The Retinal Pigment Epithelium Chemical Composition and Structure

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Abstract

Chemical and morphologic studies were carried out on bovine retinal pigment epithelial (RPE) cells that were free of contamination by outer segments or other tissues. Both melanotic and amelanotic cells were present, the latter originating from areas overlying the tapetum lucidum. Cells near the tapetum lucidum contain large melanolysosomes suggesting that melanin produced in this marginal area is destroyed by autophagy. Otherwise, bovine RPE cells resemble those of other species. The protein content of RPE was somewhat lower than that of whole retina, and both in turn contained less than half of that in rat liver. RPE has about the same lipid content as whole retina and in both tissues, 55 to 60 per cent of the total lipids are phospholipids. While both phosphatidyl ethanolamine and phosphatidyl choline were readily identified in retina and liver, only PC could be detected in pigment cells. The fatty acids of RPE phospholipids consisted of 16:0 (31 per cent), 20:4 (17 per cent), and 18:0, 18:1, and 18:2 (each approximately 14 per cent). Long-chain polyenoates such as 22:5 and 22:6, which comprise a major portion of retinal phospholipid fatty acids were not clearly detectable in RPE. The content of retinol plus retinyl ester in pigment cells was 1.9 µg per eye, or 481 µg per gram of wet tissue. The latter value is about five times higher than bovine liver and 70 times higher than whole retina. The DNA content of RPE is about the same as that of whole retina, and both are approximately three times higher than liver. Cytophotometric analyses showed, however, that RPE nuclei are diploid.