## 4 Discussion
Despite the poor blood supply of meniscal tissue, there is also evidence that MSC-like cells or multipotent meniscal fibroblasts (MFCs) can be isolated from meniscal debris [21]. Interestingly, all of the investigated native tissues rise from the lateral plate mesoderm (LPM) and are well-vascularized [42; 43]. This further supports our hypothesis, when taking the embryological development of MSCs into account [43]. All of these findings support our hypothesis that multipotent and plastic-adherent cells expressing an MSC-characteristic set of surface antigens can be isolated from the six examined tissues of the arthritic knee joint. Secondly, cells isolated from all six different tissues of an exemplary chosen donor sample co-expressed the surface antigens CD44, CD73, CD90, and CD105. The ISCT defined these surface antigens as highly characteristic, but not specific, for MSCs [15]. Our results showed that \(\geq\)95% of cells were positive for the surface markers CD44, CD90, and CD105, independent of the examined tissue source. Therefore, all cells showed an MSC-like pattern of surface antigens, separating them from tissue resident, plastic-adherent, and multipotent fibroblasts. Although all cells also expressed the surface antigen CD73, expression rates did not exceed 95% in cells, other than BM-MSCs, as defined by the minimal definition criteria for MSCs by the ISCT [15]. Earlier research has shown that different MSC-subpopulations may differ in their expression of surface antigens [20]. This has led to intense discussions regarding whether the characterization of unspecific, but characteristic, surface antigens, which may vary for different subpopulations of MSCs, should be concluded in the minimal criteria defining MSCs. In addition, it remains unclear if differences regarding the expression of characteristic surface antigens also effect the functional properties of different MSC-subpopulations. Additional research is necessary to further compare the possible differences in the expression of surface antigens and their effects on the therapeutic cell use of examined MSC-subpopulations. However, our results, regarding the expression of characteristic surface antigens, are in line with multiple earlier studies, showing that BM-MSCs, derived from the knee joint, clearly co-expressed CD44, CD73, CD90, and CD105 [31; 44]. A mixed population of chondrocytes and multipotent progenitor cells, isolated from both arthritic and hyal cartilage of the knee joint, expressed CD105 and CD166 [33; 34]. Interestingly, previous research showed that multipotent, plastic-adherent cells, isolated from the ACL, SM, or IFP of osteoarthritis and healthy knee joints, clearly co-expressed CD73, CD90, and CD105 [18; 22; 41; 45]. Finally, [PERSON] et al. and [PERSON] et al. showed that CD44, CD90, and CD105 positive cells could also be isolated from meniscal debris, which is in line with our current results [21; 44]. MSC-like cells isolated from the BM, arthritic hyal cartilage, the SM, the ACL, the IFP, and meniscal tissue showed clear trilineage differentiation potential in vitro, as defined in the ISCT's criteria for MSCs [15]. In this study, no qualitative differences were found, regarding the osteogenic and adipogenic differentiation capacity of BM-MSCs and MSC-like cells, isolated from the arthritic hyal cartilage, ACL, the IPF, the SM, and meniscal tissue. Although, there were differences in the gene expression of specific marker genes, none of these reached statistical significance. In this context, it has to be added that no quantitative measurements of oil red O and alizarin red S stainings were performed. The same applied to the chondrogenic differentiation of MSC-like cells isolated from the six different native tissues, meaning that observed differences in histological and immunohistochemistry stainings, as well as the relative expression of marker genes, are highly subjective. Differences regarding histological and immunohistochemistry stainings, as well as relative expression of marker genes, could be due to phaseal upregulation.