IL34 and synovitis: In low- and high-grade synovitis of knee OA, IL-34 expression was detectable in the synovial lining cell layer, predominantly in synoviocytes (arrows) as well as inflammatory cells (dotted arrows) and the synovial sub-lining cell layer, notably in epithelial cells (head arrows) and fibroblasts (white arrows) (Fig. 4B–D).