Osteoblast-osteoclast co-cultures: A systematic review and map of available literature

Developing a Bone-Mimicking Microenvironment: Surface Coating Method for Investigating Bone Remodeling in Vitro

To investigate bone formation and resorption in vitro, it is essential to create bone-like microenvironments on cell culture substrates. Here, we present a coating technique to create such a microenvironment on cell culture plastic (CCP) multiwell plates for studying bone remodeling in vitro. Utilizing this coating, we have developed an assay to simultaneously measure cellular mineral formation and resorption in osteoblast and osteoclast coculture models. A composite matrix of collagen type I and carbonated apatitic calcium phosphate was deposited onto CCP in a reproducible manner using a 10× simulated body fluid solution (SBF) supplemented with type I collagen. qPCR analysis and cellular imaging using fluorescence microscopy demonstrated the promotion of osteogenic differentiation, cell attachment, and proliferation of human bone-marrow-derived mesenchymal stem cells on coated substrates. Moreover, human bone-marrow-derived mononuclear cells successfully differentiated into osteoclasts and resorbed the coated substrate. Using the developed coating, an osteoblast and osteoclast coculture system was established, enabling real-time monitoring of mineral formation and resorption. By providing a controlled and physiologically relevant in vitro model, this assay facilitates the screening of therapeutic compounds, the study of bone cell interactions, and the identification of factors influencing bone remodeling, thereby enhancing translational research in bone health.

Live Cell Sorting of Differentiated Primary Human Osteoclasts Allows Generation of Transcriptomic Signature Matrix

Osteoclasts are specialized cells that degrade the bone matrix to create space for bone regeneration. During tumorigenesis, cancer cells metastasize to bone by disrupting bone's natural remodeling cycle. However, the mechanisms underlying critical bone-tumor interactions are poorly understood due to challenges in isolating osteoclasts from human bone. Thus, the conventional method to obtain osteoclasts for in vitro studies is via the differentiation of peripheral blood monocytes, which results in mixed cultures containing progenitor cells and osteoclasts of varying maturity and nuclearity. Presently, we hypothesized that the transcriptomic signatures of mature, multinucleated osteoclasts are distinct from osteoclasts with fewer nuclei. We established a live cell biomarker expression-based sorting protocol to allow purification of mature osteoclasts while maintaining viability and function. We observed that mature, multinucleated osteoclasts were transcriptomically distinct from those with fewer nuclei and that mature osteoclasts showed higher expression of genes that are associated with osteoclast fusion and function.

Osteogenic CpG Oligodeoxynucleotide, iSN40, Inhibits Osteoclastogenesis in a TLR9-Dependent Manner

A CpG oligodeoxynucleotide (CpG-ODN), iSN40, was originally identified as promoting the mineralization and differentiation of osteoblasts, independent of Toll-like receptor 9 (TLR9). Since CpG ODNs are often recognized by TLR9 and inhibit osteoclastogenesis, this study investigated the TLR9 dependence and anti-osteoclastogenic effect of iSN40 to validate its potential as an osteoporosis drug. The murine monocyte/macrophage cell line RAW264.7 was treated with the receptor activator of nuclear factor-κB ligand (RANKL) to induce osteoclast differentiation, then the effect of iSN40 on was quantified by tartrate-resistant acid phosphatase (TRAP) staining and real-time RT-PCR. iSN40 completely inhibited RANKL-induced differentiation into TRAP+ multinucleated osteoclasts by suppressing osteoclastogenic genes and inducing anti-/non-osteoclastogenic genes. Treatment with a TLR9 inhibitor, E6446, or a mutation in the CpG motif of iSN40 abolished the intracellular uptake and anti-osteoclastogenic effect of iSN40. These results demonstrate that iSN40 is subcellularly internalized and is recognized by TLR9 via its CpG motif, modulates RANKL-dependent osteoclastogenic gene expression, and ultimately inhibits osteoclastogenesis. Finally, iSN40 was confirmed to inhibit the osteoclastogenesis of RAW264.7 cells cocultured with the murine osteoblast cell line MC3T3-E1, presenting a model of bone remodeling. This study demonstrates that iSN40, which exerts both pro-osteogenic and anti-osteoclastogenic effects, may be a promising nucleic acid drug for osteoporosis.

