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Peter Gouras, Lena M. Ivert, Martha Neuringer, Julie Mattison; A Novel Type Of Lysosome Identified In The Retinal Epithelium Of Young Rhesus Monkeys. Invest. Ophthalmol. Vis. Sci. 2011;52(14):934.
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Examining the ultra-structure of melanosomes and lysosomes of monkeys can contribute to understanding why there is a massive buildup of lysosomes in aging RPE.
By EM we determined the number, size, shape and appearance of melanosomes and lysosomes in macular RPE of young (1 & 6 years) and old (24, 24, 26 & 35 years) rhesus monkeys (Macaca mulatta). We measured the basal, middle, and apical side of each RPE cell. Size was based on the long axis of all organelles. RPE at the equator and ora was examined qualitatively. Fluorescence was examined in thick sections.
Two types of lysosomes were found. Type 1, mainly in young RPE, was round and small (long axis = 0.56 µ; SD=0.18; n=42). These had a light central zone occupying only a part but sometimes most of the lysosome. Type 2, mainly in old RPE, was larger (long axis = 1.43 µ; SD=0.1; n=53) with varied shapes. Some were elliptical; others had several appendages; all had irregular borders. Melanin inclusions were embedded in a homogeneous gray matrix presumably containing lipofuscin since only old RPE fluoresced. This matrix material was absent in Type 1 lysosomes. Both types of lysosomes were more common at the macula than elsewhere in the retina. Pure melanosomes were present in both young and old RPE but to a lesser degree in the older RPE. There were few phagosomes and no sign of melanogenesis. Young RPE did not fluoresce; old RPE fluoresced intensely except at the ora where outer segments were absent..
This is the first report identifying two types of lysosomes in monkey RPE both containing melanin in various degrees of dissolution. Type 1, newly identified in young RPE, is small and round and seems to affect the degradation of small melanosomes. This process apparently starts early in life and diminishes with age as Type 1 lysosomes disappear In the RPE of old monkeys. Type 2 lysosomes, with a larger and more variegated shape, seem to be dissolving both large and small melanosomes. The lighter matrix material of type 2 lysosomes must degrade outer segment phagosomes producing RPE fluorescence. Both lysosomal types are involved in melanin breakdown leading to melanin loss with age. The results support the idea that RPE melanosomes are lysosomal and suggest that the lysosomal buildup with age involves faulty breakdown of both melananosomes and outer segment phagosomes.
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