Synaptic and Synaptoid Vesicles Constitute a Single Category of Inclusions. New Evidence From ZIO Impregnation

Abstract

Parallel observations on central synaptic and neurohaemal terminals of the same types of neurosecretory fibres in the polychaete annelid Nereis diversicolor reveal that their respective populations of inclusions exhibit identical, highly distinctive patterns of affinity for the zinc iodide-osmium tetroxide (ZIO) reagent. The method highlights the duality of possible secretory inclusions in nerve terminals. Many typical synaptic/synaptoid vesicles have ZIO-positive contents, but intermingle with unreactive vesicles. Both positively and negatively reacting vesicles contribute to the unusual dense clusters associated with sites of release of neurochemical mediators, characteristic of polychaete nervous systems. Fewer dense-cored synaptic/synaptoid vesicles have reactive cores. The larger ‘storage granules’ typically have unreactive contents, but dense deposits form within a small minority. A possible cytophysical, in contradistinction from a cytochemical, basis of affinity for ZIO is discussed. The results further support the postulated fundamental identity of synaptic and synaptoid vesicles.