Photoreceptor outer segment development: light and dark regulate the rate of membrane addition and loss

The rate of membrane addition during outer segment development and the onset of membrane loss through shedding was evaluated for Xenopus laevis embryos reared at 23 degrees C in constant light (LL), constant darkness (DD), and cyclic light (12 hr L/12 hr D; LD). Embryos were placed under these lighting conditions 1 day following fertilization at open neural plate stages and were sampled each day for 9 days. Rod and cone outer segments were present in each treatment group on day 3 of development. At the end of 10 days of development, rod outer segment volumes were 708 +/- 195, 380 +/- 40, and 297 +/- 33 micrometer3 (mean +/- S.D.) for LL, LD, and DD treatment groups, respectively, whereas mean cone outer segment volumes were 57 +/- 8, 48 +/- 8, and 35 +/- 9 micrometer3 for these same treatment groups. The loss of outer segment material through shedding was assessed by monitoring the phagosome content of the pigment epithelium. Phagosomes were present in the pigment epithelium in DD embryos as early as 6 days of development, were present at day 5 only after the onset of the light cycle in LD embryos, and were rarely observed in LL embryos. The rate of outer segment membrane elaboration in rods (determined with 3H-leucine autoradiography) was virtually identical in LL and LD (100 and 98 micrometer3/day) but was greatly reduced in DD (70 micrometer3/day). These findings indicate that the relatively rapid rate of rod outer segment volume accumulation in LL embryos is due to a reduction in membrane loss through shedding and not to a higher rate of membrane addition as compared to animals reared in LD. In contrast, the reduced rate of rod outer segment development in DD embryos is the result of both a slower rate of membrane elaboration accompanied by the early onset and enhanced rate of membrane loss through shedding in darkness.