Clinical Characteristics, Current Treatment Options, Potential Mechanisms, Biomarkers, and Therapeutic Targets in Avascular Necrosis of Femoral Head

Avascular necrosis of femoral head (AVNFH) is a debilitating disease of the young, affecting the quality of life significantly and eventually leading to total hip replacement surgery. The disease is diagnosed clinico-radiologically and MRI is the investigation of choice to diagnose the early stages of the disease. There is neither an early biomarker for detection nor is there a permanent cure for the disease and most of the patients are managed with various combinations of surgical and medical management protocols. In this review, we comprehensively address the etiopathogenesis, clinical characteristics, therapeutic procedures, bone characteristics, histopathology, multi-omic studies, finite element modeling, and systems analysis that has been performed in AVNFH. The etiology includes various factors that compromise the blood supply to the femoral head which also includes contributions by environmental and genetic factors. Multi-omic analysis has shown an association of deregulated pathways with the disease. The cell types involved include mesenchymal stem cells, osteoblasts, osteoclasts, endothelial and immune cells. Biochemical, hematological, histopathology, IHC, and other bone remodeling and degradation marker studies have been performed. A systems analysis using multi-omic data sets from published literature was carried out, the relevance of which is discussed to delineate potential mechanisms in etiopathogenesis, diagnosis, and effective management of this debilitating disease.

Strategies for the Codelivery of Osteoclasts and Mesenchymal Stem Cells in 3D-Printable Osteochondral Scaffolds

Osteochondral defects, characterized by structural compromises to articular cartilage and subchondral bone, can cause pain and lead to progressive cartilage damage and eventual osteoarthritis. Unfortunately, repairing these defects remains difficult because of the poor regenerative properties of cartilage and complex mechanical demands of the joint. As such, the field of tissue engineering aims to develop multiphasic implants that replace pathological cartilage and bone tissue and restore mechanical functionality to the joint. Recent bone physiology investigations have demonstrated that osteoclast (OC) lineage cells are inextricably involved in osteoblastic bone formation through an extensive network of anabolic signaling pathways, and so the codelivery OC and osteoblast (OB) lineage cells within scaffolds is being actively explored for bone tissue engineering purposes. However, it remains unclear how these cells can be incorporated into the design of multiphasic osteochondral scaffolds to potentially enhance subchondral bone formation and subsequent implant osseointegration. To explore this question, we examined direct surface seeding and hydrogel encapsulation as potential scaffold cellularization strategies. First, we examined how OC precursor cells and peripheral blood monocytes (PBMCs) influence early-stage bone matrix development and osteogenesis in 2D coculture. Then, we evaluated the osteogenic potential of mesenchymal stem cells (MSCs) and PBMCs cocultures encapsulated within a gelatin methacrylate (GelMA) hydrogel system. Our findings demonstrate that coculturing PBMCs with MSCs in 2D cultures significantly enhanced cell proliferation, early bone matrix deposition, and the formation of cell clusters by Day 28. However, we observed no significant difference in type I collagen deposition between GelMA hydrogel scaffolds cultured in basal and OC conditions during the same period. In addition, we found that the GelMA hydrogel system with MSC/PBMC cocultures in OC conditions exhibited decreased osteogenic activity by Day 28. Collectively, our findings support the osteogenic potential of OC-lineage cells in 2D culture conditions, and the potential benefits of surface-seeding for the codelivery of OC-lineage cells and MSCs in osteo-scaffolds for enhanced osteochondral regeneration and broader bone tissue engineering purposes.

Comparative Analysis of Osteoblastic Responses to Titanium and Alumina-Toughened Zirconia Implants: An In Vitro Study

INTRODUCTION

Osteoblastic responses play a crucial role in the success of oral implants. Enhanced proliferation of osteoblast cells is associated with reduced cell mortality and an increase in bone regeneration. This study aims to evaluate the osteoblastic responses following oral implantation.

