Fig. 4. The differentiation of cell types found in the INL depends on shh signaling. All images are confocal sections of the retina, with anterior to the top. (A) Isl1 protein (red) in a wild-type retina at 64 hpf. Note expression in the GCL, as well as in three discrete domains in the INL: amacrine cells (AC) in the proximal INL, as well as bipolar cells (BC) and horizontal cells (HC) in the distal INL. (B) Isl1 protein in a shh^–/– retina at 64 hpf. Note the overall reduction of Isl1-expressing cells, and that most of them are located in the proximal retina. Note also that these cells are disorganized, and that there is no sign of an INL or laminar organization. (C) Glutamine synthetase protein (red) in a wild-type retina at 72 hpf. Note expression in Mueller glia, which span all the retinal layers. (D) Glutamine synthetase protein in a shh^–/– retina at 72 hpf. Note the complete absence of glutamine synthetase staining from the retina. (E) PKC protein (green) in a wild-type retina at 72 hpf. Note prominent expression in bipolar cells in the INL. (F) PKC protein in a shh^–/– retina at 72 hpf. Note the complete absence of PKC staining from the retina. (G) GAD67 protein (green) in a wild-type retina at 72 hpf. Note prominent expression in amacrine cells of the INL, and in the inner plexiform layer. (H) GAD67 protein in a shh^–/– retina at 72 hpf. Note the severe reduction of amacrine cells (arrowheads) and absence of inner plexiform layer.