Morphological features of the retinal pigment epithelium in the toad, Bufo marinus

Background: The toad's (Bufo marinus) retinal pigment epithelium (RPE) has been used in many studies as a model for understanding its role and interaction with the neural retina (NR). In this study, the fine structure of the RPE is described. Methods: In vitro RPE-chorion preparations from the toad were studied after separation from the NR. Tissues were processed for light and electron microscopy. Results: The RPE consists of a single layer of cells joined by a series of tight junctions forming part of the blood-retina barrier. These cells had minimal infolding of the basal membrane and numerous microvillous processes extending from the apical surface and surrounding photoreceptor outer segments. Internally, RPE cells display a large nucleus, numerous mitochondria located at the basal membrane, extensive smooth endoplasmic reticulum, myeloid bodies, phagosomes, scattered polysomes, large lipid droplets and melanin granules. Rough endoplasmic reticulum is relatively scarce within these cells. The choriocapillaris consists of large blood vessels facing Bruch's membrane. Bruch's membrane is typical of other vertebrates, being pentalaminate over the whole retina. The endothelium of the choriocapillaris facing Bruch's membrane is typically very thin but shows few fenestrations. Conclusions: Due to the ultrastructural similarities of toad RPE with that of mammals, it can serve as an excellent model for the study of retinal detachment/reattachment and their cellular and molecular mechanisms.