Does Melanin Turnover Occur in the Eyes of Adult Vertebrates?

Abstract

This paper is a review of what is known about the turnover of melanin in iris, choroid, and retinal pigment epithelium (RPE) of the adult vertebrate eye. Differences in size and structure of choroideal and retinal pigment epithelial melanin granules are shown by electron micrographs. The classical stages of melanin synthesis, including the premelanosome, are shown in the RPE of adult hamsters that had been exposed to intense light. Degradation or synthesis of melanin also seem to occur in the melanocytes of the choroid in these animals.It is postulated that all three pigmented eye tissues (iris, RPE, and choroid) of adult vertebrates form melanin granules in vivo. However, nothing is known about the amount of this turnover.Key words: Melanosome, Imosome, Phagosome, rnsinase, Peroxidase, Radical INTRODUCTIONIn the eyes of vertebrates two types of pigment cells belonging to different cell lines are present. The retinal pigment epithelium (RPE) originates from the outer neuroectodermal sheath of the eyecup, whereas the melanocytes of the uvea (choroid and iris) immigrate from the neural crest (Heiman, 1972; Ozanics et al., 1978; reviewed by 'Ibrczynski 1982).While it is generally accepted that the key enzyme for mammalian meh-nesis is tymsinase (Lerner et al., 1950; K6rner and F'awelek, 1982), the origin of the matrix component of melanogenesis in the RPE, the premelanosome, has been subject to controversial views. At present it is generally believed that melanogenesis in RPE and choroid follows a common pathway (Feeney et al., 1965;Hearing et al., 1973; Hu and Mah, 1983; Sarna 1992). Premelanosomes should be derived either from the rough (Stanka, 1971; Stanka et al., 1981) or, smooth (Mishima et al., 1978) endoplasmic reticulum or from polysomes (Moyer, 1961). Other workers were not able to determine the origin of mekinoflaments in the eyes of black and beige mice (Lutzner and Lowrie, 1972). Alternatively, autophagic vacuoles (Novikoff et al., 1979) or phagosomes (Schraermeyer, 1992b) were taken into consideration as matrix components for melanin granule formation in the RPE. While the turnOver of melanin is high in the skin (Ortonne, 19901, the amount of melanin turnover in the eyes of adult vertebrates is completely unknown, although the melanin in the eyes is exposed to an environment that generates H202 (see Bennan, 1979) and radicals (Noell, 1980), which can inteF act with melanin.As far as data were available, the attempt is made in the present study to focus on five points indicating turnover of melanin in the three pigmented eye tissues. bover of melanin may be indicated by: (1) the demonstration of the classical morphological stages of melanogenesis like premelanosomes or early stage melanosomes, (2) the demonstration of active tyrosinase or other melanogenic enzymes, (3) the degradation of melanin granules by macrophages or lysosomes, (4) the repigmentation after injury or age related loss of melanin, or (5) incorporation of Dopa into melanin granules. MELANIN TURNOVER IN THE RETINAL PIGMENT EPITH...

Melanosome Maturation Defect in Rab38-deficient Retinal Pigment Epithelium Results in Instability of Immature Melanosomes during Transient Melanogenesis

“…However, tyrosinase activity has been detected in extracts from adult RPE (Dryja et al, 1978;Varela et al, 1995), and tyrosinase expression/activity was detected in cultured adult RPE cells (Basu et al, 1983;Dorey et al, 1990), and it was increased after incubation with rod outer segments (Schraermeyer et al, 2006). Vacuoles with the morphological characteristics of immature melanosomes have been described in adult RPE (Schraermeyer, 1993;Schraermeyer and Heimann, 1999), but pmel17 staining in adult RPE has not been reported. We were unable to detect tyrosinase expression in adult mouse RPE by cryoimmuno-EM.…”

“…The mammalian RPE does not normally turnover in the adult, and the majority of melanin synthesis in the RPE occurs during embryonic life (Carr and Siegel, 1979). There is controversy over whether there is a low level of melanin synthesis in the adult RPE (Schraermeyer, 1993). Nevertheless, the regulation of melanosome production in RPE cells clearly differs from that of melanocytes of the skin where melanosomes are constantly synthesized before being transferred to neighboring keratinocytes.…”

Subretinal Injection of Rod Outer Segments Leads to an Increase in the Number of Early-Stage Melanosomes in Retinal Pigment Epithelial Cells

“…Some evidence, however, has been presented which indicates that melanogenesis could take place in the adult RPE (for a review, see Schraermeyer [4]). The amino acid tyrosine from which melanin is synthesized is present as are the required enzymes.…”

“…They observed transitional forms between premelanosomes and melanosomes and numerous small melanin granules in RPE cells after subretinal injection of vitreous in rabbits. However, melanogenesis in the adult RPE is still an unsettled question [4,8,9].…”

Intrastriatal Transplantation of Retinal Pigment Epithelial Cells for the Treatment of Parkinson Disease: In Vivo Longitudinal Molecular Imaging with18F-P3BZA PET/CT

“…However, melanogenesis in adult RPE cells remains controversial. Some scientists believe that melanin synthesis occurs in hRPE cells throughout a person's life, albeit at a slow rate, whereas others consider that it ceases at birth (8,35). Age-related change of melanin expression in hRPE cells has thus been studied (36).…”