Abstract
Purpose :
In macaque monkey blue cone bipolar cells are postsynaptic to short (S), avoiding contact with long (L) or middle (M) wavelength-sensitive cone types. Blue cone bipolar cells are presynaptic to the small bistratified ganglion cell, a pathway that transmits a distinctive S ON, L+M OFF cone-opponent signal. In human retina the blue-cone bipolar-small bistratified ganglion cell circuit has been identified (Kim et al., PNAS,120, 2023). Our goal was to use connectomic reconstruction to identify additional major pathways that arise from the blue cone bipolar cell, characterize the circuitry of these pathways, and predict their chromatic properties.
Methods :
Eyes were acquired from a human donor (white male, 52 yrs) at time of death and immersion fixed for electron microscopy (EM). Pieces of foveal and parafoveal retina were embedded for ultrathin sectioning in the vertical plane and mounted for serial block-face scanning EM, sectioned at 50 nm (~2000 layers) and scanned at 5 nm x-y resolution. Volumes were created, neurons reconstructed, and synapses identified using TrakEM2 software.
Results :
S cones and their ON-bipolar cell partners were identified in both foveal and parafoveal samples by their distinctive synaptic relationship (Kim et al., PNAS,120, 2023). In addition to the small bistratified cells, we found blue cone bipolar cells presynaptic to the large bistratified and the ON midget ganglion cell types. ON midget ganglion cells combined synaptic input from blue cone bipolar cells (20±11% of total input; n = 7), midget bipolar cells (77±14%) with input from L or M cones, and diffuse bipolar cell types (3±4%). Large bistratified ganglion cells received combined input from blue cone bipolar (20±10% of total input; n = 2), ON-midget (33±13%), OFF-midget (16±4%) and unidentified diffuse bipolar (31±0%) cell types.
Conclusions :
In macaque monkey the canonical S vs LM (“blue-yellow”) and L vs M (“red-green”) cone-opponent pathways arise in parallel from the small bistratified and midget circuits, creating orthogonal “cardinal axes” of early chromatic processing. The distinctive, combined excitatory inputs from presumed L vs M midget and S vs LM blue cone bipolar cells to the dendritic tree of the large bistratified cell as well as ON midget ganglion cells predicts that, in human foveal retina, cone opponent signals may be more diverse than predicted from the cardinal axes.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.