Similarly, while Gdf5-lineage cells are the main progenitors that form articular cartilage during development, repair cartilage after injury in adulthood,2 and form osteophytes in osteoarthritis,11 other cells can give rise to new chondrocytes, especially ectopically in synovium after injury.2 Thus, while the Gdf5-lineage cells are the natural progenitors for FLS and articular chondrocytes, under conditions of stress, other cells in the joint supply FLS and chondrocytes in a compensatory mechanism. This evidence concerns the gene GDF5 and osteoarthritis.