<qa>  What is the potential explanation for this phenomenon?Possible explanations to this phenomenon could be decreased senescence, migration of mesenchymal stem cells from the circulation, epithelial to mesenchymal cell-transition or expansion of a stem cell pool in the synovium. FLS precursors could also migrate to the synovium through pores in cortical bone, as has been demonstrated in mice with collagen-induced arthritis. In a murine model, it has further been shown that RA FLS cells can spread via the systemic circulation from one joint and attack previously unaffected joints. 