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tissue-derived MSCs (AMSCs) showed limited or decreased effects of MSCs on chondrogenesis. [243-245] Such effect was hardly seen in co-culture studies using bone- marrow-derived MSCs (BMSCs), which may propose that, compared to BMSCs, AMSCs are less efficient in co-culture. Due to methodological heterogeneity however, a direct comparable analysis between AMSCs and BMSCs in co-culture could not be easily made. So far, only three research groups have directly compared the effect of AMSCs and BMSCs on chondrocytes in co-culture. [80, 246, 247] Unfortunately, these studies demonstrate conflicting outcomes and have never translated to animal research. Therefore, we aim to investigate whether MSCs undergo chondrogenic differentiation upon contact with chondrocytes or by trophic effects of MSCs on chondrocytes. Whether the co-culture effect is dependent on MSC-origin or a general characteristic of MSCs, is further elucidated. Therefore, chondrogenesis of human AMSCs ( h AMSCs) and BMSCs ( h BMSCs) combined with bovine articular chondrocytes ( b ACs) is compared. The xenogeneic set-up using h MSCs and b ACs will allow conclusions about the cell type responsible for chondrogenesis. As cellular interactions can be influenced or overruled by exogenous growth factors, no growth factors are added to the culture system to study cartilage formation of the co-cultures in vitro . Moreover, cartilage formation will be evaluated after immediate subcutaneous implantation of the constructs in mice. To further elucidate the interactions between MSCs and ACs, different in-vitro culture systems will be used: (1) co-culture system of h MSC/ b ACs in alginate, (2) pellet co-culture system of h MSC/ b ACs, (3) Transwell® system of singular isolated h MSCs and b ACs in alginate, and (4) conditioned media culture systems of conditioned medium of h MSCs on b ACs and vice versa. 86 CHAPTER 5