Coated Microvesicles in Neurosecretory Terminals of Posterior Pituitary Glands shed their Coats to become Smooth “Synaptic” Vesicles

Abstract

WE have suggested that microvesicles (“synaptic vesicles”) within neurosecretory terminals of posterior pituitary glands are by-products of exocytosis formed by micropinocytosis-like activity serving to recapture excess membrane, of neurosecretory granules, incorporated in the cell surface. Our view arose from the discovery of exocytotic figures in electron micrographs of these terminals and the presence, in some such figures, of indications of membrane vesiculation: caveolae with microvesicles of similar size near them^1,2. The suggestion harmonized not only with the theoretical requirement for membrane recapture after exocytosis but with the familiar characteristics of the “synaptic vesicle” population: localization in terminal regions where hormone is released; aggregates against the cell membrane, the so called “synaptoid” formations; and increased number after stimulation. The scheme, however, was not entirely satisfactory since classical “synaptic vesicles”³ have smooth membranes while those apparently formed by membrane vesiculation were often seen to be coated, as were the adjacent caveolae². While this coating supported the interpretation of such figures as micropinocytosis-like activity (coating is believed to represent the mechanism inducing membrane vesiculation^4,5) it required us to postulate that coated vesicles are somehow transformed into the smooth “synaptic” type². We here present evidence for this transformation.