## 1 Introduction  
Tissue engineering (TE) plays a major role in the future of tissue regeneration and restoration of functionality after degenerative tissue loss [1]. The triad of TE includes the use of biomaterials and growth factors, addressing specific signaling pathways to guide differentiation and development of living cells [1,2]. Currently, tissue loss and organ failure represent a growing threat to modern health and lead to significant expenses for healthcare systems worldwide [3,4]. One challenge is the steady increase in life expectancy and rising prevalence of risk factors, such as obesity, leading to increasing numbers of patients suffering from degenerative diseases, which could benefit from scientific progress in the field of regenerative medicine [4,5]. This problem is aggravated by the considerable lack of donor organs, as reported by the World Health Organization (WHO) [6].  
Multiple disorders in the field of orthopaedic surgery, such as the degenerative loss of articular cartilage in osteoarthritis, skeletal diseases, or soft tissue defects, all substantially contribute to this problem [7]. Over 10% of men and 18% of women over the age of 60 are affected by symptoms of osteoarthritis [8]. Therefore, the number of joint replacement surgeries performed is expected to sharply rise over the next decades [8,9]. Although patient satisfaction after joint arthroplasty is high, survival rates of implants are limited by time, and revision surgery remains complicated, while being linked to higher complication rates [10, 11, 12, 13].  
These circumstances have promoted research, regarding TE and other cell-based approaches for articular cartilage repair, and treatment of other disorders of the locomotive system [7]. Mesenchymal stromal cells (MCSs) can be isolated from a variety of mature or neonatal tissues and offer extensive in vitro multipotent differentiation potential [1,14]. In 2006, the International Society for Cellular Therapy (ISCT) defined MSCs as plastic-adherent cells with osteogenic, adipogenic, and chondrogenic differentiation capacity in vitro [15]. In addition, MSCs express (\(\geq\)95% positive) characteristic surface molecules CD73, CD90, and CD105 and lack the expression (\(\leq\)2% positive) of hematopoietic cell markers [15]. Bone marrow-derived MSCs (BM-MSCs) are viewed as the gold standard and have shown promising results, when used for TE and regenerative therapy in vivo [16]. However, the isolation of BM-MSCs requires invasive measures and causes airogenic damage at the extraction site, while mostly only offering a small yield of cells [17]. Interestingly, earlier studies have identified multiple other tissue sources that offer vast amounts of easily accessible MSC-like cells, while minimizing donor site morbidity. MSC-like cells derived from some of these tissue sources showed greater proliferation capacity and even better multipotent differentiation potential in vitro, when compared to BM-MSCs [18, 19, 20].  
While MSCs have been used for regenerative treatment of ligament, meniscal, or cartilage damage in the knee joint earlier, they have also been shown to reside in a variety of tissues, which form and surround the knee joint [18,21,22]. Therefore, the goal of our current in vitro study was to add further research on different sources of MSCs by proving the presence and comparing the characteristics of BM-MSCs and MSC-like cells isolated from arthritic hyaline cartilage, synovial tissue, the anterior cruciate ligament, meniscal tissue, and the infraptellar fat pad from the human arthritic knee joint [19]. Our hypothesis was that BM-MSCs and MSC-like cells isolated from all six tissues from the arthritic knee joint possess similar in vitro multipotent differentiation potential. In contrast to earlier research, we simultaneously isolated MSC-like cells from all six tissues of all respective donors, in order to allow for an optimal comparison of all cell sources.