Human Foveal Cone and RPE Cell Topographies and Their Correspondence With Foveal Shape

Abstract

Purpose: To characterize the association between foveal shape and cone and retinal pigment epithelium (RPE) cell topographies in healthy humans.

Methods: Multimodal adaptive scanning light ophthalmoscopy and optical coherence tomography (OCT) were used to acquire images of foveal cones, RPE cells, and retinal layers in eyes of 23 healthy participants with normal foveas. Distributions of cone and RPE cell densities were fitted with nonlinear mixed-effects models. A linear mixed-effects model was used to examine the relationship between cone and RPE inter-cell distances and foveal shape as obtained from the OCT scans of retinal thickness.

Results: The best-fit model to the cone densities was a power function with a nasal-temporal asymmetry. There was a significant linear relationship among cone and RPE cell spacing, foveal shape, and foveal cell topography. The model predictions of the central 10° show that the contributions of both the cones and RPE cells are necessary to account for foveal shape.

Conclusions: The results indicate that there is a strong relationship between cone and RPE cell spacing and the shape of the human adolescent and adult fovea. This finding adds to the existing evidence of the critical role that the RPE serves in fetal foveal development and through adolescence, possibly via the imposition of constraints on the number and distribution of foveal cones.

Figure 1.

Raw confocal and darkfield images showing cone and RPE cells from four eccentricities (−1 nasal, 0, 1, and 3 temporal degrees) for three representative participants (A), cropped to 200 × 200 µm. The geometry of the cone (B) and RPE (C) cell analysis in confocal and darkfield images, respectively, of the same region of retina. The centers of retinal cells were semiautomatically segmented over the entire image and Voronoi tessellations were generated for cones (red) and RPE cells (yellow). Green lines show ROIs within which summary statistics were computed. Regions outside the ROIs (low contrast) were not included in analysis. Bounded cells (those whose Voronoi vertices were entirely within the ROI) are highlighted with thicker lines. Regarding inclusion criteria for the number of cones per RPE cell analysis (D), cones (red) were only considered to be “inside” an RPE cell if their centers were within the RPE Voronoi cell. In this example, there were 19 cones (thick red lines) within the central RPE cell (yellow).

Figure 2.

Cone (A) and RPE (B) cell densities as a function of eccentricity (central ±6°) for all 23 participants (each represented by a different color) based on estimates over 50 × 50 µm and 200 × 200 µm ROIs, respectively. Cone (C) and RPE (D) ICDs (in µm) transformed to cone (E) and RPE (F) cell densities (cells/mm²) by Equation 1. Data are based on 201,188 cones and 11,954 RPE cells. Log cone (G) and RPE (H) cell density are shown for three representative participants. The black points are the ICDs (in µm) transformed to density in cells/mm². The solid lines are the asymmetric power function (Equation 2) fits for cone ICDs and asymmetric generalized exponential function (Equation 3) fits for RPE ICDs.

Figure 3.

(A) Number of cones per RPE cell as a function of eccentricity for the 12 participants whose central foveal cones were resolvable. (B) Number of cones per RPE cell within the central 0.5 × 0.5 deg² as a function of RPE cell area. Data are based on 34,173 cones and 2732 RPE cells. (C) Violin plot of cell area of cones (gray) and RPE (yellow) cells as a function of eccentricity. The black circle and solid line show the mean cell area ± SD. The width of the violin represents the associated probability of a given cell area, with wider sections corresponding to higher probabilities.

Figure 4.

Log cone ICDs (A) and log number of cones per RPE cell (B) as a function of retinal thickness for three typical participants. Each point in (A) is the ICD for a cone plotted against the retinal thickness at that cell's absolute eccentricity (i.e., nasal and temporal data have been folded together). (B) Number of cones that lie within the Voronoi cell of an RPE cell. Blue lines are linear regressions fitted to data with R² values shown for each participant. (C) Predictions of cone and RPE ICDs (Equation 4) of the foveal shape as a function of eccentricity (black points). The solid orange line is the foveal shape represented by the eccentricity-dependent retinal thickness change from OCT. (D) Horizontal spectral-domain OCT scan through the foveal center of the central ±5° with the segmented inner limiting membrane (ILM) and RPE-BrM layer overlaid (orange lines). Foveal shape is defined as the distance between these two lines. White arrows indicate the foveal center.