[Quantitative morphologic examination of the vesicle population of mixed synapses--method and significance]

Quantitative electron microscopic investigations have been performed regarding the population of clear round vesicles within the axosomatic mixed contacts between secondary vestibular and oculomotor neurons of the trout. The measurements are based on a subdivision of the axon terminal area into single fields which topographically belong to different synaptic types (active zone, gap junction) of this mixed contact. Furthermore, areas between active zones and gap junctions ("intermediate zones") and areas along the lateral membranes of the axon terminal ("border zones") have been analyzed. The following vesicle parameters have been estimated stereologically: volume density (Vvv), surface density (Svv), numerical vesicle density (Nvv) mean vesicle volume (Vv) and mean vesicle surface (Sv). In this connection, aspects of the homogeneous distribution, of number and size of the organelles hae been investigated. The results obtained show a characteristic distribution in number and size of vesicles inthe presynaptic areas of the different membrane specializations of this mixed synapses. The results further indicate that two different pools of clear round vesicles exist in this mixed synapse, clear synaptic vesicles for chemical transmission and gap junction vesicles. In relation to the synaptic vesicles, a recycling mechanism is suggested. Gap-junction vesicles may function as Ca++ transport organelles to regulate gap junction permeability. A possible role in trophic tasks of the gap junction is also discussed.