Blocking Notch activity during retinogenesis results in a loss of MG. (A and B) Treatment with DAPT at 45 hpf results in the loss of Notch activity (green) in whole-mount retinas at 72 hpf. (C and D) In cryosections, MG specifically express glutamine synthetase (GS), but after DAPT treatment the retina is completely lacking in GS expression. (E and F) DAPT treatment of double transgenic fish, ath5:gapRFP;GFAP:GFP, results in a complete loss of GFP-expressing MG in the retina of the transgenic line GFAP:GFP. (G) Treatment with DAPT at 50 hpf is too late to completely block MG specification in the central retina (arrow). G′ shows the boxed region. (H and I) There are reduced GFAP:GFP-positive cells in the central retina after Su(H) morpholino injection or treatment with Compound E, another γ-secretase inhibitor, at 96 hpf, similar to DAPT treatments. (J and K) Retinal neurons are specified and patterned properly in the SoFa fish. (L and M) There is no increase in cell death (TUNEL+ cells) in the absence of MG (mean + SEM, n = 10 retinas). Bars: (A–D) 25 µm; (E–G) 20 µm; (G’) 10 µm; (H and I) 30 µm; (J and K) 20 µm; (L) 50 µm.