Expression of meis1, meis2.2 and meis2.1 during eye development. (A-H) Dorsal views of flat-mounted embryos. meis1 and meis2.2 are expressed uniformly throughout the optic vesicle (A,E) and early-stage optic cup (B,F). After this developmental point, meis2.2 expression fades (G,H), while meis1 becomes progressively restricted to the peripheral retina (C,D). Inset in B is a flat mount of a 16-hpf meis1-YFP showing uniform YFP signal in the optic vesicle, which recapitulates meis1 expression at this stage. (I-K,M-O) Lateral views of whole-mount embryos. meis2.1 and meis2.2 are not expressed in the eye (open arrowheads) at 24 hpf (I,M). Both genes begin to be expressed in a central patch of the neural retina at about 33-36 hpf (J,N) that becomes a thin rim of cells by 48-52 hpf (K,O). (L,P) 40-μm vibratome sections of gelatin-embedded embryos show that, at 48-52 hpf, meis2.1 and meis2.2 are expressed in postmitotic ganglion cells. (Q-S) 15-μm cryostat sections of eyes from 4-dpf fish, hybridized with probes against (Q) meis1, (R) meis2.1 and (S) meis2.2. At this stage, all three genes are expressed in the GCL and in the most central region of the INL - likely amacrine cells - but at different relative levels. Also at this stage, only meis2.2 is expressed at significant levels in the ciliary margin zone (CMZ). Arrowheads indicate expression detected by in situ hybridization. hpf and dpf, hours and days post-fertilization, respectively, at 28.5°C. The meis1-YFP enhancer trap is an insertion of a YFP-retroviral vector (Laplante et al., 2006) located 1,707 bp downstream of the last exon of meis1 (chr13) and inserted in the same transcriptional direction.