GDF5 and rheumatoid arthritis: A recent study reported functionally distinct fibroblast subsets in RA synovium, with destructive fibroblasts restricted to the synovial lining layer and immune effector fibroblasts located in the synovial sub-lining.2 In our study, we show that while in normal knee joint synovium the Gdf5-lineage SF were largely confined to the synovial lining, as previously reported,5 in AIA synovitis the Gdf5-lineage SF underwent extensive proliferation and expansion throughout the synovium, indicating a derangement of the anatomical segregation of fibroblast lineages during synovitis.