Department of Anatomy, School of Medical Sciences, University of Fukui, Fukui, Japan.
Life Science Innovation Center, University of Fukui, Fukui, Japan.
J Morphol. 2023 Mar;284(3):e21559. doi: 10.1002/jmor.21559.
Vertebrate paired appendages are one of the most important evolutionary novelties in vertebrates. During embryogenesis, the skeletal elements of paired appendages differentiate from the somatic mesoderm, which is a layer of lateral plate mesoderm. However, the presence of the somatic mesoderm in the common ancestor of vertebrates has been controversial. To address this problem, it is necessary but insufficient to understand the developmental process of lateral plate mesoderm formation in lamprey (jawless vertebrates) embryos. Here, I show the presence of the somatic mesoderm in lamprey (Lethenteron camtschaticum) embryos using plastic sectioning and transmission electron microscopy analysis. During the early pharyngeal stages, the somatic mesoderm transforms from the lateral plate mesoderm in the trunk region. Soon after, when the cardiac structures were morphologically distinct, the somatic mesoderm was recognized through the cardiac to more caudal regions. These findings indicated that the somatic mesoderm evolved before the emergence of paired appendages. I also discuss the developmental changes in the body wall organization in the common ancestor of vertebrates, which is likely related to the evolution of the paired appendages.
脊椎动物的成对附肢是脊椎动物最重要的进化创新之一。在胚胎发生过程中，成对附肢的骨骼元素从体节中胚层分化而来，体节中胚层是侧板中胚层的一层。然而，脊椎动物共同祖先中体节中胚层的存在一直存在争议。为了解决这个问题，有必要但还不够了解硬骨鱼（无颌脊椎动物）胚胎侧板中胚层形成的发育过程。在这里，我使用塑料切片和透射电子显微镜分析显示了硬骨鱼（Lethenteron camtschaticum）胚胎中体节中胚层的存在。在早期咽阶段，体节中胚层从躯干部的侧板中胚层转化而来。不久之后，当心脏结构在形态上明显不同时，通过心脏到更尾部的区域识别出体节中胚层。这些发现表明，体节中胚层在成对附肢出现之前就已经进化了。我还讨论了脊椎动物共同祖先的体壁组织的发育变化，这可能与成对附肢的进化有关。
