The cells of the retinal pigment epithelium (RPE) play an essential role in the process of vision. In this microscope image, the spherical nucleus of each RPE cell is labeled with a blue fluorescent dye. The cell boundaries are demarcated with a green fluorescent dye. To perform their duties, RPE cells must pack tightly together, giving the population a remarkably regular, honeycomb-like appearance. Image courtesy of Robert Fariss, NEI.