR, and alkaline phosphate staining. In vivo experiments involved destabilizing the medial meniscus in mice to establish an OA model, assessing the therapeutic potential of Sik1. The CT scans and histological staining were used to analyze subchondral bone alterations and cartilage damage. The findings show that Sik1 downregulation correlates with advanced OA and heightened osteogenic differentiation in BMSCs. Sik1 overexpression inhibits osteogenesis-related markers in vitro and reduces cartilage damage and subchondral osteosclerosis in vivo. Mechanistically, Sik1 modulates osteogenesis and subchondral bone changes through Runx2 activity regulation. The research emphasizes Sik1 as a promising target for treating OA, suggesting its involvement in controlling bone formation and changes in the subchondral osteosclerosis.

Current Perspectives of Protein in Bone Tissue Engineering: Bone Structure, Ideal Scaffolds, Fabrication Techniques, Applications, Scopes, and Future Advances

In view of their exceptional approach, excellent inherent biocompatibility and biodegradability properties, and interaction with the local extracellular matrix, protein-based polymers have received attention in bone tissue engineering, which is a multidisciplinary field that repairs and regenerates fractured bones. Bone is a multihierarchical complex structure, and it performs several essential biofunctions, including maintaining mineral balance and structural support and protecting soft organs. Protein-based polymers have gained interest in developing ideal scaffolds as emerging biomaterials for bone fractured healing and regeneration, and it is challenging to design ideal bone substitutes as perfect biomaterials. Several protein-based polymers, including collagen, keratin, gelatin, serum albumin, etc., are potential materials due to their inherent cytocompatibility, controlled biodegradability, high biofunctionalization, and tunable mechanical characteristics. While numerous studies have indicated the encouraging possibilities of proteins in BTE, there are still major challenges concerning their biodegradability, stability in physiological conditions, and continuous release of growth factors and bioactive molecules. Robust scaffolds derived from proteins can be used to replace broken or diseased bone with a biocompatible substitute; proteins, being biopolymers, provide excellent scaffolds for bone tissue engineering. Herein, recent developments in protein polymers for cutting-edge bone tissue engineering are addressed in this review within 3-5 years, with a focus on the significant challenges and future perspectives. The first section discusses the structural fundamentals of bone anatomy and ideal scaffolds, and the second section describes the fabrication techniques of scaffolds. The third section highlights the importance of proteins and their applications in BTE. Hence, the recent development of protein polymers for state-of-the-art bone tissue engineering has been discussed, highlighting the significant challenges and future perspectives.

Bone cells influence the degradation interface of pure Mg and WE43 materials: Insights from multimodal in vitro analysis

In this study, the interaction of pure Mg and WE43 alloy under the presence of osteoblast (OB) and osteoclast (OC) cells and their influence on the degradation of materials have been deeply analyzed. Since OB and OC interaction has an important role in bone remodeling, we examined the surface morphology and dynamic changes in the chemical composition and thickness of the corrosion layers formed on pure Mg and WE43 alloy by direct monoculture and coculture of pre-differentiated OB and OC cells in vitro. Electrochemical techniques examined the corrosion performance. The corrosion products were characterized using a combination of the focused ion beam (FIB), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). Cell viability and morphology were assessed by fluorescent microscopy and SEM. Our findings demonstrate cell spread and attachment variations, which differ depending on the Mg substrates. It was clearly shown that cell culture groups delayed degradation processes with the lowest corrosion rate observed in the presence of OBOC coculture for the WE43 substrate. Ca-P enrichment was observed in the outer-middle region of the corrosion layer but only after 7 days of OBOC coculture on WE43 and after 14 days on the pure Mg specimens. STATEMENT OF SIGNIFICANCE: Magnesium metallic materials that can degrade over time provide distinct opportunities for orthopedic application. However, there is still a lack, especially in elucidating cell-material interface characterization. This study investigated the influence of osteoblast-osteoclast coculture in direct Mg-material contact. Our findings demonstrated that pre-differentiated osteoblasts and osteoclasts cocultured on Mg substrates influenced the chemistry of the corrosion layers. The cell spread and attachment were Mg substrate-dependent. The findings of coculturing bone cells directly on Mg materials within an in vitro model provide an effective approach for studying the dynamic degradation processes of Mg alloys while also elucidating cell behavior and their potential contribution to the degradation of these alloys.

Targeting Osteoblast-Osteoclast Cross-Talk Bone Homeostasis Repair Microcarriers Promotes Intervertebral Fusion in Osteoporotic Rats

Balancing osteoblast-osteoclast (OB-OC) cross-talk is crucial for restoring bone tissue structure and function. Current clinical drugs targeting either osteogenesis or osteoclastogenesis fail to effectively regulate cross-talk, impeding efficient bone repair in osteoporosis patients. Ubiquitin-specific protease 26 (USP26) is shown to coordinate OB-OC cross-talk by independently regulating β-catenin and Iκb-α. However, effective drugs for activating USP26 are still lacking. Here, they constructed bone homeostasis repair microcarriers (BHRC) that encapsulate Usp26 mRNA-loaded lipid nanoparticles (mRNA@LNP) within MMPs-responsive GelMA hydrogel microspheres. These microcarriers target the osteoporotic microenvironment and regulate OB-OC cross-talk, thereby facilitating intervertebral fusion in osteoporotic rats. Results demonstrate that mRNA@LNP exhibits uniform particle size and high transfection efficiency, while GelMA hydrogel microspheres possess excellent biocompatibility and MMP responsiveness, providing favorable cell survival space and controllable release of mRNA@LNP. The released LNP upregulates USP26 protein expression, effectively promoting osteogenesis while suppressing osteoclast formation. In vivo experiments show that injecting BHRC into the defect site of intervertebral discs in osteoporotic rats significantly promotes tail vertebrae fusion by responding to the microenvironment and regulating cell-to-cell cross-talk. Thus, the BHRC holds great potential in regulating osteoporotic homeostasis, particularly in challenging bone defects such as intervertebral fusion in osteoporotic environments.

Recent Advances in 3D Printing of Smart Scaffolds for Bone Tissue Engineering and Regeneration

The repair and functional reconstruction of bone defects resulting from severe trauma, surgical resection, degenerative disease, and congenital malformation pose significant clinical challenges. Bone tissue engineering (BTE) holds immense potential in treating these severe bone defects, without incurring prevalent complications associated with conventional autologous or allogeneic bone grafts. 3D printing technology enables control over architectural structures at multiple length scales and has been extensively employed to process biomimetic scaffolds for BTE. In contrast to inert and functional bone grafts, next-generation smart scaffolds possess a remarkable ability to mimic the dynamic nature of native extracellular matrix (ECM), thereby facilitating bone repair and regeneration. Additionally, they can generate tailored and controllable therapeutic effects, such as antibacterial or antitumor properties, in response to exogenous and/or endogenous stimuli. This review provides a comprehensive assessment of the progress of 3D-printed smart scaffolds for BTE applications. It begins with an introduction to bone physiology, followed by an overview of 3D printing technologies utilized for smart scaffolds. Notable advances in various stimuli-responsive strategies, therapeutic efficacy, and applications of 3D-printed smart scaffolds are discussed. Finally, the review highlights the existing challenges in the development and clinical implementation of smart scaffolds, as well as emerging technologies in this field.

Evaluation of in vitro osteoblast and osteoclast differentiation from stem cell: a systematic review of morphological assays and staining techniques

Background

Understanding human stem cell differentiation into osteoblasts and osteoclasts is crucial for bone regeneration and disease modeling. Numerous morphological techniques have been employed to assess this differentiation, but a comprehensive review of their application and effectiveness is lacking.

Methods

Guided by the PRISMA framework, we conducted a rigorous search through the PubMed, Web of Science and Scopus databases, analyzing 254 articles. Each article was scrutinized against pre-defined inclusion criteria, yielding a refined selection of 14 studies worthy of in-depth analysis.

Results

The trends in using morphological approaches were identified for analyzing osteoblast and osteoclast differentiation. The three most used techniques for osteoblasts were Alizarin Red S (mineralization; six articles), von Kossa (mineralization; three articles) and alkaline phosphatase (ALP; two articles) followed by one article on Giemsa staining (cell morphology) and finally immunochemistry (three articles involved Vinculin, F-actin and Col1 biomarkers). For osteoclasts, tartrate-resistant acid phosphatase (TRAP staining) has the highest number of articles (six articles), followed by two articles on DAPI staining (cell morphology), and immunochemistry (two articles with VNR, Cathepsin K and TROP2. The study involved four stem cell types: peripheral blood monocyte, mesenchymal, dental pulp, and periodontal ligament.

