• development of photoreceptors during embryogenesis
• uptake, transport, storage, metabolism and isomerization of vitamin A
• recognition, ingestion and phagocytosis of shedded photoreceptor outer segment tips
• detoxification of drugs
• synthesis of melanin and extracellular matrixes

Physiologic

• maintenance of the photoreceptor environment
• selective transport of metabolites to and from the retina to contribute to adhesion between the retina and RPE

Physical and optical

• promotion of retinal adhesion
• maintenance of the outer blood-retinal barrier
• stray light absorption by melanin to control light scatter