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Optic nerves in plethodontid salamanders (amphibia, urodela): neuroglia, fiber spectrum and myelination

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Summary

In five species of lungless salamanders, family Plethodontidae, which all show highly developed visual abilities, the ultrastructure of the optic nerve was investigated and the total number of retinal ganglion cell axons, the percentage of myelinated axons, and the volume densities of glia and axons were determined. More than 80% of all axons were smaller than 0.4 μm and only 2–3% were larger than 0.8 μm. In individual nerves the degree of myelination varied between 1 and 9% which is in the range reported for other amphibian species. The miniaturized and highly paedomorphic species Batrachoseps attenuatus was an exception because only very few or even no myelinated axons were present in the nerve, which is unique among gnathostome vertebrates. The five investigated species had total numbers of axons ranging from 26000 in Batrachoseps attenuatus to about 50000 in Plethodon jordani. These numbers are the lowest found among vertebrates with an elaborated visual system. The amount of glial material in the optic nerve varied between 25 and 50%, with larger nerves possessing more glia than smaller ones. Ultrastructural analysis revealed that the optic nerve of each species contained both astrocytes and oligodendrocytes, although often in immature form. In Batrachoseps attenuatus the glia showed features of both astrocytes and oligodendrocytes which reflect an undifferentiated state.

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  1. Rüdiger Linke

    Present address: University of Würzburg, Institute of Physiology, Röntgenring 9, D-8700, Würzburg, Federal Republic of Germany

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  1. Brain Research Institute University of Bremen, FB 2, P.O. Box 340440, D-2800, Bremen 33, Federal Republic of Germany

    Rüdiger Linke & Gerhard Roth

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  1. Rüdiger Linke
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Linke, R., Roth, G. Optic nerves in plethodontid salamanders (amphibia, urodela): neuroglia, fiber spectrum and myelination. Anat Embryol 181, 37–48 (1990). https://doi.org/10.1007/BF00189726

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