Regenerative and Anti-Inflammatory Potential of Regularly Fed, Starved Cells and Extracellular Vesicles In Vivo.

BACKGROUND: Mesenchymal stem/stromal cells (MSC) have been employed successfully in immunotherapy and regenerative medicine, but their therapeutic potential is reduced considerably by the ischemic environment that exists after transplantation. The assumption that preconditioning MSC to promote quiescence may result in increased survival and regenerative potential upon transplantation is gaining popularity. METHODS: The purpose of this work was to evaluate the anti-inflammatory and regenerative effects of human bone marrow MSC (hBM-MSC) and their extracellular vesicles (EVs) grown and isolated in a serum-free medium, as compared to starved hBM-MSC (preconditioned) in streptozotocin-induced diabetic fractured male C57BL/6J mice. RESULTS: Blood samples taken four hours and five days after injection revealed that cells, whether starved or not, generated similar plasma levels of inflammatory-related cytokines but lower levels than animals treated with EVs. Nonetheless, starved cells prompted the highest production of IL-17, IL-6, IL-13, eotaxin and keratinocyte-derived chemokines and induced an earlier soft callus formation and mineralization of the fracture site compared to EVs and regularly fed cells five days after administration. CONCLUSIONS: Preconditioning may be crucial for refining and defining new criteria for future MSC therapies. Additionally, the elucidation of mechanisms underpinning an MSC's survival/adaptive processes may result in increased cell survival and enhanced therapeutic efficacy following transplantation.

Administration of Human Non-Diabetic Mesenchymal Stromal Cells to a Murine Model of Diabetic Fracture Repair: A Pilot Study.

Individuals living with type 1 diabetes mellitus may experience an increased risk of long bone fracture. These fractures are often slow to heal, resulting in delayed reunion or non-union. It is reasonable to theorize that the underlying cause of these diabetes-associated osteopathies is faulty repair dynamics as a result of compromised bone marrow progenitor cell function. Here it was hypothesized that the administration of non-diabetic, human adult bone marrow-derived mesenchymal stromal cells (MSCs) would enhance diabetic fracture healing. Human MSCs were locally introduced to femur fractures in streptozotocin-induced diabetic mice, and the quality of de novo bone was assessed eight weeks later. Biodistribution analysis demonstrated that the cells remained in situ for three days following administration. Bone bridging was evident in all animals. However, a large reparative callus was retained, indicating non-union. µCT analysis elucidated comparable callus dimensions, bone mineral density, bone volume/total volume, and volume of mature bone in all groups that received cells as compared to the saline-treated controls. Four-point bending evaluation of flexural strength, flexural modulus, and total energy to re-fracture did not indicate a statistically significant change as a result of cellular administration. An ex vivo lymphocytic proliferation recall assay indicated that the xenogeneic administration of human cells did not result in an immune response by the murine recipient. Due to this dataset, the administration of non-diabetic bone marrow-derived MSCs did not support fracture healing in this pilot study.

Impact of Type 2 Diabetes Mellitus on Human Bone Marrow Stromal Cell Number and Phenotypic Characteristics.

Human bone marrow-derived mesenchymal stromal cells (MSCs) have been investigated in numerous disease settings involving impaired regeneration because of the crucial role they play in tissue maintenance and repair. Considering the number of comorbidities associated with type 2 diabetes mellitus (T2DM), the hypothesis that MSCs mediate these comorbidities via a reduction in their native maintenance and repair activities is an intriguing line of inquiry. Here, it is demonstrated that the number of bone marrow-derived MSCs in people with T2DM was reduced compared to that of age-matched control (AMC) donors and that this was due to a specific decrease in the number of MSCs with osteogenic capacity. There were no differences in MSC cell surface phenotype or in MSC expansion, differentiation, or angiogenic or migratory capacity from donors living with T2DM as compared to AMCs. These findings elucidate the basic biology of MSCs and their potential as mediators of diabetic comorbidities, especially osteopathies, and provide insight into donor choice for MSC-based clinical trials. This study suggests that any role of bone marrow MSCs as a mediator of T2DM comorbidity is likely due to a reduction in the osteoprogenitor population size and not due to a permanent alteration to the MSCs' capacity to maintain tissue homeostasis through expansion and differentiation.