MATERIALS AND METHODS

Osteoblast stem cells were harvested and subsequently cultivated using cell culture techniques. The osteoblastic phenotype of the extracted cells was confirmed by examining the extracellular matrix. Cell morphogenesis on functionalized biomaterial surfaces was assessed through indirect immunofluorescence staining. The cellular response was investigated in the presence of two types of implant materials: titanium (Ti) and alumina-toughened zirconia (ATZ). Cell viability and apoptosis were quantitatively assessed using MTT assays and flow cytometry, respectively.

RESULTS

The survival of osteoblastic lineage cells was moderately reduced post-implantation. Viability in the Ti implant group remained at approximately 86%, while in the ATZ group, it was observed at 75%, which is considered acceptable. Moreover, there was a significant disparity in cell survival between the two implant groups (p < 0.05). Analysis of apoptosis levels at various concentrations revealed that the rate of apoptosis was 3.6% in the control group and 18.5% in the ATZ group, indicating that apoptosis or programmed cell death in the ATZ-treated group had increased nearly four-fold (p < 0.05).

CONCLUSIONS

The findings of this study indicate a reduction in osteoblastic cell line survival following implant treatment, with titanium implants exhibiting superior performance in terms of cell survival. However, it was also noted that the incidence of apoptosis in osteoblast cells was significantly higher in the presence of zirconium-based implants.

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.

Effect of systemic atorvastatin on bone regeneration in critical-sized defects in hyperlipidemia: an experimental study

PURPOSE

Hypocholesterolemic medications similar to atorvastatin are efficient in lowering blood lipid levels; however, compared to other medications in the statin family, their impact on bone metabolism is claimed to be insufficient. The impact of atorvastatin on bone regeneration in dental implantology in individuals with hyperlipidemia who received atorvastatin in the clinic is doubtful.

METHODS

In the study, 16 male New Zealand rabbits of 6 months were used. All rabbits were fed a high-cholesterol diet for 8 weeks, and hyperlipidemia was created. It was confirmed that the total cholesterol level in rabbits was above 105 mg/dl. A critical-sized defect was created in the mandible. The defect was closed with xenograft and membrane. Oral 10 mg/kg atorvastatin was started in the experimental group, and no drug was administered in the control group. At 16th week, animals were sacrificed. For histomorphological examination, the new bone area, osteoclast, and osteoblast activities were evaluated.

RESULTS

While new bone area (45,924 µm2, p < 0.001) and AP intensities (105.645 ± 16.727, p = 0.006) were higher in the atorvastatin group than in the control group, TRAP intensities in the control group (82.192 ± 5.346, p = 0.021) were higher than that in the atorvastatin group.

CONCLUSIONS

It has been found that high blood lipid levels will adversely affect bone graft healing and the use of systemic atorvastatin contributes to bone healing. Clinicians should pay attention to the selection of surgical materials, considering the importance of questioning drug use in their patients and the risks in cases of non-use.

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.

Osteogenesis and osteoclastogenesis on a chip: Engineering a self-assembling 3D coculture

Healthy bone is maintained by the process of bone remodeling. An unbalance in this process can lead to pathologies such as osteoporosis which are often studied with animal models. However, data from animals have limited power in predicting the results that will be obtained in human clinical trials. In search for alternatives to animal models, human in vitro models are emerging as they address the principle of reduction, refinement, and replacement of animal experiments (3Rs). At the moment, no complete in vitro model for bone-remodeling exists. Microfluidic chips offer great possibilities, particularly because of the dynamic culture options, which are crucial for in vitro bone formation. In this study, a scaffold free, fully human, 3D microfluidic coculture model of bone remodeling is presented. A bone-on-a-chip coculture system was developed in which human mesenchymal stromal cells differentiated into the osteoblastic lineage and self-assembled into scaffold free bone-like tissues with the shape and dimensions of human trabeculae. Human monocytes were able to attach to these tissues and to fuse into multinucleated osteoclast-like cells, establishing the coculture. Computational modeling was used to determine the fluid flow induced shear stress and strain in the formed tissue. Furthermore, a set-up was developed allowing for long-term (35 days) on-chip cell culture with benefits including continuous fluid-flow, low bubble formation risk, easy culture medium exchange inside the incubator and live cell imaging options. This on-chip coculture is a crucial advance towards developing in vitro bone remodeling models to facilitate drug testing.

