Histological findings. Photomicrographs showing serial frontal sections 10 μm thick stained with (A to D) H & E and (E to H) safranin O. Bar = 100 μm in all sections. (I) Histological grade of knee osteoarthritis assessed by modified Mankin criteria 5 weeks after injection, according to the treatment modality. Data are mean ± standard deviation of five guinea pigs in each group. *P < 0.05. HA, hyaluronic acid; MSC, mesenchymal stem cell.