Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 60

The neurosensory retina varies in thickness, being thickest near the optic disc and tapering toward the ora ciliaris retinae. Ophthalmoscopically it is clear, and any disease usually results in increases in its transparency! The width of all layers decreases toward its periphery from the optic nerve head, but the nerve fiber layer contributes most to the variation in thickness. Most domestic animals have a central retina of approximately 200–240 μm and a peripheral retina of 100–190 μm. In animals with poorly vascularized or avascular retinas, retinal thickness rarely exceeds 140 μm, which is the proposed oxygen diffusion maximum for retinal tissue.

The retinal photoreceptors are the primary visual cells of the eye and are the first‐order neurons (Figure 1.54). Rods function in dim or reduced illumination, and cones function in bright light. The rods allow detection of shapes and motion, while the cones provide sharp visual acuity and color sensitivity. Primates and many avian and reptilian species possess cone‐rich regions completely free of rods; these are called foveae (i.e., fovea centralis) and are responsible for the perception of different hues of color, high resolution, binocular fixation, and depth perception. Domestic animals do not have foveal pits, but dogs have been shown to have a small fovea‐like structure, a fovea plana. Other domestic animals instead possess an area of high cone density called the area centralis. The area centralis, which surrounds the fovea plana, frequently occurs in a location 1.5 mm temporal and 0.6 mm superior to the optic disc in the dog. The visual streak is a region of the retina with increased ganglion cell density that occurs in a horizontal band, dorsal to the optic disc. The area centralis resides within the visual streak, and these terms are sometimes used synonymously. The photoreceptor layer contains only the outer parts of the photoreceptor cells known as the inner and outer segments (Figure 1.55); the photoreceptor nuclei are contained in the outer nuclear layer. These segments are cylindrically to conically shaped and closely packed together, with a radial orientation parallel to incoming light as it passes through the pupil.

Figure 1.54 The photoreceptor layer of the pig contains many cones (C) among the rods (R) within the area centralis, making this animal well suited for day vision. Note that the rods are uniform in width throughout their length. (Original magnification, 400×.)

Figure 1.55 Tip of the outer segment discs in a rod of a young dog. Note that the discs or lamellae are separated from each other as well from as the plasma membrane (PM). The intralamellar space is slightly dilated at their periphery (arrow). Apical villi (AV) of pigmented epithelium are found between outer segments. (Original magnification, 54 000×.)

An extremely thin limiting membrane composed of the junctional complexes between Müller cells, as well as between Müller and photoreceptor cells, forms a lateral, intercellular border between the inner segments of the photoreceptor layer and their nuclei. The function of the outer limiting membrane is somewhat speculative.

The somas, or cell bodies, of the photoreceptors are contained within outer nuclear layer. The number of rows of nuclei varies greatly according to species and location within the retina. In the central retina, the dog and cat possess the greatest depth of rows (10–15 and 12–18, respectively), whereas ungulates have fewer rows (5 in the horse and pig, 10 in the cow). The rod and cone connecting fibers link the photoreceptor nuclei to their respective inner segments, while the axons of the rod and cone nuclei extend into the outer plexiform layer to synapse with horizontal and bipolar cells.

The axons of the photoreceptors and their synaptic connections are the two components of the outer plexiform layer. The photoreceptors of the outer nuclear layer form connections with the horizontal and bipolar cells of the inner nuclear layer. Two distinct types of synapses occur in the outer plexiform layer, and each is specific to the rod spherule or the cone pedicle. The invagination of each rod spherule synaptic expansion contains two deeply inserted, horizontal cell processes laterally and one or more bipolar cell process centrally.

The inner nuclear layer is composed of the somas of horizontal, bipolar, amacrine, interplexiform (in some retinas), and Müller cells. The neurons in this layer maintain connections between the photoreceptor layer and the ganglion cell layer and are involved in modification and integration of the neural responses elicited by the stimuli. Specifically, horizontal and bipolar cells are second‐order neurons that connect with photoreceptors (first‐order neurons) and ganglion cells (third‐order neurons).

The inner plexiform layer comprises the cell processes of the inner nuclear and ganglion cell layers, at which synapses between bipolar, amacrine, and ganglion cells occur. The bipolar cells synapse in ribbon synapses with two postsynaptic elements, and hence are termed a dyad.

The ganglion cell layer contains different types of ganglion cells, neuroglial cells, and retinal blood vessels. It is the innermost cell layer of the retina and consists of a single layer of cells, except in the area centralis and visual streak, where it can be two or three cell layers thick. Three basic forms of ganglion cells have been described in the cat, and extensive research has related the morphology of these cells to their neurophysiology (see Chapter 2). α‐, β‐, and γ‐ganglion cells have been identified in the retina on the basis of dendritic fields, and these morphological types correspond with the three physiological types of ganglion cells (Y, X, and W). The size of the dendritic fields for each of the three morphological types varies with location in the retinal field.

The nerve fiber layer consists of the axons of ganglion cells gathered in the nerve fiber layer, converging to turn at right angles and coursing to the posterior pole at which the optic nerve exits the globe. The nerve fiber layer increases in thickness as it approaches the optic disc. To maintain transparency of the retina, the axons lack myelin sheaths. Large retinal vessels occur in the nerve fiber layer as well as in the ganglion cell and inner plexiform layers. The axons are of various sizes, and the large axons originate from the large ganglion cells (Y or α‐cells).

The inner limiting membrane is a true basement membrane formed by the fused terminations of Müller cells. Vitreal fibrils insert into the membrane, effectively establishing a “fusion” between the neurosensory retina and the vitreous body.