A dialysis medium refreshment cell culture set-up for an osteoblast-osteoclast coculture

Culture medium exchange leads to loss of valuable auto- and paracrine factors produced by the cells. However, frequent renewal of culture medium is necessary for nutrient supply and to prevent waste product accumulation. Thus it remains the gold standard in cell culture applications. The use of dialysis as a medium refreshment method could provide a solution as low molecular weight molecules such as nutrients and waste products could easily be exchanged, while high molecular weight components such as growth factors, used in cell interactions, could be maintained in the cell culture compartment. This study investigates a dialysis culture approach for an in vitro bone remodeling model. In this model, both the differentiation of human mesenchymal stromal cells (MSCs) into osteoblasts and monocytes (MCs) into osteoclasts is studied. A custom-made simple dialysis culture system with a commercially available cellulose dialysis insert was developed. The data reported here revealed increased osteoblastic and osteoclastic activity in the dialysis groups compared to the standard nondialysis groups, mainly shown by significantly higher alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (TRAP) activity, respectively. This simple culture system has the potential to create a more efficient microenvironment allowing for cell interactions via secreted factors in mono- and cocultures and could be applied for many other tissues.

Surfactin suppresses osteoclastogenesis via the NF-κB signaling pathway, promotes osteogenic differentiation in vitro, and inhibits oestrogen deficiency-induced bone loss in vivo

BACKGROUND

Fractures caused by osteoporosis (OP) are one of the main causes of death in the elderly, bringing a heavy burden to the country and society. The imbalance between osteoblast-mediated osteogenesis and osteoclast-mediated bone resorption is an important cause of OP. Therefore, finding drugs that can regulate this dynamic balance can be an important way to treat osteoporosis. Surfactin is a highly effective biosurfactant derived from Bacillus subtilis and it has been proven to have various pharmacological effects in previous studies, but its effect on bone metabolism remains unknown. Here, we performed a study on the role and mechanism of Surfactin in inhibiting osteoclastogenesis and its possible mechanism as well as the role in promoting osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs).

METHODS

We investigated the effect of Surfactin on osteoclast differentiation and osteogenic differentiation in vitro and in vivo. The effect of Surfactin on the activity of osteoclastogenesis and osteogenesis was verified by CCK-8 assay, quantitative Real-time polymerase chain reaction (qPCR) and Western blotting analysis were used to verify the effect of Surfactin on osteoclast and osteogenic differentiation-specific genes and proteins. The effect of Surfactin on TRAP、ALP activity and mineral deposition was verified by TRAP、ALP and ARS staining. We then used an ovariectomy-induced osteoporosis mice model to observe the effect of Surfactin in vivo.

RESULTS

Surfactin is noncytotoxic to BMMs, RAW264.7, and BMSCs. And it can effectively inhibit osteoclastogenesis and promote osteogenic differentiation. Moreover, we found that Surfactin can inhibit the differentiation of osteoclasts through the NF-κB signaling pathway. Surfactin can also alleviate bone loss in ovariectomy-induced osteoporosis mice.

CONCLUSIONS

Our results suggest that Surfactin can inhibit osteoclastogenesis through the NF-κB signaling pathway, promote the osteogenic differentiation of BMSCs, and also can effectively alleviate bone loss in ovariectomy-induced osteoporosis mice.

