These expanding membrane plates (b-c, Fig. 5 above) become detached as free floating discs inside the outer segment membrane in the case of the rods. In the case of the cones though, the outer segment discs remain attached to the outer segment membrane.

So the outer segment is a structure filled entirely with discs of folded double membranes in which are embedded the light sensitive visual pigment molecules.

Fig. 6. Drawing of rod outer segment discs (39 K jpeg image)

Vertebrate photoreceptors can respond to light by virtue of their containing a visual pigment embedded in the bilipid membranous discs that make up the outer segment. The visual pigment consists of a protein called opsin and a chromophore derived from vitamin A known as retinal. The vitamin A is manufactured from beta-carotene in the food we eat, and the protein is manufactured in the photoreceptor cell (see above). The opsin and the chromophore are bound together and lie buried in the membranes of the outer segment discs.

Fig 8. Schematic diagram of Rhodopsin in the outer segment discs (59 K jpeg image)

Fig. 9. Structural model of Rhodopsin (59 K jpeg image)

About 50% of the opsin is within the bilipid membrane connected by short protein loops outside. Each molecule of rhodopsin consists of seven of these transmembrane portions surrounding the chromophore (11-cis retinal) in the lipid bilayer. The chromophore apparently lies horizontally in the membrane and is bound at a lysine residue to helix seven (Hargrave et al. 1984, Hargrave and McDowell, 1992). Each outer segment disc, of course, contains many (thousands) visual pigment molecules. Upon absorption of a photon of light, the retinal isomerizes from the 11-cis form to an all-trans form which starts conformational changes in the molecule resulting in bleaching. Several intermediaries are formed in bleaching among them metarhodopsin II which activates the G-protein transducin and a further cascade of events summarized below (see review by Hargrave and McDowell (1992) and by Archer, 1995).

Light transduces the visual pigment via the following enzyme cascade: photons - rhodopsin - activated rhodopsin (metarhodopsin II) - a GTP binding protein (transducin) - an enzyme hydrolyzing cGMP (cGMP-phosphodiesterase) - closes a membrane bound cGMP-gated cation channel.

In the dark a steady current flows into the open channels, carried mainly by Na ions, constituting a "dark current" that partially depolarizes the photoreceptor cell. Thus, the depolarized photoreceptor releases neurotransmitter (thought to be the amino acid glutamate) from its synaptic terminals upon second-order neurons in the dark. On light stimulation the rhodopsin molecules are isomerized to the active form, the above cascade ensues, leading to closure of the cation channels of the photoreceptor membrane, stopping the dark current and causing the photoreceptor cell membrane to hyperpolarize and cease neurotransmitter release to second-order neurons (see Stryer, 1991; Yau, 1994, and Kawamura, 1995, for reviews).

The "dark current" is composed mainly of the influx of the Na+ component (80%) however, a Ca2+ component (15%) and a Mg2+ component (5%) are also present (Yau, 1994). In darkness there must be a mechanism to remove Ca2+ as well as the excess Na+, and it is thought to be done so through a sodium/calcium exchanger in the membranes of the outer segment of the photoreceptor. Ca2+, once thought to be the second messenger in linking the rhodopsin photoisomerization to the membrane events is now known to have a secondary but important regulatory role in phototransduction. Although it does not directly participate in the transduction cascade it does improve the signalling capability of the rods in speeding the recovery after illumination and down regulating of the rods sensitivity in steady illumination (Yau, 1994). The latter effect is a mechanism for light adaptation.

For it must be remembered that a photoreceptor cell does not simply detect light. It can also adapt to environmental light. For example cone photoreceptors can adapt so that our visual system can see from the dim shadows under a tree to objects in bright sunlight snow, a shift of light intensity of 7-9 log units of light intensity (Normann et al., 1991). Rod photoreceptors, thought at one time not to light adapt, are now known to adapt over a range of 2 log units of background intensity and combined with a network adaptation through the whole visual system, allows as much as 5 log units of background intensity adaptation in rod driven vision (Yau, 1994).

4. Phagocytosis of outer segments by pigment epithelium.

The stacks of discs containing visual pigment molecules in the outer segments of the photoreceptors are constantly renewed. New discs are added at the base of the outer segment at the cilium as discussed above. At the same time old discs are displaced up the outer segment and are pinched off at the tips and engulfed by the apical processes of the pigment epithelium. These discarded, spent discs become known as phagosomes in the pigment epithelial cells. They are then broken down by lysis. Photoreceptor outer segment discs are phagocytosed by the pigment epithelium in a diurnal cycle. There is a burst of disc shedding at light on in the morning, judged by increased numbers of phagosomes in the pigment epithelium shortly thereafter (Young 1971).

Click here to see an animation of the phagocytosis (956 K quicktime movie)

Fig. 11. Section through an RPE cell to show stages of outer segment phagocytosis (59 K jpeg image)

Fig. 12. Diagrammatic representation of disc shedding and phagosome formation in the pigment epithelial cell (59 K jpeg image)

Cone outer segments differ from rod outer segments in several respects. Firstly, they are shorter and more conical with a wider base and tapering shape compared with those of rods. Secondly, as mentioned above, their discs are connected to the plasma membrane throughout the extent of the outer segment, and thus they are open to extracellular space. Apical processes of the pigment epithelium phagocytize chunks of cone outer segments, just like they do the rod outer segments, but at a different time in the diurnal cycle compared with rods i.e. at light off-set compared light-onset (see original work by Young, 1971, 1976; LaVail, 1976; Steinberg et al., 1977; reviewed by Besharse, 1982).

[Light microscopy and ultrastructure] [Outer segment generation] [Visual pigments and visual transduction] [Phagocytosis of outer segments] [Different types of cones] [Morphology of S-cones] [Densities of rods and cones in human retina] [Rods and night vision] [Ultrastructure of synaptic endings] [Inter-photoreceptor contacts] [References]

Updated: June, 2007