Conclusion

This review offers a valuable resource for researchers, with Alizarin Red S and TRAP staining being the most utilized morphological procedures for osteoblasts and osteoclasts, respectively. This understanding provides a foundation for future research in this rapidly changing field.

A New Tissue Engineering Strategy to Promote Tendon-bone Healing: Regulation of Osteogenic and Chondrogenic Differentiation of Tendon-derived Stem Cells

In the field of sports medicine, repair surgery for anterior cruciate ligament (ACL) and rotator cuff (RC) injuries are remarkably common. Despite the availability of relatively effective treatment modalities, outcomes often fall short of expectations. This comprehensive review aims to thoroughly examine current strategies employed to promote tendon-bone healing and analyze pertinent preclinical and clinical research. Amidst ongoing investigations, tendon-derived stem cells (TDSCs), which have comparatively limited prior exploration, have garnered increasing attention in the context of tendon-bone healing, emerging as a promising cell type for regenerative therapies. This review article delves into the potential of combining TDSCs with tissue engineering methods, with ACL reconstruction as the main focus. It comprehensively reviews relevant research on ACL and RC healing to address the issues of graft healing and bone tunnel integration. To optimize tendon-bone healing outcomes, our emphasis lies in not only reconstructing the original microstructure of the tendon-bone interface but also achieving proper bone tunnel integration, encompassing both cartilage and bone formation. In this endeavor, we thoroughly analyze the transcriptional and molecular regulatory variables governing TDSCs differentiation, incorporating a retrospective analysis utilizing single-cell sequencing, with the aim of unearthing relevant signaling pathways and processes. By presenting a novel strategy rooted in TDSCs-driven osteogenic and chondrogenic differentiation for tendon-bone healing, this study paves the way for potential future research avenues and promising therapeutic applications. It is anticipated that the findings herein will contribute to advancing the field of tendon-bone healing and foster the exploration of TDSCs as a viable option for regenerative therapies in the future.

Advancing bone regeneration: Unveiling the potential of 3D cell models in the evaluation of bone regenerative materials

Bone, a rigid yet regenerative tissue, has garnered extensive attention for its impressive healing abilities. Despite advancements in understanding bone repair and creating treatments for bone injuries, handling nonunions and large defects remains a major challenge in orthopedics. The rise of bone regenerative materials is transforming the approach to bone repair, offering innovative solutions for nonunions and significant defects, and thus reshaping orthopedic care. Evaluating these materials effectively is key to advancing bone tissue regeneration, especially in difficult healing scenarios, making it a critical research area. Traditional evaluation methods, including two-dimensional cell models and animal models, have limitations in predicting accurately. This has led to exploring alternative methods, like 3D cell models, which provide fresh perspectives for assessing bone materials' regenerative potential. This paper discusses various techniques for constructing 3D cell models, their pros and cons, and crucial factors to consider when using these models to evaluate bone regenerative materials. We also highlight the significance of 3D cell models in the in vitro assessments of these materials, discuss their current drawbacks and limitations, and suggest future research directions. STATEMENT OF SIGNIFICANCE: This work addresses the challenge of evaluating bone regenerative materials (BRMs) crucial for bone tissue engineering. It explores the emerging role of 3D cell models as superior alternatives to traditional methods for assessing these materials. By dissecting the construction, key factors of evaluating, advantages, limitations, and practical considerations of 3D cell models, the paper elucidates their significance in overcoming current evaluation method shortcomings. It highlights how these models offer a more physiologically relevant and ethically preferable platform for the precise assessment of BRMs. This contribution is particularly significant for "Acta Biomaterialia" readership, as it not only synthesizes current knowledge but also propels the discourse forward in the search for advanced solutions in bone tissue engineering and regeneration.

Advances of 3D Cell Co-Culture Technology Based on Microfluidic Chips

Cell co-culture technology aims to study the communication mechanism between cells and to better reveal the interactions and regulatory mechanisms involved in processes such as cell growth, differentiation, apoptosis, and other cellular activities. This is achieved by simulating the complex organismic environment. Such studies are of great significance for understanding the physiological and pathological processes of multicellular organisms. As an emerging cell cultivation technology, 3D cell co-culture technology, based on microfluidic chips, can efficiently, rapidly, and accurately achieve cell co-culture. This is accomplished by leveraging the unique microchannel structures and flow characteristics of microfluidic chips. The technology can simulate the native microenvironment of cell growth, providing a new technical platform for studying intercellular communication. It has been widely used in the research of oncology, immunology, neuroscience, and other fields. In this review, we summarize and provide insights into the design of cell co-culture systems on microfluidic chips, the detection methods employed in co-culture systems, and the applications of these models.

Evaluation of Biomarkers of Bone Metabolism on Salivary Matrix in the Remodeling of Periodontal Tissue during Orthodontic Treatment

The aim of this study was to evaluate changes in the concentration of N-terminal type I collagen extension pro-peptide (PINP), tartrate-resistant acid phosphatase (TRAcP), and parathyroid hormone-related protein (PTHrP) in saliva during orthodontic treatment in order to evaluate whether changes in bone turnover marker (BTM) concentration can help highlight the effects of orthodontic mechanical loading in the absence of clinical evidence of tooth movement in terms of tooth movement. Saliva samples from 25 apparently healthy young subjects (10 females and 15 males) were collected using Salivette® (Sarstedt) with cotton swabs and the concentrations of PTHrP, TRAcP 5b, and PINP were analyzed at time 0 (T1), 25 days (T2), and at 45 days (T3). Differences in the median value of biomarker levels between baseline T1 and follow-up of the different groups (T2 and T3) were assessed using the non-parametric Mann-Whitney U test. Trough concentrations of P1NP, PTHrP, and TRAcP were 0.80 µg/L, 0.21 ng/mL, and 0.90 U/L above the method LOD. The non-parametric Mann-Whitney U test confirmed a statistically significant difference in T1 versus concentrations of T2 and T3. All subjects evaluated had a statistically significant difference between T1 vs. T3. when compared with the specific critical difference (RCV) for the analyte The results obtained demonstrate that the evaluation of BTM changes in saliva can help the evaluation of orthodontic procedures and the monitoring of biomechanical therapy.

Proteomics Analysis of Interactions between Drug-Resistant and Drug-Sensitive Cancer Cells: Comparative Studies of Monoculture and Coculture Cell Systems

Cell-cell interactions, which allow cells to communicate with each other through molecules in their microenvironment, are critical for the growth, health, and functions of cells. Previous studies show that drug-resistant cells can interact with drug-sensitive cells to elevate their drug resistance level, which is partially responsible for cancer recurrence. Studying protein targets and pathways involved in cell-cell communication provides essential information for fundamental cell biology studies and therapeutics of human diseases. In the current studies, we performed direct coculture and indirect coculture of drug-resistant and drug-sensitive cell lines, aiming to investigate intracellular proteins responsible for cell communication. Comparative studies were carried out using monoculture cells. Shotgun bottom-up proteomics results indicate that the P53 signaling pathway has a strong association with drug resistance mechanisms, and multiple TP53-related proteins were upregulated in both direct and indirect coculture systems. In addition, cell-cell communication pathways, including the phagosome and the HIF-signaling pathway, contribute to both direct and indirect coculture systems. Consequently, AK3 and H3-3A proteins were identified as potential targets for cell-cell interactions that are relevant to drug resistance mechanisms. We propose that the P53 signaling pathway, in which mitochondrial proteins play an important role, is responsible for inducing drug resistance through communication between drug-resistant and drug-sensitive cancer cells.

Engineering Innervated Musculoskeletal Tissues for Regenerative Orthopedics and Disease Modeling

Musculoskeletal (MSK) disorders significantly burden patients and society, resulting in high healthcare costs and productivity loss. These disorders are the leading cause of physical disability, and their prevalence is expected to increase as sedentary lifestyles become common and the global population of the elderly increases. Proper innervation is critical to maintaining MSK function, and nerve damage or dysfunction underlies various MSK disorders, underscoring the potential of restoring nerve function in MSK disorder treatment. However, most MSK tissue engineering strategies have overlooked the significance of innervation. This review first expounds upon innervation in the MSK system and its importance in maintaining MSK homeostasis and functions. This will be followed by strategies for engineering MSK tissues that induce post-implantation in situ innervation or are pre-innervated. Subsequently, research progress in modeling MSK disorders using innervated MSK organoids and organs-on-chips (OoCs) is analyzed. Finally, the future development of engineering innervated MSK tissues to treat MSK disorders and recapitulate disease mechanisms is discussed. This review provides valuable insights into the underlying principles, engineering methods, and applications of innervated MSK tissues, paving the way for the development of targeted, efficacious therapies for various MSK conditions.