Epimedii Folium and Ligustri Lucidi Fructus Promote Osteoblastogenesis and Inhibit Osteoclastogenesis against Osteoporosis via Acting on Osteoblast-Osteoclast Communication

Osteoblast (OB) and osteoclast (OC) play important roles in bone formation and bone resorption, which can communicate with each other through cytokine paracrine. Previous studies have confirmed that Epimedii Folium (EF) and Ligustri Lucidi Fructus (LLF) used alone or in combination can treat osteoporosis (OP) through regulating bone remodeling, but the effects of EF and LLF on osteoblastogenesis, osteoclastogenesis, and OB-OC communication are unclear. In this study, we investigated the direct and indirect effects of EF and LLF on OBs and OCs via monoculture and coculture (transwell) models of OBs and OCs. We found that the combination of EF and LLF (EF&LLF) could promote osteoblastogenesis and inhibit osteoclastogenesis directly and indirectly. In order to study the mechanisms of EF&LLF on indirectly regulating osteoblastogenesis and osteoclastogenesis, we detected the expression of cytokines by which OBs and OCs could communicate with each other. We found that EF&LLF could downregulate the expression of RANKL and M-CSF and the protein ratio of RANKL/OPG of OBs and Atp6v0d2 expression of OCs and upregulate the expression of OPG and TGF-β1 of OBs and the expression of TGF-β1, BMP-2, and IGF-1 of OCs, indicating that EF&LLF could regulate cytokine expressions of OBs/OCs to affect OB-OC communication. In addition, EF&LLF had a better effect on regulating cytokines of OBs and OCs than EF or LLF in single use. This study suggested that EF&LLF exhibited the effects of promoting osteoblastogenesis and inhibiting osteoclastogenesis via acting on OB-OC communication and provided some scientific evidences for EF&LLF against OP.

Evaluating material-driven regeneration in a tissue engineered human in vitro bone defect model

Advanced in vitro human bone defect models can contribute to the evaluation of materials for in situ bone regeneration, addressing both translational and ethical concerns regarding animal models. In this study, we attempted to develop such a model to study material-driven regeneration, using a tissue engineering approach. By co-culturing human umbilical vein endothelial cells (HUVECs) with human bone marrow-derived mesenchymal stromal cells (hBMSCs) on silk fibroin scaffolds with in vitro critically sized defects, the growth of vascular-like networks and three-dimensional bone-like tissue was facilitated. After a model build-up phase of 28 days, materials were artificially implanted and HUVEC and hBMSC migration, cell-material interactions, and osteoinduction were evaluated 14 days after implantation. The materials physiologically relevant for bone regeneration included a platelet gel as blood clot mimic, cartilage spheres as soft callus mimics, and a fibrin gel as control. Although the in vitro model was limited in the evaluation of immune responses, hallmarks of physiological bone regeneration were observed in vitro. These included the endothelial cell chemotaxis induced by the blood clot mimic and the mineralization of the soft callus mimic. Therefore, the present in vitro model could contribute to an improved pre-clinical evaluation of biomaterials while reducing the need for animal experiments.

The effects of seeding density and osteoclastic supplement concentration on osteoclastic differentiation and resorption

The bone resorbing osteoclasts are a complex type of cell essential for in vivo bone remodeling. There is no consensus on medium composition and seeding density for in vitro osteoclastogenesis, despite the importance thereof on osteoclastic differentiation and activity. The aim of this study was to investigate the relative effect of monocyte or peripheral blood mononuclear cell (PBMC) seeding density, osteoclastic supplement concentration and priming on the in vitro generation of functional osteoclasts, and to explore and evaluate the usefulness of commonly used markers for osteoclast cultures. Morphology and osteoclast formation were analyzed with fluorescence imaging for tartrate resistant acid phosphatase (TRAP) and integrin β3 (Iβ3). TRAP release was analyzed from supernatant samples, and resorption was analyzed from culture on Corning® Osteo Assay plates. In this study, we have shown that common non-standardized culturing conditions of monocyte or PBMCs had a significant effect on the in vitro generation of functional osteoclasts. We showed how increased osteoclastic supplement concentrations supported osteoclastic differentiation and resorption but not TRAP release, while priming resulted in increased TRAP release as well. Increased monocyte seeding densities resulted in more and large TRAP positive bi-nuclear cells, but not directly in more multinucleated osteoclasts, resorption or TRAP release. Increasing PBMC seeding densities resulted in more and larger osteoclasts and more resorption, although resorption was disproportionally low compared to the monocyte seeding density experiment. Exploration of commonly used markers for osteoclast cultures demonstrated that Iβ3 staining was an excellent and specific osteoclast marker in addition to TRAP staining, while supernatant TRAP measurements could not accurately predict osteoclastic resorptive activity. With improved understanding of the effect of seeding density and osteoclastic supplement concentration on osteoclasts, experiments yielding higher numbers of functional osteoclasts can ultimately improve our knowledge of osteoclasts, osteoclastogenesis, bone remodeling and bone diseases.

