Histological sections demonstrating typical histological appearance of cartilage at loci away (A-C) and close (D-F) to the central lesion of the plateau. Sections were stained with hematoxylin and Safranin O/Fast Green FCF. The higher OA grades shown by the biopsies close to the central lesion (D-F) compared with those away from the central lesion (A-C) (see Figures 2 and 3), are due to changes in different histological characteristics. Away from the lesion, the cartilage appears sometimes almost intact (A), or shows a slight reduction in Safranin O staining (red) (B and C) sometimes along with mild surface fibrillations (C). These histological features are representative of early OA-induced changes. Closer to the lesion, OA is more severe as reflected by more serious surface erosion (D), and vascular invasion of the tidemark appears (D, arrow heads). Cell clusters are another typical feature observed during OA progression (E and F, yellow arrows), as well as clefts into the transitional zone (E, star). In severely OA-damaged tissue, clefts into the radial zone (F, star), more frequent cell clustering as well as extensive loss of Safranin O staining occur (F). Bars = 100 μm. OA, osteoarthritis.