Ultrastructural and developmental features of the tessellated endoskeleton of elasmobranchs (sharks and rays)
Ultrastructural and developmental features of the tessellated endoskeleton of elasmobranchs (sharks and rays)
复制标题
软骨鱼类（鲨鱼和鳐鱼）棋盘状内骨骼的超微结构和发育特征
作者：
Ronald Seidel;Kady Lyons;M. Blumer;P. Zaslansky;P. Fratzl;J. Weaver;M. Dean
板鳃类（鲨鱼和鳐）的内骨骼主要由未矿化的软骨组成，与其他脊椎动物的骨骼有根本的不同。板鳃亚纲骨骼的进一步区别在于表面的镶嵌矿化，一层微小的多边形矿化瓦片，称为镶嵌。这种“镶嵌”已经定义了超过4亿年的板鳃类动物群，但关于板鳃类骨骼发育和超微结构的有限数据（例如镶嵌体的形状和矿物质密度如何随年龄变化）限制了我们发展镶嵌软骨生长假说的能力。使用高分辨率，二维和三维材料和结构表征技术，我们研究了圆形黄貂鱼Urobatis halleri的棋盘格软骨的个体发育系列，使我们能够定义骨骼矿化的一系列不同阶段和以前未被识别的棋盘格解剖特征。我们发现，板鳃类钙化软骨的明显的瓦片状形态在美国早期就已形成。halleri发育，在组织营养型胚胎中首先形成镶嵌体，作为孤立的、矿化组织的球形胰岛。到了亚成体阶段，镶嵌块的大小增加，并相互接触。这些接触导致了更多几何形状（直边）的形成，并形成了我们在此首次描述的两个重要的曲面解剖学特征。第一，intertesseral关节，其中相邻的镶嵌块邻接没有明显的重叠或互锁，是远比以前实现的复杂，包括一个回旋的轴承表面周围的纤维附着区。第二种是镶嵌块轮辐，是层状的、高矿物质密度的特征，从每个镶嵌块的中心向外辐射，就像车轮上的轮辐一样，从每个镶嵌块的中心到它与相邻镶嵌块的关节，可能充当镶嵌块之间关节的结构增强。随着个体发育过程中镶嵌体尺寸的增加，辐条通过添加新的薄片而延长，当用扫描电子显微镜（SEM）以反向散射模式观察时，在老年人骨骼的较大镶嵌体中产生视觉上引人注目的矿化模式。背散射SEM还显示，较大镶嵌体中心的细胞陷窝通常填充有高矿物质密度材料，这表明当镶嵌体内细胞死亡时，细胞调节的矿化抑制被中断。我们描述的许多定义超微结构的细节涉及到局部变化的组织矿物质密度和支持以前提出的增生生长机制的镶嵌。高分辨率微型计算机断层扫描数据表明，我们描述的U。尽管方晶的形状和大小有很大的变化，但Halleri在所有主要板鳃类动物中很常见。我们讨论了这些功能如何发展的假设，并将其与其他脊椎动物骨骼组织类型及其生长机制进行比较。
The endoskeleton of elasmobranchs (sharks and rays) is comprised largely of unmineralized cartilage, differing fundamentally from the bony skeletons of other vertebrates. Elasmobranch skeletons are further distinguished by a tessellated surface mineralization, a layer of minute, polygonal, mineralized tiles called tesserae. This ‘tessellation’ has defined the elasmobranch group for more than 400 million years, yet the limited data on development and ultrastructure of elasmobranch skeletons (e.g. how tesserae change in shape and mineral density with age) have restricted our abilities to develop hypotheses for tessellated cartilage growth. Using high‐resolution, two‐dimensional and three‐dimensional materials and structural characterization techniques, we investigate an ontogenetic series of tessellated cartilage from round stingray Urobatis halleri, allowing us to define a series of distinct phases for skeletal mineralization and previously unrecognized features of tesseral anatomy. We show that the distinct tiled morphology of elasmobranch calcified cartilage is established early in U. halleri development, with tesserae forming first in histotroph embryos as isolated, globular islets of mineralized tissue. By the sub‐adult stage, tesserae have increased in size and grown into contact with one another. The intertesseral contact results in the formation of more geometric (straight‐edged) tesseral shapes and the development of two important features of tesseral anatomy, which we describe here for the first time. The first, the intertesseral joint, where neighboring tesserae abut without appreciable overlapping or interlocking, is far more complex than previously realized, comprised of a convoluted bearing surface surrounded by areas of fibrous attachment. The second, tesseral spokes, are lamellated, high‐mineral density features radiating outward, like spokes on a wheel, from the center of each tessera to its joints with its neighbors, likely acting as structural reinforcements of the articulations between tesserae. As tesserae increase in size during ontogeny, spokes are lengthened via the addition of new lamellae, resulting in a visually striking mineralization pattern in the larger tesserae of older adult skeletons when viewed with scanning electron microscopy (SEM) in backscatter mode. Backscatter SEM also revealed that the cell lacunae in the center of larger tesserae are often filled with high mineral density material, suggesting that when intratesseral cells die, cell‐regulated inhibition of mineralization is interrupted. Many of the defining ultrastructural details we describe relate to local variation in tissue mineral density and support previously proposed accretive growth mechanisms for tesserae. High‐resolution micro‐computed tomography data indicate that some tesseral anatomical features we describe for U. halleri are common among species of all major elasmobranch groups despite large variation in tesseral shape and size. We discuss hypotheses about how these features develop, and compare them with other vertebrate skeletal tissue types and their growth mechanisms.