Degeneration of retinal ganglion cells. (A) Retina sections from Ctsd ko mice contained normal numbers of BRN-3A-positive ganglion cells at P10 (Ab) but reduced ganglion cell numbers at P25 (Ad) when compared with age-matched control retinas (Aa,Ac, respectively). Quantitative analyses of retina sections confirmed a significantly decreased RGC density in Ctsd ko retinas (filled bars in Ae) starting from P20 when compared with age-matched control retinas (open bars in Ae). n.s., not significant; *, p < 0.05; ***, p < 0.001, two-way ANOVA. (B) Analyses of retinal flatmounts from P25 Ctsd ko (Bb) and control mice (Ba) confirmed a significant loss of ganglion cells in the mutant at this age (Bc). ***, p < 0.001, Student’s t-test. Each bar in (A,B) represents the mean value (±SEM) of at least 6 animals. BRN-3A, brain-specific homeobox/POU domain protein 3A; cn, control; gcl, ganglion cell layer; inl, inner nuclear layer; ko, knock-out; onl, outer nuclear layer; P, postnatal day. Scale bars: 100 µm.