Landis William J, Silver Frederick H
Department of Biochemistry and Molecular Pathology, Northeastern Ohio Universities College of Medicine, Rootstown, OH 44272, USA.
Comp Biochem Physiol A Mol Integr Physiol. 2002 Dec;133(4):1135-57. doi: 10.1016/s1095-6433(02)00248-9.
The leg tendons of certain avian species normally calcify. The gastrocnemius, or Achilles, tendon of the domestic turkey, Meleagris gallopavo, is one such example. Its structure and biomechanical properties have been studied to model the adaptive nature of this tendon to external forces, including the means by which mineral deposition occurs and the functional role mineralization may play in this tissue. Structurally, the distal rounded, thick gastrocnemius bifurcates into two smaller proximal segments that mineralize with time. Mineral deposition occurs at or near the bifurcation, proceeding in a distal-to-proximal direction along the segments toward caudal and medial muscle insertions of the bird hip. Mineral formation appears mediated first by extracellular matrix vesicles and later by type I collagen fibrils. Biomechanical analyses indicate lower tensile strength and moduli for the thick distal gastrocnemius compared to narrow, fan-shaped proximal segments. Tendon mineralization here appears to be strain-induced, the muscle forces causing matrix deformation leading conceptually to calcium binding through the exposure of charged groups on collagen, release of sequestered calcium by proteoglycans, and increased diffusion. Functionally, the mineralized tendons limit further tendon deformation, reduce tendon strain at a given stress, and provide greater load-bearing capacity to the tissue. They also serve as important and efficient elastic energy storage reservoirs, increasing the amount of stored elastic energy by preventing flexible type I collagen regions from stretching and preserving muscle energy during locomotion of the animals.
某些鸟类的腿部肌腱通常会钙化。家火鸡（Meleagris gallopavo）的腓肠肌（即跟腱）就是这样一个例子。人们对其结构和生物力学特性进行了研究，以模拟这种肌腱对外力的适应性，包括矿物质沉积发生的方式以及矿化在该组织中可能发挥的功能作用。在结构上，远端呈圆形且厚实的腓肠肌会分叉成两个较小的近端部分，随着时间的推移会发生矿化。矿物质沉积发生在分叉处或其附近，沿着这些部分从远端向近端朝着鸟类髋部的尾侧和内侧肌肉附着点进行。矿物质形成似乎首先由细胞外基质小泡介导，随后由I型胶原纤维介导。生物力学分析表明，与狭窄的扇形近端部分相比，厚实的远端腓肠肌的拉伸强度和模量较低。此处的肌腱矿化似乎是应变诱导的，肌肉力量导致基质变形，从概念上讲会通过胶原上带电基团的暴露、蛋白聚糖释放螯合的钙以及增加扩散来导致钙结合。在功能上，矿化的肌腱限制了肌腱的进一步变形，在给定应力下降低了肌腱应变，并为组织提供了更大的承载能力。它们还作为重要且高效的弹性能量储存库，通过防止柔性的I型胶原区域伸展并在动物运动过程中保存肌肉能量，增加了储存的弹性能量。
