Summary
Electronmicroscopical examination of the dorsal giant fibers in the earthworms Lumbricus terrestris (L.t.) and Eisenia foetida (E.f.) reveals that their segmental septa are structures of so far unknown complexity. In both species the extracellular cleft between the two axon membranes of the septum amounts to only 65 Å (E.f.) and 75 Å (L.t.) respectively and is therefore regarded as “gap junction”. The following other structural differentiations of the septum were observed: “septate junctions” (E.f. and L.t.), “intermediate junetions” (E.f. and L.t.), densities apposed to both sides of the septum and not surrounded by a membrane (E.f.), and densities resembling “dense projections” on one side of the septum only (L.t.). In addition the septa of L.t. show vesicles on both sides which are bounded by a unit membrane and resemble the vesicles of chemically transmitting synapses in size (φ ca. 575 Å), location, accumulation, and electronoptical habitus. The significance of the findings in regard to electrotonic transmission and the possible existence of a mixed synapse is discussed.
Zusammenfassung
Die erneute elektronenmikroskopische Untersuchung der dorsalen Riesenfasern der Lumbriciden Lumbricus terrestris (L.t.) und Eisenia foetida (E.f.) zeigt, daß es sich bei den segmentalen Septen um Strukturen bisher unbekannter Komplexität handelt. Bei beiden Tierarten beträgt der Septalspalt nur 65 Å (E.f.) bzw. 75 Å (L.t.) und ist deshalb als „gap junction“ anzusprechen. Daneben fallen folgende Differenzierungen am Septum auf: „septate junctions” (L.t. und E.f.), „intermediate junctions“ (L.t. und E.f.), beidseitig am Septum gelegene und nicht von einer Membran umschlossene Membranappositionen (E.f.), sowie einseitig am Septum gelegene „dense projections“ (L.t.). Bei L.t. kommen außerdem auf beiden Seiten des Septums Vesikel vor, die von einer Elementarmembran umschlossen sind und nach Größe (φ ca. 575 Å), Lage, Haufenbildung und elektronenoptischem Habitus den Vesikeln chemisch übertragender Synapsen gleichen. Die Befunde werden hinsichtlich ihrer möglichen Bedeutung bei der elektrischen Übertragung und als Indizien für das Vorliegen einer gemischten Synapse diskutiert.
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Oesterle, D., Barth, F.G. Zur Feinstruktur einer elektrischen Synapse. Z.Zellforsch 136, 139–152 (1973). https://doi.org/10.1007/BF00307685
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DOI: https://doi.org/10.1007/BF00307685