Bone Mineral Is More Heterogeneously Distributed in the Femoral Heads of Osteoporotic and Diabetic Patients: A Pilot Study.

Osteoporosis is associated with systemic bone loss, leading to a significant deterioration of bone microarchitecture and an increased fracture risk. Although recent studies have shown that the distribution of bone mineral becomes more heterogeneous because of estrogen deficiency in animal models of osteoporosis, it is not known whether osteoporosis alters mineral distribution in human bone. Type 2 diabetes mellitus (T2DM) can also increase bone fracture risk and is associated with impaired bone cell function, compromised collagen structure, and reduced mechanical properties. However, it is not known whether alterations in mineral distribution arise in diabetic (DB) patients' bone. In this study, we quantify mineral content distribution and tissue microarchitecture (by µCT) and mechanical properties (by compression testing) of cancellous bone from femoral heads of osteoporotic (OP; n = 10), DB (n = 7), and osteoarthritic (OA; n = 7) patients. We report that though OP cancellous bone has significantly deteriorated compressive mechanical properties and significantly compromised microarchitecture compared with OA controls, there is also a significant increase in the mean mineral content. Moreover, the heterogeneity of the mineral content in OP bone is significantly higher than controls (+25%) and is explained by a significant increase in bone volume at high mineral levels. We propose that these mineral alterations act to exacerbate the already reduced bone quality caused by reduced cancellous bone volume during osteoporosis. We show for the first time that cancellous bone mineralization is significantly more heterogeneous (+26%) in patients presenting with T2DM compared with OA (non-DB) controls, and that this heterogeneity is characterized by a significant increase in bone volume at low mineral levels. Despite these mineralization changes, bone microarchitecture and mechanical properties are not significantly different between OA groups with and without T2DM. Nonetheless, the observed alterations in mineral heterogeneity may play an important tissue-level role in bone fragility associated with OP and DB bone. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Knowledge, Perceptions and Concerns of Diabetes -Associated Complications Among Individuals Living with Type 1 and Type 2 Diabetes Mellitus.

The purpose of this study was to investigate the knowledge, perceptions and concerns of individuals living with diabetes mellitus regarding the disorder and its associated long-term health complications. Individuals living with type 1 (N = 110) and type 2 (N = 100) diabetes were surveyed at the Diabetes Centre at University Hospital Galway (Ireland). A questionnaire was used to record respondent's perceptions and concerns about living with diabetes and developing associated long-term health complications, especially diabetes-induced osteopathy. Participants' responses revealed a variety of perspectives. Individuals with type 1 diabetes had a deeper understanding of the aetiology of diabetes and were more concerned about its complications than individuals with type 2 diabetes. The most recognized complications identified by the participants were retinopathy (92% type 1; 83% type 2), amputations (80% type 1; 70% type 2) and nephropathy (83% type 1; 63% type 2). Diabetes-related osteopathy was under-recognized, with 37% (type 1) and 23% (type 2) of respondents identifying bone fractures as a diabetes-related complication. Enhancing the patient awareness of this under-recognized diabetes-associated complication and ensuring that preventative measures are incorporated within health care programmes may offer methodologies to address this complication clinically.

Diabetes Mellitus and Gender Have a Negative Impact on the Outcome of Hip Fracture Surgery-A Pilot Study.

