SKELETAL ALTERATIONS IN HYPOPHYSECTOMIZED RATS .2. A HISTOMORPHOMETRIC STUDY ON TIBIAL CORTICAL BONE

Background: Pituitary hormones play an important role in bone growth, modeling, and remodeling. The purpose of this study is to examine the effect of hypophysectomy (HX) on tibial cortical bone with histomorpho metry. Methods: Forty-five female Sprague-Dawley rats, at 3 months of age, were hypophysectomized or served as intact controls, They were sa crificed at 0, 2, and 5 weeks after the surgery. Cortical bone histomo rphometry was performed on double-fluorescent-labeled 30-mcm-thick sec tions of the tibial shaft. Results: The dry weight and density of tibi al diaphysis and the cortical bone area of the tibial shaft in the HX rats were significantly lower (P<0.05) than that of the age-matched in tact rats, but did not differ between the HX and basal control rats. T he dynamic data show that the bone formation parameters (labeled surfa ce, mineral apposition rate, and bone formation rate) were profoundly decreased (P<0.01) on both the periosteal and endocortical surfaces in the HX rats as compared with the age-matched intact rats at the 2 and 5 weeks, However, the decrease in the labeled surface was much less o n the endocortical envelope than on the periosteal envelope in the HX rats, Although no significant change was detected in the medullar size between the HX and age-matched intact rats, the eroded surface on the endocortical surface was greater (P<0.05) in the HX rats than in the intact rats at either time point. Conclusions: Hypophysectomy-suppress ed, radial growth-dependent bone gain without a bone loss in the tibia l shaft of the young rat. This is associated with decreased modeling-d ependent bone formation. A greater eroded surface on the endosteum did not affect the marrow size at 5 weeks after hypophysectomy. (C) 1995 Wiley-Liss, Inc.