Hypertrophic Cartilage Research Articles

The Importance of Physioxia in Mesenchymal Stem Cell Chondrogenesis and the Mechanisms Controlling Its Response.

Articular cartilage covers the surface of synovial joints and enables joint movement. However, it is susceptible to progressive degeneration with age that can be accelerated by either previous joint injury or meniscectomy. This degenerative disease is known as osteoarthritis (OA) and it greatly affects the adult population. Cell-based tissue engineering provides a possible solution for treating OA at its earliest stages, particularly focal cartilage lesions. A candidate cell type for treating these focal defects are Mesenchymal Stem Cells (MSCs). However, present methods for differentiating these cells towards the chondrogenic lineage lead to hypertrophic chondrocytes and bone formation in vivo. Environmental stimuli that can stabilise the articular chondrocyte phenotype without compromising tissue formation have been extensively investigated. One factor that has generated intensive investigation in MSC chondrogenesis is low oxygen tension or physioxia (2–5% oxygen). In vivo articular cartilage resides at oxygen tensions between 1–4%, and in vitro results suggest that these conditions are beneficial for MSC expansion and chondrogenesis, particularly in suppressing the cartilage hypertrophy. This review will summarise the current literature regarding the effects of physioxia on MSC chondrogenesis with an emphasis on the pathways that control tissue formation and cartilage hypertrophy.

Neural Cell Adhesion Molecule Regulates Chondrocyte Hypertrophy in Chondrogenic Differentiation and Experimental Osteoarthritis

Chondrocyte hypertrophy-like change is an important pathological process of osteoarthritis (OA), but the mechanism remains largely unknown. Neural cell adhesion molecule (NCAM) is highly expressed and participate the chondrocyte differentiation of mesenchymal stem cells (MSCs). In this study, we found that NCAM deficiency leads to hypertrophic chondrocyte differentiation in murine MSCs, in which extracellular signal-regulated kinase (ERK) signaling play an important role. Moreover, in an interleukin (IL)-1β induced OA cellular model, NCAM expression is down-regulated. Interestingly, overexpression of NCAM substantially inhibits hypertrophic differentiation in the OA cellular model. Consistently, NCAM deficiency accelerates chondrocyte hypertrophy in growth-plate and articular cartilage of OA mice. In conclusion, NCAM could inhibit hypertrophic chondrocyte differentiation of MSCs by activating ERK signaling and reduce chondrocyte hypertrophy in experimental OA model, suggesting the potential utility of NCAM as a novel therapeutic target for alleviating chondrocyte hypertrophy of OA. Funding Statement: This work was supported by the grants from the National Natural Science Foundation of China (No. 81403161, U1704186, 81402416 and U1404816), Key Scientific Research Project of Colleges and Universities in Henan Province (No. 19A310020 and 16A180015), Henan Collaborative Innovation Center of Molecular Diagnosis and Laboratory Medicine (No. XTCX-2015-ZD3) and Scientific and Technological Research Project of Henan Province (No. 182102311136). Declaration of Interests: The authors declare no competing financial interest. Ethics Approval Statement: All animal experiments were approved by the Ethics Committee of Xinxiang Medical University.

Biofabrication of spatially organised tissues by directing the growth of cellular spheroids within 3D printed polymeric microchambers

Successful tissue engineering requires the generation of human scale implants that mimic the structure, composition and mechanical properties of native tissues. Here, we report a novel biofabrication strategy that enables the engineering of structurally organised tissues by guiding the growth of cellular spheroids within arrays of 3D printed polymeric microchambers. With the goal of engineering stratified articular cartilage, inkjet bioprinting was used to deposit defined numbers of mesenchymal stromal cells (MSCs) and chondrocytes into pre-printed microchambers. These jetted cell suspensions rapidly underwent condensation within the hydrophobic microchambers, leading to the formation of organised arrays of cellular spheroids. The microchambers were also designed to provide boundary conditions to these spheroids, guiding their growth and eventual fusion, leading to the development of stratified cartilage tissue with a depth-dependant collagen fiber architecture that mimicked the structure of native articular cartilage. Furthermore, the composition and biomechanical properties of the bioprinted cartilage was also comparable to the native tissue. Using multi-tool biofabrication, we were also able to engineer anatomically accurate, human scale, osteochondral templates by printing this microchamber system on top of a hypertrophic cartilage region designed to support endochondral bone formation and then maintaining the entire construct in long-term bioreactor culture to enhance tissue development. This bioprinting strategy provides a versatile and scalable approach to engineer structurally organised cartilage tissues for joint resurfacing applications.

