Pericytes are contractile, motile cells that surround the capillary. Recent studies have shown that pericytes promoted joint fibrosis and induced subchondral bone angiogenesis, indicating the role of pericytes in osteoarthritis (OA). However, whether pericytes are involved in regulating inflammatory and catabolic response, as well as fibrotic repair of cartilage is still unclear. Here we used 2D and 3D models to investigate the communication of pericytes and chondrocytes under inflammatory osteoarthritis conditions. CD34-CD146+ pericytes were isolated and sorted from human bone marrow. Human OA chondrocytes were isolated from OA joints. In 2D studies, monolayer cultured chondrocytes were treated +/- pericyte conditioned media, +/- 1ng/ml IL1β for 24h. In 3D studies, pericytes and chondrocytes were cultured within fibrin gel in 3D polyurethane scaffolds, separately or combined for 7 days, followed by treatment of +/- IL1β for another 7 days (Fig 2A). The inflammatory response, catabolic activity and expression of fibrosis markers of chondrocytes and pericytes were measured by ELISA and/or q-rtPCR. Pericytes had weak inflammatory, catabolic and fibrotic response to IL1β (data not shown). The 2D study showed that pericyte conditioned media promoted inflammation, catabolism and fibrosis markers of chondrocytes, in the absence of IL1β treatment (Figure 1). However, study in 3D showed that coculture of chondrocytes and pericytes reduced the inflammatory and catabolic response of chondrocytes to IL1β and induced fibrosis markers in chondrocytes (Figure 2). Pericytes are involved in regulating inflammatory response, catabolic response and fibrosis of chondrocytes. The opposite results from 2D and 3D experiments indicate the variety of the regulatory role of pericytes in the interaction with chondrocytes within different culture models. The underlying mechanism is under evaluation with on-going studies. Acknowledgements. This study was funded by SINPAIN project, from European Union's Horizon Europe research and innovation programme under Grant Agreement NO. 101057778. Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them. For any figures or tables, please contact the authors directly

Extensive bone defects, caused by severe trauma or resection of large bone tumors, are difficult to treat. Regenerative medicine, including stem cell transplantation, may provide a novel solution for these intractable problems and improve the quality of life in affected patients. Adipose-derived stromal/stem cells (ASCs) have been extensively studied as cell sources for regenerative medicine due to their excellent proliferative capacity and the ability to obtain a large number of cells with minimal donor morbidity. However, the osteogenic potential of ASCs is lower than that of bone marrow-derived stromal/stem cells. To address this disadvantage, our group has employed various methods to enhance osteogenic differentiation of ASCs, including factors such as bone morphogenetic protein or Vitamin D, coculture with bone marrow stem cells, VEGF transfection, and gene transfer of Runx-2 and osterix. Recently, we mined a marker that can predict the osteogenic potential of ASC clones and also investigated the usefulness of the molecule as the enhancer of osteogenic differentiation of ASCs as well as its mechanism of action. Through RNA-seq gene analysis, we discovered that GSTT1 was the most distinguished gene marker between highly osteogenic and poorly osteogenic ASC clones. Knockdown of GSTT1 in high osteogenic ASCs by siGSTT1 treatment reduced mineralized matrix formation while GSTT1 overexpression by GSTT1 transfection or GSTT1 recombinant protein treatment enhanced osteogenic differentiation of low osteogenic ASCs. Metabolomic analysis confirmed significant changes of metabolites related to bone differentiation in ASCs transfected with GSTT1. A high total antioxidant capacity, low levels of cellular reactive oxygen species and increased GSH/GSSG ratios were also detected in GSTT1- transfected ASCs. GSTT1 can be a useful marker to screen the highly osteogenic ASC clones and also a therapeutic factor to enhance the osteogenic differentiation of poorly osteogenic ASC clones

