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D. M. Allen, G. Kovalick, R. Joy; Control of Melanization of the Retinal Pigment Epithelium of the Channel Catfish, Ictalurus punctatus. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2515.
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© ARVO (1962-2015); The Authors (2016-present)
To examine transcriptional control of melanization as a factor in resistance to light damage, using pigmented and albino channel catfish, Ictalurus punctatus.
Whole eye-cups were photographed with a digital imaging system to define regions of melanization in pigmented and albino specimens. Differential interference contrast microscopy was used to assess numbers and size of melanosomes in un-fixed RPE sheets stripped from the neuro-retina. Slides of semi-thin or thin sections were prepared for conventional or transmission electron microscopy. Fish were held in 50 lux cyclic light or challenged with 6000-15,000 lux constant light. To quantify expression of tyrosinase, we used RT-PCR, with cDNA generated from RNA and primers specific to the channel catfish tyrosinase.
Channel catfish skin is counter-shaded by skin melanocytes and this pattern also shown within the retina: the dorsal retina is heavily melanized whereas mid and ventral regions are less melanized, presumably to allow use of the tapetum. Albinos display absence of pigment in skin but their ocular RPE is melanized along the same regional pattern as in normally pigmented fish; however, the number of melanosomes is reduced overall. The size of melanosomes does not vary.
Preliminary data indicate that differential tyrosinase expression corresponds to regional distribution and abundance of melanosomes in the RPE of pigmented and albino specimens. Light challenge leads to only to moderate losses of rod outer segment volume in the least melanized area of albino retinas. The possibility that tyrosinase expression is up-regulated during light challenge is currently under investigation.
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