Diffuse microdamage in bone: Direct repair without remodeling

描述(申请人提供)：微观损伤是骨骼磨损(即疲劳)的微观结构结果。骨重建可以修复骨中已报道的典型的线性微裂纹，这种能力对于保持其力学完整性是必不可少的。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。大多数其他的基质损伤过程会引起“小裂纹”类型的损伤，统称为“扩散”损伤。弥漫性损伤广泛存在于老化的骨骼中，与典型的线性微裂纹一样，它会降低骨材料的性能，但弥漫性损伤与骨生理的相关性尚不清楚。我们的初步研究表明，弥漫性损伤并不像典型的微裂缝那样激活骨重建，也不会导致目前已知的控制随后重建反应的局部骨细胞凋亡。相反，弥漫性损伤部位的骨细胞似乎保持相当健康。鉴于最近的发现显示骨细胞主动调节其周围基质，特别是通过调节局部矿物质沉积，我们认为骨具有一种不同于基于破骨细胞的重塑的基质水平的自我修复机制。在这个过程中，细小的(“弥漫性”)裂缝损伤通过骨细胞的作用进行直接修复。我们将使用在体大鼠尺骨弯曲疲劳模型在体内解决这个问题。在第一个研究中，将导致弥漫基质损伤的离散区域。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力学性能的变化。定量背向散射成像将用于检查扩散损伤区域的矿物基质完整性和局部矿物含量，拉曼光谱将用于表征晶体大小以及矿物和有机成分。在第二系列研究中，我们将确定疲劳是否选择性地引起弥漫性损伤部位骨细胞局部矿化调节分子表达的变化，这与弥漫性损伤区域局部基质矿物质完整性的恢复是一致的。 与公共健康相关：人们早就知道，骨重建可以移除和修复因骨骼磨损(即机械疲劳)而产生的典型微观裂纹(约100 5m大小)，从而帮助恢复强度和防止骨折。然而，在骨基质中还有其他类型的疲劳损伤，由小得多的裂缝(1-2.5米或更小)组成，这也大大削弱了骨骼。我们最近发现，这些细小的裂缝并不是通过骨重建来处理的。因此，它们的愈合必须涉及其他生物机制。在这些研究中，我们将测试骨中这些非常小的裂缝是否可以通过另一种不涉及骨重塑的机制随着时间的推移而愈合，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脆性。

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.