EFFECTS OF X-RAY-IRRADIATION ON TERMINAL DIFFERENTIATION AND CARTILAGE MATRIX CALCIFICATION OF RABBIT GROWTH-PLATE CHONDROCYTES IN CULTURE

The retardation of long bone growth caused by irradiation is thought t o be closely related to the impairment of growth plate function, but i ts mechanism remains unclear. In this study, we examined the effects o f irradiation on the terminal differentiation of growth plate chondroc ytes and on calcification. Chondrocytes were isolated from the growth plate of the ribs of four-week-old rabbits and inoculated at a high de nsity on a type-I collagen-coated dish. Following logarithmic prolifer ation, they reached confluence (premature chondrocytes), then matured (mature chondrocytes), and became hypertrophied (hypertrophic chondroc ytes). 10 Gy or less irradiation of the premature chondrocytes potentl y inhibited the terminal differentiation and matrix mineralization. Ir radiation inhibited chondrocyte hypertrophy and suppressed alkaline ph osphatase induction and the expression of type-X collagen without chan ging the protein composition profile of any other cell layer. Prematur e cells had the highest radiosensitivity. The sensitivity was decrease d as the cells differentiated; the effects of irradiation on hypertrop hic chondrocytes with terminal differentiation-related phenotypes were reduced. This study showed that 10 Gy or less irradiation of growth p late chondrocytes impaired terminal differentiation and mineralization . Since chondroclasts and bone marrow cells invade only to the mineral ized cartilage, the induction of calcification in cartilage matrices i s one of the most important steps in endochondral ossification. Theref ore, it is conceivable that the damage in the growth plate induced by irradiation could account for the subsequent abnormal bone and skeleta l growth.