Comparative Biomedical Sciences, Royal Veterinary College, London, UK.
J Anat. 2014 Jun;224(6):615-23. doi: 10.1111/joa.12171. Epub 2014 Mar 18.
The proportion of total limb length taken up by the individual skeletal elements (limb proportionality), varies widely between species. These diverse skeletal forms have evolved to allow for a range of limb uses and they first emerge as the embryo develops, to achieve the characteristic skeletal architecture of each species. During this time, the developing skeleton experiences mechanical loading as a result of embryonic muscle contraction. The possibility that adaptation to such mechanical input may allow embryos to coordinate the appearance of skeletal design with their expanding range of movements has so far received little attention. This is surprising, given the critical role exerted by embryo movement in normal skeletal development; stage-specific in ovo immobilisation of embryonic chicks results in joint contractures and a reduction in longitudinal bone growth in the limbs. Epigenetic mechanisms allow for selective activation of genes in response to environmental signals, resulting in the production of phenotypic complexity in morphogenesis; mechanical loading of bone during movement appears to be one such signal. It may be that 'mechanosensitive' genes under regulation of mechanical input adjust proportionality along the bone's proximo-distal axis, introducing a level of phenotypic plasticity. If this hypothesis is upheld, species with more elongated distal limb elements will have a greater dependence on mechanical input for the differences in their growth, and mechanosensitive bone growth in the embryo may have evolved as an additional source of phenotypic diversity during skeletal development.
肢体各骨骼元素在整个肢体长度中所占的比例（肢体比例）在不同物种间差异很大。这些多样化的骨骼形态是为了适应各种肢体用途而进化的，它们在胚胎发育过程中首先出现，以实现每个物种的特征性骨骼结构。在此期间，发育中的骨骼会因胚胎肌肉收缩而受到机械载荷。胚胎适应这种机械输入的可能性，可能使胚胎能够协调骨骼设计的出现与其不断扩大的运动范围，而这一点到目前为止还没有得到太多关注。这令人惊讶，因为胚胎运动在正常骨骼发育中起着至关重要的作用；鸡胚胎在特定发育阶段的体外固定会导致关节挛缩和四肢长骨生长减少。表观遗传机制允许基因对环境信号选择性激活，从而在形态发生中产生表型复杂性；运动过程中骨骼的机械加载似乎就是这样的信号之一。“机械敏感”基因可能受到机械输入的调节，从而沿骨骼的近-远轴调整比例，引入一定程度的表型可塑性。如果这一假设成立，那么具有更长的远端肢体元素的物种将更依赖于机械输入来维持其生长差异，而胚胎中的机械敏感骨生长可能已经进化成为骨骼发育过程中表型多样性的另一个来源。