Tissue repair is believed to rely on tissue-resident progenitor cell populations proliferating, migrating, and undergoing differentiation at the site of injury. During these processes, the crosstalk between mesenchymal stromal/stem cells (MSCs) and macrophages has been shown to play a pivotal role. However, the influence of extracellular matrix (ECM) remodelling in this crosstalk, remains elusive. Human MSCs cultured on tissue culture plastic (TCP) and encased within fibrin in vitro were treated with/without TNFα and IFNγ. Human monocytes were cocultured with untreated/pretreated MSCs on TCP or within fibrin. After seven days, the conditioned media (CM) were collected. Human chondrocytes were exposed to CM in a migration assay. The impact of TGFβ was assessed by adding an inhibitor (TGFβRi). Cell activity was assessed using RT-qPCR and XL-protein-profiler-array. Previously, we demonstrated that culturing human MSCs within 3D-environments significantly enhances their immunoregulatory activity in response to pro-inflammatory stimuli. In this study, monocytes were co-cultured with MSCs within fibrin, acquiring a distinct M2-like repair macrophage phenotype in contrast to TCP co-cultures. MSC/macrophage CM characterization using a protein array demonstrated differences in release of several factors, including chemokines, growth factors and ECM components. Chondrocyte migration was significantly reduced in CM from untreated MSC/monocytes co-cultures in fibrin compared to CM of untreated MSCs/monocytes on TCP. This impact on migration was not seen with chondrocytes cultured in CM of monocytes co-cultured with pretreated MSCs in fibrin. The CM of monocytes co-cultured with pretreated MSCs in fibrin up-regulates COL2A1 and SOX9 compared to TCP. Chondrogenesis and migration were TGFβ dependent. MSC/macrophage crosstalk and responsiveness to cytokines are influenced by the ECM environment, which subsequently impacts tissue-resident cell migration and chondrogenesis. The direct effects of ECM on MSC/macrophage secretory phenotype is complemented by the dynamic ECM binding and release of growth factors such as TGFβ

Abstract. Objectives. Tissue repair is believed to rely on tissue-resident progenitor cell populations proliferating, migrating, and undergoing differentiation at the site of injury. During these processes, the crosstalk between mesenchymal stromal/stem cells (MSCs) and macrophages has been shown to play a pivotal role. However, the influence of extracellular matrix (ECM) remodelling in this crosstalk, remains elusive. Methods. Human MSCs cultured on tissue culture plastic (TCP) and encased within fibrin in vitro were treated with/without TNFα and IFNγ. Human monocytes were cocultured with untreated/pretreated MSCs on TCP or within fibrin. After seven days, the conditioned media (CM) were collected. Human chondrocytes were exposed to CM in a migration assay. The impact of TGFβ was assessed by adding an inhibitor (TGFβRi). Cell activity was assessed using RT-qPCR and XL-protein-profiler-array. Results. Previously, we demonstrated that culturing human MSCs within 3D-environments significantly enhances their immunoregulatory activity in response to pro-inflammatory stimuli. In this study, monocytes were co-cultured with MSCs within fibrin, acquiring a distinct M2-like repair macrophage phenotype in contrast to TCP co-cultures. MSC/macrophage CM characterization using a protein array demonstrated differences in release of several factors, including chemokines, growth factors and ECM components. Chondrocyte migration was significantly reduced in CM from untreated MSC/monocytes co-cultures in fibrin compared to CM of untreated MSCs/monocytes on TCP. This impact on migration was not seen with chondrocytes cultured in CM of monocytes co-cultured with pretreated MSCs in fibrin. The CM of monocytes co-cultured with pretreated MSCs in fibrin up-regulates COL2A1 and SOX9 compared to TCP. Chondrogenesis and migration were TGFβ dependent. Conclusion. MSC/macrophage crosstalk and responsiveness to cytokines are influenced by the ECM environment, which subsequently impacts tissue-resident cell migration and chondrogenesis. The direct effects of ECM on MSC/macrophage secretory phenotype is complemented by the dynamic ECM binding and release of growth factors such as TGFβ. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Aims

Currently, the effect of drug treatment for osteoporosis is relatively poor, and the side effects are numerous and serious. Melatonin is a potential drug to improve bone mass in postmenopausal women. Unfortunately, the mechanism by which melatonin improves bone metabolism remains unclear. The aim of this study was to further investigate the potential mechanism of melatonin in the treatment of osteoporosis.

