Melanin Granules

Melanin has a double role in the retina: to reduce the chromatic aberration, increasing the visual acuteness, and to protect against oxidative stress by acting as a cellular antioxidant. Its concentration increases from the equator to the posterior pole reaching a peak in the macula. The increased concentration of melanosomes in the macula is due to the fact that the RPE cells here are larger and concentrated into a smaller area with respect to the smaller extra-macular RPE cells dispersed in a much larger area (Handelman and Dratz, 1986). The differential distribution of melanosomes is maintained during the first 40 years of life, but afterward a significant reduction in the melanin granules is seen in all regions of the retina (Weiter et al., 1986). To make a comparison, consider three age groups: 10-20; 21-60; and 61-100. The reduction in the quantity of melano-somes between the first and the third class is 35%. Talking in terms of cell volume, therefore, around 8% is occupied by melanin in the first two decades of life, which reduces to 6% in the second age group, and finally further diminishes, arriving at 3.5% in the third age group (Feney Burns, 1984).

Melanin is a complex heterogenic biopolymer, containing free radicals, which can be identified using electron spin resonance spectroscopy (ESR). Using this technique a 40% reduction in melanin content is observed with aging (Sarna et al., 2003). Three possible mechanisms may explain the loss of melanin from RPE cells: expulsion of the granules, lysosomal degradation, and chemical damage. The expulsion of the granules may be a possibility, notwithstanding the fact that the granules are not found in the Bruch's membrane or in the interphotoreceptor space. Lysosomal degradation is highly elevated due to its function of degrading the acromeres of the photoreceptors (Boulton and Wassel, 1998). With aging comes an increase in the number of melano-lysosomes, accompanied by a change in the appearance of the melanin granules. Notwithstanding that the morphology of the melanosomes changes following an interaction with the lysosomes, it is likely that the melanin is not degraded and that the changes derive from the degradation of the proteins of the matrix on which the melanin is deposited. The third mechanism is that of chemical degradation. The irradiation of human eyes with intense blue light induces a nonuniform photobleaching of the melanosomes. The lack of uniformity of the bleaching seems to be due to the fact that lipofuscin is also found in the complex granules of aged RPE cells and this is more photoreactive than melanin, or even may act as photosensitizers. Blue light therefore would induce oxidative photo-degradation of melanin by the formation of superoxide anion and hydrogen peroxide. On the one hand the photo-degradation of melanin (oxidation or irreversible bleaching) does not have any biological significance in tissues with high turnover (such as hair and skin); on the other hand, this event gains a high importance when it occurs in those tissues with low turnover such as in the RPE cells, which are postmitotic cells (Boulton et al., 1998).

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