Accumulation of osteoclast precursors upon induction of arthritis. Osteoclast precursors are rapidly built upon induction of arthritis in mice. This shows the junction zone as well as Haversian channels in cortical bone one day after onset of arthritis. Osteoclast precursors are stained brown for cathepsin K expression as shown in the right panels. The left panels show corresponding hematoxylin eosin stained sections. Arrows indicate bone erosion.