Osteoarthritis (OA) is mainly caused by ageing, strain, trauma, and congenital joint abnormalities, resulting in articular cartilage degeneration. During the pathogenesis of OA, the changes in subchondral bone (SB) are not only secondary manifestations of OA, but also an active part of the disease, and are closely associated with the severity of OA. In different stages of OA, there were microstructural changes in SB. Osteocytes, osteoblasts, and osteoclasts in SB are important in the pathogenesis of OA. The signal transduction mechanism in SB is necessary to maintain the balance of a stable phenotype, extracellular matrix (ECM) synthesis, and bone remodelling between articular cartilage and SB. An imbalance in signal transduction can lead to reduced cartilage quality and SB thickening, which leads to the progression of OA. By understanding changes in SB in OA, researchers are exploring drugs that can regulate these changes, which will help to provide new ideas for the treatment of OA.

Cite this article: Bone Joint Res 2023;12(9):536–545.

Osteoarthritis (OA) is a chronic degenerative joint disease characterized by progressive cartilage degradation, synovial membrane inflammation, osteophyte formation, and subchondral bone sclerosis. Pathological changes in cartilage and subchondral bone are the main processes in OA. In recent decades, many studies have demonstrated that activin-like kinase 3 (ALK3), a bone morphogenetic protein receptor, is essential for cartilage formation, osteogenesis, and postnatal skeletal development. Although the role of bone morphogenetic protein (BMP) signalling in articular cartilage and bone has been extensively studied, many new discoveries have been made in recent years around ALK3 targets in articular cartilage, subchondral bone, and the interaction between the two, broadening the original knowledge of the relationship between ALK3 and OA. In this review, we focus on the roles of ALK3 in OA, including cartilage and subchondral bone and related cells. It may be helpful to seek more efficient drugs or treatments for OA based on ALK3 signalling in future.

Though dentin matrix protein 1 (Dmp1) is known to play critical role in mediating bone mineralization, it has also been validated to be expressed in brain and helps maintain blood brain barrier (BBB). Our study aims to clarify the expression pattern of Dmp1 in mouse brain and explore whether intercellular mitochondrial transfer occurs between Dmp1 positive astrocytes (DPAs) and endothelial cells, and thus acting as a mechanism in maintaining BBB during aging. Single cell RNA sequencing (scRNAseq) of 1 month, 6 month, and 20 month old mice brain (n=1, respectively) was employed to identify Dmp1 positive cell types. Dmp1. Cre. -mGmT and Dmp1. Cre. -COX8a fluorescent mice were generated to visualize DPAs and investigate their mitochondrial activities. A 3D noncontact coculture system and mitochondrial transplantation were applied to study the role of mitochondrial transfer between astrocytes and bEnd.3 endothelial cells. Dmp1. Cre. -Mfn2. f/f. mice were generated by depleting the ER-mitochondria tethering protein Mfn2 in DPAs. Dmp1 was mainly expressed in astrocytes at different ages. GO analysis revealed that cell projection and adhesion of DPAs were upregulated. Confocal imaging on Dmp1. Cre. -mGmT mice indicated that DPAs are a cluster of astrocytes that closely adhere to blood vessels (n=3). Bioinformatics analysis revealed that mitochondrial activity of DPAs were compromised during aging. Enriched scRNAseq of fluorescent cells from Dmp1. Cre. -COX8a mice (n=2) and immunofluorescent imaging (n=3) validated the acquisition of extrinsic mitochondria in endothelial cells. 3D coculture of astrocytes and bEnd.3 and direct mitochondrial transplantation revealed the rescue effect of mitochondrial transfer on damaged bEnd.3. BBB was impaired after depleting Mfn2 in DPAs, expressing a similar phenotype with aging brain. Astrocytes that express Dmp1 play a significant role in maintaining BBB via transferring mitochondria to vascular endothelial cells. Compromised mitochondrial transfer between DPAs and endothelial cells might be the potential mechanism of impaired BBB during aging

