## 2 Materials and Methods  
### Isolation and Cultivation of Cells  
Cells were isolated from six different tissues of the human arthritic knee joint. All tissues were extracted during total knee arthroplasty (TKA) in five female patients under the age of 65 (median age 59.6), with a mean body mass index of 25.08 kg/m\({}^{2}\), suffering from moderate to severe osteoarthritis of the knee joint. Additional patient information and demographics are listed in Table 1. Bone marrow (BM) extracted from the femur, arthritic hyaline cartilage, anterior cruciate ligament (ACL), meniscus tissue, infrapatellar fat pad (IPF), and synovial membrane (SM) (see Table 2 for detailed information) were collected during primary TKA and served as donor tissues. Prior to sampling, written informed consent was obtained from every patient, in accordance with the local ethics committee of the University of Wurzburg (186/18). All methods were carried out in accordance with the relevant guidelines and regulations. Cells from the BM were isolated by manually processing the bone into little pieces, which were washed in Dulbecco's modified eagle medium (DMEM)/Ham's F12 (1:1; Life Technologies GmbH, Thermo Fisher Scientific, Dreieich, Germany) in a 50 mL test tube. The mixture was centrifuged for 5 min (min) to separate the cells from the marrow. Cell pellets at the bottom of the tube were resuspended in 10 mL of DMEM/Ham's F12 and carefully poured over a cell strainer. To further purify the solution, it was once more centrifuged, and the pellets were resuspended in 10 mL of standard cell culture medium, consisting of \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Patient Number** & **Age (Years)** & **BMI (kg/m\({}^{2}\))** & **Secondary Diagnosis** &
\begin{tabular}{c} **Osteoarthritis Grade** \\ **([PERSON] \& [PERSON])** \\ \end{tabular} & **Smoking** \\ \hline
1 & 61 & 23.62 & hypothyroidism & 3 & no \\
2 & 55 & 29.07 & hypertension, breast cancer & 4 & no \\
3 & 61 & 18.22 & - & 3 & yes \\
4 & 62 & 28.05 & - & 3 & no \\
5 & 59 & 26.44 & hypertension & 4 & yes \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Patient demographics and detailed information regarding the five donors undergoing total knee arthroplasty. Precise description of the five different patients that underwent total knee arthroplasty and served as donors for the isolation of mesenchymal stromal cell (MSC)-like cells. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Nomenclature** & **Donor Tissue** & **Surgical Extraction** \\ \hline BM-MSCs & \begin{tabular}{c} Bone marrow (BM) of the proximal \\ tibia and the femoral condyles \\ \end{tabular} & \begin{tabular}{c} The femoral condyles and the proximal tibia are removed \\ with a bone saw during surgery to replace the damaged \\ knee joint with the endoprosthesis. Bone marrow was \\ extracted from the bone using a bone curette. \\ \end{tabular} \\ \hline MSC-like cells & \begin{tabular}{c} Arthritic hyaline cartilage of the tibia \\ and the femoral condyles \\ \end{tabular} & \begin{tabular}{c} Macroscopically eroded hyaline cartilage was scraped off \\ the femoral condyles with a scalpel. \\ \end{tabular} \\ \cline{2-3}  & Inner and outer meniscus & \begin{tabular}{c} The inner and outer meniscus are removed during the \\ procedure and secured. \\ \end{tabular} \\ \cline{2-3}  & Anterior cruciate ligament (ACL) & The ACL was removed during the procedure and secured. \\ \cline{2-3}  & \begin{tabular}{c} Synoial membrane (SM) \\ \end{tabular} & \
\begin{tabular}{c} To access the knee-joint the synovial membrane was \\ removed, and a sample was secured. \\ \end{tabular} \\ \cline{2-3}  & Infrapatellar fat pad (IPF) &
\begin{tabular}{c} The infrapatellar fat pad is removed during surgery and \\ was secured. \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Detailed description of the six different tissues, derived from five individual donors, used for the isolation of BM-MSCs and MSC-like cells.