Retinal development and photoreceptor synaptic ultrastructure are altered by abnormal rearing light regimes

The effects of light rearing regimen on distal retinal development and photoreceptor ultrastructure were investigated using light and electron microscopy. Zebrafish larvae were reared in constant light, control/cyclic light (14 hr light/10 hr dark), or constant dark conditions until 4 or 8 days postfertilization (dpf). Gross retinal morphology was not altered by light rearing conditions; however, ultrastructural differences were noted both within and between age groups. Significant differences were seen in photoreceptor outer segments (OS) and synaptic ribbons, the size of cone photoreceptor mitochondria, and postsynaptic horizontal cell spinules. Larvae reared in constant dark displayed reduced pigment dispersion; OS development was delayed and cone mitochondria were smaller at 4dpf, two results that reversed by 8dpf. Photoreceptor terminals of larvae reared in all treatment conditions displayed anchored synaptic ribbons with arciform densities and no significant differences in ribbon number. Ribbons were 30- 40% longer in photoreceptor terminals within the constant light treatment. The number of horizontal cell spinules invaginating into cone terminals varied and the spinule-to-ribbon ratio was higher in control and constant light-reared tissue by more than 2x at 4dpf. By 8dpf, this ratio was significantly highest in retinas reared in control/cyclic light conditions. Taken together, these results show that abnormal light rearing conditions affect synaptic structure in distal retina. These changes suggest a mechanism for the physiological and behavioral deficits reported in zebrafish larvae grown under constant light and/or dark conditions

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The effects of rearing light level and duration differences on the optic nerve, brain, and associated structures in developing zebrafish larvae: a light and transmission electron microscope study.

The ultrastructure of the optic nerve, brain, and some associated structures of larval zebrafish, grown under three different light regimens were studied. Fish grown under cyclic light (control), constant dark (CD), and constant light (CL) were studied for 4 and 8 days postfertilization (dpf). We also studied the control and CD fish at 15 dpf. The brains of the control and CL fish were larger at 4 dpf than at 8 dpf. In all 4 dpf fish, the brain occupied the entire expanse between the two retinas and the optic nerve extended the shortest distance between the retina and the brain. The 15 dpf zebrafish had the smallest brain size. Groups of skeletal muscle cells associated with the optic nerves became visible in all older larvae. In the 15 dpf larvae, bulges and dilations in the optic nerve occurred as it reached the brain and optic chiasms occurred proximal to the brain. Electron microscopy yielded information about myelinated and unmyelinated axons in the optic nerve, the dimensions of neurotubules, neurofilaments, and myofilaments, including a unique variation in actin myofilaments, and a confirmation of reported myosin myofilament changes (but with dimensions). We also describe the ultrastructure of a sheath-like structure that is confluent over the optic nerve and the brain, which has not been described before in zebrafish. Also presented are images of associated fibroblasts, epithelial cells lining the mouth, cartilage plates, blood vessels, nerve bundles, and skeletal muscle cells, most of which have not been previously described in the literature.