Functional mesenchymal stem cell niches in adult mouse knee joint synovium in vivo

OBJECTIVE: We previously reported that human synovium contains cells that, after culture-expansion, display properties of mesenchymal stem cells (MSCs). The objective of this study was to identify MSCs in the native synovium in vivo.

METHODS: To identify stem cells in the synovium in vivo, a double nucleoside-analogue cell-labelling scheme was used in a mouse model of joint surface injury. To label slow-cycling cells, mice received IdU for 30 days followed by a 40-day wash-out period. To label cells that proliferate after injury, mice underwent knee surgery to produce an articular cartilage defect and received CldU for 4 days starting at multiple time-points after surgery. Non-operated and sham-operated joints served as controls. Knee joint paraffin sections were analyzed by double and triple immunostaining to detect nucleoside analogues, conventional MSC markers, and chondrocyte-lineage markers.

RESULTS: Long-term retaining, slow-cycling IdU-positive cells were detected in the synovium. At 4 and 8 days after injury, there was marked proliferation of IdU-positive cells, which co-stained for CldU. IdU-positive cells were non-haematopoietic, non-endothelial stromal cells, distinct from pericytes, and stained positive for MSC markers. MSCs were phenotypically heterogeneous and located in topographically distinct niches in the lining layer and the subsynovial tissue. At 12 days after injury, double-nucleoside labelled cells within synovium were embedded in cartilage-specific metachromatic extracellular matrix and co-stained positive for the chondrocyte-lineage markers Sox9 and collagen type 2.

CONCLUSION: Our findings provide first evidence of existence of resident MSCs in the knee joint synovium, which undergo proliferation and chondrogenic differentiation following injury in vivo.