Mechanical loading of mouse caudal vertebrae increases trabecular and cortical bone mass-dependence on dose and genotype.

Most in vivo studies addressing the skeletal responses of mice to mechanical loading have targeted cortical bone. To investigate trabecular bone responses also we have developed a caudal vertebral axial compression device (CVAD) that transmits mechanical loads to compress the fifth caudal vertebra via stainless steel pins inserted into the forth and sixth caudal vertebral bodies. Here, we used the CVAD in C57BL/6 (B6) and C3H/Hej (C3H) female mice (15 weeks of age) to investigate whether the effect of regular bouts of mechanical stimulation on bone modeling and bone mass was dependent on dose and genotype. A combined micro-computed tomographic and dynamic histomorphometric analysis was carried out at the end of a 4-week loading regimen (3,000 cycles, 10 Hz, 3 x week) for load amplitudes of 0N, 2N, 4N and 8N. Significant increases in trabecular bone mass of 9 and 21% for loads of 4N and 8N, respectively, were observed in B6 mice. A significant increase of 10% in trabecular bone mass occurred for a load of 8N in the C3H strain. For other loads, no significant increases were detected. Both mouse strains exhibited substantial increases in trabecular bone formation rates for all loads, B6: 111% (2N), 86% (4N), 164% (8N), C3H: 41% (2N), 38% (4N), 141% (8N). Significant decreases in osteoclast number of 146 and 93% for a load of 8N were detected in B6 and C3H mice, respectively. These findings demonstrate that the effect of loading on the structural and functional parameters of bone is dose and genotype dependent. The caudal vertebral loading model established here is proposed for further studies addressing the molecular processes involved in the skeletal responses to mechanical stimuli.