BMP immunohistochemistry in ankylosing enthesitis. (A–C) The first phase of ankylosing enthesitis is characterized by enthesial cell proliferation and cartilage differentiation; immunoreactivity for BMP2 is detected in spindle-shaped enthesial fibroblast-like cells (arrowheads, inset in B) and in more rounded early chondroblast-like cells (arrows) (A; B, detail of boxed area in A). At this stage of the disease process, no cartilage hypertrophy is seen. (C) Negative control staining using IgG. (D–I) Cartilage formation in ankylosing enthesitis shows BMP7-positive proliferating and prehypertrophic chondrocyte-like cells (D; E, detail of boxed area in D; arrows indicate zone of positive cells) and BMP6-positive hypertrophic chondrocyte-like cells (G; H, detail of boxed area in G; arrows indicate positive cells). No BMP7 expression is seen in hypertrophic chondrocyte-like cells (E, arrowheads). (F and I) Negative control staining using IgG. Scale bars: 200 μm in A and D; 100 μm in G; 80 μm in E and F; 50 μm in B and C; 25 μm in H and I.