Background The apical organ is the most prominent neural structure in spiralian larvae. Although it has been thoroughly investigated in larvae of the class Pilidiophora in phylum Nemertea, studies on its structure in other nemertean larvae are limited. Most adult hoplonemertean worms have a frontal organ located in a position corresponding to that of the larval apical organ. The development and sensory function of the frontal organ has not been thoroughly characterized to date. Results The apical organ in the early rudiment stage of Quasitetrastemma stimpsoni larvae consists of an apical plate enclosed by ducts of frontal gland cells and eight apical neurons. The apical plate is abundantly innervated by neurites of apical neurons. During the late rudiment stage, the larval apical organ has external innervation from below by two subapical-plate neurons, along with 11 apical neurons, and its plate contains serotonin-like immunoreactive (5-HT-lir) cells. In the vermicular stage (free-swimming juvenile), the number of apical neurons is reduced, and their processes are resorbed. Serotonin is detected in the apical plate with no visible connection to apical neurons. In adult worms, the frontal organ has a small apical pit with openings for the frontal gland ducts. The organ consists of 8 to 10 densely packed 5-HT-lir cells that form the roundish pit. Conclusions Although the ultrastructure of the Q. stimpsoni larval apical organ closely resembles that of the apical organ of Polycladida larvae, the former differs in the presence of flask-shaped neurons typical of Spiralia. Significant differences in the structure of the apical organs of hoplonemertean and pilidia larvae point to two different paths in the evolutionary transformation of the ancestral apical organ. Ultrastructural and immunoreactive analyses of the apical organ of a hoplonemertean larva in the late rudiment and vermicular stages and the frontal organ of the adult worms identified common morphological and functional features. Thus, we hypothesize that the larval apical organ is modified during morphogenesis to form the adult frontal organ, which fulfills a sensory function in the hoplonemertean worm. This unique developmental trait distinguishes the Hoplonemertea from other nemertean groups.

中文翻译：

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hoplonemertean Quasitetrastemma stimpsoni 成虫的额部感觉器官是否起源于幼虫的顶端器官？


背景 顶端器官是螺旋幼虫最突出的神经结构。尽管它已在纽虫门的毛毛纲幼虫中进行了彻底的研究，但对其在其他纽虫幼虫中的结构的研究仍然有限。大多数成体全肢虫有一个额器官，其位置与幼虫的顶端器官相对应。迄今为止，额叶器官的发育和感觉功能尚未得到彻底的表征。结果 Quasitetrastemma stimpsoni 幼虫发育早期的顶端器官由额腺细胞导管包围的顶板和 8 个顶端神经元组成。顶板由顶神经元的神经突充分支配。在发育后期，幼虫顶端器官由两个顶端下板神经元和 11 个顶端神经元从下方进行外部神经支配，并且其板含有血清素样免疫反应性 (5-HT-lir) 细胞。在蠕虫阶段（自由游动的幼体），顶端神经元的数量减少，并且它们的突起被吸收。在顶板中检测到血清素，与顶​​神经元没有明显的连接。在成虫中，额器官有一个小顶端凹坑，上面有额腺导管的开口。该器官由 8 至 10 个密集的 5-HT-lir 细胞组成，形成圆形凹坑。结论 尽管 Q. stimpsoni 幼虫顶端器官的超微结构与多枝幼虫顶端器官的超微结构非常相似，但前者的不同之处在于存在螺旋虫典型的烧瓶状神经元。跳毛虫和皮毛虫幼虫顶端器官结构的显着差异表明祖先顶端器官的进化转变有两条不同的路径。 对处于萌芽阶段和蠕虫阶段的全线虫幼虫的顶端器官和成虫的额器官进行超微结构和免疫反应分析，确定了共同的形态和功能特征。因此，我们假设幼虫顶端器官在形态发生过程中发生修饰，形成成虫额叶器官，该器官在跳线虫中履行感觉功能。这种独特的发育特征将Hoplonemertea 与其他纽虫类群区分开来。