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Fig. 4. Hypothetical cone cycle in the retina and rod cycle in the retina/retinal
pigment epithelium (RPE) of the chicken eye. Chicken retina stores
11-cis retinyl esters and chicken RPE stores all-trans
retinyl esters. Light bleaches photopigments in the retina, leading to the
accumulation of 11-cis retinyl esters in the retina and
all-trans retinyl ester in the RPE. In the dark, 11-cis and
all-trans retinyl ester pools deplete while visual pigments
regenerate. The amount and the rates of accumulation/depletion of
11-cis retinyl esters in the retina correspond to the bleaching and
regeneration of cone pigments, providing support for a cone visual cycle in
the chicken retina. The amount and the rates of accumulation/depletion of
all-trans retinyl esters in the RPE correspond to the bleaching and
regeneration of rod pigments, thus supporting a rod visual cycle in the
chicken RPE/retina. The types of retinal cell where 11-cis retinoids
are synthesized and stored, as well as the biochemical mechanism of
isomerization, are not known. The method to partition these two visual cycles
remains to be studied.
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