Diffuse microdamage in bone: Direct repair without remodeling

描述（由申请人提供）：微观损伤是骨骼磨损（即疲劳）的微观结构后果。骨重塑可以修复骨中典型的线性微裂缝，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。其他基体损伤过程大多是“小裂纹”型损伤，统称为“弥漫性”损伤。弥漫性损伤在老化的骨骼中被广泛观察到，像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那样激活骨重塑，也不会导致局部骨细胞凋亡，而现在已知的骨细胞凋亡可以控制随后的重塑反应。相反，弥漫性损伤部位的骨细胞似乎保持相当健康。鉴于最近发现骨细胞积极调节其周围基质，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，不同于基于破骨细胞的重塑。在这个过程中，小的（“弥漫性”）裂纹损伤通过骨细胞的作用直接修复。我们将使用大鼠体内尺骨弯曲疲劳模型来解决这个问题。在最初的研究中，会引起弥漫性基体损伤的离散区域。弥漫性损伤局部区域和非疲劳骨相应区域的力学性能变化将使用扫描声学显微镜进行测量。定量后向散射成像将用于检查矿物基质的完整性和漫射损伤区域的局部矿物含量，拉曼光谱用于表征晶体尺寸、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，从而恢复弥漫性损伤区域的局部基质矿物完整性。

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.