Aims

Circular RNA (circRNA) is involved in the regulation of articular cartilage degeneration induced by inflammatory factors or oxidative stress. In a previous study, we found that the expression of circStrn3 was significantly reduced in chondrocytes of osteoarthritis (OA) patients and OA mice. Therefore, the aim of this paper was to explore the role and mechanism of circStrn3 in osteoarthritis.

Aims

The effects of remnant preservation on the anterior cruciate ligament (ACL) and its relationship with the tendon graft remain unclear. We hypothesized that the co-culture of remnant cells and bone marrow stromal cells (BMSCs) decreases apoptosis and enhances the activity of the hamstring tendons and tenocytes, thus aiding ACL reconstruction.

Aims

The purpose of this study was to explore a simple and effective method of preparing human acellular amniotic membrane (HAAM) scaffolds, and explore the effect of HAAM scaffolds with juvenile cartilage fragments (JCFs) on osteochondral defects.

Aims

The use of 3D-printed titanium implant (DT) can effectively guide bone regeneration. DT triggers a continuous host immune reaction, including macrophage type 1 polarization, that resists osseointegration. Interleukin 4 (IL4) is a specific cytokine modulating osteogenic capability that switches macrophage polarization type 1 to type 2, and this switch favours bone regeneration.

Cell sheets are manufactured from a high-density cell layer stabilized by its own freshly produced extracellular matrix (ECM). They could serve as versatile scaffolds for tissue repair. Unfortunately, their production often remains time-consuming requiring weeks of culturing. Ligament cell sheets are so far barely available. Regarding musculoskeletal tissues exposed to high repetitive biomechanical forces, the stability of cell sheets is insufficient. It could help to combine them with a biomechanical competent scaffold e.g. produced by an embroidering technique. Hence, we wanted to (1) develop a very rapid strategy to produce ACL ligamentocyte sheets within 24 h by using a thermoresponsive polymer surface, (2) use the sheets for scaffold seeding and (3) reflect the fibrocartilaginous transition zone of an ACL enthesis by combining sheets of ligamentocytes with chondrocytes or chondrogenic precursor cells as a strategy for directed seeding of two cell types on topologically different scaffold areas. Different cell numbers of lapine ACL ligamentocytes (L-ACLs), lapine articular chondrocytes (L-ACs) and human mesenchymal stromal cells (H-MSCs) were used for sheet formation. Experiments were performed with novel, self-assembled poly(glycidyl ether) (PGE) brushes based on random glycidyl methyl ether and ethyl glycidyl ether copolymers on polystyrene 12-well cell culture plates, which allow rapid sheet formation within 24 h. Uncoated plates served as controls. Temperature-triggered detachment was performed by 10 min incubation with PBS at ambient temperature before treatment with fresh warm PBS for 5 min at 37 degrees Celsius. Harvested cell sheets were transferred on polyglycolic acid (PGA) or embroidered poly-lactic acid / poly-co-caprolactone (PLA/P[LA-CL]) scaffolds, functionalized with collagen foam and fluorine gas treatment (prepared at the IPF in Dresden and the FILK in Freiberg). Cell distribution, growth, vitality and synthesis of ECM components were monitored up to 7 days. Cell numbers required for sheet preparation (3.9 cm2) depended strongly on the cell type (L-ACLs: 0.395 mio/cm2, L-AC: 0.342 mio/cm2, H-MSCs: 0.131 mio/cm2) and was highest for L-ACLs. The majority of cells survived sheet assembly, detachment, transfer onto the scaffolds and culturing. Cells migrated from the sheets into the scaffolds and spread through the scaffolds. L-ACLs and L-ACs produced ECM and maintained their phenotypes (type II collagen and sulfated glycosaminoglycans in L-AC sheets, decorin and tenascin C in L-ACL sheets). The presence and distribution of two cell types in scaffold cocultures (L-ACLs and H-MSCs) was proven by anti-human vimentin labeling. Hence, the PGE brush surface allows rapid formation (24 h) of cell sheets

