Not all bone nanocomposites are equal: structure-water-micromechanics of osteocytic and anosteocytic fishbone material

脊椎动物的骨骼是由骨头构成的，骨头是一种纳米复合材料，包括矿化的胶原原纤维和水，以及少量的非胶原蛋白。由于存在一种累积微损伤的自我修复机制，并且能够适应不断变化的载荷，骨骼可以在很长一段时间内承受反复出现的机械载荷而不会失败（re/modeling）。在广泛研究的哺乳动物骨骼中，re/modeling被认为是由骨基质腔隙中的骨细胞（骨细胞）调节和激活的，并通过骨小管进行交流。因此，令人感兴趣的是，一大群高级硬骨鱼的骨材料完全缺乏骨细胞（anosteocytic bone）。在之前的资助期内，我们发现了两种骨类型之间的一些差异，这可能有助于理解它们如何能够在有骨细胞和没有骨细胞的情况下进行重建/建模。我们建议进一步探索和了解medaka和斑马鱼骨类型之间结构-力学功能关系的差异，在第一个资助期进行体内研究。为此，新的基于计算机的实验将探索在研究的第一部分中已经收集的数据。我们将量化组成、微观结构和透水性的差异，并通过三维原位加载实验和有限元分析来比较它们的力学变形响应。在第一个资助期，尽管矿化胶原纤维的成分非常相似，但我们观察到水在肌成细胞和骨细胞骨ECM区域的扩散存在意想不到的差异。我们将研究这种差异的原因，以及水扩散可能是骨细胞骨应力转导的假设。骨脱水产生的应力也将用于裂缝倾向差异的研究。通过结合所有提出的方法，我们希望提供对几何、纹理和骨细胞和骨细胞的水合状态之间相互作用的见解，因此，为骨骼如何应对机械应力，避免损伤提供一个新的视角。

The skeletons of vertebrates are made of bone, a nanocomposite material comprising mineralized collagen fibrils and water, as well as small amounts of non-collagenous proteins. Bone can withstand recurrent mechanical loads over a long period of time without failing due to the existence of a mechanism of self-repair of accumulated microdamage and by being able to adapt to changing loads (re/modeling). In widely studied mammalian bone, re/modeling is thought to be regulated and activated by bone cells (osteocytes) residing in lacunae within the bone matrix, communicating via canaliculi. It is therefore intriguing that the bone material of a large group of advanced teleosts is totally devoid of osteocytes (anosteocytic bone). In the previously funded period, we found several differences between the two bone types that might help understand how they may be able to re/model with and without osteocytes. We propose to further explore and understand the differences in the structure-mechanical function relations between medaka and zebrafish bone types, studied in-vivo in the first funding period. To this end, new computer-based experiments will explore data already collected in the first part of the study. We will quantify differences in composition, microstructure and water permeability, and their mechanical deformation response will be compared by 3D in situ loading experiments along with FE analysis. In the first funding period, we observed an unexpected difference between water diffusion across the ECM regions in anosteocytic and osteocytic bone, despite a very similar composition of mineralized collagen fibers. We will investigate the cause of this difference, as well as the hypothesis that water diffusion is the likely agent of stress transduction in osteocytic bone. Stress generated by bone dehydration will also be used to study crack propensity differences. By combining all the proposed methods, we expect to provide insights into the interplay between geometry, texture, and hydration state of the anosteocytic and osteocytic bone and, therefore, give a new perspective of how bones cope with mechanical stress, circumventing damage.