Osteogenic mechanism of chlorogenic acid and its application in clinical practice

Natural polyphenols may have a role in counteracting oxidative stress, which is associated with aging and several bone-related diseases. Chlorogenic acid (CGA) is a naturally occurring polyphenolic compound formed by the esterification of caffeic and quininic acids with osteogenic, antioxidant, and anti-inflammatory properties. This review discusses the potential of CGA to enhance osteogenesis by increasing the osteogenic capacity of mesenchymal stem cells (MSCs), osteoblast survival, proliferation, differentiation, and mineralization, as well as its ability to attenuate osteoclastogenesis by enhancing osteoclast apoptosis and impeding osteoclast regeneration. CGA can be involved in bone remodeling by acting directly on pro-osteoclasts/osteoblasts or indirectly on osteoclasts by activating the nuclear factor kB (RANK)/RANK ligand (RANKL)/acting osteoprotegerin (OPG) system. Finally, we provide perspectives for using CGA to treat bone diseases.

Aggregation of human osteoblasts unlocks self-reliant differentiation and constitutes a microenvironment for 3D-co-cultivation with other bone marrow cells

Skeletal bone function relies on both cells and cellular niches, which, when combined, provide guiding cues for the control of differentiation and remodeling processes. Here, we propose an in vitro 3D model based on human fetal osteoblasts, which eases the study of osteocyte commitment in vitro and thus provides a means to examine the influences of biomaterials, substances or cells on the regulation of these processes. Aggregates were formed from human fetal osteoblasts (hFOB1.19) and cultivated under proliferative, adipo- and osteoinductive conditions. When cultivated under osteoinductive conditions, the vitality of the aggregates was compromised, the expression levels of the mineralization-related gene DMP1 and the amount of calcification and matrix deposition were lower, and the growth of the spheroids stalled. However, within spheres under growth conditions without specific supplements, self-organization processes occur, which promote extracellular calcium deposition, and osteocyte-like cells develop. Long-term cultivated hFOB aggregates were free of necrotic areas. Moreover, hFOB aggregates cultivated under standard proliferative conditions supported the co-cultivation of human monocytes, microvascular endothelial cells and stromal cells. Overall, the model presented here comprises a self-organizing and easily accessible 3D osteoblast model for studying bone marrow formation and in vitro remodeling and thus provides a means to test druggable molecular pathways with the potential to promote life-long bone formation and remodeling.

Investigating the Effects and Mechanisms of Cyclomorusin on Osteoclasts in a High Glucose Environment

Diabetes mellitus is an endocrine disease characterized by prolonged hyperglycemia. Prolonged high blood sugar levels interfere with the differentiation and maturation process of OBs and OCs, leading to the onset of osteoporosis. However, OCs differentiation and maturation is a complex regulatory process. In this study, we used a co-culture system of RAW264.7 and MC3T3-E1 cells under HG concentration to explore the effect of CYM on OCs in a HG environment. The effects of CYM on the formation and function of OCs were observed using TRAP-positive cell counts and bone resorption pits. Then, mRNA and protein expression levels of OCs-related genes were detected by real-time qPCR and western blotting. The results showed that CYM had an inhibitory effect on OCs differentiation and bone resorption, reduced mRNAs expression of OCs-associated genes, and downregulated RANKL/RANK/TRAF6 pathway that mediates OCs differentiation. CYM could be a promising natural compound against diabetic osteoporosis.

The interplay of rheumatoid arthritis and osteoporosis: exploring the pathogenesis and pharmacological approaches

Rheumatoid arthritis (RA) and osteoporosis are two chronic disorders that are often seen together. RA is an autoimmune disorder that causes pain and inflammation in the joints, while osteoporosis is a disorder in which the bones become weak and fragile. Risk factors for bone loss in RA include disease activity, longer disease duration, erosive disease, autoantibody positivity, and joint damage leading to impaired physical activity. Recent research has shown that there is a complex interplay between immune cells, cytokines, and bone remodeling processes in both RA and osteoporosis. The bone remodeling process is regulated by cytokines and immune system signaling pathways, with osteoclasts activated through the RANK/RANKL/OPG pathway and the Wnt/DKK1/sclerostin pathway. Understanding these mechanisms can aid in developing targeted therapies for treatment of osteoporosis in RA patients. Current pharmacological approaches include anti-osteoporotic drugs such as bisphosphonates, denosumab, teriparatide, abaloparatide, raloxifene, and romosozumab. Conventional disease-modifying antirheumatic drugs such as methotrexate and biologicals including TNF inhibitors, IL-6 inhibitors, rituximab, and abatacept lower disease activity in RA and can improve bone metabolism by reducing inflammation but have limited impact on bone mineral density. This review will shed light on the relationship between osteoporosis and rheumatoid arthritis as well as the various factors that influence the onset of osteoporosis in RA patients. We also explore several treatment approaches to effectively managing osteoporosis in RA patients.

Establishment and validation of an efficient method for the 3D culture of osteoclasts in vitro

INTRODUCTION

Osteoclasts (OCs) play a crucial role in maintaining bone health. Changes in OC activity are linked to different bone diseases, making them an intriguing focus for research. However, most studies on OCs have relied on 2D cultures, limiting our understanding of their behavior. Yet, there's a lack of knowledge regarding platforms that effectively support osteoclast formation in 3D cultures.

METHODS

In our investigation, we explored the capacity of collagen and GelMA hydrogels to facilitate osteoclast development in 3D culture settings. We assessed the osteoclast development by using different hydrogels and cell seeding strategies and optimizing cell seeding density and cytokine concentration. The osteoclast development in 3D cultures was further validated by biochemical assays and immunochemical staining.

RESULTS

Our findings revealed that 0.3 % (w/v) collagen was conducive to osteoclast formation in both 2D and 3D cultures, demonstrated by increased multinucleation and higher TRAP activity compared to 0.6 % collagen and 5 % to 10 % (w/v) GelMA hydrogels. Additionally, we devised a "sandwich" technique using collagen substrates and augmented the initial macrophage seeding density and doubling cytokine concentrations, significantly enhancing the efficiency of OC culture in 3D conditions. Notably, we validated osteoclasts derived from macrophages in our 3D cultures express key osteoclast markers like cathepsin K and TRAP.

CONCLUSIONS

To conclude, our study contributes to establishing an effective method for cultivating osteoclasts in 3D environments in vitro. This innovative approach not only promises a more physiologically relevant platform to study osteoclast behavior during bone remodeling but also holds potential for applications in bone tissue engineering.

CLINICAL SIGNIFICANCE

This study introduces an efficient method for cultivating osteoclasts in 3D environments in vitro. It offers a more physiologically relevant platform to investigate osteoclast behavior and holds promise to advance research in bone biology and regenerative dentistry.

Cell Instructive Behavior of Composite Scaffolds in a Co-Culture of Human Mesenchymal Stem Cells and Peripheral Blood Mononuclear Cells

The in vitro evaluation of 3D scaffolds for bone tissue engineering in mono-cultures is a common practice; however, it does not represent the native complex nature of bone tissue. Co-cultures of osteoblasts and osteoclasts, without the addition of stimulating agents for monitoring cellular cross-talk, remains a challenge. In this study, a growth factor-free co-culture of human bone marrow-derived mesenchymal stem cells (hBM-MSCs) and human peripheral blood mononuclear cells (hPBMCs) has been established and used for the evaluation of 3D-printed scaffolds for bone tissue engineering. The scaffolds were produced from PLLA/PCL/PHBV polymeric blends, with two composite materials produced through the addition of 2.5% w/v nanohydroxyapatite (nHA) or strontium-substituted nanohydroxyapatite (Sr-nHA). Cell morphology data showed that hPBMCs remained undifferentiated in co-culture, while no obvious differences were observed in the mono- and co-cultures of hBM-MSCs. A significantly increased alkaline phosphatase (ALP) activity and osteogenic gene expression was observed in co-culture on Sr-nHA-containing scaffolds. Tartrate-resistant acid phosphatase (TRAP) activity and osteoclastogenic gene expression displayed significantly suppressed levels in co-culture on Sr-nHA-containing scaffolds. Interestingly, mono-cultures of hPBMCs on Sr-nHA-containing scaffolds indicated a delay in osteoclasts formation, as evidenced from TRAP activity and gene expression, demonstrating that strontium acts as an osteoclastogenesis inhibitor. This co-culture study presents an effective 3D model to evaluate the regenerative capacity of scaffolds for bone tissue engineering, thus minimizing time-consuming and costly in vivo experiments.

