Abstract
Purpose: :
To determine the spatial and temporal pattern of cone packing during Marmoset monkey foveal development.
Methods: :
Cone density was determined in the central retina using wholemounts, DIC imaging and a new hexagonal quantification method. The retinas ranged in age between fetal day 125 and 6 years. The wholemounts were labeled with rod markers so that the foveal center was located reliably. Cone number was sampled in 25µm steps in the fovea and 100µm steps in the parafovea and expressed as cones x 10³/mm² (K).
Results: :
In wholemount counts, two weeks before birth cone density had a flat distribution of 20-30K across central retina. At postnatal (P) 1 day, density began to rise around but not in the foveal center and this continued to reach a parafoveal peak of 45-55K by P10 days. Between this age and P33 days, there was an inversion such that foveal cone density rose rapidly, reaching 283K by P3 months and 600K by P5.4 months. Peak density then diminished slightly so that at P6 months and older, foveal cone density peaked at 440K and then dropped to 60K at 300 µm from the foveal center. Counts done on sections over the same age range showed the same pattern of low density up to P10 days, a rapid rise to P30-90 days, followed by a small decrease into adulthood.This rise in cone density was accompanied 1) by a reduction in the amount of Muller cell cytoplasm which surrounded each foveal cone, 2) by the increased stacking of foveal cone nuclei into a mound 6-10 deep and 3) by the progressive reduction in distance from the foveal center to the first rods. Calculations find that maximum packing of the original 20K cone monolayer is limited by nuclear diameter, which restricts the final cone density to 300K can be achieved.
Conclusions: :
Marmosets reach adult peak cone density by P3-6 months. If this time point is expressed as % of life expectancy, Marmosets reach adult levels at 3.5%, while macaques take 6% and humans take 8%. This rapid postnatal maturation of the fovea may be required by the early weaning and arboreal environment of Marmosets. Rapid foveal development also makes Marmosets a good model for postnatal environmental and experimental manipulations.
Keywords: photoreceptors • retinal development • macula/fovea