Structural development and gravity

Changes in contical bone formation found in rats on the model and wheather they are due to hind limb unloadng or involve a stress response was studied. Variables known to effect bone histomorphometry and physiology in that model are defined. The variables studied were: (1) changes in bone formation with age in two commonly used rat strains; (2) effects of cold stress on bone formation and apposition rates relative to changes in the same parameter in rats on the model. Body mass, growth rates, and tibial radographic and histomorphometric measurements were compared in Sprague-Dawley (S/D) and Fischer 344 (F) rats aged 6 to 68 weeks. Histomorphometric measurements of rat tibia from the juvenile to the adult period were studied, and tibial growth rates relative to both age and weight were compared. The physiological effects of unweighting the hind-limbs with those of cold stress in rats studied after 1 or 3 weeks of exposure. Adrenal weights, thymus weights, and corticosteroid levels were used as indictors of environmental stress.