Advances in the roles of runt-related transcription factor 2 in skeletal development and differentiation of mesenchymal stem cells

Runt-related transcription factor 2 (Runx2) is a multifunctional transcription factor that belongs to the Runx family, which plays a key role in the process of ontogenesis and tumorigenesis. As an important role in regulating the development and maturation of osteoblasts and chondrocytes, Runx2 contributes to the commitment of mesenchymal stem cells into osteoblasts lineage at the initial stage and inhibits the osteoblasts maturation and the transition of osteoblasts to osteocytes. Moreover, this transcriptional factor acts to promote the chondrocytes proliferation, hypertrophy and the surrounding vascular invasion, which facilitates endochondral ossification. Also, Runx2 boosts articular permanent cartilage hypertrophy and affects homeostasis of the cartilage. Runx2 has an impact on the differentiation of mesenchymal stem cells in vitro and acts as the target gene of stress and hypoxia in regulating osteogenic and chondrogenic differentiation. This review summarized recent research progress in the role of Runx2 in skeletal development and the differentiation of mesenchymal stem cells in vitro. Key words: Runt-related transcription factor 2; Osteoblast; Chondrocyte; Differentiation of mesenchymal stem cells; Review

Comparison of the effect of growth factors on chondrogenesis of canine mesenchymal stem cells.

Mesenchymal stem cells (MSCs) are proposed to be useful in cartilage regenerative medicine, however, canine MSCs have been reported to show poor chondrogenic capacity. Therefore, optimal conditions for chondrogenic differentiation should be determined by mimicking the developmental process. We have previously established novel and superior canine MSCs named bone marrow peri-adipocyte cells (BM-PACs) and the objective of this study was to evaluate the effects of growth factors required for in vivo chondrogenesis using canine BM-PACs. Spheroids of BM-PACs were cultured in chondrogenic medium containing 10 ng/ml transforming growth factor-β1 (TGF-β1) with or without 100 ng/ml bone morphogenetic protein-2 (BMP-2), 100 ng/ml growth differentiation factor-5 (GDF-5) or 100 ng/ml insulin-like growth factor-1 (IGF-1). Chondrogenic differentiation was evaluated by the quantification of glycosaminoglycan and Safranin O staining for proteoglycan production. The expression of cartilage matrix or hypertrophic gene/protein was also evaluated by qPCR and immunohistochemistry. Spheroids in all groups were strongly stained with Safranin O. Although BMP-2 significantly increased glycosaminoglycan production, Safranin O-negative outer layer was formed and the mRNA expression of COL10 relating to cartilage hypertrophy was also significantly upregulated (P<0.05). GDF-5 promoted the production of glycosaminoglycan and type II collagen without increasing COL10 mRNA expression. The supplementation of IGF-1 did not significantly affect cartilaginous and hypertrophic differentiation. Our results indicate that GDF-5 is a useful growth factor for the generation of articular cartilage from canine MSCs.

Prefabrication of a large pedicled bone graft by engineering the germ for de novo vascularization and osteoinduction

Large and complex bone defects represent challenging clinical scenarios, typically requiring autologous vascularized bone transplants. In order to bypass the numerous associated limitations, here we aimed at ectopically prefabricating a bone graft surrogate with vascular pedicle.A hollow cylinder of devitalized cancellous bone was used to define the space of a large bone substitute. This space was filled with devitalized pellets of engineered hypertrophic cartilage as bone-inducing material, in combination or not with stromal vascular fraction (SVF) of adipose tissue as source of osteoprogenitors and endothelial cells. Vascularization of the space was targeted through axial insertion of an arterio-venous (AV) bundle. Constructs were subcutaneously implanted in nude rats for 12 weeks and analyzed for bone formation and vascularization by histology and microtomography.Retrieved constructs were extensively vascularized in all conditions, with vessels sprouting from the AV bundle and reaching a higher density in the axially central volume. Bone tissue was formed through remodeling of hypertrophic cartilage, and quantitatively correlated with de novo vascularization.Our study demonstrates feasibility to prefabricate large, pedicled bone grafts in predefined shapes. The combination of an AV bundle with engineered hypertrophic cartilage provided a germ for the coupled processes of vascularization and bone formation. The demonstrated osteoinductivity of devitalized hypertrophic cartilage offers the opportunity of implementing the proposed regenerative surgery strategy through off-the-shelf materials.

