Fig. 1

Detection of follistatin and activin and their receptors on synovial fibroblasts. a Activin A staining of RA synovial tissue compared to b OA synovial tissue (representative stainings, n = 4 each). c Activin A staining at the site of cartilage invasion in RA (n = 4). d mRNA of ACVR1 was detectable in all 5 RASF and 3 OASF and mRNA of ACVR2A in all 4 RA- and 3 OASF; here, LS174T cells are shown as a positive control. Negative control: water instead of RNA. 18S rRNA served as the loading control. e Immunocytochemistry for ACVR2A and ACVR1B protein confirmed the presence of both receptors on cultured RASF. Positive control: mesenchymal marker vimentin, negative control: matched isotype control. f Follistatin expression was limited to single cells in RA and OA synovial tissue (n = 3) when compared to the g vimentin staining showing the distribution of mesenchymal cells including synovial fibroblasts as well as h β2-microglobulin, an MHC class I subunit expressed by almost all nucleated cells. i H/E staining of the tissue. 100-fold magnification