UV cones in tbx2b mutants are dysmorphic.
A) Retinal cryosection from a sws1:EGFP adult zebrafish; UV cones are magenta, red-green double cones are immunostained with the specific marker, zpr1 (yellow), and nuclei are stained with the fluorescent dye, Hoechst (grey). The central panel is an overlay of the epifluorescent channel (UV cones) and transmitted light (differential interference contrast). Nuclei of red-green double cones (arrows), and blue cones (not indicated), are radially elongated and displaced apical to the outer limiting membrane (dashed line), whereas nuclei of UV cones are triangular in shape (circles) and positioned basal to the outer limiting membrane. B, C) In tbx2b; sws1:EGFP mutants, the morphology of red-green double cones (zpr1, yellow) and their nuclear position (white arrows) is normal, whereas the nuclei of UV cones (circles) are displaced apically beyond the outer limiting membrane (dashed line). One UV cone (black arrow) is collapsed. D, E) 3D volume renderings of UV cones near the retinal margin in wild-type, sws1:EGFP fish. Like all photoreceptors UV cones have a single, bulbous axonal terminal (cone pedicle, arrow). D) Many UV cones in tbx2b; sws1:EGFP mutant adults have a bifurcated axon and two pedicles (arrows). Scale bar: 20 μm (A,B,C).