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R. Tarboush, G. B. Chapman, V. P. Connaughton; A Light and Electron Microscopy Study of the Zebrafish Distal Retina. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5945.
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© ARVO (1962-2015); The Authors (2016-present)
To examine the organization and ultrastructure of the photoreceptors and their synapses in the outer plexiform layer of the zebrafish retina.
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 transmission electron microscopy. Thin (0.25-1µm) and ultrathin (50nm) sections were examined.
Photoreceptors showed a typical ordered tier arrangement and pigment granules surrounded all outer segments (OS). Photoreceptor terminals were organized into a single row in the outer plexiform layer (OPL). Overall, the OPL was narrow (vitreal-scleral dimension) in comparison to the inner plexiform layer (IPL). Rods were easily distinguished from cones based on OS morphology, though both contained megamitochondria within the inner segments. We found four major ultrastructual differences between rods and cones: (1) the membranes of the cone lamellar disks were fused to the plasma membrane, while the rod lamellar membranes were not, (2) cone pedicles typically had multiple synaptic ribbons, while rod spherules typically had 1-2 ribbons, (3) rod synaptic ribbons were longer than ribbons found in cone pedicles, and (4) the cytoplasm of rod spherules was more electron-dense than the cone cytoplasm. Two lateral elements were typically observed postsynaptic to each ribbon. These elements have an electron-lucent cytoplasm with electron-dense material along their plasma membrane, suggesting they are processes from horizontal cells. Central elements were also observed at some of the ribbon synapses. Between ribbons electron-dense regions were observed along the photoreceptor plasma membrane. Some of these have the morphology of basal junctions. At other regions, bead-like densities were observed on the intracellular side of processes invaginating photoreceptor terminals. These processes are similar to horizontal cell spinules reported in developing zebrafish and goldfish, with the membrane densities indicating sites of probable synaptic inputs to the terminals. Electron dense regions were also observed between the adjacent processes invaginating the photoreceptor terminals.
The organization of the zebrafish outer retina is similar to that of other vertebrates, a finding that further supports its validity as a model for studying the vertebrate retina.
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