PECAM1 and rheumatoid arthritis: Vessel formation was enhanced by a greater amount by RA synovial fluid compared with OA synovial fluid, according to hemoglobin content of the Matrigel plugs measured using the Drabkin’s method and immunohistochemistry (IHC) staining of vessel marker (CD31 and VEGF) expression (Fig. 1F–H) and the quantitative data of the positive new vessels for VEGF and CD31 immunohistochemistry showed in Fig. 1I.