cGMP levels in rd/rd retina. (A) Comparison of cGMP levels in rd/+ control and rd/rd posterior central retina. Chickens were light adapted for 2 hr under laboratory illumination (1,600 lux) before sacrifice. The shaded area from embryonic day 18 (E18) to posthatch day 7 (P7) is the time during which photoreceptor outer segment development and elongation occurs. Each value represents the mean ±SD of 6–11 eyes. Filled symbols, control retina; open symbols, rd/rd retina. (B) cGMP levels in microdissected layers of retinas taken from 1- to 2-day posthatch chickens. Assays were carried out on seven retinal layers taken from central retina (CHOR, choroid; PE/OS, pigment epithelium plus outer segments; OS/OD, outer segments plus oil droplets; IS/ONL/OPL, inner segments plus outer nuclear layer plus outer plexiform layer; INL, inner nuclear layer; IPL, inner plexiform layer; GC/NFL, ganglion cells plus nerve fiber layer). Each value represents the mean ±SD of three to four eyes (10–30 sections per eye). All measurements were made in duplicate. Open bars, rd/+ control retina; filled bars, rd/rd retina.