Diabetes mellitus (DM) is associated with an elevated risk of post-operative complications. The impact it has on patients living with DM following hip fracture surgery (HFS) is not completely understood and may represent a predictor of increased mortality. This study investigates the impact of DM, gender, American Society of Anaesthesiologists (ASA) grade, and fracture location, on the outcome of HFS in Ireland. The Hospital Inpatient Enquiry (HIPE) database records all fragility hip fractures within Galway University Hospital. Retrospective data collection was performed over a 3-year period. Data collected included patient age, gender, date of HFS, anatomical fracture location, type of operation, ASA grade, DM status, and mortality. A database of 650 individuals was created including 461 females and 189 males, with an average group age of 80.2 ± 9.3 years. Results showed a significantly higher incidence of hip fractures in males with DM (19.57%) than females with DM (12.36%) (χ2 test, p = 0.020). Cox regression survival analysis indicated that DM status and ASA grade were the two main independent predictors of patient survival following HFS. Nevertheless, when examining the combined impact of gender and DM status on survival after HFS, results showed that survival post HFS differed significantly with gender and presence of DM (log-rank test, p < 0.001), with males with DM performing worse than females with DM (p = 0.021) or males without DM (p = 0.001). This gender and disease-associated outcome should prompt an early multi-disciplinary team approach to the management of hip fractures in patients with DM. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:834-842, 2020.

Impact of Diabetes Mellitus on Bone Health.

Long-term exposure to a diabetic environment leads to changes in bone metabolism and impaired bone micro-architecture through a variety of mechanisms on molecular and structural levels. These changes predispose the bone to an increased fracture risk and impaired osseus healing. In a clinical practice, adequate control of diabetes mellitus is essential for preventing detrimental effects on bone health. Alternative fracture risk assessment tools may be needed to accurately determine fracture risk in patients living with diabetes mellitus. Currently, there is no conclusive model explaining the mechanism of action of diabetes mellitus on bone health, particularly in view of progenitor cells. In this review, the best available literature on the impact of diabetes mellitus on bone health in vitro and in vivo is summarised with an emphasis on future translational research opportunities in this field.

Hypoxia Activates the PTHrP -MEF2C Pathway to Attenuate Hypertrophy in Mesenchymal Stem Cell Derived Cartilage.

Articular cartilage lacks an intrinsic repair capacity and due to the ability of mesenchymal stem cells (MSCs) to differentiate into chondrocytes, MSCs have been touted as a cellular source to regenerate damaged cartilage. However, a number of prevailing concerns for such a treatment remain. Generally, administration of MSCs into a cartilage defect results in poor regeneration of the damaged cartilage with the repaired cartilage consisting primarily of fibro-cartilage rather than hyaline cartilage. Methods that improve the chondrogenic potential of transplanted MSCs in vivo may be advantageous. In addition, the proclivity of MSC-derived cartilage to undergo hypertrophic differentiation or form bone in vivo also remains a clinical concern. If MSC-derived cartilage was to undergo hypertrophic differentiation in vivo, this would be deleterious in a clinical setting. This study focuses on establishing a mechanism of action by which hypoxia or low oxygen tension can be used to both enhance chondrogenesis and attenuate hypertrophic differentiation of both MSC and ATDC5 derived chondrocytes. Having elucidated a novel mechanism of action, the subsequent goals of this study were to develop an in vitro culture regime to mimic the beneficial effects of physiological low oxygen tension in a normoxic environment.

Biodistribution and retention of locally administered human mesenchymal stromal cells: Quantitative polymerase chain reaction-based detection of human DNA in murine organs.

