, Volume 164, Issue 3, pp 357-369

Scanning electron microscope observations on the muscle innervation of Oikopleura dioica fol (appendicularia, tunicata) with notes on the arrangement of connective tissue fibres

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Critical point dried and fractured appendicularia of the species Oikopleura dioica have been examined in the scanning electron microscope. The dorsal nerve cord with ganglion cells and peripheral nerve fibres could easily be observed. Thick peripheral nerve fibres leave the nerve cord as bilateral pairs at constant intervals along the tail. Most of these fibres branch from the naked nerve cord, but some evidently originate in ganglion perikarya bulging out from the nerve cord itself. These paired peripheral nerves always have elaborate end-arborizations on the medial surface of the lateral muscle cells. They are accordingly interpreted as motor axons. Some thinner peripheral nerve fibres originate at irregular intervals from both the nerve cord and the ganglion cells. Due to the numerous extracellular fibrils that connect the bilateral layers of the epidermal fins and the muscle cells to each other, these thin nerve fibres can seldom be traced to their termination. A few ones can, however, be traced ventrally between the notochord and the muscle cells and seem to end in singular bulb-like expansions. Clusters of synaptic vesicles are present in transmission electron micrographs of such nerves, and they are accordingly believed to carry efferent impulses. The extracellular fibrils are arranged in a highly ordered pattern with thick bundles crossing the gap between the structures to be interconnected and with numerous radiating insertions on the surface of the tissues.

The author wish to express his gratitude to the Norwegian Research Council for Science and the Humanities for financial support (grant no. C. 21, 30-8), to Mr. R. Haakonsen and Mr. R. Jensen for the photographic work, to Mrs. Jorunn Spurkeland for typing the manuscript, to Dr. Q. Bone for his kind and constructive revision of the manuscript, and to the directors and staffs of Laboratoire Arago, Banuyls sur mer, France, Biologische Anstalt, Helgoland, and Biological Station, Espegrend, Norway for research facilities and supply of material.