Background::Previous studies have reported that mitochondrial dysfunction participates in the pathological process of osteoarthritis (OA). However, studies that improve mitochondrial function are rare in OA. Mitochondrial transfer from mesenchymal stem cells (MSCs) to OA chondrocytes might be a cell-based therapy for the improvement of mitochondrial function to prevent cartilage degeneration. This study aimed to determine whether MSCs can donate mitochondria and protect the mitochondrial function and therefore reduce cartilage degeneration.Methods::Bone-marrow-derived mesenchymal stromal cells (BM-MSCs) were harvested from the marrow cavities of femurs and tibia in young rats. OA chondrocytes were gathered from the femoral and tibial plateau in old OA model rats. BM-MSCs and OA chondrocytes were co-cultured and mitochondrial transfer from BM-MSCs to chondrocytes was identified. Chondrocytes with mitochondria transferred from BM-MSCs were selected by fluorescence-activated cell sorting. Mitochondrial function of these cells, including mitochondrial membrane potential (Δψm), the activity of mitochondrial respiratory chain (MRC) enzymes, and adenosine triphosphate (ATP) content were quantified and compared to OA chondrocytes without mitochondrial transfer. Chondrocytes proliferation, apoptosis, and secretion ability were also analyzed between the two groups.Results::Mitochondrial transfer was found from BM-MSCs to OA chondrocytes. Chondrocytes with mitochondrial from MSCs (MSCs + OA group) showed increased mitochondrial membrane potential compared with OA chondrocytes without mitochondria transfer (OA group) (1.79 ± 0.19 n vs. 0.71 ± 0.12, n t = 10.42, n P < 0.0001). The activity of MRC enzymes, including MRC complex I, II, III, and citrate synthase was also improved ( n P < 0.05). The content of ATP in MSCs + OA group was significantly higher than that in OA group (161.90 ± 13.49 n vs. 87.62 ± 11.07 nmol/mg, n t = 8.515, n P < 0.0001). Meanwhile, we observed decreased cell apoptosis (7.09% ± 0.68% n vs.15.89% ± 1.30%, n t = 13.39, n P < 0.0001) and increased relative secretion of type II collagen (2.01 ± 0.14 n vs.1.06 ± 0.11, n t = 9.141, n P = 0.0008) and proteoglycan protein (2.08 ± 0.20 n vs. 0.97 ± 0.12, n t = 8.227, n P = 0.0012) in MSCs + OA group, contrasted with OA group.n Conclusions::Mitochondrial transfer from BM-MSCs provided protection for OA chondrocytes against mitochondrial dysfunction and degeneration through improving mitochondrial function, cell proliferation, and inhibiting apoptosis in chondrocytes. This finding may offer a new therapeutic direction for OA.“,”Background::Previous studies have reported that mitochondrial dysfunction participates in the pathological process of osteoarthritis (OA). However, studies that improve mitochondrial function are rare in OA. Mitochondrial transfer from mesenchymal stem cells (MSCs) to OA chondrocytes might be a cell-based therapy for the improvement of mitochondrial function to prevent cartilage degeneration. This study aimed to determine whether MSCs can donate mitochondria and protect the mitochondrial function and therefore reduce cartilage degeneration.Methods::Bone-marrow-derived mesenchymal stromal cells (BM-MSCs) were harvested from the marrow cavities of femurs and tibia in young rats. OA chondrocytes were gathered from the femoral and tibial plateau in old OA model rats. BM-MSCs and OA chondrocytes were co-cultured and mitochondrial transfer from BM-MSCs to chondrocytes was identified. Chondrocytes with mitochondria transferred from BM-MSCs were selected by fluorescence-activated cell sorting. Mitochondrial function of these cells, including mitochondrial membrane potential (Δψm), the activity of mitochondrial respiratory chain (MRC) enzymes, and adenosine triphosphate (ATP) content were quantified and compared to OA chondrocytes without mitochondrial transfer. Chondrocytes proliferation, apoptosis, and secretion ability were also analyzed between the two groups.Results::Mitochondrial transfer was found from BM-MSCs to OA chondrocytes. Chondrocytes with mitochondrial from MSCs (MSCs + OA group) showed increased mitochondrial membrane potential compared with OA chondrocytes without mitochondria transfer (OA group) (1.79 ± 0.19 n vs. 0.71 ± 0.12, n t = 10.42, n P < 0.0001). The activity of MRC enzymes, including MRC complex I, II, III, and citrate synthase was also improved ( n P < 0.05). The content of ATP in MSCs + OA group was significantly higher than that in OA group (161.90 ± 13.49 n vs. 87.62 ± 11.07 nmol/mg, n t = 8.515, n P < 0.0001). Meanwhile, we observed decreased cell apoptosis (7.09% ± 0.68% n vs.15.89% ± 1.30%, n t = 13.39, n P < 0.0001) and increased relative secretion of type II collagen (2.01 ± 0.14 n vs.1.06 ± 0.11, n t = 9.141, n P = 0.0008) and proteoglycan protein (2.08 ± 0.20 n vs. 0.97 ± 0.12, n t = 8.227, n P = 0.0012) in MSCs + OA group, contrasted with OA group.n Conclusions::Mitochondrial transfer from BM-MSCs provided protection for OA chondrocytes against mitochondrial dysfunction and degeneration through improving mitochondrial function, cell proliferation, and inhibiting apoptosis in chondrocytes. This finding may offer a new therapeutic direction for OA.