Deletion of Runx2 in condylar chondrocytes disrupts TMJ tissue homeostasis.

Runt-related transcription factor-2 (Runx2) is essential for chondrocyte maturation during cartilage development and embryonic mandibular condylar development. The process that chondrocytes, especially a subgroup of hypertrophic chondrocytes (HC), could transform into bone cells in mandibular condyle growth makes chondrocytes crucially important for normal endochondral bone formation. To determine whether Runx2 regulates postnatal condylar cartilage growth and tissue homeostasis, we deleted Runx2 in chondrocytes in postnatal mice and assessed the consequences on temporomandibular joint (TMJ) cartilage growth and remodeling. The cell lineage tracing data provide information demonstrating the role of chondrocytes in subchondral bone remodeling. The histologic and immunohistochemical data showed that Runx2 deficiency caused condylar tissue disorganization, including loss of HC and reduced hypertrophic zone, reduced proliferative chondrocytes, and decreased cartilage matrix production. Expression of Col10a1, Mmp13, Col2a1, Aggrecan, and Ihh was significantly reduced in Runx2 knockout mice. The findings of this study demonstrate that Runx2 is required for chondrocyte proliferation and hypertrophy in TMJ cartilage and postnatal TMJ cartilage growth and homeostasis, and that Runx2 may play an important role in regulation of chondrocyte-derived subchondral bone remodeling.

Ultrasonographic Assessment of Femoral Cartilage in Individuals With Anterior Cruciate Ligament Reconstruction: A Case-Control Study.

Developing osteoarthritis is common after anterior cruciate ligament reconstruction (ACLR). Monitoring changes in femoral cartilage size after ACLR may be a way to detect the earliest structural alterations before the radiographic onset of osteoarthritis. Diagnostic ultrasonography (US) offers a clinically accessible and valid method for evaluating anterior femoral cartilage size. To compare the US measurements of anterior femoral cross-sectional area and cartilage thickness between limbs in individuals with a unilateral ACLR and between the ACLR limbs of these individuals and the limbs of uninjured control participants. Case-control study. Research laboratory. A total of 20 volunteers with an ACLR (37.0 ± 26.6 months after surgery) and 28 uninjured volunteers. We used US to assess anterior femoral cartilage cross-sectional area and thickness (ie, medial, lateral, and intercondylar) in the ACLR and contralateral limbs of participants with ACLR and unilaterally in the reference limbs of uninjured participants. The ACLR limb presented with greater anterior femoral cartilage cross-sectional area (96.68 ± 22.68 mm2) than both the contralateral (85.69 ± 17.57 mm2, t19 = 4.47; P < .001) and uninjured (84.62 ± 15.89 mm2, t46 = 2.17; P = .04) limbs. The ACLR limb presented with greater medial condyle thickness (2.61 ± 0.61 mm) than both the contralateral (2.36 ± 0.47 mm, t19 = 2.78; P = .01) and uninjured limbs (2.22 ± 0.40 mm, t46 = 2.69; P = .01) and greater lateral condyle thickness (2.46 ± 0.65 mm) than the uninjured limb (2.12 ± 0.41 mm, t46 = 2.20; P = .03). Anterior femoral cartilage cross-sectional area and thickness assessed via US were greater in the ACLR limb than in the contralateral and uninjured limbs. Greater thickness and cross-sectional area may have been due to cartilage swelling or hypertrophy after ACLR, which may affect the long-term health of the joint.

Eldecalcitol is more effective in promoting osteogenesis than alfacalcidol in Cyp27b1-knockout mice.

