Cone mosaics in the embryonic, larval, and adult zebrafish retina.
A) Schematic of photoreceptor packing in the apical plane of the adult retina, showing cones of red, green, blue, and UV (magenta) spectral sensitivities, and smaller rods (black). The 12-cell repeating motif of the cone mosaic pattern is outlined by the yellow rectangle. B) Regular pattern of cones visualized in a flat-mount retinal preparation from an adult, double transgenic zebrafish (mi2009 line) in which UV and blue cones express different fluorescent reporters (pseudocolored magenta and blue) under the control of UV and blue opsin promoters, respectively. Apical boundaries of cells are delineated by ZO-1 immunostaining (yellow). Horizontal rows of alternating blue and UV cones alternate with horizontal rows of unlabeled cone profiles representing red-green double cone pairs. C) Rod photoreceptors are visualized in a flat-mount retinal preparation from an adult transgenic zebrafish (kj2 line) in which the rod opsin promoter drives expression of a reporter gene (pseudocolored cyan). Apical boundaries of cells are delimited by ZO-1 immunostaining (yellow). Rods are largely excluded from the vertical columns of contiguous cones, and instead occupy the spaces between adjacent columns. D) Schematic of the hemispherical retinal epithelium. Retinal neurogenesis in the circumferential germinal zone at the peripheral margin adds annuli of new retinal neurons, such that the age of retinal cells is a direct function of their distance from the periphery. The ordered cone pattern illustrated in panels A–C is in the peripheral retina, whereas the central retina, surrounding the pole of the hemisphere, exhibits the disordered embryonic/larval pattern (panel E), with a transition zone in-between (panel G). The annular ligament (not shown) roughly encircles the germinal zone. E) Apical retinal surface of a double transgenic mi2009 zebrafish at 4 days post-fertilization, showing the packing of cone cells that differentiated during embryonic development, before progressive addition of cones at the retinal margin begins: UV cones are magenta, blue cones are blue, ZO-1 immunostaining (yellow) outlines cell profiles at the level of the OLM. F) Average orientational order parameter (Fig. 2 and Methods) for embryonic and adult retina. (Three and six regions of ∼20 by 15 cone cells were used to calculate the values for embryonic and adult retina, respectively.) G) Transition from disordered cell packing in the larval remnant (left side) to ordered packing (right side) in a flat-mount retina of an adult double transgenic zebrafish, mi2009 labeling blue and UV (magenta) cones, with cell boundaries visualized with ZO-1 immunostaining (yellow). The curved, dashed line segment traces a cone column. H) High magnification views of the angles at which three cone-cone interfaces meet (ZO-1 in yellow).