Ultrastructural modifications of the developing retina associated with ambient light condition

The effects of light rearing regimen on the ultrastructure of retinal photoreceptors and the synapses they make in the outer plexiform layer (OPL) were investigated using light and electron microscopy. Zebrafish larvae were reared in constant light, 14 hours light/10 hours dark (control), or constant dark conditions for 4, 8, or 15 days after fertilization. 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 and outer segments (OS), ribbons in photoreceptor terminals, and post-synaptic horizontal cell processes. In particular, the development of OS was delayed and cone mitochondria were smaller in larvae reared in constant dark. Photoreceptor terminals of larvae reared in all treatments had normal synaptic ribbons with arciform densities and no significant differences in the numbers of ribbons. However, ribbons were 30-40% longer in the retinas maintained in constant light than in either control 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) also varied with rearing light condition. The ratio of spinules-to-ribbons was used as a measure of synaptic functionality and was higher in cyclic and constant light than in constant dark by more than two-fold. By 8 days post-fertilization (dpf), the ratio was significantly highest in cyclic light. Taken together, these results show that abnormal light rearing conditions affect synaptic structure and function in retinal photoreceptors. These changes in synapses might explain the physiological deficits reported in zebrafish larvae grown under constant light and/or constant dark conditions. The findings also support the idea that the environment and cellular activity can modify neuronal and synaptic morphology.