Figure 1.

Scanning block-face electron microscopy demonstrates an S-OFF midget circuit in macaque retina. A, Image of a single layer from the EM volume composed of a vertical section taken 400 μm from the foveal center along the foveal slope. At this central location, ganglion-cell bodies stack up to six cell bodies thick. HFL, Henle fiber layer; OPL, outer plexiform layer; INL, inner nuclear layer; GCL, ganglion cell layer. B, Profiles of volume-rendered processes in another vertical layer. In the OPL, OFF midget bipolar profiles are in yellow and blue cone (S ON) bipolar cells are in blue; at the top a pedicle identified as from an S cone is shown in light blue (arrowhead) alongside a few scattered rod pedicles (dark red). Below in the INL are OFF midget bipolar cells (yellow) and blue cone (S ON) bipolar cells (blue). In the GCL, some midget ganglion-cell bodies are shown in magenta and pink. C, Zoomed view of three pedicles in the OPL: the S cone in B (blue), flanked by two L/M cones (green and red). Blue cone (S ON) bipolar dendritic arbors (dark blue profile) form the invaginating central element and are encircled by the flat dendritic arbors (yellow profiles) of an OFF midget bipolar cell. D, Zoomed view of the IPL showing axons and axon terminals of OFF midget bipolar cells stratified near the outer border of the IPL (yellow profiles, arrow) and blue cone (S ON) bipolar cells stratified near the inner border of the IPL (blue profiles, arrows). A single ON midget bipolar terminal (orange profile; arrow) is also present. Midget ganglion-cell dendritic profiles in the GCL are shown in magenta. E, Complete reconstruction of four neighboring cone pedicles from one S cone and three L/M cones. The S-cone pedicle is smaller than the L/M cones and lacks the distinctive telodendria that connect adjacent L/M cones. F, Zoomed view of reconstructed S-cone pedicle in partial transparency, showing ribbon synapses (red) and the single dendritic arbor of an OFF midget bipolar cell in dense contact with the pedicle synaptic face. G, Zoomed out view of the entire reconstructed OFF midget bipolar circuit (yellow) linked to the S-cone pedicle (blue) shown in F. The axon terminal exclusively contacts the compact dendritic arbor of an OFF midget ganglion cell (orange profile). H, Zoomed view of the OFF midget bipolar axon terminal synaptic relation (yellow) with its associated OFF midget ganglion cell (orange) shown in partial transparency with synaptic ribbons (red). The ganglion cell for this particular private-line circuit was at the edge of the volume and could not be completely reconstructed.