Calcium (Ca) absorption from the intestinal tract is promoted by active vitamin D (1α,25D3). Vitamin D not only promotes Ca homeostasis, but it also inhibits bone resorption and promotes osteogenesis, thus playing a role in the maintenance of normal bone metabolism. Because 1α,25D3 plays an important role in osteogenesis, vitamin D formulations, such as alfacalcidol (ALF) and eldecalcitol (ELD), are used for treating osteoporosis. While it is known that, in contrast to ALF, ELD is an active ligand that directly acts on bone, the reason for its superior osteogenesis effects is unknown. Cyp27b1-knockout mice (Cyp27b1–/–mice) are congenitally deficient in 1α,25D3 and exhibit marked hypocalcemia and high parathyroid hormone levels, resulting in osteodystrophy involving bone hypocalcification and growth plate cartilage hypertrophy. However, because the vitamin D receptor is expressed normally in Cyp27b1–/–mice, they respond normally to 1α,25D3. Accordingly, in Cyp27b1–/–mice, the pharmacological effects of exogenously administered active vitamin D derivatives can be analyzed without being affected by 1α,25D3. We used Cyp27b1–/–mice to characterize and clarify the superior osteogenic effects of ELD on the bone in comparison with ALF. The results indicated that compared to ALF, ELD strongly induces ECaC2, calbindin-D9k, and CYP24A1 in the duodenum, promoting Ca absorption and decreasing the plasma concentration of 1α,25D3, resulting in improved osteogenesis. Because bone morphological measurements demonstrated that ELD has stronger effects on bone calcification, trabecular formation, and cancellous bone density than ALF, ELD appears to be a more effective therapeutic agent for treating postmenopausal osteoporosis, in which cancellous bone density decreases markedly. By using Cyp27b1–/–mice, this study was the first to succeed in clarifying the osteogenic effect of ELD without any influence of endogenous 1α,25D3. Furthermore, ELD more strongly enhanced bone mineralization, trabecular proliferation, and cancellous bone density than did ALF. Thus, ELD is expected to show an effect on postmenopausal osteoporosis, in which cancellous bone mineral density decreases markedly. In the future, this study may enable the development of next-generation active vitamin D derivatives with higher affinity for bone than ELD.

Changes in ephrin gene expression during bone healing identify a restricted repertoire of ephrinsmediating fracture repair.

To identify the repertoire of ephrin genes that might regulate endochondral bone fracture repair, we examined changes in ephrin ligand and receptor (Eph) gene expression in fracture callus tissues during bone fracture healing. Ephrin and Eph proteins were then localized in the fracture callus tissues present when changes in gene expression were observed. Ephrin gene expression was widespread in fracture tissues, but the repertoire of ephrin genes with significant changes in expression that might suggest a regulatory role in fracture callus development was restricted to the ephrin A family members Epha4, Epha5 and the ephrin B family member Efnb1. After 3 weeks of healing, Epha4 fracture expression was downregulated from 1.3- to 0.8-fold and Epha5 fracture expression was upregulated from 1.2- to 1.5-fold of intact contralateral femur expression, respectively. Efnb1 expression was downregulated from 1.5- to 1.2-fold after 2 weeks post-fracture. These ephrin proteins were localized to fracture callus prehypertrophic chondrocytes and osteoblasts, as well as to the periosteum and fibrous tissues. The observed positive correlation between mRNA levels of EfnB1 with Col10 and Epha5 with Bglap, together with colocalized expression with their respective proteins, suggest that EfnB1 is a positive mediator of prehypertrophic chondrocyte development and that Epha5 contributes to osteoblast-mediated mineralization of fracture callus. In contrast, mRNA levels of Epha4 and Efnb1 correlated negatively with Bglap, thus suggesting a negative role for these two ephrin family members in mature osteoblast functions. Given the number of family members and widespread expression of the ephrins, a characterization of changes in ephrin gene expression provides a basis for identifying ephrin family members that might regulate the molecular pathways of bone fracture repair. This approach suggests that a highly restricted repertoire of ephrins, EfnB1 and EphA5, are the major mediators of fracture callus cartilage hypertrophy and ossification, respectively, and proposes candidates for additional functional study and eventual therapeutic application.

Targeting anti-chondrogenic factors for the stimulation of chondrogenesis: A new paradigm in cartilage repair.