Aims

Ageing-related incompetence becomes a major hurdle for the clinical translation of adult stem cells in the treatment of osteoarthritis (OA). This study aims to investigate the effect of stepwise preconditioning on cellular behaviours in human mesenchymal stem cells (hMSCs) from ageing patients, and to verify their therapeutic effect in an OA animal model.

Aims. Proliferation, migration, and differentiation of anterior cruciate ligament (ACL) remnant and surrounding cells are fundamental processes for ACL reconstruction; however, the interaction between ACL remnant and surrounding cells is unclear. We hypothesized that ACL remnant cells preserve the capability to regulate the surrounding cells’ activity, collagen gene expression, and tenogenic differentiation. Moreover, extracorporeal shock wave (ESW) would not only promote activity of ACL remnant cells, but also enhance their paracrine regulation of surrounding cells. Methods. Cell viability, proliferation, migration, and expression levels of Collagen-I (COL-I) A1, transforming growth factor beta (TGF-β), and vascular endothelial growth factor (VEGF) were compared between ACL remnant cells untreated and treated with ESW (0.15 mJ/mm. 2. , 1,000 impulses, 4 Hz). To evaluate the subsequent effects on the surrounding cells, bone marrow stromal cells (BMSCs)’ viability, proliferation, migration, and levels of Type I Collagen, Type III Collagen, and tenogenic gene (Scx, TNC) expression were investigated using coculture system. Results. ESW-treated ACL remnant cells presented higher cell viability, proliferation, migration, and increased expression of COL-I A1, TGF-β, and VEGF. BMSC proliferation and migration rate significantly increased after coculture with ACL remnant cells with and without ESW stimulation compared to the BMSCs alone group. Furthermore, ESW significantly enhanced ACL remnant cells’ capability to upregulate the collagen gene expression and tenogenic differentiation of BMSCs, without affecting cell viability, TGF-β, and VEGF expression. Conclusion. ACL remnant cells modulated activity and differentiation of surrounding cells. The results indicated that ESW enhanced ACL remnant cells viability, proliferation, migration, and expression of collagen, TGF-β, VEGF, and paracrine regulation of BMSC proliferation, migration, collagen expression, and tenogenesis. Cite this article: Bone Joint Res 2020;9(8):457–467

Objectives. Up to 10% of fractures result in undesirable outcomes, for which female sex is a risk factor. Cellular sex differences have been implicated in these different healing processes. Better understanding of the mechanisms underlying bone healing and sex differences in this process is key to improved clinical outcomes. This study utilized a macrophage–mesenchymal stem cell (MSC) coculture system to determine: 1) the precise timing of proinflammatory (M1) to anti-inflammatory (M2) macrophage transition for optimal bone formation; and 2) how such immunomodulation was affected by male versus female cocultures. Methods. A primary murine macrophage-MSC coculture system was used to demonstrate the optimal transition time from M1 to M2 (polarized from M1 with interleukin (IL)-4) macrophages to maximize matrix mineralization in male and female MSCs. Outcome variables included Alizarin Red staining, alkaline phosphatase (ALP) activity, and osteocalcin protein secretion. Results. We found that 96 hours of M1 phenotype in male cocultures allowed for maximum matrix mineralization versus 72 hours in female cocultures. ALP activity and osteocalcin secretion were also enhanced with the addition of IL-4 later in male versus female groups. The sex of the cells had a statistically significant effect on the optimal IL-4 addition time to maximize osteogenesis. Conclusion. These results suggest that: 1) a 72- to 96-hour proinflammatory environment is critical for optimal matrix mineralization; and 2) there are immunological differences in this coculture environment due to sex. Optimizing immunomodulation during fracture healing may enhance and expedite the bone regeneration response. These findings provide insight into precise immunomodulation for enhanced bone healing that is sex-specific. Cite this article: K. Nathan, L. Y. Lu, T. Lin, J. Pajarinen, E. Jämsen, J-F. Huang, M. Romero-Lopez, M. Maruyama, Y. Kohno, Z. Yao, S. B. Goodman. Precise immunomodulation of the M1 to M2 macrophage transition enhances mesenchymal stem cell osteogenesis and differs by sex. Bone Joint Res 2019;8:481–488. DOI: 10.1302/2046-3758.810.BJR-2018-0231.R2

