Conical Electron Tomography of a Chemical Synapse: Vesicles Docked to the Active Zone are Hemi-Fused

We have used thin sectioning and conical electron tomography to determine the three-dimensional structure of synaptic vesicles that were associated (docked) at release sites of the presynaptic membrane, called active-zones. Vesicles docked at the active zone occupied a strategic location: they formed regions of contact with the plasma membrane on one side and with that of one or more vesicles located deeper within the presynaptic terminal on the other side. The region of contact with the active zone measured ∼15 nm in diameter (∼2% of the vesicle’s surface) and contained a smaller ∼6 nm region where the proximal leaflets merged (hemi-fused). Hemi-fusion was only observed on the side of vesicles in contact with the active zone; at the side of contact between neighboring vesicles, the membranes were not hemi-fused. Approximately three-fourths of the docked vesicles contained hemi-fused regions. Vesicles fully fused to the active zone (exhibiting pores that appeared as interruptions of a single membrane) were less frequently observed (∼1 of 10 hemi-fused vesicles). In conclusion, our observations in cortical synapses strengthen the hypothesis that hemi-fusion is a stable intermediary that precedes full fusion and release.