Photoreceptor Organelles in Animals

Abstract

Summary 1. The relatively high refractive index of rod and cone outer segments causes them to show wave‐guide properties. 2. Rod outer segments have a positive intrinsic birefringence and a negative form birefringence. The former results from oriented lipid molecules, the latter from the disk structure of the outer segment. 3. Electron microscopy of outer segments shows that the disks are formed from infoldings of the surface membrane, each disk consisting of two folds adhering together on their outer surfaces. Studies of other cell membranes have suggested that their lipid molecules form a continuous or discontinuous bimolecular leaflet which is sandwiched between protein layers, and this structure probably applies to each of the disk membranes. 4. From the lipid content of rod outer segments a rough estimate of the thickness of a continuous lipid layer in each disk membrane can be made. This is only a little smaller than the requirements of a bimolecular leaflet. 5. Rhodopsin is a lipoprotein whose lipid content forms a considerable fraction of the total outer‐segment lipid. This and other data suggest that rhodopsin may be a structural component of the disk membrane. 6. The initial effect of light on rhodopsin is probably to isomerize II‐cis to all‐trans retinene, and various thermal rearrangements follow which eventually cause the detachment of retinene from opsin. The absorption spectrum of rhodopsin is similar in situ and in solution, but the absorption spectra of the photoproducts are somewhat different. 7. Rhodopsin is strongly dichroic in the rod, indicating that the retinene molecule lies mainly in the plane of the disks. 8. Rhabdomeres, the photoreceptor organelles of many higher invertebrates, are formed from close‐packed microvilli. They show a negative birefringence which probably results from the lipid molecules within the microvillar membrane. 9. All well‐authenticated rhabdomere visual pigments have a retinene chromophore, and in many features the photochemical processes resemble those of vertebrate rhodopsin. The extraction processes for vertebrate and invertebrate rhodopsins are also similar. 10. The question of whether insect rhabdomeres show dichroism is still open, but there is good evidence for dichroism in some cephalopod rhabdomeres. This is probably the physiological basis for the polarized light discrimination shown by octopus. 11. Some possible mechanisms for the transmission of the light stimulus within photoreceptor organelles are discussed.

Rods and Cones

“…Useful early reviews are those of SJOSTRAND (1959bSJOSTRAND ( , 1963, , MOODY and ROBERTSON (1960), FERNANDEZ-MORAN (1961), andMOODY (1964). More recent views of the general molecular architecture of cell membranes are those of ROBERTSON (1966), KORN (1966), Lucy (1968), andCHERRY (1968).…”

Localization of vitamin A in the eye of a pulmonate snail.

“…Heretofore, it has been assumed that the microvilli of rhabdomeric eyes8 are photoreceptoral organelles because of their anatomical position (e.g., immediately beneath a lens); their fine structure (e.g., extensive membranous surfaces in orderly arrays); spectrophotometric identification of rhodopsin in heads, eves, and rhabdomeric fractions from arthropods and mollusks (see ref. 9 for references); and lastly because of loss of spectral sensitivity due to vitamin A deficiency." We have made preliminary studies on extracts of eyes of II.…”

Comparative Physiology and Evolution of Vision in Invertebrates

“…Rhabdomeric (microvilli) membrane is dichroic, i.e., its absorption depends on the direction of the electric vector E oflinearly polarized light (MOODY, 1964;BORN and WOLF, 1970). Measurements of dichroism indicate the degree to which the absorbing dipoles (chromophore group) are aligned within the visual membrane.…”