Figure 2311

Schematic diagram of the ultrastructure of rod and cone cells. The outer segments of the rods and cones are closely associated with the adjacent pigment epithelium.

ports the activity of the photoreceptors. The outer segment is considered to be a highly modified cilium because it is joined to the inner segment by a short connecting stalk containing a basal body (Fig. 23.12a).

With the TEM, 600 to 1000 regularly spaced horizontal discs are seen in the outer segment (Fig. 23.12). In rods, these discs are membrane-bounded structures measuring about 2 /jcm in diameter. They are enclosed within the plasma membrane of the outer segment (Fig. 23.12a). The parallel membranes of the discs are about 6 nm thick and are continuous at their ends. The central enclosed space is about 8 nm across. In both rods and cones, the membranous discs are formed from repetitive transverse infolding of the plasma membrane in the region of the outer segment near the cilium. Autoradiographic studies have demonstrated that rods form new discs by infolding of the plasma membrane throughout their lifespan. Discs are formed in cones in a similar manner but are not replaced on a regular basis.

Rod discs lose their continuity with the plasma membrane from which they are derived soon after they are formed. They then pass like a stack of plates, proximally to distally, along the length of the cylindrical portion of the outer segment until they are eventually shed and phagocy-tosed by the pigment epithelial cells. Thus, each rod disc is a membrane-enclosed compartment within the cytoplasm. Discs within the cones retain their continuity with the plasma membrane (Fig. 23.12b).

Each rod and cone photoreceptor consists of three parts:

• Outer segment of the photoreceptor is roughly cylindrical or conical (hence, the descriptive name rod or cone). This portion of the photoreceptor is intimately related to microvilli projecting from the adjacent pigment epithelial cells.

• Connecting stalk contains a cilium composed of nine peripheral microtubule doublets extending from a basal body. The connecting stalk appears as the constricted region of the cell that joins the inner to the outer segment. In this region, a thin, tapering process called the calyceal process extends from the distal end of the inner segment to surround the proximal portion of the outer segment (see Fig. 23.11).

• Inner segment is divided into an outer ellipsoid and an inner myoid portion. This segment contains a typical complement of organelles associated with a cell actively synthesizing proteins. A prominent Golgi apparatus, rER, and free ribosomes are concentrated in the myoid region. Mitochondria are most numerous in the ellipsoid region. Microtubules are distributed throughout the inner segment. In the outer ellipsoid portion, cross-striated fibrous rootlets may extend from the basal body among the mitochondria.

The outer segment is the site of photosensitivity, and the inner segment contains the metabolic machinery that sup-

Rod cells contain the visual pigment rhodopsin; cone cells contain the visual pigment iodopsin

Rhodopsin (visual purple) in rod cells initiates the visual stimulus when it is bleached by light. Rhodopsin is present in globular form on the outer surface of the lipid bilayer (on the cytoplasmic side) of the membranous discs. In the cone cells, the visual pigment on the membranous discs is the photopigment iodopsin. Each cone cell is specialized to respond maximally to one of three colors: red, green, or blue. Both rhodopsin and iodopsin, contain a membrane-bound subunit called an opsin and a second component called a chromophore. The opsin of rods is scotopsin; the opsins of cones are photopsins. The chromophore of rods is a vitamin A-derived carotenoid called retinal. Thus, an adequate intake of vitamin A is essential for normal vision. Prolonged dietary deficiency of vitamin A leads to the inability to see in dim light ("night blindness").

The interior of the discs of cones is continuous with the extracellular space

The basic difference in the structure of the rod and cone discs, i.e., continuity with the plasma membrane, is correlated with the slightly different means by which the visual pigments are renewed in rods and cones. Newly synthesized rhodopsin is incorporated into the membrane of the rod disc as the disc is being formed at the base of the outer segment.

cilium calyceal process basal body

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