Moderate Physical Exercise Results in Increased Cell Activity in Articular Cartilage of the Knee Joint in Rats

Abstract

Background: Moderate exercise regimens have shown minor positive effects on matrix turnover in articular cartilage (AC), while effects at cellular level, e.g. proliferation, are scarcely described. Aim: The aim of this study was to investigate the effects of moderate exercise on cell proliferation and recruitment of cells possibly active in regeneration in different regions of cartilage in the rat knee joint. Methods: Eighteen rats were orally given 5-bromo-2-deoxyuridine (BrdU) for 14 days for in vivo DNA labeling. Nine rats underwent treadmill training for 50 min/day, 5 days/week (exercise group), and 9 rats served as controls (no exercise). Animals were sacrificed after 14, 56 and 105 days, and knee joints were harvested. BrdU+ cells were visualized immunohistochemically (IHC) and counted in AC, posterior stem cell niche (PN), potential migration route (PMR; area between PN and the AC border), potential migration area (PMA; region between PN and AC including PN) and epiphyseal cartilage plate (EP) of the tibia and femur. Results: Compared to controls, in the exercise group BrdU+ cells/mm² were increased on days 14 (p = 0.022) and 105 (p = 0.045) in AC of the tibia and on day 105 (p = 0.014) in AC of the femur. BrdU+ cell numbers were increased in the PMR region of the tibia on days 14 (p = 0.023) and 105 (p = 0.0018) and in the PMR region of the femur on day 105 (p = 0.0099) as well as in the PMA region of the tibia (p = 0.0008) and femur (p = 0.0080) on day 105. No significant differences in BrdU+ cells/mm² were seen in PN or EP between the groups at any time point. Regarding collagen 2A1 expression and proteoglycan accumulation, no significant differences between groups were detected. Conclusions: The results indicate increased cell activity in AC in response to physical exercise and may help to understand the complexity of AC regeneration in the normal mammal knee joint.