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DISCUSSION Combining chondrocytes and MSCs holds great promise for cell-based cartilage repair as it reduces the required number of chondrocytes and diminishes many disadvantages of individually used cell types leading to enhanced cartilage matrix formation with low hypertrophic differentiation. In line with former research, h AMSC/ b ACs and h BMSC/ b ACs produced similar or even improved quantities of cartilage matrix components as constructs containing b ACs only, both in vitro and in vivo . Moreover, hypertrophic gene expression ( COL10 ) was not affected by h BMSCs, but slightly enhanced by h AMSCs. However, constructs containing either h AMSC/ b ACs or h BMSC/ b ACs appeared stable and did not calcify in vivo . This suggests that 80% of b ACs can be replaced by either h AMSCs or h BMSCs without influencing cartilage matrix production nor stability. Therefore, mixed-cell-cultures of MSCs and chondrocytes could be very valuable for cell-based cartilage repair, as appropriate numbers of cells are more easily acquired from bone-marrow aspirates or adipose tissue than from cartilage biopsies. The cellular mechanism responsible for enhanced cartilage production in co-culture is however still debated. Numerous cellular communication pathways have been hypothesized in order to explain the beneficial effect in co-cultures [73]. We found no evidence that cartilage formation was the consequence of chondrogenic lineage differentiation of h MSCs, as stated by others [80, 253-262]. In contrast, cartilage matrix clearly originated from b ACs, which suggests a predominantly trophic role for h MSCs in these constructs: both h AMSCs and h BMSCs improved b AC-proliferation as well as b AC-sGAG-formation. This confirms previous studies were the co-culture effect has been ascribed to ACs, whose cartilage-forming capacity and proliferation activity appears to enhance in the presence of MSCs. [74, 246, 263-268] The trophic and paracrine function of MSCs herein appeared essential rather than MSCs actively undergoing chondrogenic differentiation. We show that this is a general feature that applies to both AMSCs and BMSCs. To date, only three studies have compared the trophic effect of several MSC-sources - such as AMSCs and BMSCs - on ACs in co-culture. [80, 246, 247] Unfortunately, these studies demonstrate conflicting outcomes and have never translated to animal research. Therefore, to our knowledge, we are the first to systematically compare the cartilage forming capacity of either h AMSC/ b ACs and h BMSC /b ACs in vitro and in vivo . In vitro , h BMSC/ b ACs contained significantly more cartilage matrix components than h AMSC/ b ACs. Cartilage formation after 8 weeks of subcutaneous implantation was, however, not different in constructs containing h AMSC/ b ACs and h BMSC/ b ACs, although large donor variations were observed, in particular in h AMSC/ b ACs. Our results support a general trophic or immunomodulatory role for h AMSCs and h BMSCs on b ACs in co-culture, as stated by Wu [246] and Maumus et al [247] . Although both cell sources share comparable immunomodulatory modalities, they do not necessarily behave the same. In monocultures there are clear differences observed between h AMSCs and h BMSCs. For instance, they possess distinctive proliferation capacities and a dissimilar potential to chondrogenically differentiate. [40] Moreover, both cell sources secrete different subsets of paracrine factors: compared to h BMSCs, h AMSCs secrete significantly more VEGF- D [269], IGF-1 [269, 270], IL-8 [269] and IL-6 [269, 271], and significantly less SDF-1 [272] and 102 CHAPTER 5