Trauma and age-related cartilage disorders represent a major global cause of morbidity, resulting in chronic pain and disability in patients. A lack of effective therapies, together with a rapidly aging population, creates an impressive clinical and economic burden on healthcare systems. In this scenario, experimental therapies based on transplantation or in situ stimulation of skeletal Mesenchymal Stem/progenitor Cells (MSCs) have raised great interest for cartilage repair. Nevertheless, the challenge of guiding MSC differentiation and preventing cartilage hypertrophy and calcification still needs to be overcome. While research has mostly focused on the stimulation of cartilage anabolism using growth factors, several issues remain unresolved prompting the field to search for novel solutions. Recently, inhibition of anti-chondrogenic regulators has emerged as an intriguing opportunity. Anti-chondrogenic regulators include extracellular proteins as well as intracellular transcription factors and microRNAs that act as potent inhibitors of pro-chondrogenic signals. Suppression of these inhibitors can enhance MSC chondrogenesis and production of cartilage matrix. We here review the current knowledge concerning different types of anti-chondrogenic regulators. We aim to highlight novel therapeutic targets for cartilage repair and discuss suitable tools for suppressing their anti-chondrogenic functions. Further effort is needed to unveil the therapeutic perspectives of this approach and pave the way for effective treatment of cartilage injuries in patients. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Induced pluripotent stem cells for cartilage repair: current status and future perspectives.

The establishment of cartilage regenerative medicine is an important clinical issue, but the search for cell sources able to restore cartilage integrity proves to be challenging. Human mesenchymal stromal cells (MSCs) are prone to form epiphyseal or hypertrophic cartilage and have an age-related limited proliferation. On the other hand, it is difficult to obtain functional chondrocytes from human embryonic stem cells (ESCs). Moreover, the ethical issues associated with human ESCs are an additional disadvantage of using such cells. Since their discovery in 2006, induced pluripotent stems cells (iPSCs) have opened many gateways to regenerative medicine research, especially in cartilage tissue engineering therapies. iPSCs have the capacity to overcome limitations associated with current cell sources since large numbers of autologous cells can be derived from small starting populations. Moreover, problems associated with epiphyseal or hypertrophic-cartilage formation can be overcome using iPSCs. iPSCs emerge as a promising cell source for treating cartilage defects and have the potential to be used in the clinical field. For this purpose, robust protocols to induce chondrogenesis, both in vitro an in vivo, are required. This review summarises the recent progress in iPSC technology and its applications for cartilage repair.

CaMKII inhibition in human primary and pluripotent stem cell-derived chondrocytes modulates effects of TGFβ and BMP through SMAD signaling

Upregulation of calcium/calmodulin-dependent kinase II (CaMKII) is implicated in the pathogenesis of osteoarthritis (OA) and reactivation of articular cartilage hypertrophy. However, direct inhibition of CaMKII unexpectedly augmented symptoms of OA in animal models. The role of CaMKII in OA remains unclear and requires further investigation. Analysis of CaMKII expression was performed in normal human and OA articular chondrocytes, and signaling mechanisms were assessed in articular, fetal and Pluripotent Stem Cell (PSC)-derived human chondrocytes using pharmacological (KN93), peptide (AC3-I) and small interfering RNA (siRNA) inhibitors of CaMKII. Expression levels of phospho-CaMKII (pCaMKII) were significantly and consistently increased in human OA specimens. BMP2/4 activated expression of pCaMKII as well as COLII and COLX in human adult articular chondrocytes, and also increased the levels and nuclear localization of SMADs1/5/8, while TGFβ1 showed minimal or no activation of the chondrogenic program in adult chondrocytes. Targeted blockade of CaMKII with specific siRNAs decreased levels of pSMADs, COLII, COLX and proteoglycans in normal and OA adult articular chondrocytes in the presence of both BMP4 and TGFβ1. Both human fetal and PSC-derived chondrocytes also demonstrated a decrease of chondrogenic differentiation in the presence of small molecule and peptide inhibitors of CaMKII. Furthermore, immunoprecipitation for SMADs1/5/8 or 2/3 followed by western blotting for pCaMKII showed direct interaction between SMADs and pCaMKII in primary chondrocytes. Current study demonstrates a direct role for CaMKII in TGF-β and BMP-mediated responses in primary and PSC-derived chondrocytes. These findings have direct implications for tissue engineering of cartilage tissue from stem cells and therapeutic management of OA.

