Mashanov Vladimir S, Zueva Olga R, Heinzeller Thomas, Aschauer Beate, Dolmatov Igor Yu
Institute of Marine Biology FEB RAS, Palchevsky 17, 690041 Vladivostok, Russia.
Evol Dev. 2007 May-Jun;9(3):244-56. doi: 10.1111/j.1525-142X.2007.00157.x.
In adult echinoderms, the nervous system includes the ectoneural and hyponeural subsystems. The former has been believed to develop from the ectoderm, whereas the latter is considered to be mesodermal in origin. However, this view has not been substantially supported by embryological examinations. Our study deals with the developmental origin of the nervous system in the direct-developing sea cucumber Eupentacta fraudatrix. The rudiment of the adult nervous system develops from ectodermally derived cells, which ingress into the primary body cavity from the floor of the vestibule. At the earliest stages, only the rudiment of the ectoneural nerve ring is laid down. The radial nerve cords and tentacular nerves grow out from this subcutaneous rudiment. The ectoneural cords do not develop simultaneously but make their appearance in the following order: unpaired mid-ventral cord, paired dorsal lateral cords, and ventral lateral cords. These transitional developmental stages probably recapitulate the evolution of the echinoderm body plan. The holothurian hyponeural subsystem, as other regions of the metazoan nervous system, has an ectodermal origin. It originally appears as a narrow band of tissue, which bulges out of the basal region of the ectoneural neuroepithelium. Our data combined with those of other workers strongly suggest that the adult nervous tissue in echinoderms develops separately from the superficial larval system of ciliary nerves. Therefore, our data are neither in strict accordance with Garstang's hypothesis nor do they allow to refuse it. Nevertheless, in addition to ciliary bands, other areas of neurogenetic epidermis must be taken into account.
在成年棘皮动物中，神经系统包括外神经和下神经子系统。前者一直被认为是由外胚层发育而来，而后者被认为起源于中胚层。然而，这一观点并未得到胚胎学检查的充分支持。我们的研究涉及直接发育的海参伪刺参神经系统的发育起源。成年神经系统的原基由外胚层来源的细胞发育而来，这些细胞从前庭底部进入初级体腔。在最早阶段，只形成了外神经环的原基。桡神经索和触手神经从这个皮下原基长出。外神经索并非同时发育，而是按以下顺序出现：不成对的腹中线索、成对的背外侧索和腹外侧索。这些过渡性发育阶段可能概括了棘皮动物身体结构的进化过程。海参的下神经子系统与后生动物神经系统的其他区域一样，起源于外胚层。它最初表现为一条狭窄的组织带，从外神经神经上皮的基部区域突出。我们的数据与其他研究者的数据相结合，有力地表明棘皮动物的成年神经组织与幼虫表面的纤毛神经系统是分开发育的。因此，我们的数据既不完全符合加斯唐的假说，也不允许拒绝该假说。然而，除了纤毛带之外，神经发生表皮的其他区域也必须予以考虑。
