Department of Biomaterials, Max Planck Institute of Colloids and Interfaces, Am Mühlenberg 1, 14476 Potsdam, Germany.
Visual Data Analysis Department, Zuse Institute Berlin, Takustrasse 7, 14195 Berlin, Germany.
Bone. 2020 May;134:115264. doi: 10.1016/j.bone.2020.115264. Epub 2020 Feb 11.
In most vertebrates the embryonic cartilaginous skeleton is replaced by bone during development. During this process, cartilage cells (chondrocytes) mineralize the extracellular matrix and undergo apoptosis, giving way to bone cells (osteocytes). In contrast, sharks and rays (elasmobranchs) have cartilaginous skeletons throughout life, where only the surface mineralizes, forming a layer of tiles (tesserae). Elasmobranch chondrocytes, unlike those of other vertebrates, survive cartilage mineralization and are maintained alive in spaces (lacunae) within tesserae. However, the functions of the chondrocytes in the mineralized tissue remain unknown. Applying a custom analysis workflow to high-resolution synchrotron microCT scans of tesserae, we characterize the morphologies and arrangements of stingray chondrocyte lacunae, using lacunar morphology as a proxy for chondrocyte morphology. We show that the cell density is comparable in unmineralized and mineralized tissue and that cells maintain similar volume even when they have been incorporated into tesserae. Our findings support previous hypotheses that elasmobranch chondrocytes, unlike those of other taxa, do not proliferate, hypertrophy or undergo apoptosis during mineralization. Tessera lacunae show zonal variation in their shapes, being flatter further from and more spherical closer to the unmineralized cartilage matrix, and larger in the center of tesserae. The lacunae show pronounced organization into parallel layers and strong orientation toward neighboring tesserae. Tesserae also exhibit local variation in lacunar density, with the density considerably higher near pores passing through the tesseral layer, suggesting pores and cells interact, and that pores may contain a nutrient source. We propose that the different lacunar types reflect the stages of the tesserae formation process, while also representing local variation in tissue architecture and cell function. Lacunae are linked by small passages (canaliculi) in the matrix to form elongated series at the tesseral periphery and tight clusters in the center of tesserae, creating a rich connectivity among cells. The network arrangement and the shape variation of chondrocytes in tesserae indicate that cells may interact within and between tesserae and manage mineralization differently from chondrocytes in other vertebrates, perhaps performing analogous roles to osteocytes in bone.
在大多数脊椎动物中，胚胎的软骨骨骼在发育过程中被骨骼取代。在此过程中，软骨细胞（chondrocytes）矿化细胞外基质并发生细胞凋亡，为成骨细胞（osteocytes）让路。相比之下，鲨鱼和鳐鱼（软骨鱼纲）终生拥有软骨骨骼，只有表面矿化，形成一层瓦片（tesserae）。与其他脊椎动物的软骨细胞不同，软骨鱼纲的软骨细胞在软骨矿化过程中存活下来，并在瓦片内的腔隙（lacunae）中保持存活。然而，矿化组织中的软骨细胞的功能仍然未知。通过对瓦片的高分辨率同步加速器微 CT 扫描应用定制的分析工作流程，我们以腔隙形态作为软骨细胞形态的替代指标，对黄貂鱼软骨细胞腔隙的形态和排列进行了表征。我们发现，未矿化组织和矿化组织中的细胞密度相当，并且即使细胞已经被整合到瓦片中，细胞也保持相似的体积。我们的研究结果支持先前的假设，即与其他分类群的软骨细胞不同，软骨鱼纲的软骨细胞在矿化过程中不会增殖、肥大或发生细胞凋亡。瓦片腔隙的形状在其形状上呈现出带状变化，远离未矿化软骨基质的腔隙更平，更接近未矿化软骨基质的腔隙更圆，在瓦片的中心更大。腔隙呈现出明显的平行层组织化和强烈的朝向相邻瓦片的取向。瓦片还表现出局部的腔隙密度变化，在穿过瓦片层的孔附近密度明显较高，表明孔和细胞相互作用，并且孔可能包含营养源。我们提出，不同的腔隙类型反映了瓦片形成过程的不同阶段，同时也反映了组织结构和细胞功能的局部变化。腔隙通过基质中的小通道（canaliculi）连接，在瓦片的周边形成拉长的系列，在瓦片的中心形成紧密的簇，在细胞之间形成丰富的连接。软骨细胞在瓦片中的网络排列和形状变化表明，细胞可能在瓦片内和瓦片之间相互作用，并以不同于其他脊椎动物软骨细胞的方式管理矿化，也许在功能上类似于骨中的成骨细胞。
