Denisova Sofia A, Shchenkov Sergei V
Department of Invertebrate Zoology, Faculty of Biology, Saint Petersburg State University, Saint Petersburg, Russia.
J Morphol. 2020 Jul;281(7):765-777. doi: 10.1002/jmor.21137. Epub 2020 May 5.
The biology of free-living and parasitic Platyhelminthes is diverse. Taking into account the widespread prevalence of parasitic flatworms, Digenea is the least studied group regarding the fine structure of nervous system especially of the cercarial life stage. Here, we present a description of the fine structure of central nervous system (CNS) and two types of uniciliate sensory papillae of xiphidiocercaria Cercaria parvicaudata (Microphalloidea, Renicolidae). The present study documents that C. parvicaudata has a complex nervous system that includes a well-developed ganglion with a cortex of perikarya and glia-like sheaths, myelin-like structures within one of the dorsal nerve cords and four types of polarized synapses between neurites. Different types of neurons in the CNS could not be distinguished on ultrastructural level due to high similarity in their fine structure. Shared polarized synapses with high electron density of presynaptic components are numerous in the neuropile and nerve cords of this larva. Within the larval body, we detected specialized "support" processes that relate to different tissues. Some "support" processes are also closely related to the nervous system of C. parvicaudata, where they are considered as glia-like structures. In this case, the fine structure of glia-like "support" cells of C. parvicaudata differs from those described as glia-like cells in adult flatworms. We suggest a wide prevalence of glia-like cells among cercariae, as well as the fact that glia-like structures in digenean nervous systems can develop from various nonneuronal tissues.
自由生活和寄生的扁形动物的生物学特性多种多样。考虑到寄生扁虫的广泛流行，就神经系统的精细结构，尤其是尾蚴生活阶段而言，复殖吸虫是研究最少的类群。在此，我们描述了短小尾蚴（微口科，肾居科）中枢神经系统（CNS）的精细结构以及两种类型的单纤毛感觉乳头。本研究记录表明，短小尾蚴具有复杂的神经系统，包括一个发育良好的神经节，其具有核周体皮质和神经胶质样鞘，一条背神经索内有髓鞘样结构，以及神经突之间有四种类型的极化突触。由于中枢神经系统中不同类型神经元的精细结构高度相似，在超微结构水平上无法区分它们。在该幼虫的神经纤维网和神经索中，具有高电子密度突触前成分的共享极化突触数量众多。在幼虫体内，我们检测到了与不同组织相关的特殊“支持”突起。一些“支持”突起也与短小尾蚴的神经系统密切相关，在那里它们被视为神经胶质样结构。在这种情况下，短小尾蚴的神经胶质样“支持”细胞的精细结构与成年扁虫中描述为神经胶质样细胞的结构不同。我们认为神经胶质样细胞在尾蚴中广泛存在，以及复殖吸虫神经系统中的神经胶质样结构可以从各种非神经组织发育而来这一事实。
