Notch signaling is active in the nascent and mature MG in the developing retina. (A and A’) MG span the entirety of the three neural layers from the inner limiting membrane (ILM) to the outer nuclear layer (OLM). (B) Using Cralbp as an early marker for MG specification in cryosections, we see expression on the apical side of the retina at 48 hpf. (C and D) This expression increases in the INL by 60 hpf (C), and was strongly expressed at 72 hpf in mature MG (D). (E) In whole-mount samples at 35 hpf, many cells within the nascent retina have Notch activity (green), especially on the apical side of the retina (arrowheads). (F) Notch activity at 55 hpf appears in radial cells on the apical side of the retina (arrowheads). (G) Notch activity is restricted to MG at 72 hpf. (H–J) The Notch-positive cells in the mature retina are MG, as shown by the overlap in the TP1:Venus and GFAP:dTomato transgenic constructs (arrows). (K) The transplantation strategy to generate clones of TP1:Venus (green) embryos into the ath5:gapRFP host (magenta). (L–P) Panels from a time-lapse movie of a clone in a living zebrafish retina (n = 6). (L) Notch is active in multiple cells in the clone at 38 hpf. Notch activity is progressively restricted until a single cell maintains Notch on the apical side of the retina at 42 hpf (M). Notch activity increases in this cell at 48 hpf (N), and at 55 hpf (O) it migrates basally and will position itself within the INL and begin to mature by 62 hpf (P). Figures always show the apical side of the retina at the top and basal at the bottom. OPL, outer plexiform layer; IPL, inner plexiform layer. Bars, 10 µm.