ACP5 and rheumatoid arthritis: Figure 7A shows the quantification of bone density (front and hind paws) in Hounsfield units and representative images of hind paws. Arthritis induction significantly reduced bone density while osteostatin normalized these values. The differentiation of osteoclasts is a key process driving major erosive lesions in RA [17]. Osteoclast area was assessed by tartrate-resistant acid phosphatase (TRAP) staining of mice ankle sections. Arthritic control mice showed a significant enhancement of TRAP staining when compared with non-arthritic animals (Figure 7B).