Chondrocytes embedded in the epiphyseal growth plates of long bones undergo autophagy prior to the induction of osteogenesis

Bone growth takes place through the activities of chondrocytes embedded in the epiphyseal growth plate. Stress conditions in the plate can promote the autophagic response through the modulation of genes controlling metabolite utilization. mTOR plays a critical role in autophagy serving as the sensor that integrates metabolic and growth factor signals. Ongoing studies indicate that terminal chondrocytes exhibit autophagic characteristics. Morphologically, the arrested cells contain double membrane vacuoles; there is a loss of membrane structure, limited staining and organelle destruction. Since the life history of the growth plate chondrocyte is very short, even minor disturbances in the metabolic state can result in gross impairment of growth. We contend that the induction of the autophagic response, permits the terminally differentiated cells to survive the brief rigors of the harsh local microenvironment. Whether chondrocytes can recover from this state, and possibly participate in osteogenesis, is not known at this time.