Melanosome in Retinal Pigment Epithelium: Distribution, Type and Activity of Acid Phosphatase

ABSTRACT

Distribution of melanosome, types of melanosome in retinal pigment epithelium(RPE) and activity of acid phosphatase in melanosome were studied in rabbits. The eyes were observed using electron microscopy and enzyme cytochemical electron microscopy. The majority of melanosomes were located near the apical region of RPE. So melanosomes can absorb light that passes through the neural retina rapidly and remove free radicals that develop in RPE effectively, thereby benefiting the optical system and protecting visual outer segments. Melanosomes in RPE were classified as two types; ellipsoid and spherical or oval. Ellipsoid types were loccated parallel to the apical process and spherical or oval types, vertically or obliquely to apical process. Mature and immature melanosome showed mostly positive responses for activity of acid phosphatase, indicating that melanosome commonly incorporates into the lysosomal system of the retinal pigment epithelium. But some melanosomes showed negative responses for activity of acid phosphatse, suggesting that melanosome has a stable and inert property. The observed premelanosome showed negative response. Two types of melanosomerelated complex granules were identified melanosome with a cortex of enzymereactive material(melanolysosome) and melanosome with a cortex of lipofuscin(melanolipofuscin). They were located near or below the melanosomes in the cytoplasm. These findings indicate that a relationship between melanosome and the lysosomal system of the retinal pigment epithelium exists, and suggest that melanosome undergoes modification or degradation there. Also, the findings of premelanosome and positive responses for the activity of acid pnosphatase indicate that melanosome continues to be synthesized at a low rate in adult eye.