Retina, Choroid, Sclera

Chapter 11 Retina, Choroid, Sclera



INTRODUCTION


The retina is a multicellular layer 100 to 250μm thick that lines the posterior segment of the eye. It is sandwiched between the vitreous and retinal pigment epithelium (RPE) and choroid, from which its outer layers receive nutrition. The normal retina is almost transparent; the cellular arrangement is such that light can pass through the retina with limited impediment to reach the photoreceptors (rods and cones) that absorb light and convert it into electrochemical energy. The photoreceptors are adjacent to the RPE. Some light is absorbed by the retinal layers, however, which becomes evident when the retina undergoes degeneration and becomes thinner. In dogs and cats some of the light incident on the retina is absorbed as it passes through the photoreceptor layer. The light then hits the tapetum cellulosum in the choroid, which acts as a mirror and reflects the light back via the retina, where more is absorbed to maximize the light-gathering power of the eye. For this to be possible, little light must be absorbed in the intervening RPE cell layer, and it is for this reason that the RPE is largely nonpigmented over the area of the tapetal fundus. The tapetal fundus occupies the upper hemisphere of the fundus. The nontapetal fundus occupies the lower hemisphere of the posterior segment and also surrounds the periphery of the tapetal fundus superiorly, nasally, and temporally.


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Figure 11-97 Rod-cone dysplasia type 1 in a 14-month-old Irish setter—the same dog as shown in Figures 11-98 and 11-99. This photograph shows the peripheral tapetum and marked tapetal hyperreflectivity and vessel attenuation.



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Figure 11-99 Rod-cone dysplasia type 1 in a 14-month-old Irish setter—the same dog as shown in Figures 11-97 and 11-98. Tapetal hyperreflectivity, vessel thinning, optic atrophy, and glial proliferations at edge of the nerve (gliosis) can be seen.





The layered structure of the retina, RPE, choroid, and sclera can be reduced to a schematic to illustrate how in various areas of the normal fundus the ophthalmoscopic view appears as it does. The appearance of and variations in the fundus owe much to the pigmentation in the eye as a whole, which can vary considerably among species and individual animals or even between the two eyes of an animal.



NORMAL FUNDUS IN DOGS



Normal Tapetum in Dogs


When a dog’s tapetal fundus is viewed ophthalmoscopically, retinal vessels are superimposed on the highly colored tapetum. The only visible components of the retina are the blood vessels. The RPE in this region is nonpigmented to allow light to reach and be reflected by the tapetum. Therefore, the RPE layer is not visible either. The tapetum is visible as the aggregate of numerous tapetal cells that overlap one another to make a layer of variable thickness, depending on the location in the posterior segment, and varies between eyes according to the amount of pigment in the eye because the tapetum consists of modified pigment cells. The tapetum may be gray, violet, blue, green, yellow, or reddish orange. The surface area of the posterior segment covered by the tapetum also may vary, tending to occupy a larger area in larger dog breeds (and including the optic nerve in the tapetal area) and a smaller area (usually more on the temporal area of the fundus) in small breeds. Some dogs have no tapetum at all. The border of the tapetal and nontapetal fundus may be sharply defined (usually more likely in short-coated breeds) or irregular with interlocking of tapetal and nontapetal regions (in longer-coated animals). Dogs of same breeds often have the same tapetal appearance and often color.


Because the tapetum acts as a mirror to reflect light for a second pass through the retina, the underlying choroid and sclera in the tapetal area of the fundus are not usually visible (at least if the tapetum is well developed).



































































































Normal Nontapetum in Dogs


In the nontapetum of a normally pigmented dog the RPE is pigmented. The ophthalmoscopic appearance of the nontapetum is brown to black, with retinal vessels overlying the RPE.





































































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Figure 11-84 Australian shepherd from Figure 11-83. Some areas of RPE are normally pigmented; other areas are nonpigmented, and choroidal vessels can be seen.





Jul 31, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Retina, Choroid, Sclera
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