## 4 Discussion  
MSCs have emerged as a promising part of cell-based regenerative treatment of traumatic or degenerative orthopaedic diseases, such as cartilage defects, osteoarthritis, or ligament damage [28, 29]. Tissues other than BM have been shown to offer vast amounts of MSC-like cells, with multipotent differentiation potential, while requiring less invasive surgery for the isolation of cells [18, 30]. Figure 6: Histological and immunohistochemical analysis of chondrogenesis in cell-pellets after 21 d of pellet culture. Induced chondrogenesis pellets, containing cells from bone marrow (BM), hyaline cartilage, the meniscus, the anterior cruciate ligament (ACL), the synovial membrane (SM) and the infrapatellar fat pad (IFP), were incubated with chondrogenic differentiation medium for 21 days (d). Controls were maintained in cell culture medium under the same conditions. After 21 d, alcian blue staining (**a**) was performed for the detection of proteoglycans. Immunohistochemical stainless of collagen type II (COL II) (**b**) and collagen type X (COL X) (**c**) were performed on pellet sections cells from BM (1), hyaline cartilage (2), the meniscus (3), the ACL (4), the SM (5), and the IPF (6) after incubation in chondrogenic differentiation medium for 21 d. Positive staining for COL II (b) and COL X (c) appeared brown. Chondrogenic assays were performed with all samples (five donors, six different MSC population each), and we show one representative donor of each staining. Representative samples were captured at low (50\(\times\); black bar = 300 \(\upmu\)m) and high (200\(\times\); black bar = 300 \(\upmu\)m) magnification. In this present in vitro study, our goal was to detect and characterize less-well-known MSC-like cells, isolated from meniscal tissue, i.e., the ACL, IPF, and SM, during primary TKA of the arthritic knee joint. This study adds further research on the different MSC-subpopulations, mentioned above, by proving their presence and comparing their qualitative differentiation capacity in vitro. Our hypothesis was that BM-MSCs and MSC-like cells isolated from all other tissues of the arthritic knee joint possess multipotent differentiation potential, which we confirmed by our investigations. Primarily, this study showed that multipotent and plastic-adherent cells could be isolated from all six examined tissues, i.e., BM, arthritic hyaline cartilage, meniscal tissue, the IPF, the SM, and the ACL, of the arthritic knee joint of five female patients undergoing primary TKA. Interestingly, CPD between isolation from native tissues and primary passage was significantly higher in cells derived from BM, when compared to cells derived from cartilage, the ACL, and the IPF. The presence and characteristics of BM-MSCs, which are viewed as the gold standard for TE and reside in the endosteal or perivascular niche, have been thoroughly examined in earlier studies [25; 31; 32]. In addition, [PERSON] et al. and [PERSON] et al. isolated MSC-like cells from the superficial zone of arthritic or healthy hyaline cartilage [33; 34]. Earlier research also proved the presence of MSC-like cells in the SM of the knee joint [35; 36]. Inside the SM, MSC-like cells are believed to reside within the intimal layer and small blood vessels, located within the subintimal tissue [37]. Since synovial blood vessels act as a connection between the BM and synovial fluid, it is speculated that BM-MSCs may migrate from the BM to form MSC-like cells in the synovial fluid and SM of diarthroidal joints [37]. The IPF and the SM of the knee joint serve as a functional unit, which is why the IPF is often referred to as the adipose synovium [38]. Consequently, earlier studies proved the presence of MSC-like cells within the IPF [18; 39]. We, and others, also showed that MSC-like cells can be isolated from the ACL, where they are believed to be located within small sinusoids, fascicles, or the endothelial lining of small blood vessels [22; 40; 41].