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Ultrastructure of the circumoral nerve ring and the radial nerve cords in holothurians (Echinodermata)

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Abstract

The circumoral nerve ring and the radial nerve cords (RNCs) of Eupentacta fraudatrix and Pseudocnus lubricus (Holothuroidea) were examined as an example of holothurian nervous tissue. The RNC is composed of outer ectoneural and inner hyponeural layers, which are interconnected with one another via short neural bridges. The circumoral nerve ring is purely ectoneural. Both ectoneural and hyponeural components are epithelial tubes with a thick neuroepithelium at one side. A thin ciliated non-neuronal epithelium complements the neuroepithelium to form a tube, thereby enclosing the epineural and hyponeural canals. The whole of the ectoneural and hyponeural subsystems is separated from the surrounding tissue by a continuous basal lamina. The nerve ring and the ectoneural and hyponeural parts of the radial nerves are all neuroepithelia composed of supporting cells and neurons. Supporting cells are interpreted as being glial cells. Based on ultrastructural characters, three types of neurons can be distinguished: (1) putative primary sensory neurons, whose cilium protrudes into the epineural or hyponeural canal; (2) non-ciliated neurons with swollen rough endoplasmic reticulum cisternae; (3) monociliated neurons that are embedded in the trunk of nerve fibers. Different types of synapses occur in the neuropile area. They meet all morphological criteria of classical chemical synapses. Vacuolated cells occur in the neuroepithelium of E. fraudatrix, but are absent in P. lubricus; their function is unknown. The cells of the non-neuronal epithelia that overlie the ectoneural and hyponeural canals are hypothesized to belong to the same cell type as the supporting cells of the neuroepithelium.

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Acknowledgments

We are grateful to Philip Lambert of the Royal British Columbia Museum Victoria, Canada, for providing us with fixed specimens of juvenile P. lubricus and to Beate Aschauer (LMU, Munich) for her inestimable technical assistance. We are especially indebted to Prof. I.C. Wilkie (Glasgow Caledonian University, Scotland) for his critical reading of the manuscript. The work was supported by the grant of the Russian Foundation for Basic Research (project no. 05-04-48715) to I.Yu. Dolmatov and by the DAAD scholarship to V.S. Mashanov.

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Authors and Affiliations

  1. Institute of Marine Biology FEB RAS, Palchevsky Str. 17, 690041, Vladivostok, Russia

    Vladimir S. Mashanov, Olga R. Zueva & Igor Yu. Dolmatov

  2. Ludwig-Maximilians-Universität München, Pettenkoferstrasse 11, 80336, München, Germany

    Thomas Heinzeller

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  1. Vladimir S. Mashanov
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  2. Olga R. Zueva
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  3. Thomas Heinzeller
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  4. Igor Yu. Dolmatov
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Correspondence to Vladimir S. Mashanov.

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Mashanov, V.S., Zueva, O.R., Heinzeller, T. et al. Ultrastructure of the circumoral nerve ring and the radial nerve cords in holothurians (Echinodermata). Zoomorphology 125, 27–38 (2006). https://doi.org/10.1007/s00435-005-0010-9

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  • DOI: https://doi.org/10.1007/s00435-005-0010-9

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