Transverse distribution of macromolecules in the main body of the AZM at resting active zones.

<p><b>A,D)</b> Two regions 10 nm apart along the depth axis of the reconstructed volume of an active zone sectioned near its transverse plane. Each of the images was formed by the summation of three 1.2 nm thick serial virtual slices made through the volume in the same plane as the section. <b>B,E)</b> The same regions shown in <b>A</b> and <b>D</b> with the summed outlines of the AZM macromolecules in each of the virtual slices overlaid. <b>C,F)</b> Surface models, 10 nm thick, of the AZM macromolecules shown in <b>A</b> and <b>D</b> derived from segmentation of eight adjacent virtual slices and rotated to the transverse plane. <b>G,H,I)</b> Surface models generated as for those in <b>C</b> and <b>F</b>. Tissue used for each of the surface models was from a different frog. The tissue was chemically fixed except for that used for the surface model in <b>I,</b> which was fixed by rapid freezing. Ribs (yellow gold) extend from beams (brown gold) in the superficial layer of AZM, spars (red) extend from steps (gray) in the intermediate layer, and booms (purple) extend from masts (dark green) in the deep layer, while the ribs, spars and booms connect to synaptic vesicles (dark blue) docked on the presynaptic membrane (pale blue). Topmasts (light green) in <b>C,F,G</b> and <b>I</b> link masts to undocked vesicles near the active zone’s midline. The linkage of some topmasts to masts is incomplete in <b>C,F,</b> and <b>H</b> because discontinuous staining made segmentation uncertain or the topmast extended beyond the edge of the tissue section (arrowheads). Scale bars = 50 nm.