A current challenge in mesenchymal stem cell (MSC)-based cartilage repair is to solve donor and tissue-dependent variability of MSC cultures and to prevent chondrogenic cells from terminal differentiation like in the growth plate. The aim of this study was to select the best source for MSC which could promise stable cartilage formation in the absence of hypertrophy and ectopic in vivo mineralization. We hypothesized that MSC from synovium are superior to bone marrow- and adipose tissue-derived MSC since they are derived from a joint tissue. MSC were characterized by flow cytometry. MSC pellets were cultured under chondrogenic conditions and differentiation was evaluated by histology, gene expression analysis, and determination of alkaline phosphatase activity (ALP). After chondrogenic induction, pellets were transplanted subcutaneously into SCID mice. MSC from bone marrow, adipose tissue, and synovium revealed similar COL2A1/COL10A1 mRNA levels after chondrogenic induction and were positive for collagen-type-X. Bone marrow-derived and adipose tissue-derived MSC showed significantly higher ALP activity than MSC from synovium. Low ALP-activity before transplantation of pellets correlated with marginal calcification of explants. Surprisingly, non-mineralizing transplants specifically lost their collagen-type II, but not collagen-type I deposition in vivo, or were fully degraded. In conclusion, the lower donor-dependent ALP activation and reduced mineralization of synovium-derived heterotopic transplants did not lead to stable ectopic cartilage as known from articular chondrocytes, but correlated with fibrous dedifferentiation or complete degeneration of MSC pellets. This emphasizes that beside appropriate induction of differentiation, locking of MSC in the desired differentiation state is a major challenge for MSC-based repair strategies.

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Human mesenchymal stromal cells derived from bone marrow (BMSC) and adipose tissue (ATSC) represent a valuable source of progenitor cells for cell therapy and tissue engineering. While ectopic bone formation is a standard activity of human BMSC on calcium phosphate ceramics, the bone formation capacity of human ATSC has so far been unclear. The objectives of this study were to assess the therapeutic potency of ATSC for bone formation in an ectopic mouse model and determine molecular differences by standardized comparison with BMSC. Although ATSC contained less CD146(+) cells, exhibited better proliferation and displayed similar alkaline phosphatase activity upon osteogenic in vitro differentiation, cells did not develop into bone-depositing osteoblasts on β-TCP after 8weeks in vivo. Additionally, ATSC expressed less BMP-2, BMP-4, VEGF, angiopoietin and IL-6 and more adiponectin mRNA, altogether suggesting insufficient osteochondral commitment and reduced proangiogenic activity. Chondrogenic pre-induction of ATSC/β-TCP constructs with TGF-β and BMP-6 initiated ectopic bone formation in >75% of samples. Both chondrogenic pre-induction and the osteoconductive microenvironment of β-TCP were necessary for ectopic bone formation by ATSC pointing towards a need for inductive conditions/biomaterials to make this more easily accessible cell source attractive for future applications in bone regeneration.