Factors regulating condylar cartilage growth under repeated load application

Mechanical loading can influence the biological behavior of the bone-associated cells leading to adaptive changes in skeletal mass and architecture. SOX9 and PTHrP genes are known to regulate chondrocyte differentiation and delay maturation, ultimately control the endochondral bone formation. To investigate the effects of repeated mechanical loading on bone, 280 Sprague-Dawley rats were used in this experiment. The animals were randomly allocated into experimental and control groups. Repeated mechanical loading was applied through a bite-jumping device in the experimental group. The experimental animals were sacrificed on 10 different time points together with the matched control. Total RNA was extracted from the mandibular condylar cartilage for PTHrP and SOX9 genes quantification using real-time RTPCR. Results showed that PTHrP expression was increased and reached a peak level on the seventh day after mechanical loading was given. Repeated mechanical loading triggered a significant increase of PTHrP expression leading to another peak increment. The expression of SOX9 was highly correlated with the PTHrP expression, and its pattern of expression was similar to that of PTHrP after repeated mechanical loading. In conclusions, repeated mechanical loading on the condyle triggers the expression of PTHrP and SOX9, which in turn promotes condylar cartilage growth.