Pathological progression of osteoarthritis: a perspective on subchondral bone

Osteoarthritis (OA) is a degenerative bone disease associated with aging. The rising global aging population has led to a surge in OA cases, thereby imposing a significant socioeconomic burden. Researchers have been keenly investigating the mechanisms underlying OA. Previous studies have suggested that the disease starts with synovial inflammation and hyperplasia, advancing toward cartilage degradation. Ultimately, subchondral-bone collapse, sclerosis, and osteophyte formation occur. This progression is deemed as "top to bottom." However, recent research is challenging this perspective by indicating that initial changes occur in subchondral bone, precipitating cartilage breakdown. In this review, we elucidate the epidemiology of OA and present an in-depth overview of the subchondral bone's physiological state, functions, and the varied pathological shifts during OA progression. We also introduce the role of multifunctional signal pathways (including osteoprotegerin (OPG)/receptor activator of nuclear factor-kappa B ligand (RANKL)/receptor activator of nuclear factor-kappa B (RANK), and chemokine (CXC motif) ligand 12 (CXCL12)/CXC motif chemokine receptor 4 (CXCR4)) in the pathology of subchondral bone and their role in the "bottom-up" progression of OA. Using vivid pattern maps and clinical images, this review highlights the crucial role of subchondral bone in driving OA progression, illuminating its interplay with the condition.

Advances in 3D bioprinting for regenerative medicine applications

Biofabrication techniques allow for the construction of biocompatible and biofunctional structures composed from biomaterials, cells and biomolecules. Bioprinting is an emerging 3D printing method which utilizes biomaterial-based mixtures with cells and other biological constituents into printable suspensions known as bioinks. Coupled with automated design protocols and based on different modes for droplet deposition, 3D bioprinters are able to fabricate hydrogel-based objects with specific architecture and geometrical properties, providing the necessary environment that promotes cell growth and directs cell differentiation towards application-related lineages. For the preparation of such bioinks, various water-soluble biomaterials have been employed, including natural and synthetic biopolymers, and inorganic materials. Bioprinted constructs are considered to be one of the most promising avenues in regenerative medicine due to their native organ biomimicry. For a successful application, the bioprinted constructs should meet particular criteria such as optimal biological response, mechanical properties similar to the target tissue, high levels of reproducibility and printing fidelity, but also increased upscaling capability. In this review, we highlight the most recent advances in bioprinting, focusing on the regeneration of various tissues including bone, cartilage, cardiovascular, neural, skin and other organs such as liver, kidney, pancreas and lungs. We discuss the rapidly developing co-culture bioprinting systems used to resemble the complexity of tissues and organs and the crosstalk between various cell populations towards regeneration. Moreover, we report on the basic physical principles governing 3D bioprinting, and the ideal bioink properties based on the biomaterials' regenerative potential. We examine and critically discuss the present status of 3D bioprinting regarding its applicability and current limitations that need to be overcome to establish it at the forefront of artificial organ production and transplantation.

Fibroblasts and endothelial cells interplay drives hypertrophic scar formation: Insights from in vitro and in vivo models

Hypertrophic scar formation is influenced by the intricate interplay between fibroblasts and endothelial cells. In this study, we investigated this relationship using in vitro and in vivo models. Clinical observations revealed distinct morphological changes and increased vascularity at pathological scar sites. Further analysis using OCTA, immunohistochemistry, and immunofluorescence confirmed the involvement of angiogenesis in scar formation. Our indirect co-culture systems demonstrated that endothelial cells enhance the proliferation and migration of fibroblasts through the secretion of cytokines including VEGF, PDGF, bFGF, and TGF-β. Additionally, a suspended co-culture multicellular spheroid model revealed molecular-level changes associated with extracellular matrix remodeling, cellular behaviors, inflammatory response, and pro-angiogenic activity. Furthermore, KEGG pathway analysis identified the involvement of TGF-β, IL-17, Wnt, Notch, PI3K-Akt, and MAPK pathways in regulating fibroblasts activity. These findings underscore the critical role of fibroblasts-endothelial cells crosstalk in scar formation and provide potential targets for therapeutic intervention. Understanding the molecular mechanisms underlying this interplay holds promise for the development of innovative approaches to treat tissue injuries and diseases.

Bioactive Self-Regulated Liquified Microcompartments to Bioengineer Bone-Like Microtissues

Designing a microenvironment that drives autonomous stromal cell differentiation toward osteogenesis while recapitulating the complexity of bone tissue remains challenging. In the current study, bone-like microtissues are created using electrohydrodynamic atomization to form two distinct liquefied microcapsules (mCAPs): i) hydroxypyridinone (HOPO)-modified gelatin (GH mCAPs, 7.5% w/v), and ii) HOPO-modified gelatin and dopamine-modified gelatin (GH+GD mCAPs, 7.5%+1.5% w/v). The ability of HOPO to coordinate with iron ions at physiological pH allows the formation of a semipermeable micro-hydrogel shell. In turn, the dopamine affinity for calcium ions sets a bioactive milieu for bone-like microtissues. After 21 days post encapsulation, GH and GH+GD mCAPs potentiate autonomous osteogenic differentiation of mesenchymal stem cells accompanied by collagen type-I gene upregulation, increased alkaline phosphatase (ALP) expression, and formation of mineralized extracellular matrix. However, the GH+GD mCAPs show higher levels of osteogenic markers starting on day 14, translating into a more advanced and organized mineralized matrix. The GH+GD system also shows upregulation of the receptor activator of nuclear factor kappa-B ligand (RANK-L) gene, enabling the autonomous osteoclastic differentiation of monocytes. These catechol-based mCAPs offer a promising approach to designing multifunctional and autonomous bone-like microtissues to study in vitro bone-related processes at the cell-tissue interface, angiogenesis, and osteoclastogenesis.

Temporal dynamics of immune-stromal cell interactions in fracture healing

Bone fracture repair is a complex, multi-step process that involves communication between immune and stromal cells to coordinate the repair and regeneration of damaged tissue. In the US, 10% of all bone fractures do not heal properly without intervention, resulting in non-union. Complications from non-union fractures are physically and financially debilitating. We now appreciate the important role that immune cells play in tissue repair, and the necessity of the inflammatory response in initiating healing after skeletal trauma. The temporal dynamics of immune and stromal cell populations have been well characterized across the stages of fracture healing. Recent studies have begun to untangle the intricate mechanisms driving the immune response during normal or atypical, delayed healing. Various in vivo models of fracture healing, including genetic knockouts, as well as in vitro models of the fracture callus, have been implemented to enable experimental manipulation of the heterogeneous cellular environment. The goals of this review are to (1): summarize our current understanding of immune cell involvement in fracture healing (2); describe state-of-the art approaches to study inflammatory cells in fracture healing, including computational and in vitro models; and (3) identify gaps in our knowledge concerning immune-stromal crosstalk during bone healing.

Biomimetic mineralization of platelet lysate/oxidized dextran cryogel as a macroporous 3D composite scaffold for bone repair

Scaffold development approaches using autologous sources for tissue repair are of great importance in obtaining bio-active/-compatible constructs. Platelet-rich plasma (PRP) containing various growth factors and platelet lysate (PL) derived from PRP are autologous products that have the potential to accelerate the tissue repair response by inducing a transient inflammatory event. Considering the regenerative capacity of PRP and PL, PRP/PL-based scaffolds are thought to hold great promise for tissue engineering as a natural source of autologous growth factors and a provider of mechanical support for cells. Here, a bio-mineralized PRP-based scaffold was developed using oxidized dextran (OD) and evaluated for future application in bone tissue engineering. Prepared PL/OD scaffolds were incubated in simulated body fluid (SBF) for 7, 14 and 21 d periods. Mineralized PL/OD scaffolds were characterized using Fourier transform infrared spectroscopy, x-ray diffraction spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis, porosity and compression tests. SEM and energy-dispersive x-ray spectroscopy analyses revealed mineral accumulation on the PL/OD scaffold as a result of SBF incubation.In vitrocytotoxicity andin vitrohemolysis tests revealed that the scaffolds were non-toxic and hemocompatible. Additionally, human osteoblasts (hOBs) exhibited good attachment and spreading behavior on the scaffolds and maintained their viability throughout the culture period. The alkaline phosphatase activity assay and calcium release results revealed that PL/OD scaffolds preserved the osteogenic properties of hOBs. Overall, findings suggest that mineralized PL/OD scaffold may be a promising scaffold for bone tissue engineering.

