ACTA1 and Arthritis: Consistent with this, immunofluorescent staining of joints for the endothelial cell marker, isolectin IB4, and α-smooth muscle actin (α-SMA), a marker of perivascular mural cells, revealed a high degree of vascularisation of the soft tissues of joints of both Hsd11b1MKO and control mice following K/BxN serum transfer arthritis (Fig. 3A).