Figure 239

Schematic drawing of the layers of the retina. The interrelationship of the neurons is indicated. Light enters the retina and passes through the inner layers of the retina before reaching the photoreceptors of the rods and cones that are closely associated with the pigment epithelium.

incident light

inner limiting membrane cells outer plexiform inner segment of cone and rod limiting membrane outer segment of cone and rod cone pigment epithelium

5. Outer plexiform layer—contains the processes of retinal rods and cones and processes of the horizontal, amacrine, and bipolar cells that connect to them

6. Inner nuclear layer—contains the cell bodies (nuclei) of horizontal, amacrine, bipolar, and Miiller's cells

7. Inner plexiform layer—contains the processes of horizontal, amacrine, bipolar, and ganglion cells that connect to each other

8. Ganglion cell layer—contains the cell bodies (nuclei) of ganglion cells

9. Layer of optic newe fibers—contains processes of ganglion cells that lead from the retina to the brain

10. Inner limiting membrane—composed of the basal lamina of Miiller's cells

Each of the layers is more fully described in the following sections (see corresponding numbers).

The cells of the RPE (layer 1) have extensions that surround the processes of the rods and cones

The RPE is a single layer of cuboidal cells about 14 /xm wide and 10 to 14 /xm tall. The cells rest on Bruch's membrane of the choroid layer. The pigment cells are tallest in the fovea and adjacent regions, which accounts for the darker color of this region.

Adjacent RPE cells are connected by a junctional complex consisting of gap junctions and elaborate zonulae oc-cludentes and adherentes. This junctional complex is the site of the "blood-retina barrier."

The pigment cells have cylindrical sheaths on their apical surface that are associated with, but do not directly contact, the tip of the photoreceptor processes of the adjacent rod and cone cells. Complex cytoplasmic processes project for a short distance between the photoreceptors of the rods and cones. Numerous elongated melanin granules, unlike those found elsewhere in the eye, are present in

A potential space exists in the retina as a vestige of the space between the apical surfaces of the two epithelial layers of the optic cup. If this space expands, the neural retina separates from the pigment epithelium. This condition is called retinal detachment. If not corrected, blindness results. More commonly, as the vitreous body ages (in the sixth and seventh decades of life), it tends to shrink and pull away from the neural retina, which causes single or multiple tears in the neural retina. Retinal detachment is repaired by cryosurgery or laser surgery to prevent visual loss.

many of these processes. They aggregate on the side of the cell nearest the rods and cones and are the most prominent feature of the cells. The nucleus with its many convoluted infoldings is located near the basal plasma membrane adjacent to Bruch's membrane. The cells also contain material phagocytosed from the processes of the photoreceptors in the form of lamellar debris contained in residual bodies or phagosomes. A supranuclear Golgi apparatus and an extensive network of smooth endoplasmic reticulum (sER) surround the melanin granules and residual bodies that are present in the cytoplasm.

The RPE serves several important functions including

• Absorption of light passing through the neural retina to prevent reflection and resultant glare.

• Isolation of the retinal cells from blood-borne substances. It serves as a major component of the blood-retina barrier via tight junctions between RPE cells.

• Participation in restoration of photosensitivity to visual pigments that were dissociated in response to light. The metabolic apparatus for visual pigment resynthesis is present in the RPE cells.

• Phagocytosis and disposal of membranous discs from the rods and cones of the retinal photoreceptor cells.

The rods and cones of the photoreceptor cell (layer 2) extend from the outer layer of the neural retina to the pigment epithelium

The rods and cones are the outer segments of photoreceptor cells whose nuclei form the outer nuclear layer of the retina (Figs. 23.10 and 23.11). The light that reaches the photoreceptors must first pass through all of the internal layers of the neural retina. The rods and cones are arranged in a palisade manner; therefore, in the light microscope, they appear as vertical striations.

The retina contains approximately 120 million rods and 7 million cones. The rods are about 2 /xm thick and 50 /xm long (ranging from about 60 /xm at the fovea to 40 /xm peripherally). The cones vary in length from 85 /xm at the fovea to 25 /xm at the periphery of the retina.

Functionally, the rods are more sensitive to light and are the receptors used during periods of low light intensity (e.g., at dusk or at night). The visual image provided is one composed of gray tones ("a black and white picture"). In contrast, the cones exist in three classes: L, VI, and S (long-, middle-, and short-wavelength sensitive, respectively) that cannot be distinguished morphologically. They are less sensitive to low light and have maximal sensitivity to the red, green, or blue region of the visual spectrum. They provide a visual image composed of color, which is believed to permit better visual acuity. The specificity of the cones provides a functional basis to explain color blindness that is believed to result from the lack of red-, green-, or (much less commonly) blue-sensitive cones.

ROD CONE

synaptic lamella inner rod fiber outer rod fiber inner segment'

outer segment-

inner rod fiber outer rod fiber inner segment'

liÈ—calyceal process nucleus outer limiting membrane

Golgi apparatus mitochondria liÈ—calyceal process outer segment-

nucleus outer limiting membrane

Golgi apparatus mitochondria

pedicle inner cone fiber inner segment outer segment pedicle inner cone fiber inner segment outer segment pigment epithelial cells

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