Objectives

Mesenchymal stem cells (MSCs) are of growing interest in terms of bone regeneration. Most preclinical trials utilize bone-marrow-derived mesenchymal stem cells (bMSCs), although this is not without isolation and expansion difficulties. The aim of this study was: to compare the characteristics of bMSCs and adipose-derived mesenchymal stem cells (AdMSCs) from juvenile, adult, and ovarectomized (OVX) rats; and to assess the effect of human parathyroid hormone (hPTH) 1-34 on their osteogenic potential and migration to stromal cell-derived factor-1 (SDF-1).

Osteoarthritis (OA) is a degenerative disease with a strong inflammatory component. Intra-articular (IA) injections of mesenchymal stem cells (MSCs) modulate local inflammation, although the lack of engraftment suggests that they undergo apoptosis. The aim of this study is to investigate the fate of IA-delivered MSCs in an animal model of OA and to assess the role of apoptosis in vitro. Collagenase-induced OA (CIOA) was performed on C57BL/6 mice and 2×10∧5 GFP+ MSCs were IA-injected in the animals. 3 days later, knee joints were digested into a single-cell suspension and MSCs retrieved by cell sorting. Conditioned medium (CM) of retrieved cells was tested on murine macrophages and cytokine secretion was measured. Apoptosis of MSCs was induced in vitro with staurosporine (STS) and evaluated by Annexin V/Sytox Blue staining; activation of caspases was measured by FLICA assays. Murine lymphocytes were cocultured with apoptotic MSCs and their proliferation measured by quantification of Cell Trace Violet. 1.63% of injected cells were retrieved and proliferated in culture. Their CM significantly modulated activation of macrophages, with greater effects from OA-induced MSCs. STS induced apoptosis with activation of Caspase 3/7. Apoptotic MSCs significantly prevented the proliferation of murine lymphocytes. MSCs can be administered and retrieved from murine knees. Retrieval yield is low, consistent with previous studies. MSCs were licensed from the OA joint to produce an immunosuppressive milieu that modulated macrophages ex vivo. In vitro, apoptosis increased the immunomodulatory potential of MSCs. This suggests that apoptosis may contribute to the therapeutic effects of MSCs in OA

