Deep time evolution of bone cells and the implications for bone metabolism in vertebrate history

大多数脊椎动物具有细胞骨，其中骨细胞（骨细胞）被封闭在骨基质内的洞穴样腔隙中，并且通过在高度复杂的小管（小管）网络中延伸的树突状细胞过程彼此连接。最近的研究表明，这种所谓的腔隙-小管网络在骨代谢中起着关键作用，如根据应变变化协调骨表面的建模和重塑，检测骨中的微裂纹并诱导其修复，以及在矿物质体内平衡中发挥重要作用。此外，已经表明，每单位骨体积的骨细胞数量与骨生长速率成比例，并且它们的密度和形状根据在其形成期间作用于特定骨的局部应变环境而变化。鉴于现存脊椎动物骨代谢的多种功能，推断骨细胞分布和形态的模式允许对化石脊椎动物生物学的新见解。然而，尽管骨化石中的骨陷窝小管网络通常保存完好，但骨细胞的进化历史却知之甚少。在拟议的项目中，将在整个脊椎动物历史上定量和定性地研究腔隙管网络，以获得对骨细胞进化的深入时间观点，从第一个具有细胞骨的脊椎动物，无颌骨介类动物，到基底有颌脊椎动物（盾皮动物和棘皮动物），硬骨鱼类鱼类，茎四足动物，两栖动物和脊椎动物。化石骨将通过光学显微镜、扫描电子显微镜（SEM）和同步加速器X射线相衬断层扫描进行研究。重点在于骨陷窝的密度和排列、骨陷窝的形状和大小、骨小管的形态以及骨陷窝小管系统所嵌入的骨基质的类型。从化石分类群获得的结果将与来自已知代谢率的现存脊椎动物的骨骼学薄切片以及现代骨细胞生物学的数据进行比较。这将允许推断代谢率，骨骼建模和重塑，以及作用于长期灭绝动物骨骼的局部应变机制，并将在身体大小，个体发育年龄和栖息地（例如，水生对陆生）。将使用不同的度量标准对脊椎动物不同谱系中的腔隙管特征的系统发育信号进行测试。这个项目的结果将是重要的，我们了解整个脊椎动物的历史骨代谢的发展，特别是对主要的进化转变的背景。

Most vertebrates have cellular bone in which bone cells (osteocytes) are enclosed in cave-like lacunae within the bone matrix and are connected to each other via dendritic cell processes that extend in a highly complex network of tubules (canaliculi). Recent studies have shown that this so called lacunocanalicular network plays a pivotal role in bone metabolism, like orchestrating modeling and reshaping of bone surfaces according to strain changes, detecting micro-cracks in bone and inducing their repair, and playing a significant role in mineral homoeostasis. Furthermore, it has been shown that the number of osteocytes per unit of bone volume is proportional to bone growth rate, and their density and shape varies depending on the local strain environment that acts on a certain bone during its formation. Given the multitude of functions in bone metabolism of extant vertebrates, it is inferred that patterns of osteocyte distribution and morphology allow new insights into the biology of fossil vertebrates. However, despite the fact that the lacunocanalicular network is usually well preserved in fossil bone, the evolutionary history of osteocytes is poorly known. In the proposed project, the lacunocanalicular network will be investigated quantitatively and qualitatively throughout vertebrate history to gain a deep time perspective on the evolution of osteocytes, spanning the range from the first vertebrates with cellular bone, the jawless osteostracans, to basal jawed vertebrates (placoderms and acanthodians), osteichthyan fishes, stem-tetrapods, amphibians, and amniotes. Fossil bone will be investigated by light microscopy, scanning electron microscopy (SEM), and synchrotron X-ray phase contrast tomography. The focus will lie on lacunar density and arrangement, lacunar shape and size, morphology of canaliculi, as well as the type of bone matrix in which the lacunocanalicular system is embedded. The results obtained from fossil taxa will be compared with osteological thin sections derived from extant vertebrates for which the metabolic rate is known, and with data from modern bone cell biology. This will allow to draw inferences about metabolic rate, bone modeling and remodeling, and local strain regimes that acted on the skeleton in long-extinct animals and will be regarded in the context of body size, ontogenetic age, and habitat (e.g., aquatic versus terrestrial). A test of phylogenetic signal of lacunocanalicular characters in different lineages of vertebrates will be conducted using different metrics. The results of this project will be important for our understanding of the development of bone metabolism throughout vertebrate history and especially against the background of major evolutionary transitions.