Macroscopic findings. Macroscopic photographs (bird's-eye view) of the articular surface of the tibial plateau 5 weeks after injection of each reagent in Hartley strain guinea pigs, stained with India ink: (A) PBS group, (B) hyaluronic acid (HA) group, (C) PBS + mesenchymal stem cell (MSC) group, (D) HA + MSC group. Arrowheads: stained lesions. (E) Macroscopic osteoarthritis (OA) scores in the same animal groups evaluated 5 weeks after injection. Data are mean ± standard deviation of five guinea pigs in each group. *P < 0.05.