We developed a 3D vascularized bone remodeling model embedding human osteoblast and osteoclast precursors and endothelial cells in a mineralized matrix. All the cells included in the model exerted their function, resulting in a vascularized system undergoing mineralized matrix remodeling. Bone remodeling is a dynamic process relying on the balance between the activity of osteoblasts and osteoclasts which are responsible for bone formation and resorption, respectively. This process is also characterized by a tight coupling between osteogenesis and angiogenesis, indicating the existence of a complex cross-talk between endothelial cells and bone cells. We have recently developed microscale in vitro hydrogel-based models, namely the 3D MiniTissue models, to obtain bone-mimicking microenvironments including a 3D microvascular network formed by endothelial cell self-assembly [1–2]. Here, we generated a vascularized 3D MiniTissue bone remodeling model through the coculture of primary human cells in a 3D collagen/fibrin (Col/Fib) matrix enriched with CaP nanoparticles (CaPn) to mimic bone mineralized matrix. Human umbilical vein endothelial cells (HUVECs), bone marrow mesenchymal stem cells (BMSCs), osteoblast (OBs) and osteoclast (OCs) precursors were cocultured in plain and CaPn-enriched Col/Fib according to the following experimental conditions: a) HUVECs-BMSCs; b) OBs-OCs; c) HUVECs-BMSCs-OBs-OCs. Undifferentiated BMSCs were used to support HUVECs in microvascular network formation. BMSCs and peripheral blood mononuclear cells were respectively pre-differentiated into OB and OC precursors through 7 days of culture in osteogenic or osteoclastogenic medium. Needle-shaped CaPn (Ø ∼20 nm, length ∼80 nm) were added to a collagen/fibrinogen solution. Cells were resuspended in a thrombin solution and then mixed with plain or CaPn-enriched collagen/fibrinogen. The cell-laden mix was injected in U-shaped PMMA masks and let to polymerize to generate constructs of 2×2×5 mm. 3. Samples were cultured for 10 days. Microvascular network formation was evaluated by confocal microscopy. OB differentiation was analyzed by quantification of Alkaline Phosphatase (ALP) and cell-mediated mineralization. OC differentiation was assessed by Tartrate-Resistant Acid Phosphatase (TRAP) and cell-mediated phosphate release quantification. HUVECs developed a robust 3D microvascular network and BMSCs differentiated into mural cells supporting vasculogenesis. The presence of CaPn enhanced OB and OC differentiation, as demonstrated by the significantly higher ALP and TRAP levels and by the superior cell-mediated mineralization and phosphate release measured in CaPn-enriched than in plain Col/Fib. The coculture of OBs and OCs with HUVECs and BMSCs further enhanced ALP and TRAP levels, indicating that the presence of HUVECs and BMSCs positively contributed to OB and OC differentiation. Remarkably, higher values of ALP and TRAP activity were measured in the tetraculture in CaPn-enriched Col/Fib compared to plain Col/Fib, indicating that also in the tetraculture the mineralized matrix stimulated OB and OC differentiation. The 3D MiniTissue bone remodeling model developed in this study is a promising platform to investigate bone cell and endothelial cell cross-talk. This system allows to minimize the use of cells and reagents and is characterized by a superior ease of use compared to other microscale systems, such as microfluidic models. Finally, it represents a suitable platform to test drugs for bone diseases and can be easily personalized with patient-derived cells further increasing its relevance as drug screening platform

Mesenchymal stem cells (MSCs) reside around blood vessels in all organs. This reservoir of progenitors can be ‘recruited’ in response to injury. The ability to manipulate stem cells therapeutically within injured tissue provides an attractive alternative to transplantation. Stem cells are regulated by neighbouring cells. We hypothesized that endothelial cells (ECs) influence MSC differentiation into bone and fat. MSCs were sorted from fat using fluorescent activated sorting. Their capacity to differentiate into bone, fat and cartilage was used to confirm MSC phenotype. MSCs and ECs were cultured in two-dimensions (standard culture dishes) and three-dimensions (vascular networks suspended in gel). Cocultures were exposed to osteogenic and adipogenic media. The role of EC-released factors on MSC differentiation was determined using a system in which cells share media but do not contact. Wnt pathway modulators were used to investigate the role of Wnt signalling. MSCs differentiated into bone, fat and cartilage. MSCs and ECs integrated in two- and three-dimensions. MSCs and ECs formed vessel-like structures in three-dimensions. When cultured with ECs, MSC differentiation to bone was accelerated while differentiation to fat was inhibited. This effect on osteogenesis was maintained when cells shared media but did not contact. Coculture with Wnt modulators confirmed that this effect is in part, mediated through Wnt signalling. Our data suggest that ECs influence MSC differentiation. Therapeutic targeting of EC-MSCs signalling may enable manipulation of MSCs in vivo avoiding the need for cell transplantation. This could enable trauma and orthopaedic patients who have healthy resident stem cells to self-repair