Oxygen generating biomaterials at the forefront of regenerative medicine: advances in bone regeneration

Globally, an annual count of more than two million bone transplants is conducted, with conventional treatments, including metallic implants and bone grafts, exhibiting certain limitations. In recent years, there have been significant advancements in the field of bone regeneration. Oxygen tension regulates cellular behavior, which in turn affects tissue regeneration through metabolic programming. Biomaterials with oxygen release capabilities enhance therapeutic effectiveness and reduce tissue damage from hypoxia. However, precise control over oxygen release is a significant technical challenge, despite its potential to support cellular viability and differentiation. The matrices often used to repair large-size bone defects do not supply enough oxygen to the stem cells being used in the regeneration process. Hypoxia-induced necrosis primarily occurs in the central regions of large matrices due to inadequate provision of oxygen and nutrients by the surrounding vasculature of the host tissues. Oxygen generating biomaterials (OGBs) are becoming increasingly significant in enhancing our capacity to facilitate the bone regeneration, thereby addressing the challenges posed by hypoxia or inadequate vascularization. Herein, we discussed the key role of oxygen in bone regeneration, various oxygen source materials and their mechanism of oxygen release, the fabrication techniques employed for oxygen-releasing matrices, and novel emerging approaches for oxygen delivery that hold promise for their potential application in the field of bone regeneration.

Functional and analytical recapitulation of osteoclast biology on demineralized bone paper

Osteoclasts are the primary target for osteoporosis drug development. Recent animal studies revealed the crucial roles of osteoblasts in regulating osteoclastogenesis and the longer lifespans of osteoclasts than previously thought with fission and recycling. However, existing culture platforms are limited to replicating these newly identified cellular processes. We report a demineralized bone paper (DBP)-based osteoblast culture and osteoclast assay platform that replicates osteoclast fusion, fission, resorption, and apoptosis with high fidelity and analytical power. An osteoid-inspired DBP supports rapid and structural mineral deposition by osteoblasts. Coculture osteoblasts and bone marrow monocytes under biochemical stimulation recapitulate osteoclast differentiation and function. The DBP-based bone model allows longitudinal quantitative fluorescent monitoring of osteoclast responses to bisphosphonate drug, substantiating significantly reducing their number and lifespan. Finally, we demonstrate the feasibility of humanizing the bone model. The DBP-based osteo assay platforms are expected to advance bone remodeling-targeting drug development with improved prediction of clinical outcomes.

CXCL chemokines-mediated communication between macrophages and BMSCs on titanium surface promotes osteogenesis via the actin cytoskeleton pathway

The refined functional cell subtypes in the immune microenvironment of specific titanium (Ti) surface and their collaborative role in promoting bone marrow mesenchymal stem cells (BMSCs) driven bone integration need to be comprehensively characterized. This study employed a simplified co-culture system to investigate the dynamic, temporal crosstalk between macrophages and BMSCs on the Ti surface. The M2-like sub-phenotype of macrophages, characterized by secretion of CXCL chemokines, emerges as a crucial mediator for promoting BMSC osteogenic differentiation and bone integration in the Ti surface microenvironment. Importantly, these two cells maintain their distinct functional phenotypes through a mutually regulatory interplay. The secretion of CXCL3, CXCL6, and CXCL14 by M2-like macrophages plays a pivotal role. The process activates CXCR2 and CCR1 receptors, triggering downstream regulatory effects on the actin cytoskeleton pathway within BMSCs, ultimately fostering osteogenic differentiation. Reciprocally, BMSCs secrete pleiotrophin (PTN), a key player in regulating macrophage differentiation. This secretion maintains the M2-like phenotype via the Sdc3 receptor-mediated cell adhesion molecules pathway. Our findings provide a novel insight into the intricate communication and mutual regulatory mechanisms operating between BMSCs and macrophages on the Ti surface, highlight specific molecular events governing cell-cell interactions in the osteointegration, inform the surface design of orthopedic implants, and advance our understanding of osteointegration.

Hydrogels and Bioprinting in Bone Tissue Engineering: Creating Artificial Stem-Cell Niches for In Vitro Models

Advances in bioprinting have enabled the fabrication of complex tissue constructs with high speed and resolution. However, there remains significant structural and biological complexity within tissues that bioprinting is unable to recapitulate. Bone, for example, has a hierarchical organization ranging from the molecular to whole organ level. Current bioprinting techniques and the materials employed have imposed limits on the scale, speed, and resolution that can be achieved, rendering the technique unable to reproduce the structural hierarchies and cell-matrix interactions that are observed in bone. The shift toward biomimetic approaches in bone tissue engineering, where hydrogels provide biophysical and biochemical cues to encapsulated cells, is a promising approach to enhancing the biological function and development of tissues for in vitro modeling. A major focus in bioprinting of bone tissue for in vitro modeling is creating dynamic microenvironmental niches to support, stimulate, and direct the cellular processes for bone formation and remodeling. Hydrogels are ideal materials for imitating the extracellular matrix since they can be engineered to present various cues whilst allowing bioprinting. Here, recent advances in hydrogels and 3D bioprinting toward creating a microenvironmental niche that is conducive to tissue engineering of in vitro models of bone are reviewed.

Recent advancement in vascularized tissue-engineered bone based on materials design and modification

Bone is one of the most vascular network-rich tissues in the body and the vascular system is essential for the development, homeostasis, and regeneration of bone. When segmental irreversible damage occurs to the bone, restoring its vascular system by means other than autogenous bone grafts with vascular pedicles is a therapeutic challenge. By pre-generating the vascular network of the scaffold in vivo or in vitro, the pre-vascularization technique enables an abundant blood supply in the scaffold after implantation. However, pre-vascularization techniques are time-consuming, and in vivo pre-vascularization techniques can be damaging to the body. Critical bone deficiencies may be filled quickly with immediate implantation of a supporting bone tissue engineered scaffold. However, bone tissue engineered scaffolds generally lack vascularization, which requires modification of the scaffold to aid in enhancing internal vascularization. In this review, we summarize the relationship between the vascular system and osteogenesis and use it as a basis to further discuss surgical and cytotechnology-based pre-vascularization strategies and to describe the preparation of vascularized bone tissue engineered scaffolds that can be implanted immediately. We anticipate that this study will serve as inspiration for future vascularized bone tissue engineered scaffold construction and will aid in the achievement of clinical vascularized bone.

Functionally Tailored Metal-Organic Framework Coatings for Mediating Ti Implant Osseointegration

Owing to their mechanical resilience and non-toxicity, titanium implants are widely applied as the major treatment modality for the clinical intervention against bone fractures. However, the intrinsic bioinertness of Ti and its alloys often impedes the effective osseointegration of the implants, leading to severe adverse complications including implant loosening, detachment, and secondary bone damage. Consequently, new Ti implant engineering strategies are urgently needed to improve their osseointegration after implantation. Remarkably, metalorganic frameworks (MOFs) are a class of novel synthetic material consisting of coordinated metal species and organic ligands, which have demonstrated a plethora of favorable properties for modulating the interfacial properties of Ti implants. This review comprehensively summarizes the recent progress in the development of MOF-coated Ti implants and highlights their potential utility for modulating the bio-implant interface to improve implant osseointegration, of which the discussions are outlined according to their physical traits, chemical composition, and drug delivery capacity. A perspective is also provided in this review regarding the current limitations and future opportunities of MOF-coated Ti implants for orthopedic applications. The insights in this review may facilitate the rational design of more advanced Ti implants with enhanced therapeutic performance and safety.

The Impact of Culture Variables on a 3D Human In Vitro Bone Remodeling Model: A Design of Experiments Approach

Human in vitro bone remodeling models, using osteoclast-osteoblast cocultures, can facilitate the investigation of human bone remodeling while reducing the need for animal experiments. Although current in vitro osteoclast-osteoblast cocultures have improved the understanding of bone remodeling, it is still unknown which culture conditions support both cell types. Therefore, in vitro bone remodeling models can benefit from a thorough evaluation of the impact of culture variables on bone turnover outcomes, with the aim to reach balanced osteoclast and osteoblast activity, mimicking healthy bone remodeling. Using a resolution III fractional factorial design, the main effects of commonly used culture variables on bone turnover markers in an in vitro human bone remodeling model are identified. This model is able to capture physiological quantitative resorption-formation coupling along all conditions. Culture conditions of two runs show promising results: conditions of one run can be used as a high bone turnover system and conditions of another run as a self-regulating system as the addition of osteoclastic and osteogenic differentiation factors is not required for remodeling. The results generated with this in vitro model allow for better translation between in vitro studies and in vivo studies, toward improved preclinical bone remodeling drug development.

