α-Smooth muscle actin (α-sma) staining in healthy and arthritic synovial tissue. (a) No α-sma staining was found in the lining layer of healthy synovial tissue. (c) In synovial biopsies from rheumatoid arthritis patients, cells positive for α-sma expression were found in the lining layer. Relative isotype controls are shown in (b,d). Arrows indicate α-sma positive cells from the lining layer.