Identification and distribution of coated vesicles in the retinal pigment epithelium of man and rabbit.

Coated pits (CPs) and coated vesicles (CVs) were investigated in the retinal pigment epithelium (RPE) of man and rabbit. In both species CPs and CVs show the typical coat of bristles radiating from their convex surface. CPs originate only from invaginations of the basal and apical cell membrane. The bipolar distribution of CPs is reflected in the distribution of CVs, which are most concentrated near the basal and apical cell boundaries. Neither fusion of CVs with the cytoplasmic organelles nor the presence of receptosomes could be demonstrated. The distribution patterns of CPs, CVs, Golgi apparatus, and mitochondria are very similar in RPE of man and rabbit. Moreover, as compared with other ocular tissues. CVs found in RPE differ both in size and distribution from those found in the pigment epithelium of the pars plana ciliaris and in trabecular endothelial cells. This peculiarity might indicate that receptor-mediated endocytosis via CVs plays a role in the transport across RPE of substances that are specific to this epithelium. Some analogies between the specificity of size and distribution of CVs in RPE and the process of internalization and transport of retinal are suggested.