Cortical bone maturation in young hypophysectomized rats

Cortical bone growth and maturation were studied in young growing rats (62 days) that were hypophysectomized. After 10 days, femur bones were obtained for morphological and biochemical analysis of the middiaphysis. Hypophysectomy significantly decreased (42%) normal body growth demonstrated by reductions in body weight. Bone morphology data indicate that hypophysectomy significantly inhibits the circumferential growth of the middiaphysis as well as region-specific thickness compared with the control group. The effects of hypophysectomy on cortical bone mineral composition and density were significantly greater in the hypophysectomized rats. Bone density and total mineral density were significantly greater, 17% and 24%, respectively, in the hypophysectomized group compared with control animals. Bone collagen maturation, measured by the amount of the mature collagen cross-link hydroxylysylpyridinoline, in the hypophysectomized animals was significantly greater (130%). Also, collagen concentration and content per unit volume were significantly greater, 24% and 70%, respectively, in the hypophysectomized group. Our findings suggest that "normal" growth and maturation of cortical bone in rapidly growing animals is dependent upon the modulation of pituitary hormones. These data also suggest that an increased rate of mature collagen cross-link formation may be modified by the turnover rate of collagen and other matrix constituents. In addition, hormonal mechanisms associated with a suppression of bone growth may enhance the probability of mature collagen cross-link formation.