First evidence of globuli ossei in bird (Aves, Spheniciformes). Implications on paleohistology and bird behaviour

ABSTRACTGlobuli ossei are subspherical structures of endochondral origin that are inserted in the hypertrophic cartilage of long bones, and are related to paedomorphic phenomena. These structures are here described for the first time in Aves, concretely in a postcranial bone of an extinct Spheniciformes from Antarctica (Delphinornis arctowskii). Although it is difficult to infer the significance of the presence of these structures, they appear not to be related to adaptations to aquatic life or depend on the small size or ontogenetic stage of the specimen. A pathological origin for these structures lacks evidence sustaining this hypothesis. The data provided in this contribution reveals that the presence of globuli ossei in vertebrate long bones is not always linked with adaptations to aquatic life and/or paedomorphic processes.

Bone and joint structural impairments in acromegaly

Elevated serum levels of growth hormone (GH) and insulin-like growth factor 1 (IGF-1) in patients with acromegaly result in intensified bone turnover, as evidenced by increased levels of bone remodeling markers and higher risk of low-traumatic vertebral fractures. However, it was repeatedly observed that bone mineral density (BMD) is normal or even increased in patients with acromegaly (including the active stage of the disease). Increased secretion of GH/IGF-1 causes structural changes in the vertebrae and peripheral joints (osteophyte formation and cartilage hypertrophy), resulting in pain and various deformities of the articular system. These changes are known under the common name «acromegalic arthropathy». It is quite specific complication of the disease. Skeletal complications of acromegaly can persist even after radical treatment, i.e., their course and progression, presumably, do not necessarily depend on the remission level of the underlying disease. This review summarizes the current understanding of the pathophysiology, clinical presentation, diagnosis, and treatment of osteo-articular complications of acromegaly.

Molecular characterization of mesenchymal stem cells in human osteoarthritis cartilage reveals contribution to the OA phenotype

Adult human articular cartilage harbors a population of CD166+ mesenchymal stem cell-like progenitors that become more numerous during osteoarthritis (OA). While their role is not well understood, here we report that they are indeed part of cellular clusters formed in OA cartilage, which is a pathological hallmark of this disease. We hypothesize that these cells, termed OA mesenchymal stem cells (OA-MSCs), contribute to OA pathogenesis. To test this hypothesis, we generated and characterized multiple clonally derived stable/immortalized human OA-MSC cell lines, which exhibited the following properties. Firstly, two mesenchymal stem cell populations exist in human OA cartilage. While both populations are multi-potent, one preferentially undergoes chondrogenesis while the other exhibits higher osteogenesis potential. Secondly, both OA-MSCs exhibit significantly higher expression of hypertrophic OA cartilage markers COL10A1 and RUNX2, compared to OA chondrocytes. Induction of chondrogenesis in OA-MSCs further stimulated COL10A1 expression and MMP-13 release, suggesting that they contribute to OA phenotypes. Finally, knocking down RUNX2 is insufficient to inhibit COL10A1 in OA-MSCs and also requires simultaneous knockdown of NOTCH1 thereby suggesting altered gene regulation in OA stem cells in comparison to chondrocytes. Overall, our findings suggest that OA-MSCs may drive pathogenesis of cartilage degeneration and should therefore be a novel cell target for OA therapy.

An endochondral ossification approach to early stage bone repair: Use of tissue-engineered hypertrophic cartilage constructs as primordial templates for weight-bearing bone repair.

Mimicking endochondral ossification to engineer constructs offers a novel solution to overcoming the problems associated with poor vascularisation in bone repair. This can be achieved by harnessing the angiogenic potency of hypertrophic cartilage. In this study, we demonstrate that tissue-engineered hypertrophically primed cartilage constructs can be developed from collagen-based scaffolds cultured with mesenchymal stem cells. These constructs were subsequently implanted into femoral defects in rats. It was evident that the constructs could support enhanced early stage healing at 4weeks of these weight-bearing femoral bone defects compared to untreated defects. This study demonstrates the value of combining knowledge of development biology and tissue engineering in a developmental engineering inspired approach to tissue repair.

The Potential for Synovium-derived Stem Cells in Cartilage Repair.

