In situ hybridisation of matrix metalloproteinase (MMP) mRNA expression in a Crohn’s disease fistula. Sections of a paraffin embedded fistula specimen with acute inflammation and granulation tissue (A–C, G–I) and chronic inflammation with fibrosis (D–F) were hybridised with antisense MMP-2 (A, D), MMP-3 (B, E), and MMP-9 (C, F) or sense MMP-2 (G), MMP-3 (H), and MMP-9 (I). In acute inflammation with granulation tissue, MMP-2 mRNA expression was observed in large mononuclear macrophage-like cells and endothelial cells (A). MMP-3 mRNA expression was seen in large mononuclear macrophages-like cells (B) while MMP-9 mRNA expression was seen in granulocytes and small mononuclear leucocytes (data not shown) (C). In chronic inflammation with fibrosis, MMP-2 and MMP-3 was seen in large mononuclear macrophage-like cells and in fibroblasts (D, E) while MMP-9 was seen in sporadic small mononuclear leucocytes and granulocytes (data not shown). Magnification ×100 (A–F) and ×20 (G–I).