Transgenic zebrafish engineered for blue cone ablation were labeled for apoptosis using TUNEL to examine death of target photoreceptors (blue cones) and any effects on adjacent photoreceptors. (A, B) Zebrafish larvae that had received either vehicle control (DMSO) or prodrug metronidazole (MTZ) treatment, respectively, sacrificed at 9 days post fertilization (dpf). Blue cones expressing mCherry are pseudocoloured in magenta, TUNEL labelling is represented in green, nuclei are in cyan. Intact blue cones are not detectable in the prodrug treated larvae. (C) Quantification of the number of TUNEL-positive cells per section within the ONL. The number of TUNEL+ mCherry- cells did not differ significantly between treatment groups, indicating no detectable off-target cell death. The number of TUNEL+ mCherry+ cells differed significantly between groups, as was expected since the prodrug treatment is designed to induce apoptosis in the mCherry+ blue cones of this transgenic model. Scale bars are 100μm. *** = p<0.001. n = 12 & 11 fish for the DMSO & MTZ treatment groups, respectively.