Purpose Osteoarthritis (OA) represents a relevant social and economic burden worldwide. “Mesenchymal stem cells” or, as recently proposed, “medicinal signaling cells” (MSCs) have been recently introduced as injective treatments for OA with the aim of restoring joint homeostasis. The aim of this review is to provide the reader with the tools necessary to interpret the currently available clinical data, focusing on the MSC mechanisms of action which might help to clarify what we should expect from this treatment. Methods Clinical studies reporting MSC injections for the treatment of knee OA, either freshly isolated or culture-expanded cells, have been included and commented in relation to the supposed therapeutic effect that MSCs might exert giving their supposed mode of actions. Results The majority of the studies reports significant improvements in terms of pain and knee function compared to baseline values, up to 24 months of follow-up. Although these data support the expected therapeutic effect of this therapy giving the features of these cells, only 14% of the studies present a control group and more than one-third of them report the results on less than ten patients. Conclusions Despite the constant presence of positive and satisfactory results in the studies analyzed, the complexity of MSC metabolism and related therapeutic effects as well as the weakness of most of the studies do not allow withdrawing definitive conclusions about the superiority of one tissue source over another, as well as about the best cell dose and the long-term durability of the effects of these procedures. Given the high potential value of these therapies in the treatment of OA, further studies accurately designed, carefully defining the type of patients to be included and pursuing minimal standard requirements in terms of follow-up, number of patients, and types of measurements should be conducted to finally assess the efficacy of MSC-based injective treatments.

Osteochondral defects are difficult to treat because the articular cartilage and the subchondral bone have dissimilar characteristics and abilities to regenerate. Bioinspired scaffolds are designed to mimic structural and biological cues of the native osteochondral unit, supporting both cartilaginous and subchondral bone repair and the integration of the newly formed osteochondral matrix with the surrounding tissues. The aim of this review is to outline fundamental requirements and strategies for the development of biomimetic scaffolds reproducing the unique and multifaceted anatomical structure of the osteochondral unit. Recent progress in preclinical animal studies using bilayer and multilayer scaffolds, together with continuous gradient scaffolds will be discussed and placed in a translational perspective with data emerging from their clinical application to treat osteochondral defects in patients.

Stem cells hold great promise in tissue engineering for repairing tissues damaged by disease or injury. Mesenchymal stem cells (MSCs) are multipotent cells able to proliferate and differentiate into multiple mesodermal tissues such as bone, cartilage, muscle, tendon, and fat. We have previously reported that the low-affinity nerve growth factor receptor (L-NGFR or CD271) defines a subset of cells with high proliferative, clonogenic, and multipotential differentiation ability in adult bone marrow (BM). It has been recently shown that adipose tissue is an alternative source of adult multipotent stem cells and human adipose-derived stem cells, selected by plastic adherence (PA hASCs), have been extensively characterized for their functional potentials in vitro. In this study, immunoselected L-NGFR⁺ and CD34⁺ subpopulations have been analyzed and compared with the PA hASCs. Phenotypic profile of freshly purified subpopulations showed an enrichment in the expression of some stem cell markers; indeed, a great percentage of L-NGFR⁺ cells co-expressed CD34 and CD117 antigens, whereas the endothelial-committed progenitor markers KDR and P1H12 were mainly expressed on CD34⁺ cells. Differently from PA hASCs, the immunoseparated fractions showed high increments in cell proliferation, and the fibroblast colony-forming activity (CFU-F) was maintained throughout the time of culture. Furthermore, the immunoselected populations showed a greater differentiative potential toward adipocytes, osteoblasts, and chondrocyte-like cells, compared to PA hASCs. Our data suggest that both CD34⁺ and L-NGFR⁺ hASCs can be considered alternative candidates for tissue engineering and regenerative medicine applications.

The synovium of osteoarthritis (OA) patients can be characterized by an abnormal accumulation of macrophages originating from extravasated monocytes. Since targeting monocyte extravasation may represent a promising therapeutic strategy, our aim was to develop an organotypic microfluidic model recapitulating this process. Synovium and cartilage were modeled by hydrogel-embedded OA synovial fibroblasts and articular chondrocytes separated by a synovial fluid channel. The synovium compartment included a perfusable endothelialized channel dedicated to monocyte injection. Monocyte extravasation in response to chemokines and OA synovial fluid was quantified. The efficacy of chemokine receptor antagonists, RS-504393 (CCR2 antagonist) and Cenicriviroc (CCR2/CCR5 antagonist) in inhibiting extravasation was tested pre-incubating monocytes with the antagonists before injection. After designing and fabricating the chip, culture conditions were optimized to achieve an organotypic model including synovial fibroblasts, articular chondrocytes, and a continuous endothelial monolayer expressing intercellular adhesion molecule-1 and vascular cell adhesion molecule-1. A significantly higher number of monocytes extravasated in response to the chemokine mix (p < 0.01) and OA synovial fluid (p < 0.01), compared to a control condition. In both cases, endothelium pre-activation enhanced monocyte extravasation. The simultaneous blocking of CCR2 and CCR5 proved to be more effective (p < 0.001) in inhibiting monocyte extravasation in response to OA synovial fluid than blocking of CCR2 only (p < 0.01). The study of extravasation in the model provided direct evidence that OA synovial fluid induces monocytes to cross the endothelium and invade the synovial compartment. The model can be exploited either to test molecules antagonizing this process or to investigate the effect of extravasated monocytes on synovium and cartilage cells.