Effects of spaceflight and simulated weightlessness on longitudinal bone growth

Indirect measurements have suggested that spaceflight impairs bone elongati on in rats. To test this possibility, our laboratory measured, by the fluor ochrome labeling technique, bone elongation that occurred during a spacefli ght experiment. The longitudinal growth rate (LGR) in the tibia of rats in spaceflight experiments (Physiological Space Experiments 1, 3, and 4 and Ph ysiological-Anatomical Rodent Experiment 3) and in two models of skeletal u nloading (hind-limb elevation and unilateral sciatic neurotomy) were calcul ated. The effects of an 11 day spaceflight on gene expression of cartilage matrix proteins in rat growth plates were also determined by northern analy sis and are reported for the first time in this study. Measurements of long itudinal growth indicate that skeletal unloading generally did not affect L GR, regardless of age, strain, gender, duration of unloading, or method of unloading. There was, however, one exception with 34% suppression in LGR de tected in slow-growing, ovariectomized rats skeletally unloaded for 8 days by hind-limb elevation. This detection of reduced LGR by hind-limb elevatio n is consistent with changes in steady-state mRNA levels for type II collag en (-33%) and for aggrecan (-53%) that were detected in rats unloaded by an 11 day spaceflight, The changes detected in gene expression raise concern that spaceflight may result in changes in the composition of extracellular matrix, which could have a negative impact on conversion of growth-plate ca rtilage into normal cancellous bone by endochondral ossification. (Bone 27: 535-640; 2000) (C) 2000 by Elsevier Science Inc. All rights reserved.