EFFECT OF SUSPENSION ON MOUSE BONE MICROHARDNESS

Antiorthostatic (hindlimb) suspension of mice results in a considerabl e reduction of bone formation at the femur mid-diaphysis. Comparisons with appropriate control groups indicate that this reduction is attrib utable to the unloading aspect of the model, and not to physiological stress or changes in feeding. Microhardness measurements of bone are u sed to provide information on site-specific mineralization and structu ral properties. The microhardness of femora formed during suspension i s significantly less than that formed in the bone of control mice. The se differences are observed both along the endocortical (11%) and peri osteal (8%) perimeters. The microhardness of bone formed prior to the experimental period (''extant bone'') is not different in comparing su spended and control mice, and increased microhardness values for these areas are observed in comparison to baseline controls. Mice used to c ontrol for the physiological stress and feeding portions of the suspen sion model do not demonstrate reduced microhardness. Thus, the limb un loading effects of suspension, not the induced stress or feeding chang es, cause a reduction in microhardness. As microhardness is positively related to mineralization in these bones, it appears that the reduced mineralization accompanying suspension unloading may contribute to co mpromised structural properties of the bone formed.