Younossi-Hartenstein A, Jones M, Hartenstein V
Department of Molecular Cell and Developmental Biology University of California Los Angeles, Los Angeles, California 90095, USA.
J Comp Neurol. 2001 May 21;434(1):56-68. doi: 10.1002/cne.1164.
The nervous system of temnocephalid flatworms consists of the brain and three pairs of longitudinal connectives extending into the trunk and tail. The connectives are crosslinked by an invariant number of regularly spaced commissures. Branches of the connectives innervate the tentacles of the head and the sucker organ in the tail. A set of nerve rings encircling the pharynx and connected to the brain and connectives constitute the pharyngeal nervous system. The nervous system is formed during early embryogenesis when the embryo represents a multilayered mesenchymal mass of cells. Gastrulation and the formation of separate epithelial germ layers that characterize most other animal groups are absent. The brain arises as a bilaterally symmetric condensation of postmitotic cells in the deep layers of the anterior region of the embryonic mesenchyme. The pattern of axon outgrowth, visualized by labeling with anti-acetylated tubulin (acTub) antibody, shows marked differences from the pattern observed in other flatworm taxa in regard to the number of neurons that express the acTub epitope. Acetylated tubulin is only expressed in neurons that form long axon tracts. In other flatworm species, such as the typhloplanoid Mesostoma and the polyclad Imogine, which were investigated by us with the acTub antibody (Hartenstein and Ehlers [2000] Dev. Genes Evol. 210:399-415; Younossi-Hartenstein and Hartenstein [2000] Dev. Genes Evol. 210:383-398), only a small number of "pioneer neurons" become acTub positive during the embryonic period. By contrast, in temnocephalids, most, if not all, neurons express acTub and form long, large-diameter axons. Initially, the brain commissure, pharyngeal nerve ring, and the connectives are laid down. Commissural tracts and tentacle nerves branching off the connectives appear later. We speculate that the precocious differentiation of the nervous system may be related to the fact that temnocephalids move by muscle action, and possess a massive and complex muscular system when they hatch. In addition, they have muscular specializations such as the anterior tentacles and the posterior sucker that are used as soon as they hatch. By contrast, juveniles of Mesostoma and larvae of polyclads move predominantly by ciliary action that may not require a complex neural circuitry for coordination.
切头虫扁形虫的神经系统由脑和三对纵向神经索组成，这些神经索延伸至躯干和尾部。神经索通过数量恒定、间隔规律的连合相互交联。神经索的分支支配头部的触手和尾部的吸盘器官。一组环绕咽部并与脑和神经索相连的神经环构成了咽神经系统。神经系统在胚胎发育早期形成，此时胚胎是一个多层间充质细胞团。原肠胚形成以及形成大多数其他动物类群所特有的独立上皮胚层的过程并不存在。脑由胚胎间充质前部深层有丝分裂后的细胞双侧对称聚集形成。通过用抗乙酰化微管蛋白（acTub）抗体标记观察到的轴突生长模式，在表达acTub表位的神经元数量方面，与在其他扁形虫类群中观察到的模式有显著差异。乙酰化微管蛋白仅在形成长轴突束的神经元中表达。在其他扁形虫物种中，例如我们用acTub抗体研究过的盲游虫类的中口虫属（Mesostoma）和多肠目虫类的伊莫吉纳属（Imogine）（哈滕施泰因和埃勒斯[2000]《发育基因与进化》210:399 - 415；尤诺西 - 哈滕施泰因和哈滕施泰因[2000]《发育基因与进化》210:383 - 398），在胚胎期只有少数“先驱神经元”变为acTub阳性。相比之下，在切头虫扁形虫中，大多数（如果不是全部的话）神经元表达acTub并形成长的、大直径的轴突。最初，脑连合、咽神经环和神经索形成。连合束和从神经索分支出来的触手神经稍后出现。我们推测，神经系统的早熟分化可能与切头虫扁形虫通过肌肉运动这一事实有关，并且在孵化时拥有庞大而复杂的肌肉系统。此外，它们具有肌肉特化结构，如前部触手和后部吸盘，孵化后立即就会用到。相比之下，中口虫属的幼体和多肠目虫类的幼虫主要通过纤毛运动，这可能不需要复杂的神经回路来协调。
