(a) Bone is destroyed by a proliferative and invasive synovium, which is called pannus. It originates from adjacent synovial tissue and invades the cartilages and bones. (b) Magnified view of the pannus-bone interface. The pannus-bone interface is lined with mature osteoclasts (arrows). Various inflammatory cells and stromal cells comprise the invading pannus. (c) Schematic depiction of the pannus-cartilage-bone structure. Inflammatory cells such as B cells, T cells, macrophages, monocytes, and fibroblast-like synoviocytes accumulate in the pannus. For metabolic support, intensive angiogenesis is usually followed. Excessive provision of RANKL from the accumulated cells in the pannus enhances osteoclastogenesis, resulting in the erosion of bone at the pannus-bone interface.