BACKGROUND: Determining the distributive fate and retention of a cell therapy product after administration is an essential part of characterizing it's biosafety profile. Therefore, regulatory guidelines stipulate that biodistribution assays are a requirement prior to advancing a cell therapy to the clinic. Here the development of a highly sensitive quantitative polymerase chain reaction (qPCR)-based method of tracking the biodistribution and retention of human mesenchymal stromal cells (hMSCs) in mice, rats or rabbits is described. METHODS: A primer-probe-based qPCR assay was developed to detect and quantify human Alu sequences in a heterogeneous sample of human DNA (hDNA) and murine DNA from whole organ genomic DNA extracts. The assay measures the amount of genomic hDNA by amplifying a 31-base pair sequence of the human Alu (hAlu) repeat sequence, thus enabling the detection of 0.1 human cells in 1.5 × 106 heterogeneous cells. RESULTS: Using this assay we investigated the biodistribution of 3 × 105 intramuscularly injected hMSCs in Balb/c nude mice. Genomic DNA was extracted from murine organs and hAlu sequences were quantified using qPCR analysis. After 3 months, hDNA ranging from 0.07%-0.58% was detected only at the injection sites and not in the distal tissues of the mice. DISCUSSION: This assay represents a reproducible, sensitive a method of detecting hDNA in rodent and lapine models. This manuscript describes the method employed to generate preclinical biodistribution data that was accepted by regulatory bodies in support of a clinical trial application.

Diabetic Bone Fracture Repair: A Progenitor Cell-Based Paradigm.

Long bone fractures in diabetics are slower to heal, have an increased risk of developing non-union and demonstrate greater potential of infection and perioperative complications compared to non-diabetics. The causative aberrant bone mineral density and insufficient bone microstructure of diabetic patients are thought to result from altered osteoblast and osteocyte function, increased bone marrow adiposity, decreased progenitor osteo- and chondral differentiation potential and increased pro-inflammatory cytokine circulation. It is therefore reasonable to hypothesize that the root cause of faulty diabetic bone homeostasis and fracture repair is a reduced population of bone marrow progenitor cells and/or their decreased osteochondral capacity complicated by their repressed neo-vascular potential. The potential of transplanted mesenchymal stem cells with a scaffold to support callus formation through the creation of de novo bone in hyperglycemia has been reported. However, there are minimal supporting pre-clinical and clinical investigations confirming these findings. Clinical trials have instead examined mesenchymal stem cell transplantation to slow disease progression, support .-cell viability and function and restore glucose homeostasis while the direct application of allogenic non-diabetic mesenchymal stem cells at the site of orthopaedic injury remains un-investigated. Here, the literature supporting the application of mesenchymal stem cells in diabetic fracture repair is reviewed including the process of dysfunctional diabetic fracture healing, osteoblast dysregulation and the effect of the hyperglycaemic environment on progenitor cell number and performance with a view to translating the preclinical knowledge base to the administration of mesenchymal stem cells in diabetic fracture repair.

Pullulan: a new cytoadhesive for cell-mediated cartilage repair.

INTRODUCTION: Local delivery of mesenchymal stem cells (MSCs) to the acutely injured or osteoarthritic joint retards cartilage destruction. However, in the absence of assistive materials the efficiency of engraftment of MSCs to either intact or fibrillated cartilage is low and localization is further reduced by natural movement of the joint surfaces. It is hypothesised that enhanced engraftment of the delivered MSCs at the cartilage surface will increase their reparative effect and that the application of a bioadhesive to the degraded cartilage surface will provide improved cell retention. Pullulan is a structurally flexible, non-immunogenic exopolysaccharide with wet-stick adhesive properties and has previously been used for drug delivery via the wet surfaces of the buccal cavity. In this study, the adhesive character of pullulan was exploited to enhance MSC retention on the damaged cartilage surface. METHODS: MSCs labeled with PKH26 were applied to pullulan-coated osteoarthritic cartilage explants to measure cell retention. Cytocompatability was assessed by measuring the effects of prolonged exposure to the bioadhesive on MSC viability and proliferation. The surface phenotype of the cells was assessed by flow cytometry and their multipotent nature by measuring osteogenic, adipogenic and chrondrogenic differentiation. Experiments were also carried out to determine expression of the C-type lectin Dectin-2 receptor. RESULTS: MSCs maintained a stable phenotype following exposure to pullulan in terms of metabolic activity, proliferation, differentiation and surface antigen expression. An increase in osteogenic activity and Dectin-2 receptor expression was seen in MSCs treated with pullulan. Markedly enhanced retention of MSCs was observed in explant culture of osteoarthritic cartilage. CONCLUSIONS: Pullulan is a biocompatible and effective cytoadhesive material for tissue engraftment of MSCs. Prolonged exposure to pullulan has no negative impact on the phenotype, viability and differentiation potential of the cells. Pullulan dramatically improves the retention of MSCs at the fibrillated surface of osteoarthritic articular cartilage. Pullulan causes an upregulation in expression of the Dectin-2 C-type lectin transmembrane complex.

