Schematic representations of mesenchymal stem cells (MSCs) and their niches in synovium identified in mice using a double-nucleoside analogue cell-labelling scheme . (A) Schematic drawing of an uninjured control synovial joint. (B) Details of the dashed box in (A), showing cell populations in the synovium of uninjured joints. Iododeoxyuridine (IdU)-retaining cells (green) were located in both the synovial lining (SL) and the subsynovial tissue (SST). Subsets of IdU-positive cells displayed an MSC phenotype. IdU-negative cells (blue) included haematopoietic lineage cells (HC), endothelial cells (EC), pericytes (PC), and other cell types of unknown phenotype. (C) Schematic drawing of a synovial joint 12 days after articular cartilage injury in mice (arrowhead). (D) Details of the dashed box in (C), showing cell populations in the synovium. Proliferating cells were detected in both the synovial lining and the subsynovial tissue and were either double positive for IdU and chlorodeoxyuridine (CIdU; orange) or single positive for CIdU (red). Subsets of cells positive for IdU and CIdU and cells positive only for IdU (green) expressed chondrocyte lineage markers. The boxed areas in (B) and (D) show cell phenotypes. B, bone; C, cartilage; SC, synovial cavity; SM, synovial membrane. Reproduced from Kurth et al., Arthritis Rheum 2011 .