βGeo staining pattern of DBA/2JR3/+ mice showing distinct patterns of ganglion cell loss. (A) A retinal lobe of a young mouse showing normal staining for βGeo activity. (B) A retinal lobe from an older mouse (9 months), showing regional loss of ganglion cells in a sector of retina with adjacent regions of normal retina. (C) A retinal lobe from an old mouse (11.5 months) showing diffuse loss of ganglion cells generally uniformly across the retina. In this cohort of mice, the pattern of cell loss seen in the example in (B) was exhibited principally in middle-aged mice (8–9 months), with early signs of degeneration. Bar = 0.5 mm.