Assembling the Puzzle Pieces. Insights for in Vitro Bone Remodeling

As a highly dynamic organ, bone changes during throughout a person's life. This process is referred to as 'bone remodeling' and it involves two stages - a well-balanced osteoclastic bone resorption and an osteoblastic bone formation. Under normal physiological conditions bone remodeling is highly regulated that ensures tight coupling between bone formation and resorption, and its disruption results in a bone metabolic disorder, most commonly osteoporosis. Though osteoporosis is one of the most prevalent skeletal ailments that affect women and men aged over 40 of all races and ethnicities, currently there are few, if any safe and effective therapeutic interventions available. Developing state-of-the-art cellular systems for bone remodeling and osteoporosis can provide important insights into the cellular and molecular mechanisms involved in skeletal homeostasis and advise better therapies for patients. This review describes osteoblastogenesis and osteoclastogenesis as two vital processes for producing mature, active bone cells in the context of interactions between cells and the bone matrix. In addition, it considers current approaches in bone tissue engineering, pointing out cell sources, core factors and matrices used in scientific practice for modeling bone diseases and testing drugs. Finally, it focuses on the challenges that bone regenerative medicine is currently facing.

Low-intensity pulsed ultrasound regulates osteoblast-osteoclast crosstalk via EphrinB2/EphB4 signaling for orthodontic alveolar bone remodeling

Background: The limited regenerative potential of periodontal tissue remains a challenge in orthodontic treatment, especially with respect to alveolar bone remodeling. The dynamic balance between the bone formation of osteoblasts and the bone resorption of osteoclasts controls bone homeostasis. The osteogenic effect of low-intensity pulsed ultrasound (LIPUS) is widely accepted, so LIPUS is expected to be a promising method for alveolar bone regeneration. Osteogenesis is regulated by the acoustic mechanical effect of LIPUS, while the cellular perception, transduction mode and response regulation mechanism of LIPUS stimuli are still unclear. This study aimed to explore the effects of LIPUS on osteogenesis by osteoblast-osteoclast crosstalk and the underlying regulation mechanism. Methods: The effects of LIPUS on orthodontic tooth movement (OTM) and alveolar bone remodeling were investigated via rat model by histomorphological analysis. Mouse bone marrow mesenchymal stem cells (BMSCs) and bone marrow monocytes (BMMs) were purified and used as BMSC-derived osteoblasts and BMM-derived osteoclasts, respectively. The osteoblast-osteoclast co-culture system was used to evaluate the effect of LIPUS on cell differentiation and intercellular crosstalk by Alkaline phosphatase (ALP), Alizarin Red S (ARS), tartrate-resistant acid phosphatase (TRAP) staining, real-time quantitative PCR, western blotting and immunofluorescence. Results: LIPUS was found to improve OTM and alveolar bone remodeling in vivo, promote differentiation and EphB4 expression in BMSC-derived osteoblasts in vitro, particularly when cells were directly co-cultured with BMM-derived osteoclasts. LIPUS enhanced EphrinB2/EphB4 interaction between osteoblasts and osteoclasts in alveolar bone, activated the EphB4 receptor on osteoblasts membrane, transduced LIPUS-related mechanical signals to the intracellular cytoskeleton, and gave rise to the nuclear translocation of YAP in Hippo signaling pathway, thus regulating cell migration and osteogenic differentiation. Conclusions: This study shows that LIPUS modulates bone homeostasis by osteoblast-osteoclast crosstalk via EphrinB2/EphB4 signaling, which benefits the balance between OTM and alveolar bone remodeling.

In vitro biological evaluation of epigallocatechin gallate (EGCG) release from three-dimensional printed (3DP) calcium phosphate bone scaffolds

Three-dimensional printed (3DP) tricalcium phosphate (TCP) scaffolds can guide bone regeneration, especially for patient-specific defect repair applications in low-load bearing sites. Epigallocatechin gallate (EGCG), a green tea compound, has gained attention as a safer alternative treatment for bone disorders. The 3DP TCP scaffold is designed for localized EGCG delivery, which can enhance in vitro osteogenic ability, anti-osteoclastogenic activity, vascularization formation, and chemoprevention. In the cocultures of human bone marrow-derived mesenchymal stem cells (hMSCs) and monocytes (THP-1), EGCG release enhances osteogenic differentiation of hMSCs at day 16 compared to the control; this is indicated by a 2.8- and 4.0-fold upregulation of Runt-related transcription factor 2 (Runx2) and bone gamma-carboxyglutamic acid-containing protein (BGLAP), the early and late osteoblast differentiation marker expressions. However, EGCG significantly downregulates the receptor activator of nuclear factor-κB ligand (RANKL) expression by 7.0-fold, indicating that EGCG suppresses RANKL-induced osteoclast maturation. EGCG also stimulates endothelial tube formation at as early as 3 hours when human umbilical vein endothelial cells (HUVECs) grow on Matrigel. It reduces human osteosarcoma MG-63 cell viability by 66% compared to the control at day 11. An in vitro release kinetics study demonstrates that EGCG shows a ∼64% release within a day followed by a sustained release in the physiological environment (pH 7.4) because its phenolic hydroxyl groups are easily deprotonated at physiological pH. These findings contribute to developing a multifunctional scaffold for the treatment of low load-bearing patient-specific bone defects after trauma and tumor excision.

Bone-on-a-Chip: Biomimetic Models Based on Microfluidic Technologies for Biomedical Applications

With the increasing importance of preclinical evaluation of newly developed drugs or treatments, in vitro organ or disease models are necessary. Although various organ-specific on-chip (organ-on-a-chip, or OOC) systems have been developed as emerging in vitro models, bone-on-a-chip (BOC) systems that recapitulate the bone microenvironment have been less developed or reviewed compared with other OOCs. The bone is one of the most dynamic organs and undergoes continuous remodeling throughout its lifetime. The aging population is growing worldwide, and healthcare costs are rising rapidly. Since in vitro BOC models that recapitulate native bone niches and pathological features can be important for studying the underlying mechanism of orthopedic diseases and predicting drug responses in preclinical trials instead of in animals, the development of biomimetic BOCs with high efficiency and fidelity will be accelerated further. Here, we review recently engineered BOCs developed using various microfluidic technologies and investigate their use to model the bone microenvironment. We have also explored various biomimetic strategies based on biological, geometrical, and biomechanical cues for biomedical applications of BOCs. Finally, we addressed the limitations and challenging issues of current BOCs that should be overcome to obtain more acceptable BOCs in the biomedical and pharmaceutical industries.

A Bone-Penetrating Precise Controllable Drug Release System Enables Localized Treatment of Osteoporotic Fracture Prevention via Modulating Osteoblast-Osteoclast Communication

Improving local bone mineral density (BMD) at fracture-prone sites of bone is a clinical concern for osteoporotic fracture prevention. In this study, a featured radial extracorporeal shock wave (rESW) responsive nano-drug delivery system (NDDS) is developed for local treatment. Based on a mechanic simulation, a sequence of hollow zoledronic acid (ZOL)-contained nanoparticles (HZNs) with controllable shell thickness that predicts various mechanical responsive properties is constructed by controlling the deposition time of ZOL and Ca²⁺ on liposome templates. Attributed to the controllable shell thickness, the fragmentation of HZNs and the release of ZOL and Ca²⁺ can be precisely controlled with the intervention of rESW. Furthermore, the distinct effect of HZNs with different shell thicknesses on bone metabolism after fragmentation is verified. In vitro co-culture experiments demonstrate that although HZN2 does not have the strongest osteoclasts inhibitory effect, the best pro-osteoblasts mineralization results are achieved via maintaining osteoblast-osteoclast (OB-OC) communication. In vivo, the HZN2 group also shows the strongest local BMD enhancement after rESW intervention and significantly improves bone-related parameters and mechanical properties in the ovariectomy (OVX)-induced osteoporosis (OP) rats. These findings suggest that an adjustable and precise rESW-responsive NDDS can effectively improve local BMD in OP therapy.