Changes in immunological profile of allogeneic mesenchymal stem cells after differentiation: should we be concerned?

Mesenchymal stem cells (MSCs) are an adult stromal cell population possessing potent differentiation capacity and a potential for use across major histocompatibility complex barriers. Although allogeneic MSCs have potent immunosuppressive properties, evidence also suggests that they elicit a weak allogeneic immune response. However, the effect of induced differentiation on the immunosuppressive ability and immunogenicity of allogeneic MSCs is a potential obstacle when applying MSCs in tissue replacement therapies. These concerns will be explored in this review, with particular emphasis on changes in the cell surface expression of immunogenic markers, changes in the secretion of immunosuppressive molecules and in vivo functional benefits of the cell therapy. We review the literature from a translational point of view, focusing on pre-clinical studies that have utilised and analysed the effects of allogeneic immune responses on the ability of allogeneic MSCs to regenerate damaged tissue in models of bone, heart and cartilage defects.

From isolation to implantation: a concise review of mesenchymal stem cell therapy in bone fracture repair.

Compromised bone-regenerating capability following a long bone fracture is often the result of reduced host bone marrow (BM) progenitor cell numbers and efficacy. Without surgical intervention, these malunions result in mobility restrictions, deformities, and disability. The clinical application of BM-derived mesenchymal stem cells (MSCs) is a feasible, minimally invasive therapeutic option to treat non-union fractures. This review focuses on novel, newly identified cell surface markers in both the mouse and human enabling the isolation and purification of osteogenic progenitor cells as well as their direct and indirect contributions to fracture repair upon administration. Furthermore, clinical success to date is summarized with commentary on autologous versus allogeneic cell sources and the methodology of cell administration. Given our clinical success to date in combination with recent advances in the identification, isolation, and mechanism of action of MSCs, there is a significant opportunity to develop improved technologies for defining therapeutic MSCs and potential to critically inform future clinical strategies for MSC-based bone regeneration.

Growth differentiation factor-5 enhances in vitro mesenchymal stromal cell chondrogenesis and hypertrophy.

The regenerative potential for adult bone marrow-derived mesenchymal stromal cells (MSCs) has been extensively investigated in the setting of arthritic disease and focal cartilage defects. In vitro chondrogenic differentiation of MSCs is regularly accomplished by the widely used pellet culture system where MSCs are maintained in high-density pellets to mimic mesenchymal condensation during development. Supplementation of chondrogenic MSC pellet cultures with growth differentiation factor-5 (GDF-5), a highly regulated gene in the chondrogenic phase of endochondral ossification (EO), was investigated here under the hypothesis that GDF-5 will enhance the chondrogenic differentiation of MSCs, thereby supporting their entry into ossification. The supplementation of chondrogenic MSC pellets with the recombinant human GDF-5 protein significantly enhanced MSC chondrogenic differentiation, as demonstrated by enhanced collagen type II and sulfated glycosaminoglycan (GAG) incorporation into the extracellular matrix. Increased P-SMADs 1-5-8 were observed in pellets treated with GDF-5 and transforming growth factor (TGF)-ß 3 when compared to the pellets treated with TGF-ß 3 alone, demonstrated by immunostaining and western blot analysis of the chondrogenic pellet extract. A concurrent increase in alkaline phosphatase, collagen types I and X, and osteopontin secretion indicated a transition of these cultures to hypertrophy. Together, these data support the application of GDF-5 to enhance MSC chondrogenic differentiation and hypertrophy as a precursor to EO.

