Heterogeneity in ultrastructure and elemental composition of perinuclear lens retrodots.

PURPOSE To unravel the cataractogenic process(es) leading to the birefringent lenticular bodies known as perinuclear retrodots. METHODS Ten human lenses containing biomicroscopically verified perinuclear retrodots were systematically screened and analyzed using scanning electron microscopy and energy dispersive x-ray microanalysis to verify their ultrastructure and elemental composition. RESULTS Three types of retrodots were distinguished, different in size, ultrastructure, and origin. Two of them contained calcium phosphate, the third probably contained calcium oxalate. All three types were separated from surrounding normal fibers and the crystalline inclusions were sequestered within membrane-lined bodies. CONCLUSIONS Because of these observations and data found in the literature it is postulated that elevated free calcium is the initiating factor in the formation of retrodots, trapped by either oxalate or phosphate and sequestered in the retrodots. It is suggested that the oxalate is derived from ascorbate because of impaired protection against oxidative stress in the older lens. Phosphoric acid is believed to be released by calcium-induced hydrolysis of membrane phospholipids.