School of Veterinary Sciences, University of Bristol, Bristol BS40 5DU, UK.
Bone. 2013 May;54(1):113-7. doi: 10.1016/j.bone.2013.01.029. Epub 2013 Jan 26.
Experiments to investigate bone's physiological adaptation to mechanical loading frequently employ models that apply dynamic loads to bones in vivo and assess the changes in mass and architecture that result. It is axiomatic that bones will only show an adaptive response if the applied artificial loading environment differs in a significant way from that to which the bones have been habituated by normal functional loading. It is generally assumed that this normal loading is similar between experimental groups. In the study reported here we found that this was not always the case. Male and female 17-week-old C57BL/6 mice were housed in groups of six, and a single episode (40 cycles) of non-invasive axial loading, engendering 2,200 με on the medial surface of the proximal tibiae in sample mice, was applied to right tibiae on alternate days for two weeks. This engendered an adaptive increase in bone mass in females, but not males. Observation revealed the main difference in behaviour between males and females was that males were involved in fights 1.3 times per hour, whereas the females never fought. We therefore housed all mice individually. In females, there was a similar significant osteogenic response to loading in cortical and trabecular bone of both grouped and individual mice. In contrast, in males, adaptive increases in the loaded compared with non-loaded control bones was only apparent in animals housed individually. Our interpretation of these findings is that the frequent vigorous fighting that occurs between young adult males housed in groups could be sufficient to engender peak strains and strain rates that equal or exceed the stimulus derived from artificial loading. This indicates the importance of ensuring that physical activity is consistent between groups. Reducing the background level of the naturally engendered strain environment allows adaptive responses to artificial loading to be demonstrated at lower loads.
为了研究骨骼对机械加载的生理适应，实验中常采用在体施加动态载荷的模型，并评估由此产生的质量和结构变化。不言而喻，如果施加的人工加载环境与骨骼通过正常功能加载所习惯的环境在很大程度上不同，骨骼只会产生适应性反应。通常认为，实验组之间的正常加载是相似的。在本研究中，我们发现情况并非总是如此。将 17 周龄的雄性和雌性 C57BL/6 小鼠分组饲养，每组 6 只。对样本小鼠的胫骨内侧表面施加 40 个循环的非侵入性轴向加载，使胫骨近端的内侧表面产生 2200με 的应变，隔天交替对右侧胫骨施加一次加载，持续两周。这导致雌性的骨量适应性增加，但雄性没有。观察发现，雄性和雌性之间的主要行为差异在于雄性每小时参与打斗 1.3 次，而雌性从不打架。因此，我们将所有小鼠单独饲养。在雌性中，无论是分组饲养还是单独饲养的小鼠，皮质骨和松质骨对加载均有类似的显著成骨反应。相比之下，在雄性中，与未加载对照骨相比，加载骨的适应性增加仅在单独饲养的动物中明显。我们对这些发现的解释是，分组饲养的年轻成年雄性之间频繁发生的激烈争斗可能足以产生与人工加载衍生的刺激相等或超过的峰值应变和应变速率。这表明确保组间身体活动一致的重要性。降低自然产生的应变环境的背景水平可以允许在较低的负载下显示对人工加载的适应性反应。
