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Michelle Ramsey, Brian D. Perkins; Basal Bodies Exhibit Polarized Localization in Zebrafish Photoreceptors. Invest. Ophthalmol. Vis. Sci. 2012;53(14):766.
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In vertebrate photoreceptors, the light-sensitive outer segment is attached to the inner segment by a connecting cilium. The connecting cilium extends from a basal body, which is docked at the apical surface of the inner segment. In numerous cell types, basal bodies, and therefore cilia, are asymmetrically positioned at the surface of the cell according to planar polarity (i.e. side-to-side) cues. Disrupting this pattern can disrupt cilia formation or lead to cilia dysfunction. Cilia dysfunction can lead to retinal degeneration; however, the localization of cilia within this axis has not been investigated in vertebrate photoreceptors. In the zebrafish retina, cone photoreceptors form an organized mosaic that is ideal for investigating patterns in cilia positioning. We hypothesized that basal bodies would exhibit a polarized localization in photoreceptors.
Basal bodies were immunolabeled with gamma-tubulin antibodies and viewed in adult retina flatmounts. For each cone cell type, basal body position along the circumference of each cell was determined by measuring the angular localization of the basal body in relation the location of the optic nerve. The average angular localization for each cone subtype was calculated across multiple fields. Measurements were taken at variable distances from the optic nerve to determine if positioning is consistent across the retina.
In blue, red, and green sensitive cones, basal bodies localized asymmetrically on the cell edge nearest to the optic nerve. In contrast, no patterning was seen in the ultraviolet cones. Basal body patterning was less organized in cells near the larval remnant, which is the region of earliest formed photoreceptors surrounding the optic nerve.
These results establish the asymmetrical localization of basal bodies in blue, red, and green sensitive cones in adult zebrafish retinas. This pattern suggests an active cellular mechanism controlling its establishment. The disorganization of the UV sensitive cones could indicate that this mechanism requires cell-cell contact at the apical surface, as UV sensitive cones are at a lower level than the other cones. This mechanism may not be active during early eye development since photoreceptors nearest the optic nerve did not exhibit the same strong patterned asymmetry.
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