ULTRASTRUCTURAL EVIDENCE FOR A SEPARATE, SMALL SYNAPTIC VESICLE (SSV)PATHWAY IN LIGATED BOVINE SPLENIC NERVES, INCUBATED IN-VITRO

In sympathetic nerves the tubules of the axonal reticulum make up the immature elements of the neurosecretory apparatus. The formation of th e mature large dense granules occurs via a less dense tubular intermed iate, representing the maturing part. At a terminal small synaptophysi n-positive vesicles are found intermingled with the dense granules. Th e biogenesis of these clear, small synaptic vesicles and their relatio nship with dense granules remains to be determined. In search for the small synaptic vesicles we undertook a careful ultrastructural examina tion of the axons proximal to a ligation in bovine splenic nerve incub ated in vitro for 3 h. The distended axons were crowded with tubules, granule-tubular elements and dense granules. Occasionally homogeneous clusters of small, uniform vesicles were detected. They were shown to be positive for synaptophysin and were negative for dopamine-beta-hydr oxylase, a marker for the granular pathway. The clusters of small vesi cles could be found in close spatial relationship with the maturing an d mature elements of granular secretion. Our findings argue for the pr esence of two separate neurosecretory pathways in sympathetic nerves a nd favour the idea that both small synaptic vesicles and dense granule s are a differentiation product of the axonal reticulum. This confirma tion can explain the biogenesis of small synaptic vesicles and dense g ranules both in the cell body and at the nerve terminal.