Quantification of the neurons in the rod pathway of human retina

Abstract

Purpose : The rod pathway serves scotopic vision, and involves multiple rod photoreceptors converging onto rod bipolar cells, which in turn converge onto AII amacrine cells. AII amacrine cells are proposed as a target for optogenetic restoration of vision in retinal disease, therefore it is important to understand the distribution and density of AII cells and their connections in human retina.

Methods : Post mortem human eye tissue was obtained within 3.5 hours of death from the Lions NSW Eye Bank at the Sydney Eye Hospital with ethical approval by The University of Sydney Human Research Ethics committee. Retinal pieces of defined eccentricities were embedded in Agarose. Vertical sections were cut at 100 mm thickness along the horizontal meridian using a Vibratome. Sections were then processed for immunofluorescence to label rod photoreceptors, rod bipolar cells (protein kinase C alpha), and AII amacrine cells (calretinin). Sections were imaged with a Zeiss confocal microscope and z-stacks were taken along the entire section. The densities of rods, rod bipolar cells, and AII amacrine cells was determined along the horizontal meridian for eccentricities 0 to ~14 mm.

Results : Rods first appear at 300 mm from the center of the fovea. The rod peak density (at 3 mm) is ~200,000 cells/mm²; density then gradually declines to ~72,000 cells/mm² at 13 mm eccentricity. Rod bipolar cell density follows a similar pattern with a first appearance at about 300 mm eccentricity and peak at 3 mm eccentricity with ~22,000 cells/mm². Rod bipolar density falls to ~8500 cells/mm² at 8 mm. Calretinin positive (presumed AII) amacrine cells appear at about 200 mm, have peak density at 1 mm with ~12,000 cells/mm² and decline to ~6,000 cells/mm² at eccentricities of 6 mm and above. Therefore at 3 mm there is an average convergence of 9:4:1 between rods, rod bipolar and AII amacrine cells. At 1 mm, where the AII density is highest, the convergence is 10:1:1.

Conclusions : We found convergence from rods to RBCs to AII amacrine cells throughout the retina. The spatial resolution of the rod pathway is limited by the peak density of AII amacrine cells. Our results indicate rod pathway resolution would peak outside the fovea at 1 mm (~4 degrees eccentricity).

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.