Tuning the resorption-formation balance in an in vitro 3D osteoblast-osteoclast co-culture model of bone

The aim of the present study was to further improve an in vitro 3D osteoblast (OB) - osteoclast (OC) co-culture model of bone by tuning it towards states of formation, resorption, and equilibrium for their future applications in fundamental research, drug development and personalized medicine. This was achieved by varying culture medium composition and monocyte seeding density, the two external parameters that affect cell behavior the most. Monocytes were seeded at two seeding densities onto 3D silk-fibroin constructs pre-mineralized by MSC-derived OBs and were co-cultured in one of three different media (OC stimulating, Neutral and OB stimulating medium) for three weeks. Histology showed mineralized matrix after co-culture and OC markers in the OC medium group. Scanning Electron Microscopy showed large OC-like cells in the OC medium group. Micro-computed tomography showed increased formation in the OB medium group, equilibrium in the Neutral medium group and resorption in the OC medium group. Culture supernatant samples showed high early tartrate resistant acid phosphatase (TRAP) release in the OC medium group, a later and lower release in the Neutral medium group, and almost no release in the OB medium group. Increased monocyte seeding density showed a less-than-proportional increase in TRAP release and resorption in OC medium, while it proportionally increased TRAP release in Neutral medium without affecting net resorption. The 3D OB-OC co-culture model was effectively used to show an excess of mineral deposition using OB medium, resorption using OC medium, or an equilibrium using Neutral medium. All three media applied to the model may have their own distinct applications in fundamental research, drug development, and personalized medicine.

Human Platelet Lysate as Alternative of Fetal Bovine Serum for Enhanced Human In Vitro Bone Resorption and Remodeling

Introduction

To study human physiological and pathological bone remodeling while addressing the principle of replacement, reduction and refinement of animal experiments (3Rs), human in vitro bone remodeling models are being developed. Despite increasing safety-, scientific-, and ethical concerns, fetal bovine serum (FBS), a nutritional medium supplement, is still routinely used in these models. To comply with the 3Rs and to improve the reproducibility of such in vitro models, xenogeneic-free medium supplements should be investigated. Human platelet lysate (hPL) might be a good alternative as it has been shown to accelerate osteogenic differentiation of mesenchymal stromal cells (MSCs) and improve subsequent mineralization. However, for a human in vitro bone model, hPL should also be able to adequately support osteoclastic differentiation and subsequent bone resorption. In addition, optimizing co-culture medium conditions in mono-cultures might lead to unequal stimulation of co-cultured cells.

Methods

We compared supplementation with 10% FBS vs. 10%, 5%, and 2.5% hPL for osteoclast formation and resorption by human monocytes (MCs) in mono-culture and in co-culture with (osteogenically stimulated) human MSCs.

Results and Discussion

Supplementation of hPL can lead to a less donor-dependent and more homogeneous osteoclastic differentiation of MCs when compared to supplementation with 10% FBS. In co-cultures, osteoclastic differentiation and resorption in the 10% FBS group was almost completely inhibited by MSCs, while the supplementation with hPL still allowed for resorption, mostly at low concentrations. The addition of hPL to osteogenically stimulated MSC mono- and MC-MSC co-cultures resulted in osteogenic differentiation and bone-like matrix formation, mostly at high concentrations.

Conclusion

We conclude that hPL could support both osteoclastic differentiation of human MCs and osteogenic differentiation of human MSCs in mono- and in co-culture, and that this can be balanced by the hPL concentration. Thus, the use of hPL could limit the need for FBS, which is currently commonly accepted for in vitro bone remodeling models.