Does the retinal pigment epithelium polarize the choriocapillaris?

Morphometric evaluation of choroidal capillaries from spontaneously hypertensive (SHR) rats with anatomically normal and severely dystrophic retinas reveals a marked polarization of endothelial fenestrations, endothelial cytoplasmic thickness, and endothelial cell nuclei. The mean density of fenestrations per unit endothelial length is greatest and the cytoplasmic thickness and number of nuclei is least in capillary sectors closest to the retinal pigment epithelium (RPE), with a statistically significant, continuous decrease in density of fenestrations and an increase in endothelial thickness and nuclear numbers in sectors progressively farther from the RPE. Abnormal capillaries derived from the retinal circulation and growing within the RPE also have fenestrations, but show no anatomic polarization. These results are consistent with the recently proposed hypothesis that a diffusible, trophic factor, produced by the RPE and secreted from its basal surface, modulates the structure of nearby vascular endothelial cells. However, the endothelial cells themselves probably also influence the structure of the RPE, since complex infoldings of the RPE plasma membrane, identical to those normally present in its basal surface, are also extensively present surrounding the abnormal intra-RPE capillaries.