More severe RGC loss in Isl1 and Brn3b compound null mice. (A–H) Immunostaining of adult whole-mount retinas with anti-BRN3A (A–D) and SMI32 (E–H). Compared with control (A,E), Isl1-null (B,F), and Brn3b-null (C,G), a more severe loss of BRN3A+ RGCs (red) and SMI32+ axon bundles (green) is observed in Isl1/Brn3b-null retina (D,H). (I–L) Ventral views of brains reveal optic nerves (arrows). Compared with the control (I), optic nerves are reduced in Isl1-nulls (J) and Brn3b-nulls (K). The optic nerves are barely detectable in Isl1/Brn3b-null mice (L). (M–O) X-Gal staining of Brn3b-lacZ-expressing RGCs in E13.5 retina sections shows that compared with the control (M), the genesis and migration of RGCs are undisturbed in Brn3b-null (N) and Isl1/Brn3b-null (O) retinas. Scale bar: 50 μm in D; 200 μm in H; 800 μm in L and 100 μm in O.