Enhancing the mesenchymal stem cell therapeutic response: cell localization and support for cartilage repair.

Articular cartilage is a complex, multilayered biological composite material, comprised of chondrocytes encapsulated in a water-based glycosaminoglycan matrix reinforced with collagen fibers. Once damaged by osteoarthritis or traumatic injury, this aneural, avascular tissue has little self-repair capacity. Over the last 20 years, cell therapies and tissue-engineering strategies have shown significant promise for the repair or regeneration of damaged cartilage. In particular, mesenchymal stem cells (MSCs) have great potential owing to their ability to create a reparative environment. Despite the fact that there have been great strides in the design and development of three-dimensional scaffolds, there is an upper limit to the number of viable cells that can be delivered using current approaches. To this end, this review examines current strategies for optimizing MSC localization, evaluates their limitations, and looks to other technologies to devise a combinatorial strategy for the creation of an MSC-seeded composite structure that addresses both the mechanical and biological property requirements for enhanced cartilage repair.

Delayed fracture healing in growth differentiation factor 5-deficient mice: a pilot study.

BACKGROUND: Growth differentiation factor-5 (GDF-5) is a key regulator of skeletogenesis and bone repair and induces bone formation in spinal fusions and nonunion applications by enhancing chondrocytic and osteocytic differentiation and stimulating angiogenesis. Elucidating the contribution of GDF-5 to fracture repair may support its clinical application in complex fractures. QUESTIONS/PURPOSE: We therefore asked whether the absence of GDF-5 during fracture repair impaired bone healing as assessed radiographically, histologically, and mechanically. METHODS: In this pilot study, we performed tibial osteotomies on 10-week-old male mice, stabilized by intramedullary and extramedullary nailing. Healing was assessed radiographically and histologically on Days 1 (n = 1 wild-type; n = 5 bp [brachopodism]), 5 (n = 3 wild-type; n = 3 bp), 10 (n = 6 wild-type; n = 3 bp), 14 (n = 6 wild-type; n = 6 bp), 21 (n = 6 wild-type; n = 6 bp), 28 (n = 7 wild-type; n = 6 bp), and 56 (n = 6 wild-type; n = 6 bp) after fracture. After 10 (n = 7 wild-type; n = 7 bp contralateral and n = 3 bp fractured tibiae), 14 (n = 6 wild-type; n = 6 bp), 21 (n = 6 wild-type; n = 6 bp), 28 (n = 6 wild-type; n = 3 bp), and 56 (n = 8 wild-type; n = 6 bp) days, the callus cross-sectional area was calculated. We characterized the mechanical integrity of the healing fracture by yield stress and Young's modulus at 28 (n = 6 wild-type; n = 3 bp) and 56 (n = 8 wild-type; n = 6 bp) days postfracture. RESULTS: The absence of GDF-5 impaired cartilaginous matrix deposition in the callus and reduced callus cross-sectional area. After 56 days, the repaired bp fracture was mechanically comparable to that of controls. CONCLUSIONS: Although GDF-5 deficiency did not compromise long-term fracture healing, a delay in cartilage formation and remodeling supports roles for GDF-5 in the early phase of bone repair. CLINICAL RELEVANCE: Local delivery of GDF-5 to clinically difficult fractures may simulate cartilage formation in the callus and support subsequent remodeling.

Mesenchymal chondroprogenitor cell origin and therapeutic potential.

