Atake Oghenevwogaga J, Eames B Frank
Department of Anatomy, Physiology, and Pharmacology, University of Saskatchewan, Saskatoon, SK, Canada.
Front Genet. 2021 Dec 22;12:762042. doi: 10.3389/fgene.2021.762042. eCollection 2021.
The impregnation of biominerals into the extracellular matrix of living organisms, a process termed biomineralization, gives rise to diverse mineralized (or calcified) tissues in vertebrates. Preservation of mineralized tissues in the fossil record has provided insights into the evolutionary history of vertebrates and their skeletons. However, current understanding of the vertebrate skeleton and of the processes underlying its formation is biased towards biomedical models such as the tetrapods mouse and chick. Chondrichthyans (sharks, skates, rays, and chimaeras) and osteichthyans are the only vertebrate groups with extant (living) representatives that have a mineralized skeleton, but the basal phylogenetic position of chondrichthyans could potentially offer unique insights into skeletal evolution. For example, bone is a vertebrate novelty, but the internal supporting skeleton (endoskeleton) of extant chondrichthyans is commonly described as lacking bone. The molecular and developmental basis for this assertion is yet to be tested. Subperichondral tissues in the endoskeleton of some chondrichthyans display mineralization patterns and histological and molecular features of bone, thereby challenging the notion that extant chondrichthyans lack endoskeletal bone. Additionally, the chondrichthyan endoskeleton demonstrates some unique features and others that are potentially homologous with other vertebrates, including a polygonal mineralization pattern, a trabecular mineralization pattern, and an unconstricted perichordal sheath. Because of the basal phylogenetic position of chondrichthyans among all other extant vertebrates with a mineralized skeleton, developmental and molecular studies of chondrichthyans are critical to flesh out the evolution of vertebrate skeletal tissues, but only a handful of such studies have been carried out to date. This review discusses morphological and molecular features of chondrichthyan endoskeletal tissues and cell types, ultimately emphasizing how comparative embryology and transcriptomics can reveal homology of mineralized skeletal tissues (and their cell types) between chondrichthyans and other vertebrates.
生物矿物质渗入生物体的细胞外基质，这一过程被称为生物矿化，它在脊椎动物体内产生了多种矿化（或钙化）组织。矿化组织在化石记录中的保存为了解脊椎动物及其骨骼的进化历史提供了线索。然而，目前对于脊椎动物骨骼及其形成过程的理解偏向于生物医学模型，如四足动物小鼠和鸡。软骨鱼类（鲨鱼、鳐鱼、魟鱼和银鲛）和硬骨鱼类是仅有的具有现存（活着的）矿化骨骼代表的脊椎动物类群，但软骨鱼类的基础系统发育地位可能为骨骼进化提供独特的见解。例如，骨骼是脊椎动物的新特征，但现存软骨鱼类的内部支撑骨骼（内骨骼）通常被描述为缺乏骨骼。这一论断的分子和发育基础尚未得到验证。一些软骨鱼类内骨骼中的软骨下组织表现出骨骼的矿化模式、组织学和分子特征，从而挑战了现存软骨鱼类缺乏内骨骼骨骼的观点。此外，软骨鱼类的内骨骼表现出一些独特的特征，以及其他一些可能与其他脊椎动物同源的特征，包括多边形矿化模式、小梁矿化模式和无收缩的脊索鞘。由于软骨鱼类在所有其他具有矿化骨骼的现存脊椎动物中处于基础系统发育地位，因此对软骨鱼类的发育和分子研究对于阐明脊椎动物骨骼组织的进化至关重要，但迄今为止，此类研究仅有少数开展。本综述讨论了软骨鱼类内骨骼组织和细胞类型的形态学和分子特征，最终强调了比较胚胎学和转录组学如何能够揭示软骨鱼类与其他脊椎动物之间矿化骨骼组织（及其细胞类型）的同源性。
