Early changes in the knee of healer and non‐healer mice following non‐invasive mechanical injury

In this study, we examined early time‐dependent changes in articular cartilage and synovium in response to tibial compression and sought the plausible origin of cells that respond to compression in the healer (LGXSM‐6) and non‐healer (LGXSM‐33) recombinant inbred mouse strains. The right knee of 13‐week old male mice was subjected to tibial compression using 9N axial loading. The contralateral left knee served as a control. Knees were harvested at 5, 9, and 14 days post‐injury. Histological changes in cartilage and synovium, immunofluorescence pattern of CD44, aggrecan, type‐II collagen, cartilage oligomeric matrix protein and the aggrecan neo‐epitope NITEGE, and cell apoptosis (by TUNEL) were examined. We used a double nucleoside analog cell‐labeling strategy to trace cells responsive to injury. We showed that tibial compression resulted in rupture of anterior cruciate ligament, cartilage matrix loss and chondrocyte apoptosis at the injury site. LGXSM‐33 showed higher synovitis and ectopic synovial chondrogenesis than LGXSM‐6 with no differences for articular cartilage lesions. With loading, an altered pattern of CD44 and NITEGE was observed: cells in the impacted area underwent apoptosis, cells closely surrounding the injured area expressed CD44, and cells in the intact area expressed NITEGE. Cells responding to injury were found in the synovium, subchondral bone marrow and the Groove of Ranvier. Taken together, we found no strain differences in chondrocytes in the early response to injury. However, the synovial response was greater in LGXSM‐33 indicating that, at early time points, there is a genetic difference in synovial cell reaction to injury.