Mesenchymal progenitor cells, a multipotent adult stem cell population, have the ability to differentiate into cells of connective tissue lineages, including fat, cartilage, bone and muscle, and therefore generate a great deal of interest for their potential use in regenerative medicine. During development, endochondral bone is formed from a template of cartilage that transforms into bone; however, mature articular cartilage remains in the articulating joints, where its principal role is reducing friction and dispersing mechanical load. Articular cartilage is prone to damage from sports injuries or ageing, which regularly progresses to more serious joint disorders, such as osteoarthritis. Osteoarthritis is a degenerative joint disease characterized by the thinning and eventual wearing of articular cartilage, and affects millions of people worldwide. Due to low chondrocyte motility and proliferative rates, and complicated by the absence of blood vessels, cartilage has a limited ability to self-repair. Current pharmaceutical and surgical interventions fail to generate repair tissue with the mechanical and cellular properties of native host cartilage. The long-term success of cartilage repair will therefore depend on regenerative methodologies resulting in the restoration of articular cartilage that closely duplicates the native tissue. For cell-based therapies, the optimal cell source must be readily accessible with easily isolated, abundant cells capable of collagen type II and sulfated proteoglycan production in appropriate proportions. Although a cell source with these therapeutic properties remains elusive, mesenchymal chondroprogenitors retain their expansion capacity with the promise of reproducing the structural or biomechanical properties of healthy articular cartilage. As current knowledge regarding chondroprogenitors is relatively limited, this review will focus on their origin and therapeutic application.

Mesenchymal stem cells and osteoarthritis: remedy or accomplice?

Multipotent mesenchymal stromal or stem cells (MSCs) are likely to be agents of connective tissue homeostasis and repair. Because the hallmark of osteoarthritis (OA) is degeneration and failure to repair connective tissues it is compelling to think that these cells have a role to play in OA. Indeed, MSCs have been implicated in the pathogenesis of OA and, in turn, progression of the disease has been shown to be therapeutically modulated by MSCs. This review discusses current knowledge on the potential of both marrow- and local joint-derived MSCs in OA, the mode of action of the cells, and possible effects of the osteoarthritic niche on the function of MSCs. The use of stem cells for repair of isolated cartilage lesions and strategies for modulation of OA using local cell delivery are discussed as well as therapeutic options for the future to recruit and appropriately activate endogenous progenitors and/or locally systemically administered MSCs in the early stages of the disease. The use of gene therapy protocols, particularly as they pertain to modulation of inflammation associated with the osteoarthritic niche, offer an additional option in the treatment of this chronic disease. In summary, elucidation of the etiology of OA and development of technologies to detect early disease, allied to an increased understanding of the role MSCs in aging and OA, should lead to more targeted and efficacious treatments for this debilitating chronic disease in the future.

Chicken embryo as a model for regenerative medicine.

Although the chick embryo, including its extraembryonic membranes, has long been used as a model for developmental biology, its potential as a model for the repair and regeneration of adult human tissues is often overlooked. The chick offers a well-defined profile of intercellular and intracellular signaling pathways regulating the development of nearly every organ system in conjunction with great flexibility for chimeric and transgenic experiments. Depending upon the system of interest, the chick can either directly reflect the human condition, as in spinal cord repair or in chorioallantoic membrane wound healing, and therefore act as an in vivo model for repair, or mirror our aspired therapy as in limb generation or otic restoration and therefore act as our guide. With these unique opportunities, the chick embryo is certainly a model to be considered when aiming to develop a regenerative therapy for human applications.

Correlation of GDF5 and connexin 43 mRNA expression during embryonic development.

Growth/differentiation factor 5 (GDF5) regulates connexin expression and enhances embryonic chondrogenesis in a gap junction-dependent manner, suggesting that GDF5 action on developmental skeletogenesis is coordinated with gap junction activities. The results shown here demonstrate concordance between the mRNA expression profiles of GDF5 and the gap junction gene, Cx43, in the mouse embryonic limb, spine, and heart, consistent with coordinated functions for these gene products during developmental organogenesis.