CONCLUSIONES Y RECOMENDACIONES

The two kinds o f photoreceptors found in the vertebrate retina are morphologically and functionally distinct from each other. The rod photoreceptors possess thin cylindrical processes, function in dim light and are responsible for scotopic (night) vision and furthermore do not perceive colour. The cone photoreceptors are generally conical in shape, function in bright light (photopic) and are responsible for colour vision. There are on average 92 million rods and 4.6 million cones in the human eye. Individual variations in the density of both rods and cones occur in different regions in the eye. The greatest variability occurs near the fovea and at the very periphery o f the retina (i.e. at the ora serrata).

1.8.2.1 Rods

Rods numbers far exceed that of cone photoreceptors and are at their greatest concentration in mid periphery o f the retina. Rod concentration decreases as approaching the centre where they are completely absent in the fovea centralis. Extending from their cell bodies, the photoreceptors have two morphologically distinct regions: the inner and outer segments. In rods the inner and outer segments are 40-60 pm long throughout the retina. The slender outer segment (25-28 pm long and 1-1.5 pm in diameter) and the

slightly thicker inner segment of rods do not show much variation in morphology from the fovea to the periphery (Stryer, 1988). The major functions of the rod cell are highly compartmentalised. The function of the rod outer segment (ROS), which lies embedded in the interphotoreceptor matrix just internal to the retinal pigment epithelium, is the conversion o f light energy into electrical impulses. The outer segments o f the rods are cylindrical in shape and contain stacks of flattened double lamellae in the form of discs that are formed by the basal invagination of the outer segment plasma membrane. There are about 1000 discs in each human rod outer segment and the discs contain 90% o f the visual pigment, rhodopsin, whereas the remaining pigment is scattered on the surface of the plasmalemma. Rhodopsin has the greatest sensitivity for blue-green light (Àmax 493nm) and allows scotopic (dim light) vision. The discs in the ROS are continually renewed throughout life by the “disc shedding” process in which the discs at the very apex o f rods are removed in a light triggered rhythmic pattern and phagocytised by the RPE cells thus maintaining the outer segments at a relatively uniform length (Young and Bok, 1969; Bok, 1985). The outer segment is connected to the inner segment by a modified cilium, which functions in transmitting cellular components from the inner segment and cell nucleus to the discs and their plasma membrane. The rod inner segment is cylindrical and is composed of a finely granular cytoplasm. There are two histologically discernible regions: an outer eosinophilic ellipsoid and an inner basophilic myoid. The ellipsoid contains a large number of long and slender mitochondria and the cytoplasm contains smooth endoplasmic reticulum, neurotubules, free ribosomes and glycogen granules. The myoid, which is the site o f major protein synthesis for new outer segment discs, contains high concentration of free ribosomes, a rough endoplasmic reticulum, glycogen granules and a golgi apparatus. The cytoplasm also contains microtubules and microfilaments that are arranged in parallel to the long axis o f the cell and extend to the level o f external limiting membrane and of the synapse, respectively. A thin cytoplasmic process (the outer fibre) connects the inner segment to the cell body that contains the nucleus. The rods synapse with second order neurons i.e. the bipolar and horizontal cells, through round or oval cytoplasmic expansions o f 1 gm in diameter known as spherules at the synaptic body. The spherule contains numerous presynaptic vesicles filled with acetylcholine, mitochondria and neurotubules. Both horizontal and

bipolar cells make contact with the rod spherule. A horizontal cell only contacts a rod spherule once, but several different horizontal cells may contact each spherule. One to four bipolar cells contact an individual spherule at separate points and each bipolar cell may contact upto hundreds at the periphery of the retina.

1.8.2.2 Cones

The density o f cones is maximal, with an average of 199, 000 cones per mm^ at the fovea, where the visual resolution is at its greatest but this number reduces to 4500 per mm^ towards the periphery. The morphology of cones differs depending on their location in the retina (Tripathy and Tripathy, 1984). Cone length is maximal (80 pm) at the fovea and gradually reduces to 40 pm at the periphery. Moreover, cones appear rod­ like at the fovea and the inner region of the outer segment becomes wider towards the periphery. Cones located in the periphery have a conical shaped outer segment with the apex pointing towards the RPE cell layer. Ultrastructurally, the cone outer segments have more discs than (1000-1200 per cone) than do rod outer segments and are also arranged differently to rods. Unlike the rod discs, cone discs are attached to each other as well as to the surface plasma membrane and are not detached easily. Disc shedding also takes place in the cone photoreceptor cells in a process in which the cone outer segments remodel themselves after each shedding event, since most apical cone outer segments have a constant but smaller diameter than the basal ones (Hogan et al., 1974; Anderson and Fisher, 1976).

Photopic (colour) vision originates in the cones by the presence of trichromatic pigments. Each cone contains one of three different iodopsin molecules that absorb light at three distinct peaks at 440 nm (blue), 540 nm (green) and 577 nm (orange) referred to as S- (short), M- (medium), and L- (long) wavelength cones, respectively (Dacey and Lee, 1994). Cone synapses have cytoplasmic expansions known as pedicles that are larger (5 pm in the fovea) than the rod spherules and are pyramidal in shape. At the fovea only one bipolar cell contacts one cone pedicle along with two different horizontal cells.