Double labeling for osteocalcin and tartrate-resistant acidic phosphatase for visualization of osteoblasts and osteoclasts. Paraffin-embedded sections of hind paws from arthritic mice transgenic for human tumor necrosis factor (hTNFtg) were stained by TRAP to determine osteoclasts (black arrows; purple-colored cells with three or more nuclei). In the same sections, osteoblasts were visualized by in-situ hybridization for osteocalcin (green arrows; deep blue-stained cells). (a) Overview of a representative section, original magnification × 100. (b) Close-up view of the compartment of subchondral bone erosion, original magnification × 400. (c) Cortical bone channels underneath subchondral bone erosions, original magnification × 400. (d) Inflamed endosteal bone surface distant from bone erosion, original magnification × 400.