Targeted disruption of Isl1 results in the developmental loss of RGCs. (A–H) Immunostaining of retina sections with anti-BRN3B shows that in Isl1-null retina, BRN3B+ RGCs are generated and positioned properly at E13.5 (A,E) and E15.5 (B,F). However, a significant loss of BRN3B+ RGCs is detected at E17.5 (C,G) and E18.5 (D,H). Inserts show the enlarged views of the central retina. (I,J) Anti-activated caspase3 immunostaining shows an increase in apoptotic cells (arrows) in the GCL of Isl1-null retina at E17.5. (K) Quantification of apoptotic cells in developing retina by averaging activated caspase3 positive cells in 5 sections per retina. The difference of apoptotic cell numbers in wild type and Isl1-null retina is insignificant at E13.5 to E15.5. However, in Isl1-nulls, the apoptotic cells are significantly increased from E16.5 and are peaked at E17.5. Each histogram represents the mean ± s.d. n=3–4 for each genotype per stage. (**t-test P<0.01; *P<0.05;). Scale bar: 100 μm in A–D; 25 μm in I.