Fine-structural and morphometrical studies on the segmental septa of the giant fibers in the earthworm, Eisenia foetida

Summary

The fine structure of junctional specializations on the segmental septa in the median and lateral giant fibers of the earthworm (Eisenia foetida) was examined. Eight morphologically different septal domains were identified; a gap junction, a junction with hemispherical hollow structures, a chemical synapse-like junction, intermediate type and punctum adherens type junctions, a junction with adjoining vesicular layers, an area flanked by flattened membranous sacs, a non specialized area, and an area consisting of widely separated membranes with interposed glial processes. The area of each domain was measured by a cytometrical technique using quasi-serial sections. The gap junction occupied 3% and 0.2% of the septal area of the median and lateral giant fibers, respectively. Junctions with hemisperical hollow structures, characteristic of the earthworm giant fibers, occupied 2.5% and 13.9% of the median and lateral giant fibers, respectively. Various membrane domains except the gap junction, the junction with hemispherical hollow structures, and the chemical synapse-like junction accounted for similar proportional areas in both median and lateral giant fibers.

The functional implications of these junctional specializations, especially the gap junction, are discussed.