Bone cells in the synovial and subchondral compartment in rheumatoid arthritis. (A) Hematoxylin and eosin-stained section of a subchondral bone specimen with histologic evidence of erosions and intra-marrow inflammation. The different distribution of osteoclasts and osteoblasts is shown. Areas with osteoclast accumulation are visible on both the synovial (i) and the subchondral (ii) side of the joint. Osteoblasts and bone lining cells are instead located on the endosteal surface (iii). st, synovial tissue; b, bone; m, marrow. (B, C) Immunostainings for the osteoclast marker tartrate-resistant acid phosphatase (TRAP) showing TRAP-positive multinucleated osteoclasts resorbing bone from the synovium (B) as well as from the subchondral bone (C). (D) Immunostaning for parathytoid hormone receptor (PTH-R) showing PTH-R-positive cuboid osteoblasts and bone-lining cells in the subchondral compartment. (A) Original magnification ×10; (B-D) original magnification ×100.