Maintenance and health of rod receptors of vertebrates is thought to depend on turnover; new visual membrane (disks) is added to the base of the rod outer segment while old disks are shed from the tips (Young, 1978) and phagocytosed by RPE (retinal pigment epithelial cells).


Although most of my research focused on the fruit fly Drosophila, two research collaborations dovetailed intimately with this model of the phagolysosomal maintenance in the vertebrate: (1) We studied the fluorescence properties of the ³indigestible residue² in the RPE, the so-called aging pigment lipofuscin (Eldred et al, 1982). Also, we found that the sensitivity recovery of vitamin A deprived rats after vitamin A replenishment followed a time course expected for the rebuilding of a new outer segment (Katz et al, 1993).


Invertebrates also display turnover of visual membrane; in mosquito (White & Lord, 1975), crab (Nassel & Waterman, 1979) and horseshoe crab (Chamberlain & Barlow, 1984), rhabdoms are large in the dark and small in the light. Presumably, internalized vesicles are re-endocytosed into the forming multivesicular body (MVB) in crayfish, reversing the apparent p- vs. e-face half membrane leaflets (Eguchi & Waterman, 1976). In crayfish, internalized membrane was traced from MVBs through various stages in lysosomal degradation such as multilaminate bodies (Hafner et al, 1980). These few selected references represent the tip of the iceberg in a vast literature on invertebrate turnover.


I brought Drosophila into the turnover literature. MVBs were conspicuous as were coated pits, coated vesicles and acid phosphatase positive primary lysosomes (Stark et al, 1988); interestingly, coated pits were internalized from the plasmalemma as well as from the base of the rhabdomere. As expected, coated pits could not pinch off to coated vesicles in the shibire (dynamin) mutant at the restrictive temperature (Sapp et al, 1991b). Unlike other invertebrate models, the circadian rhythm of visual pigment level, sensitivity, MVB count, and rhabdomere cross sectional area resulted in small, not massive, changes (Chen et al, 1992).




Chamberlain SC, Barlow RB, Jr. (1984) Transient membrane shedding in Limulus photoreceptors: control mechanisms under natural lighting. J Neurosci 4(11): 2792-2810


Chen D-M, Christianson JS, Sapp RJ, Stark WS (1992) Visual receptor cycle in normal and period mutant Drosophila: Microspectrophotometry, electrophysiology, and ultrastructural morphometry. Vis Neurosci 9: 125-135


Eguchi E, Waterman TH (1976) Freeze-etch and histochemical evidence for cycling in crayfish photoreceptor membranes. Cell Tiss Res 169: 419-434


Eldred GE, Miller GV, Stark WS, Burns LF (1982) Lipofuscin: resolution of discrepant fluorescence data. Science 216: 757-759


Hafner GS, Hammond-Scoltis G, Tokarski T (1980) Diurnal changes of lysosome-related bodies in the crayfish photoreceptor cells. Cell Tissue Res 206(2): 319-332


Katz ML, Chen D-M, Stientjes HJ, Stark WS (1993) Photoreceptor recovery in retinoid-deprived rats after vitamin A replenishment. Exp Eye Res 56: 671-682


Nassel DR, Waterman TH (1979) Massive diurnally modulated photoreceptor membrane turnover in crab light and dark adaptation. J Comp Physiol A 131: 205-216


Sapp RJ, Christianson JS, Stark WS (1991b) Turnover of membrane and opsin in visual receptors of normal and mutant Drosophila. J Neurocytol 20: 597-608


Stark WS, Sapp RJ, Schilly D (1988) Rhabdomere turnover and rhodopsin cycle: maintenance of retinula cells in Drosophila melanogaster. J Neurocytol 17: 499-509


White RH, Lord E (1975) Diminution and enlargement of the mosquito rhabdom in light and darkness. J Gen Physiol 65(5): 583-598


Young RW (1978) Rhythmic shedding of rod and cone membranes. Invest Ophth Vis Sci 17(2): 105-116