Syntaxin 4 is highly restricted within both plexiform layers. A. Low magnification image of syntaxin 4 labeling. Labeling in the outer plexiform layer (OPL) is present in two sets of puncta. One set of small individual puncta in the distal OPL and a second set of puncta found in larger clusters in the proximal OPL. In the inner plexiform layer (IPL), syntaxin 4 puncta form a prominent, narrow stratum (brackets) at the border with the inner nuclear layer (INL) and in sparsely distributed individual puncta deeper in the IPL. Labeling for syntaxin 4 also is present in blood vessels (BV). Box indicates area shown in panel B. B: High magnification image showing syntaxin 4 puncta in the OPL. Small puncta appear as singlets or doublets in the distal OPL (arrows). In the proximal OPL, syntaxin 4 labeling is found in large clusters of puncta (arrowheads). C: Conventional fluorescence image showing a series of syntaxin 4 puncta ascending through the INL (arrowheads). The stratum of syntaxin-4 positive puncta at the INL/IPL border is also visible (brackets). Panels shown in A and B are from a projection of 46 optical sections of 10.7 μm total thickness. ONL, outer nuclear layer; GCL, ganglion cell layer. Scale bars = 20 μm for A; 10 μm for B and C.