Closed-loop vasculature network design for bioprinting large, solid tissue scaffolds

Vascularization is an indispensable requirement for fabricating large solid tissues and organs. The natural vasculature derived from medical imaging modalities for large tissues and organs are highly complex and convoluted. However, the present bioprinting capabilities limit the fabrication of such complex natural vascular networks. Simplified bioprinted vascular networks, on the other hand, lack the capability to sustain large solid tissues. This work proposes a generalized and adaptable numerical model to design the vasculature by utilizing the tissue/organ anatomy. Starting with processing the patient's medical images, organ structure, tissue-specific cues, and key vasculature tethers are determined. An open-source abdomen magnetic resonance image dataset was used in this work. The extracted properties and cues are then used in a mathematical model for guiding the vascular network formation comprising arterial and venous networks. Next, the generated three-dimensional networks are used to simulate the nutrient transport and consumption within the organ over time and the regions deprived of the nutrients are identified. These regions provide cues to evolve and optimize the vasculature in an iterative manner to ensure the availability of the nutrient transport throughout the bioprinted scaffolds. The mass transport of six components of cell culture media-glucose, glycine, glutamine, riboflavin, human serum albumin, and oxygen was studied within the organ with designed vasculature. As the vascular structure underwent iterations, the organ regions deprived of these key components decreased significantly highlighting the increase in structural complexity and efficacy of the designed vasculature. The numerical method presented in this work offers a valuable tool for designing vascular scaffolds to guide the cell growth and maturation of the bioprinted tissues for faster regeneration post bioprinting.

High Bone Mass Disorders: New Insights From Connecting the Clinic and the Bench

Monogenic high bone mass (HBM) disorders are characterized by an increased amount of bone in general, or at specific sites in the skeleton. Here, we describe 59 HBM disorders with 50 known disease-causing genes from the literature, and we provide an overview of the signaling pathways and mechanisms involved in the pathogenesis of these disorders. Based on this, we classify the known HBM genes into HBM (sub)groups according to uniform Gene Ontology (GO) terminology. This classification system may aid in hypothesis generation, for both wet lab experimental design and clinical genetic screening strategies. We discuss how functional genomics can shape discovery of novel HBM genes and/or mechanisms in the future, through implementation of omics assessments in existing and future model systems. Finally, we address strategies to improve gene identification in unsolved HBM cases and highlight the importance for cross-laboratory collaborations encompassing multidisciplinary efforts to transfer knowledge generated at the bench to the clinic. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

The Gradual Release of Alendronate for the Treatment of Critical Bone Defects in Osteoporotic and Control Rats

Purpose

Osteoporosis is a severe health problem with social and economic impacts on society. The standard treatment consists of the systemic administration of drugs such as bisphosphonates, with alendronate (ALN) being one of the most common. Nevertheless, complications of systemic administration occur with this drug. Therefore, it is necessary to develop new strategies, such as local administration.

Methods

In this study, emulsion/dispersion scaffolds based on W/O emulsion of PCL and PF68 with ALN, containing hydroxyapatite (HA) nanoparticles as the dispersion phase were prepared using electrospinning. Scaffolds with different release kinetics were tested in vitro on the co-cultures of osteoblasts and osteoclast-like cells, isolated from adult osteoporotic and control rats. Cell viability, proliferation, ALP, TRAP and CA II activity were examined. A scaffold with a gradual release of ALN was tested in vivo in the bone defects of osteoporotic and control rats.

Results

The release kinetics were dependent on the scaffold composition and the used system of the poloxamers. The ALN was released from the scaffolds for more than 22 days. The behavior of cells cultured in vitro on scaffolds with different release kinetics was comparable. The difference was evident between cell co-cultures isolated from osteoporotic and control animals. The PCL/HA scaffold show slow degradation in vivo and residual scaffold limited new bone formation inside the defects. Nevertheless, the released ALN supported bone formation in the areas surrounding the residual scaffold. Interestingly, a positive effect of systemic administration of ALN was not proved.

Conclusion

The prepared scaffolds enabled tunable control release of ALN. The effect of ALN was proved in vitro and in in vivo study supported peri-implant bone formation.

Health-Promoting Activities and Associated Mechanisms of Polygonati Rhizoma Polysaccharides

Polygonati Rhizoma, a typical homology of medicine and food, possesses remarkable anti-fatigue, anti-aging, metabolic regulatory, immunomodulatory, anti-inflammatory, neuroprotective, anti-diabetes, and anti-cancer effects. Among bioactive phytochemicals in Polygonati Rhizoma, polysaccharides play important roles in the health-promoting activities through the mechanisms mentioned above and potential synergistic effects with other bioactives. In this review, we briefly introduce the updated biosynthesis of polysaccharides, the purification method, the structure characterization, and food applications, and discuss in detail the biological activities of Polygonati Rhizoma polysaccharides and associated mechanisms, aiming at broadening the usage of Polygonati Rhizoma as functional food and medicine.

Tomographic volumetric bioprinting of heterocellular bone-like tissues in seconds

Tomographic volumetric bioprinting (VBP) has recently emerged as a powerful tool for rapid solidification of cell-laden hydrogel constructs within seconds. However, its practical applications in tissue engineering requires a detailed understanding of how different printing parameters (concentration of resins, laser dose) affect cell activity and tissue formation. Herein, we explore a new application of VBP in bone tissue engineering by merging a soft gelatin methacryloyl (GelMA) bioresin (<5 kPa) with 3D endothelial co-culture to generate heterocellular bone-like constructs with enhanced functionality. To this, a series of bioresins with varying concentrations of GelMA and lithium Phenyl(2,4,6-trimethylbenzoyl)phosphinate (LAP) photoinitiator were formulated and characterized in terms of photo-reactivity, printability and cell-compatibility. A bioresin with 5% GelMA and 0.05% LAP was identified as the optimal formulation for VBP of complex perfusable constructs within 30 s at high cell viability (>90%). The fidelity was validated by micro-computed tomography and confocal microscopy. Compared to 10% GelMA, this bioresin provided a softer and more permissive environment for osteogenic differentiation of human mesenchymal stem cells (hMSCs). The expression of osteoblastic markers (collagen-I, ALP, osteocalcin) and osteocytic markers (podoplanin, Dmp1) was monitored for 42 days. After 21 days, early osteocytic markers were significantly increased in 3D co-cultures of hMSCs with human umbilical vein endothelial cells (HUVECs). Additionally, we demonstrate VBP of a perfusable, pre-vascularized model where HUVECs self-organized into an endothelium-lined channel. Altogether, this work leverages the benefits of VBP and 3D co-culture, offering a promising platform for fast scaled biofabrication of 3D bone-like tissues with unprecedented functionality. STATEMENT OF SIGNIFICANCE: This study explores new strategies for ultrafast bio-manufacturing of bone tissue models by leveraging the advantages of tomographic volumetric bioprinting (VBP) and endothelial co-culture. After screening the properties of a series of photocurable gelatin methacryloyl (GelMA) bioresins, a formulation with 5% GelMA was identified with optimal printability and permissiveness for osteogenic differentiation of human mesenchymal stem cells (hMSC). We then established 3D endothelial co-cultures to test if the heterocellular interactions may enhance the osteogenic differentiation in the printed environments. This hypothesis was evidenced by increased gene expression of early osteocytic markers in 3D co-cultures after 21 days. Finally, VBP of a perfusable cell-laden tissue construct is demonstrated for future applications in vascularized tissue engineering.

Poly (Ether-Ether-Ketone) for Biomedical Applications: From Enhancing Bioactivity to Reinforced-Bioactive Composites-An Overview

The global orthopedic market is forecasted to reach US$79.5 billion by the end of this decade. Factors driving the increase in this market are population aging, sports injury, road traffic accidents, and overweight, which justify a growing demand for orthopedic implants. Therefore, it is of utmost importance to develop bone implants with superior mechanical and biological properties to face the demand and improve patients' quality of life. Today, metallic implants still hold a dominant position in the global orthopedic implant market, mainly due to their superior mechanical resistance. However, their performance might be jeopardized due to the possible release of metallic debris, leading to cytotoxic effects and inflammatory responses in the body. Poly (ether-ether-ketone) (PEEK) is a biocompatible, high-performance polymer and one of the most prominent candidates to be used in manufacturing bone implants due to its similarity to the mechanical properties of bone. Unfortunately, the bioinert nature of PEEK culminates in its diminished osseointegration. Notwithstanding, PEEK's bioactivity can be improved through surface modification techniques and by the development of bioactive composites. This paper overviews the advantages of using PEEK for manufacturing implants and addresses the most common strategies to improve the bioactivity of PEEK in order to promote enhanced biomechanical performance.

