Histopathology of tarsal joints day 0 to 7. A) Day 0 to 4: overview of inflammation in the tarsal joint showing equal severity of inflammation in peripheral tissues and synovium (arrows). (B-C) Day 0 to 4: bone erosion and OC activity (arrow in B, day 3) dominates. Proliferation and collagen deposition takes place (star in C, day 0) in the periosteum and results in osteoid deposition (arrows in C, day 3). (D) Day 4 to 7: the joint architecture is severely distorted (day 7). OC, osteoclast.