Synaptic vesicle populations in saccular hair cells reconstructed by electron tomography

We used electron tomography to map the three-dimensional architecture of th e ribbon-class afferent synapses in frog saccular hair cells. The synaptic body (SB) at each synapse was nearly spherical (468 +/- 65 nm diameter; mea n +/- SD) and was covered by a monolayer of synaptic vesicles (34.3 nm diam eter; 8.8% coefficient of variation), many of them tethered to it by simila r to 20-nm-long filaments, at an average density of 55% of close-packed (37 6 +/- 133 vesicles per SE). These vesicles could support similar to 900 mse c of exocytosis at the reported maximal rate, which the cells can sustain f or at least 2 sec, suggesting that replenishment of vesicles on the SE is n ot rate limiting. Consistent with this interpretation, prolonged K+ depolar ization did not deplete vesicles on the SE. The monolayer of SB-associated vesicles remained after cell lysis in the presence of 4 mM Ca2+, indicating that the association is tight and Ca2+ resistant. The space between the SE and the plasma membrane contained numerous vesicles, many of which (simila r to 32 per synapse) were in contact with the plasma membrane, This number of docked Vesicles could support maximal exocytosis for at most similar to 70 msec. Additional docked vesicles were seen within a few hundred nanomete rs of the synapse and occasionally at greater distances. The presence of om ega profiles on the plasma membrane around active zones, in the same locati ons as coated pits and coated vesicles labeled with an extracellular marker , suggests that local membrane recycling may contribute to the three- to 14 -fold greater abundance of vesicles in the cytoplasm (not associated with t he SE) near synapses than in nonsynaptic regions.