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V. P. Connaughton, R. Tarboush, G. B. Chapman; Ultrastructure of Adult Zebrafish Photoreceptors. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4122.
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© ARVO (1962-2015); The Authors (2016-present)
To describe the organization, morphological characteristics, and synaptic structure of photoreceptors in the outer retina of adult zebrafish.
Whole eyes from 3 adult fish were preserved in primary fixative composed of 5% glutaraldehyde, 2.5% glutaraldehyde/0.18M sodium cacodylate, or 2.5% glutaraldehyde/0.135M cacodylate buffer/1% tannic acid. After washing and post-fixation, samples were dehydrated through an ethanol series, and resin-embedded in preparation for TEM. For light microscopy, thin (0.25-1µm) sections were stained with an aqueous mixture of 1% methylene blue, 1% azure A and 1% sodium borate. For TEM, ultrathin (50nm) sections were and stained with uranyl acetate in 50% ethanol and 0.4% lead citrate and examined.
Zebrafish photoreceptors show a typical ordered tier arrangement, with ROS occupying a more sclerad position than COS. Rods were easily distinguished from cones based on OS morphology, though both contained large, darkly staining megamitochondria within the IS. CIS have two populations of morphologically distinct mitochondria: very large, electron-dense, distally located mitochondria and smaller, electron lucent mitochondria. The difference in size between mitochondrial types was significant (p<0.001). Rod spherules and cone pedicles terminated at the same level in the OPL, forming a single row of terminals. Synaptic ribbons in rod spherules were significantly longer than ribbons in cone pedicles (p<0.001). Further analysis of terminals identified four synaptic relationships at cone pedicles: (1) invaginating contacts postsynaptic to cone ribbons that formed dyad, triad, and quadrad synapses (2) presumed gap junctions that connected adjacent postsynaptic processes invaginating into cone terminals (3) basal junctions located away from synaptic ribbons and (4) gap junctions between adjacent photoreceptor terminals.
Adult zebrafish photoreceptors are easily distinguishable at the ultrastructural level and the OPL, though narrow (< 1um thick), has complex synaptic connections.
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