Diffuse microdamage in bone: Direct repair without remodeling

描述（由申请人提供）：微观损坏是磨损和撕裂的微观结构后果（即，疲劳）。骨重建可以修复已报道的骨中典型的线性微裂纹，这种能力对于维持其机械完整性至关重要。然而，最近的研究表明，线性微裂纹只是骨骼疲劳引起的一系列基质损伤类型之一。大多数其他基体损伤过程导致“小裂纹”型损伤，统称为“扩散”损伤。在老化骨骼中广泛观察到弥散性损伤，并且像典型的线性微裂纹一样，它降低了骨材料的性能，但弥散性损伤与骨生理学的相关性尚不清楚。我们的初步研究表明，弥漫性损伤不会像典型的微裂纹那样激活骨重建，也不会引起局部骨细胞凋亡，这是目前已知的控制随后的重建反应。相反，弥漫性损伤部位的骨细胞似乎保持相当健康。鉴于最近的发现表明骨细胞积极调节其周围的基质，特别是通过调节局部矿物质沉积，我们提出，骨具有基质水平的“自我修复”机制，这是从破骨细胞为基础的重塑不同。在这个过程中，小的（“扩散”）裂纹损伤通过骨细胞的作用进行直接修复。我们将使用大鼠在体尺骨弯曲疲劳模型来解决这个问题。在第一项研究中，将诱导离散区域的扩散基质损伤。将使用扫描声学显微镜测量弥漫性损伤局部区域和非疲劳骨中相应区域的机械性能变化。定量背散射成像将用于检查矿物基质的完整性和扩散损伤区域的局部矿物含量，拉曼光谱用于表征晶体大小以及矿物和有机成分。在第二系列的研究中，我们将确定疲劳是否选择性地elerminate局部矿化调节分子的表达变化的骨细胞内弥漫性损伤部位，符合恢复局部基质矿物质的完整性在弥漫性损伤区域。 公共卫生相关性：人们早就知道，骨重建可以去除和修复由磨损引起的典型的微观（约100 5 m大小）裂纹（即，机械疲劳），从而帮助恢复强度并防止骨折。然而，在骨基质中存在其他类型的疲劳损伤，包括小得多的裂纹（1- 2.5米或更小），这些裂纹也大大削弱了骨。我们最近发现这些小裂缝不会通过骨骼重塑来处理。因此，它们的愈合必须涉及其他生物机制。在这些研究中，我们将测试骨骼中这些非常小的裂缝是否可以通过另一种机制随着时间的推移而愈合，而不是不涉及骨骼重塑，我们还将研究骨细胞，生活在骨基质中的骨细胞，如何参与这些非常小的裂缝的直接修复并防止骨骼脆弱。

DESCRIPTION (provided by applicant): Microscopic damage is the microstructural consequence of wear and tear (i.e., fatigue) in bone. Bone remodeling can repair the typical linear microcracks that have been reported in bone, and this capability is essential for maintenance of its mechanical integrity. However, recent studies demonstrate that linear microcracks are just one of a range of matrix damage types that result from fatigue in bone. Most other matrix damage processes cause "small crack"-type damage, which are collectively referred to as "diffuse" damage. Diffuse damage is widely observed in the aging skeleton, and like typical linear microcracks it degrades bone material properties, yet the relevance of diffuse damage to bone physiology is not known. Our preliminary studies show that diffuse damage does not activate bone remodeling as do typical microcracks, nor does it cause the local osteocyte apoptosis now known to control the subsequent remodeling response. Rather, osteocytes at diffuse damage sites appear to remain quite healthy. In view of recent discoveries showing osteocytes actively regulate their surrounding matrix, particularly by regulating local mineral deposition, we propose that bone possesses a matrix level "self-repair" mechanism that is distinct from osteoclast-based remodeling. In this process, small ("diffuse") crack damage undergoes direct repair through the actions of osteocytes. We will use the rat in vivo ulnar bending fatigue model to address this question in vivo. In the first studies, discrete regions of diffuse matrix damage will be induced. Changes in the mechanical properties of local regions of diffuse damage and of corresponding areas in non-fatigued bone will be measured using Scanning Acoustic Microscopy. Quantitative back-scattered imaging will be used to examine mineral matrix integrity and the local mineral content in diffuse damage regions, and Raman spectroscopy used to characterize crystal size and mineral and organic composition. In the second series of studies, we will determine whether fatigue selectively elicits changes in the expression of local mineralization-regulating molecules by osteocytes within diffuse damage sites, consistent with a restore local matrix mineral integrity in diffuse damage regions. PUBLIC HEALTH RELEVANCE: It has long been known that bone remodeling can remove and repair the typical microscopic (~100 5m size) cracks that result from wear and tear (i.e., mechanical fatigue) in bone, and thereby help restore strength and prevent fracture. However, there are other types of fatigue damage in the matrix of bone, comprised of much smaller cracks (1-2 5m or smaller) that also weaken bone substantially. We recently discovered that these small cracks are not dealt with by bone remodeling. Thus, their healing must involve other biological mechanisms. In these studies, we will test whether these very small cracks in bone can heal over time through another mechanism than does not involve bone remodeling, and we will also examine how osteocytes, the bone cells that live buried within the bone matrix, might participate in the direct repair of these very small cracks and prevent bone fragility.