Basal infolding density in the normal pigmented rat.

A variation in retinal pigment epithelial (RPE) basal infolding (BI) density in normal rats was observed and quantified. Serial electron micrographs of the midequatorial region of all quadrants from both eyes of litter-mate pigmented rats, 58 days old, were prepared. The lengths of BI and Bruch's membrane (BM) were determined by tracing enlarged micrographs to a Zeiss MOP-3 analyser. BI density per unit length of BM was assessed over the choriocapillary (CC) lumen, nucleated cytoplasma and intercapillary septum (ICS), and expressed as the dimensionless ratio BI/BM. Fenestrae were counted. The average BI/BM was 8.7; the BI/BM over the ICS, 6.1, was significantly lower than over the CC, 10.7 (P less than 0.001). BI/BM over the lumen and nucleated CC cytoplasm was the same. The number of fenestrae was not strongly associated with BI density (R2: 0.33). These data support the notion that the density of BI is related to diffusion within BM and is proportional to the distance from the CC lumen.