Histological examination at early repair processes in osteochondral defects. Safranin O staining, immunostaining for type II collagens, and IgG staining as controls at (a–c) 3 weeks, (d–f) 6 weeks, and (g–i) 12 weeks after the implantation. At 3 weeks, the implanted tissue had not differentiated into hyaline cartilage yet. At 6 and 12 weeks, the joint surface area of the implanted tissue was stained with safranin O and immunostained for type II collagen. Endochondral bone formation was observed at the peripheral area at 6 weeks and developed toward the central area at 12 weeks. (j–l) A control rabbit without any implantation at 12 weeks. The defect was filled with fibrous tissue that did show a constant staining for safranin O and immunostaining for type II collagen, especially at the surface and central areas. Scale bar = 1 mm. Downwards arrows indicate the borders of the defect. Representative subjects of two or three experiments for each time course are shown.