Fig. 2.

Histological features of OA. The normal synovium has a thin (1–2 cells thick) lining layer and a vascularized, loose connective tissue sublining layer. OA synovium demonstrates features of synovial villous hyperplasia (#), lining hyperplasia (arrows), increased vascularity (+), and perivascular mononuclear cell (inflammatory) infiltration. In OA articular cartilage, loss of cells and matrix is accompanied by areas of cell clusters. There is thickening of the calcified zone and duplication of the tidemark, which normally separates the articular cartilage from the underlying calcified cartilage. The subchondral bone is also thickened, and vascular invasion, which can extend through the tidemark and into the base of the articular cartilage, is seen. OA = osteoarthritis. Note. From “Osteoarthritis: a disease of the joint as an organ” by Loeser, et al., 2012. Arthritis Rheumatology, 64, p. 1697–1707. Copyright 2012, The American College of Rheumatology. Reprinted with permission.