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Victoria P. Connaughton, Rania Tarboush, George B. Chapman; Light-Rearing Conditions Alter Photoreceptor Ultrastructure and Synaptic Morphology in the OPL. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2666.
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To examine the effects of light rearing regimen on the ultrastructure of retinal photoreceptors and their synapses in the outer plexiform layer (OPL).
Zebrafish larvae were reared in constant light, 14 hours light/10 hours dark, or constant dark conditions until ages 4 or 8 days postfertilization (dpf). After this time, tissue was processed for light and electron microscope analysis.
Although the gross morphology of the retina and its layers were not influenced by light rearing conditions, significant differences were seen in the ultrastructure of photoreceptor mitochondria, outer segments (OS), ribbons in photoreceptor terminals, and post-synaptic horizontal cell processes. The development of OS was delayed and cone mitochondria were smaller in larvae reared in constant dark. Photoreceptor terminals of larvae reared in constant dark, cyclic light, and constant light had normal synaptic ribbons with arciform densities, with no significant differences in the numbers of ribbons across the three light conditions. However, ribbons were 30-40% longer in constant light than in cyclic light or constant dark conditions. The numbers of horizontal cell neurites that invaginate into the cone terminal during the light phase of the diurnal cycle (spinules) varied with light rearing condition. The ratio of spinules-to-ribbons, a measure of synaptic functionality, was higher in cyclic and constant light than in constant dark by more than two-fold. By 8dpf, the spinule-to-ribbon ratio was highest in cyclic light reared larvae.
Abnormal rearing light conditions affect synaptic structure and probably function. These changes in synapses might explain the visual physiological and behavioral deficits reported in zebrafish larvae grown under constant light and dark conditions and support the idea that environment and cellular activity can modify neuronal and synaptic morphology.
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