Calcification or dedifferentiation: requirement to lock mesenchymal stem cells in a desired differentiation stage

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.