HA is detected in the endosteum and the marrow sinusoidal but not arteriolar endothelium. Staining of normal human BM sections was performed using the biotinylated HA binding protein (bHABP) and avidin-biotin horseradish peroxidase counterstained with Mayer hematoxylin. Arrows indicate the sinusoidal endothelium (A), the endosteal cells (B), and the arteriolar endothelium (C). Preincubation with hyaluronidase prevented HA labeling of sinusoidal endothelium by bHABP (D). Original magnification, × 1500 (A,C-D); and × 1000 (B).