Kidney tonifying traditional Chinese medicine: Potential implications for the prevention and treatment of osteoporosis

The aging global population is increasingly affected by osteoporosis (OP), which is one of the most significant threats to the elderly. Moreover, its prevention and treatment situations have become increasingly severe. Therefore, it is imperative to develop alternatives or complementary drugs for preventing and treating osteoporosis. Kidney tonifying traditional Chinese medicine (KTTCM) has been used for the treatment of osteoporosis for a long time. Pharmacological studies have shown that kidney tonifying traditional Chinese medicine can promote osteoblasts, inhibit osteoclasts, and regulate the level of estrogen and plays vital roles in stimulating osteogenesis, restraining adipogenesis of marrow mesenchymal stem cells (MSCs), regulating the metabolism of calcium and phosphorus, and inhibiting oxidative stress. These effects are mediated by OPG/RANKL/RANK, BMP/Smads, MAPKs, and Wnt/β-catenin systems. To develop a safe, synergistic, effective, and homogenized TCM formula with robust scientific evidence to provide faster and more economical alternatives, the anti-osteoporosis ingredients and pharmacological mechanisms of kidney tonifying traditional Chinese medicine are recapitulated from the perspective of molecular and cell biology, and the safety and toxicity of kidney tonifying traditional Chinese medicine have also been reviewed in this paper.

Cyclic pressure-induced cytokines from gingival fibroblasts stimulate osteoclast activity: Clinical implications for alveolar bone loss in denture wearers

Purpose The involvement of oral mucosa cells in mechanical stress-induced bone resorption is unclear. The aim of this study was to investigate the effects of cyclic pressure-induced cytokines from oral mucosal cells (human gingival fibroblasts: hGFs) on osteoclast activity in vitro.Methods Cyclic pressure at 50 kPa, which represents high physiologic occlusal force of dentures on the molar area, was applied to hGFs. NFAT-reporter stable RAW264.7 preosteoclasts (NFAT/Luc-RAW cells) were cultured in conditioned medium collected from hGF cultures under cyclic pressure or static conditions. NFAT activity and osteoclast formation were determined by luciferase reporter assay and TRAP staining, respectively. Cyclic pressure-induced cytokines in hGF culture were detected by ELISA, real-time RT-PCR, and cytokine array analyses.Results Conditioned media from hGFs treated with 48 hours of cyclic pressure significantly induced NFAT activity and increased multinucleated osteoclast formation. Furthermore, the cyclic pressure significantly increased the bone resorption activity of RAW264.7 cells. Cyclic pressure significantly increased the expression of major inflammatory cytokines including IL-1β/IL-1β, IL-6/IL-6, IL-8/IL-8 and MCP-1/CCL2 in hGFs compared to hGFs cultured under static conditions, and it suppressed osteoprotegerin (OPG/OPG) expression. A cytokine array detected 12 cyclic pressure-induced candidates. Among them, IL-8, decorin, MCP-1 and ferritin increased, whereas IL-28A and PDGF-BB decreased, NFAT activation of NFAT/Luc-RAW cells.Conclusions These results suggest that cyclic pressure-induced cytokines from hGFs promote osteoclastogenesis, possibly including up-regulation of IL-1β, IL-6, IL-8 and MCP-1, and down-regulation of OPG. These findings introduce the possible involvement of GFs in mechanical stress-induced alveolar ridge resorption, such as in denture wearers.

Bone-on-a-chip platforms and integrated biosensors: Towards advanced in vitro bone models with real-time biosensing

Bone diseases, such as osteoporosis and bone defects, often lead to structural and functional deformities of the patient's body. Understanding the complicated pathophysiology and finding new drugs for bone diseases are in dire need but challenging with the conventional cell and animal models. Bone-on-a-chip (BoC) models recapitulate key features of bone at an unprecedented level and can potentially shift the paradigm of future bone research and therapeutic development. Nevertheless, current BoC models predominantly rely on off-chip analysis which provides only endpoint measurements. To this end, integrating biosensors within the BoC can provide non-invasive, continuous monitoring of the experiment progression, significantly facilitating bone research. This review aims to summarize research progress in BoC and biosensor integrations and share perspectives on this exciting but rudimentary research area. We first introduce the research progress of BoC models in the study of bone remodeling and bone diseases, respectively. We then summarize the need for BoC characterization and reported works on biosensor integration in organ chips. Finally, we discuss the limitations and future directions of BoC models and biosensor integrations as next-generation technologies for bone research.

Temporomandibular Joint Osteoarthritis: Pathogenic Mechanisms Involving the Cartilage and Subchondral Bone, and Potential Therapeutic Strategies for Joint Regeneration

The temporomandibular joint (TMJ) is a specialized synovial joint that is crucial for the movement and function of the jaw. TMJ osteoarthritis (TMJ OA) is the result of disc dislocation, trauma, functional overburden, and developmental anomalies. TMJ OA affects all joint structures, including the articular cartilage, synovium, subchondral bone, capsule, ligaments, periarticular muscles, and sensory nerves that innervate the tissues. The present review aimed to illustrate the main pathomechanisms involving cartilage and bone changes in TMJ OA and some therapeutic options that have shown potential restorative properties regarding these joint structures in vivo. Chondrocyte loss, extracellular matrix (ECM) degradation, and subchondral bone remodeling are important factors in TMJ OA. The subchondral bone actively participates in TMJ OA through an abnormal bone remodeling initially characterized by a loss of bone mass, followed by reparative mechanisms that lead to stiffness and thickening of the condylar osteochondral interface. In recent years, such therapies as intraarticular platelet-rich plasma (PRP), hyaluronic acid (HA), and mesenchymal stem cell-based treatment (MSCs) have shown promising results with respect to the regeneration of joint structures or the protection against further damage in TMJ OA. Nevertheless, PRP and MSCs are more frequently associated with cartilage and/or bone repair than HA. According to recent findings, the latter could enhance the restorative potential of other therapies (PRP, MSCs) when used in combination, rather than repair TMJ structures by itself. TMJ OA is a complex disease in which degenerative changes in the cartilage and bone develop through intricate mechanisms. The regenerative potential of such therapies as PRP, MSCs, and HA regarding the cartilage and subchondral bone (alone or in various combinations) in TMJ OA remains a matter of further research, with studies sometimes obtaining discrepant results.

Nanotopographical cues for regulation of macrophages and osteoclasts: emerging opportunities for osseointegration

Nanotopographical cues of bone implant surface has direct influences on various cell types during the establishment of osseointegration, a prerequisite of implant bear-loading. Given the important roles of monocyte/macrophage lineage cells in bone regeneration and remodeling, the regulation of nanotopographies on macrophages and osteoclasts has arisen considerable attentions recently. However, compared to osteoblastic cells, how nanotopographies regulate macrophages and osteoclasts has not been properly summarized. In this review, the roles and interactions of macrophages, osteoclasts and osteoblasts at different stages of bone healing is firstly presented. Then, the diversity and preparation methods of nanotopographies are summarized. Special attentions are paid to the regulation characterizations of nanotopographies on macrophages polarization and osteoclast differentiation, as well as the focal adhesion-cytoskeleton mediated mechanism. Finally, an outlook is indicated of coordinating nanotopographies, macrophages and osteoclasts to achieve better osseointegration. These comprehensive discussions may not only help to guide the optimization of bone implant surface nanostructures, but also provide an enlightenment to the osteoimmune response to external implant.

Silk fibroin scaffolds: A promising candidate for bone regeneration

It remains a big challenge in clinical practice to repair large-sized bone defects and many factors limit the application of autografts and allografts, The application of exogenous scaffolds is an alternate strategy for bone regeneration, among which the silk fibroin (SF) scaffold is a promising candidate. Due to the advantages of excellent biocompatibility, satisfying mechanical property, controllable biodegradability and structural adjustability, SF scaffolds exhibit great potential in bone regeneration with the help of well-designed structures, bioactive components and functional surface modification. This review will summarize the cell and tissue interaction with SF scaffolds, techniques to fabricate SF-based scaffolds and modifications of SF scaffolds to enhance osteogenesis, which will provide a deep and comprehensive insight into SF scaffolds and inspire the design and fabrication of novel SF scaffolds for superior osteogenic performance. However, there still needs more comprehensive efforts to promote better clinical translation of SF scaffolds, including more experiments in big animal models and clinical trials. Furthermore, deeper investigations are also in demand to reveal the degradation and clearing mechanisms of SF scaffolds and evaluate the influence of degradation products.