Retinas from 1-year-old Berkeley SS mice that were heterozygous or homozygous for the rd1 mutation. (A, C, E) Heterozygous for the rd1 mutation; (B, D, F) homozygous for the rd1 mutation. Retinas were incubated for enzyme histochemical demonstration of ADPase activity and then flat-embedded in JB-4. The ADPase activity in viable blood vessels appears white with dark-field illumination (A, B) and black with bright-field illumination (C, D). (A) The normal mouse retina has a spoke wheel pattern of blood vessels that emanates from the optic nerve head (center). (B) The Berkeley mouse that was homozygous for rd1 had a much attenuated retinal vasculature and was missing the dense, deep capillary system. (C) There was no pigment present in the retina of the mouse heterozygous for rd1 with bright-field illumination. (D) The Berkeley rd1/rd1 mouse had many areas that were pigmented, suggesting that the retinal pigment epithelial (RPE) cells had migrated into the sensory retina (arrow). (E) When the normal-appearing retina in panels A and C was sectioned, the 2 nuclear layers of retina were apparent, and photoreceptor outer segments were present below the outer nuclear layer (bottom). The retinal vasculature in the inner retina appeared normal, and a vessel is seen connecting the superficial and deep capillary systems of the retina in this field. (F) The Berkeley rd1/rd1 mouse had no outer nuclear layer or photoreceptor outer or inner segments (bottom), a disorganized inner nuclear layer (bottom), and RPE cells (arrow) had migrated into the retina and ensheathed a dilated blood vessel.