Genes implicated in limb development and bone and joint formation,^1,2 particularly members of the segment polarity family that encode components of the hedgehog and wingless/Wnt signalling pathways, have recently been identified in inflamed synovial tissues,^3,4 prompting speculation about their role in the pathogenesis of rheumatoid arthritis (RA) (this issue, 6). In the limb bud of the embryo these genes are associated with primitive mesenchymal cells, which can be identified by their expression of heterodimeric surface membrane molecules that bind members of the transforming growth factor β super family, including bone morphogenetic protein receptors (BMPR), endoglin, anaplastic lymphoma kinase 1, and transforming growth factor β receptors.^5,6 Postnatal bone marrow has similar mesenchymal progenitor cells (MPC) that provide the reticular stroma which supports haemopoiesis, and when appropriately stimulated MPC can give rise to bone, cartilage, fat, muscle, or fibrous tissues.^6,7 RA synovium also contains cells with phenotypic and functional characteristics of MPC.^8,9