## 1 Introduction  
Osteoarthritis (OA) is a very common degenerative disease affecting the articular cartilage in both human [1] and veterinary medicine [2,3]. This condition affects 15% of the world population, amounting to colossal health-care costs [4], and has a great impact on a patient's quality of life [5]. The current therapeutic approaches focus on preventing or at least delaying the structural and functional changes of OA. The use of stem cell-based therapies and Platelet Rich Plasma (PRP) for repair and regeneration in OA has become a new avenue of treatment as an alternative to the more aggressive therapies [6, 7, 8, 9, 10]. Stem cells may be of embryonic origin (ESCs) or adult (ASCs). Adult stem cells have a much lower capacity than embryonic stem cells to self-renew and differentiate along multiple lineage pathways. However, adult stem cells are immuncompatible, and their use is not restricted by the ethical concerns associated with embryo-derived cells. Apart from the ethical concerns embryonic cells are known to raise, they have also been shown to demonstrate uncontrolled growth [11,12]. In contrast, adult stem cells, including mesenchymal stem cells (MSCs), are a very good option as they are present in a number of postnatal organs and connective tissues, and are not subject to the same restrictions as the embryonic cells [13]. Mesenchymal stem cells can be easily isolated from many adult tissues such as bone marrow, placenta [14], umbilical cord [15], skeletal muscle [16], synovium [17], synovial fluid [18], and adipose tissue [8,19] among others. An increasing amount of studies are using adipose tissue derived mesenchymal cells (aMSCs) in the treatment of OA, as large quantities are easily harvested with little donor site morbidity or patient discomfort, as well as demonstrating the ability to differentiate into chondrocytes, osteocytes and adipocytes [20]. It has been shown that autologous stem cells have an affinity for damaged joint tissue; recent studies have confirmed that stem cells have the ability to localize and participate in the repair of damaged joint structures [21]. Recently, it was published that aMSC therapy was found to be an appropriate treatment for hip joints in dogs, improving the dog's gait and ability to live a more normal life [22]. One of the most popular methods used to biologically enhance healing in the fields of orthopedic surgery and sports medicine includes the use of autologous blood products, particularly, PRP [23]. PRP is defined as the volume of autologous plasma having a platelet concentration over baseline [24, 25, 26],and is also referred to as plasma rich in growth factors (PRGF), PRP-gel, platelet gel or platelet clot [27; 28; 29]. Under normal circumstances, platelets are the first cell type to arrive at the tissue injury site and are particularly active in the early inflammatory phases [30]. Several studies describe the use of PRP as an effective and safe method in the treatment of pain and joint dysfunction in OA. The use of PRGF (PRGF-Endoret; BTI Biotechnology Institute, Vitoria, Spain), an autologous PRP characterized by the absence of leukocytes and proinflammatory cytokines and the presence of a specific dose of platelets and growth factors [28], has been demonstrated to achieve significant improvement in humans with knee OA [5; 31] and also in dogs with OA [32]. Currently, the effectiveness of these two therapies has been demonstrated for the treatment of OA, however in the literature, there are no studies comparing the results of these two treatments with each other.