Department of Zoology, Institute of Biology, University of Kassel, Kassel, Germany.
J Morphol. 2021 Sep;282(9):1298-1312. doi: 10.1002/jmor.21386. Epub 2021 Jun 23.
The tardigrade brain has been the topic of several neuroanatomical studies, as it is key to understanding the evolution of the central nervous systems in Panarthropoda (Tardigrada + Onychophora + Arthropoda). The gross morphology of the brain seems to be well conserved across tardigrades despite often disparate morphologies of their heads and cephalic sensory structures. As such, the general shape of the brain and its major connections to the rest of the central nervous system have been mapped out already by early tardigradologists. Despite subsequent investigations primarily based on transmission electron microscopy or immunohistochemistry, characterization of the different regions of the tardigrade brain has progressed relatively slowly and open questions remain. In an attempt to improve our understanding of different brain regions, we reinvestigated the central nervous system of the heterotardigrade Echiniscus testudo using anti-synapsin and anti-acetylated α-tubulin immunohistochemistry in order to visualize the number and position of tracts, commissures, and neuropils. Our data revealed five major synapsin-immunoreactive domains along the body: a large unitary, horseshoe-shaped neuropil in the head and four neuropils in the trunk ganglia, supporting the hypothesis that the dorsal brain is serially homologous with the ventral trunk ganglia. At the same time, the pattern of anti-synapsin and anti-tubulin immunoreactivity differs between the ganglia, adding to the existing evidence that each of the four trunk ganglia is unique in its morphology. Anti-tubulin labeling further revealed two commissures within the central brain neuropil, one of which is forked, and additional sets of extracerebral cephalic commissures associated with the stomodeal nervous system and the ventral cell cluster. Furthermore, our results showing the innervation of each of the cephalic sensilla in E. testudo support the homology of subsets of these structures with the sensory fields of eutardigrades.
缓步动物的大脑一直是几个神经解剖学研究的主题，因为它是理解泛节肢动物（缓步动物+有爪动物+节肢动物）中枢神经系统进化的关键。尽管它们的头部和头胸部感觉结构形态常常不同，但大脑的大体形态似乎在缓步动物中得到了很好的保守。因此，早期的缓步动物学家已经勾勒出了大脑的大致形状及其与中枢神经系统其余部分的主要连接。尽管随后的研究主要基于透射电子显微镜或免疫组织化学，但缓步动物大脑不同区域的特征描述进展相对缓慢，仍存在一些悬而未决的问题。为了尝试增进我们对不同脑区的理解，我们使用抗突触素和抗乙酰化α-微管蛋白免疫组织化学，重新研究了异缓步动物 Echiniscus testudo 的中枢神经系统，以可视化束、连合和神经节的数量和位置。我们的数据显示，沿身体有五个主要的突触素免疫反应区：头部的一个大的、马蹄形的神经节和体神经节中的四个神经节，支持了背侧脑与腹侧体神经节是系列同源的假说。同时，抗突触素和抗微管蛋白免疫反应的模式在神经节之间存在差异，这增加了现有的证据，即四个体神经节在形态上都是独特的。抗微管蛋白标记进一步显示中央脑神经节内有两个连合，其中一个是叉状的，还有额外的一组与口道神经系统和腹细胞群相关的颅神经连合。此外，我们的研究结果显示缓步动物 E. testudo 中每个头感器的神经支配，支持了这些结构的子集与真缓步动物感觉区的同源性。
