 The 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 that actively synthesize proteins. A prominent Golgi apparatus, rER, and free ribosomes are concentrated in the myoid region. Mitochondria are most numerous in the ellipsoid region. Micro-tubules 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 supports the activity of the photoreceptor cells. The outer segment is considered a highly modified cilium because it is joined to the inner segment by a short connecting stalk containing a basal body (Fig. 24.12a).
With the TEM, 600 to 1,000 regularly spaced horizontal discs are seen in the outer segment (Fig. 24.12). In rods, these
FIGURE 24.11 • 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.
discs are membrane-bounded structures measuring about 2 m in diameter. They are enclosed within the plasma membrane of the outer segment (Fig. 24.12a). 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 life span. 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 phagocytosed 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. 24.12b).
Rod cells contain the visual pigment rhodopsin; cone cells contain the visual pigment iodopsin.
Rhodopsin (also called 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 defciency 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—that is, 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. It then takes several days for the disc to reach the tip of the outer segment. In contrast, although visual proteins are constantly produced in retinal cones, the proteins are incorporated into cone discs located anywhere in the outer segment.