Articular cartilage defects often result in pain, loss of function and finally osteoarthritis. Developing cell-based therapies for cartilage repair is a major goal of orthopaedic research. Autologous chondrocyte implantation is currently the gold standard cell-based surgical procedure for the treatment of large, isolated, full thickness cartilage defects. Several disadvantages such as the need for two surgical procedures or hypertrophic regenerative cartilage, underline the need for alternative cell sources. Mesenchymal stem cells, particularly synovium-derived mesenchymal stem cells, represent a promising cell source. Synovium-derived mesenchymal stem cells have attracted considerable attention since they display great chondrogenic potential and less hypertrophic differentiation than mesenchymal stem cells derived from bone marrow. The aim of this review was to summarize the current knowledge on the chondrogenic potential for synovial stem cells in regard to cartilage repair purposes. A literature search was carried out identifying 260 articles in the databases up to January 2017. Several in vitro and initial animal in vivo studies of cartilage repair using synovia stem cell application showed encouraging results. Since synvoium-derived stem cells are located in the direct vicinity of cartilage and cartilage lesions these cells might even contribute to natural cartilage regeneration. The only one published human in vivo study with 10 patients revealed good results concerning postoperative outcome, MRI, and histologic features after a two-stage implantation of synovial stem cells into an isolated cartilage defect of the femoral condyle. Synovium-derived stem cells possess great chondrogenic potential and showed encouraging results for cartilage repair purposes. Furthermore, synovial stem cells play an important role in joint homeostasis and possibly in natural cartilage repair. Further studies are needed to elucidate the interplay of synovial stem cells and chondrocytes, and the promising role of synovium-derived stem cells in cartilage tissue engineering.

BMP signaling-driven osteogenesis is critically dependent on Prdx-1 expression-mediated maintenance of chondrocyte prehypetrophy.

During endochondral ossification, cartilage template is eventually replaced by bone. This process involves several well characterized, stereotypic, molecular and cellular changes in the cartilage primordia. These steps involve transition from resting to proliferative and then pre-hypertrophic to finally hypertrophic cartilage. BMP signaling is necessary and sufficient for osteogenesis. However, the specific step(s) of endochondral ossification in which BMP signaling plays an essential role is not yet known. In this study we have identified Prdx1, a known scavenger of ROS, to be expressed in pre-hypertrophic chondrocytes in a BMP signaling-dependent manner. We demonstrate that BMP signaling inhibition increases ROS levels in osteogenic cells. Further, Prdx1 regulates osteogenesis in vivo by helping maintenance of Ihh expressing pre-hypertrophic cells, in turn regulating these cells' transition into hypertrophy. Therefore, our data suggests that one of the key roles of BMP signaling in endochondral ossification is to maintain pre-hypertrophic state.

3D printed microchannel networks to direct vascularisation during endochondral bone repair

Bone tissue engineering strategies that recapitulate the developmental process of endochondral ossification offer a promising route to bone repair. Clinical translation of such endochondral tissue engineering strategies will require overcoming a number of challenges, including the engineering of large and often anatomically complex cartilage grafts, as well as the persistence of core regions of avascular cartilage following their implantation into large bone defects. Here 3D printing technology is utilized to develop a versatile and scalable approach to guide vascularisation during endochondral bone repair. First, a sacrificial pluronic ink was used to 3D print interconnected microchannel networks in a mesenchymal stem cell (MSC) laden gelatin-methacryloyl (GelMA) hydrogel. These constructs (with and without microchannels) were next chondrogenically primed in vitro and then implanted into critically sized femoral bone defects in rats. The solid and microchanneled cartilage templates enhanced bone repair compared to untreated controls, with the solid cartilage templates (without microchannels) supporting the highest levels of total bone formation. However, the inclusion of 3D printed microchannels was found to promote osteoclast/immune cell invasion, hydrogel degradation, and vascularisation following implantation. In addition, the endochondral bone tissue engineering strategy was found to support comparable levels of bone healing to BMP-2 delivery, whilst promoting lower levels of heterotopic bone formation, with the microchanneled templates supporting the lowest levels of heterotopic bone formation. Taken together, these results demonstrate that 3D printed hypertrophic cartilage grafts represent a promising approach for the repair of complex bone fractures, particularly for larger defects where vascularisation will be a key challenge.