Histopathology of osteochondral defects using Masson’s trichrome, alcian blue, and immunohistochemical staining of type II collagen. In animal no. 1, the articular surface was smooth and fibrocartilage developed on the subchondral bone at the implanted site (A, B, E, F), whereas the surface was irregular and fibrous tissue lay over the subchondral bone at the control site (C, D, G, H). At the implanted site in animal no. 2, the subchondral bone was symmetrically reconstructed and was covered by matrix including hyaline cartilage, which was suggested by the clusters (arrowhead) and columnar clusters (arrow) of cells (I, J, M, N). On the other hand, smooth and continuous surface was restored due to fibrocartilage formation, but subchondral bone was absent in the bottom half of the defect, at the control site in animal no. 2 (K, L, O, P). Black dotted lines indicate the areas of osteochondral defects immediately after the surgery. Masson’s trichrome staining sections (B, D, J, L) were enlarged from red dotted square in the images A, C, I, and K, respectively. The insert images in sections B, D, J, and L were enlarged from white dotted square in images B, D, J, and L, respectively. The bars in the insert images indicate 50 μm.