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Choroid The choroid is the posterior portion of the uveal coat. It is composed primarily of blood vessels and pigmented support tissues. It is the main source of nutrition for the outer layers of the retina. In most domestic animals, the anterior margin of the choroid joins the ciliary body along a...

Choroid The choroid is the posterior portion of the uveal coat. It is composed primarily of blood vessels and pigmented support tissues. It is the main source of nutrition for the outer layers of the retina. In most domestic animals, the anterior margin of the choroid joins the ciliary body along a regular, non-serrated junction called the ora ciliaris retinae. In primates, the junction is irregular and serrated and termed the ora serrata. The choroid tends to thicken along the posterior pole, becoming thinner toward the globe equator. For morphologic discussions, the choroid is divided externally to internally, as shown in Fig. 2.61. A. The canine choroid (C) consists of the suprachoroidea (1), large-vessel layer (2), medium-sized vessel and tapetum layer (3), and choriocapillaris (4). R, retina; BV, blood vessel. * denotes a nerve within the sclera (S). B. SEM shows a close-up view of the outer choroid, where the suprachoroidea (Su) forms fine collagenous attachments (arrows) with lamina fusca of the sclera (S). (Original magnification, 850×.) 1) the suprachoroidea 2) the large‐vessel layer 3) the medium‐sized vessel and tapetum layer, and 4) the choriocapillaris. The tapetal layer varies among species, and it is absent in pigs, squirrels, rodents, rabbits, kangaroos, llamas, alpacas, and many nonhuman primates. Suprachoroidea The suprachoroidea consists of elastic, heavily pigmented connective tissue that forms a transition between the sclera and the choroid. Branching collagen lamellae traverse the potential suprachoroidal space to fasten the choroid to the lamina fusca of the inner sclera (see Fig. 2.61). The suprachoroidea also functions as the posterior component for uveoscleral outflow. Aqueous humor that has moved along this narrow junction of the sclera and choroid diffuses into the sclera and, subsequently, the systemic circulation. The layers of melanocytes and fibrocytes and the interspersing collagen and elastic fibers may produce resistance to uveoscleral drainage, even though a cellular barrier has not been found. The long posterior ciliary nerves and arteries course their way anteriorly in the suprachoroidea along the horizontal meridian. In birds, this region also contains a layer of nonvascular smooth muscle cells. This layer is believed to be innervated by somatostatin‐expressing neurons located within the avian choroid. The function of this band or layer of smooth muscle within the suprachoroidea is unknown, but its location suggests that it could play a significant role in uveoscleral outflow and IOP regulation. Large-Vessel Layer Immediately internal to the suprachoroidea is a vascular plexus embedded in loose connective tissue containing melanocytes and fibrocytes. This plexus is composed mostly of large veins and scattered arteries (Fig. 2.62). SEM corrosion cast of the choroidal vasculature in the dog. Large arrows indicate collateralization of the intrascleral plexus (ISP) with the large choroidal veins (V). Small arrows indicate choroidal arteries. (Original magnification, 10×.) These veins merge centripetally into four or more prominent vortex veins located obliquely near the globe equator between the horizontal and vertical meridians. In cross section, the veins are cavern‐like (indent), occupying 50% or more of the total volume of the choroid (see Fig. 2.61 and Fig. 2.63). SEM of the posterior canine eye shows the choroid (C) is composed mostly of large, cavernous veins (V) that drain the choriocapillaris (arrow), which nourishes the outer retina (R). S, sclera. (Original magnification, 25×.) They frequently collateralize with each other and communicate anteriorly with anterior ciliary veins, the intrascleral venous plexus, and the iridal veins The large arteries, which are much fewer in number, are mostly branches of the short posterior ciliary arteries, which enter the globe in the vicinity of the optic nerve and supply the retina, optic nerve, and choroid. To a lesser extent, the choroid also receives blood from the long posterior ciliary arteries and the anterior ciliary arteries In addition to providing the major source of oxygen and nutrients for the retina, the large vessels may act as a “cooling system,” dissipating the heat produced from light absorption. The osmotic pressure created by high levels of plasma proteins in the choroidal tissue fluid might also assist in keeping the retina attached to the retinal pigment epithelium (RPE), by allowing retinal fluids to pass into the choroid, then subsequently into the suprachoroidea, sclera, and episcleral tissues. In the avian eye, the large veins are directly associated with another bed of vessels, which have been referred to as sinusoids, lacunae, or most recently, lymphatic vessels. These vessels comprise a major portion of the choroida volume, and they serve as a large reservoir that may assist in IOP regulation by removal of fluid from the blood vessels. Morphologically, the vessels are similar to lymphatic vessels, with a fenestrated endothelium but no innervation or defined, basal lamina‐associated smooth muscle. These vessels are mostly located next to the suprachoroidea, and a portion of them extends into adjacent large veins as the latter exit the choroid (Fig. 2.64). The outer choroid in the avian (i.e., chicken) eye consists of sinusoids (S) that extend to the suprachoroidea (arrows) and adjacent “cup” of hyaline cartilage. A, arteries; V, veins. (Original magnification, 200×.) The combination of these accessory vessels in the outer choroid and the previously discussed thin band of smooth muscle within the suprachoroidea distinguishes the avian from the mammalian choroid. Medium-Sized Vessel and Tapetum Layer A small layer of medium‐sized vessels and pigmented reticular connective tissue lies internal to the large‐vessel layer. The medium sized vessels, especially the arteries, dichotomously branch, radiating slightly inward in a fanlike manner from the larger vessels. The cells surrounding these vessels consist of melanocytes and fibrocytes. In heavily pigmented individuals, the melanocyte is the predominant cell type. In most domestic animals, the melanocyte possesses characteristically oval‐to‐round melanin granules ranging from 0.1 to 4.0 μm in diameter. The extracellular space consists of loosely arranged bundles of collagen interspaced with numerous elastic fibers. Non-myelinated nerve fibers in the choroid are predominantly associated with the vascular system, and most of the nerves are associated with the arterial system. They arrive in the choroid as short ciliary nerves around the optic nerve and provide numerous collaterals in the posteroanterior route. They follow the short posterior ciliary artery branches, and they give off anastomotic branches. Most of the nerve endings provide motor input to the smooth muscles of the arteries. !!!!!!NB: In most domestic animals, the dorsal portion of the choroid at the medium‐sized vessel layer contains a layer of reflective tissue called the tapetum lucidum. The tapetum is roughly triangular in shape when viewed funduscopically, and it varies in color (Fig. 2.65). The tapetum lucidum (T), which is always located dorsally, usually ends along the horizontal plane next to or including the optic nerve head (ON). In this specimen, a portion of the retina and choroid (*) have been removed to demonstrate the posterior eyewall, the sclera (S). Feline globe. It reflects light that has passed through the retina and thus restimulates the photoreceptor cells. Animals without a tapetum lucidum have diurnal habits (activity during daytime, with a period of sleeping or other inactivity at night) and red or orange to pale gray (depending on the amount of choroidal pigmentation) fundic reflections. The red‐to‐orange backgrounds result from reflection of light from choroidal blood vessels in less pigmented individuals. The tapetal layer is composed of regularly arranged collagenous fibers in herbivores (i.e., the tapetum fibrosum in horses, cattle, sheep, and goats) and of specific polyhedral cells, or iridocytes, containing reflecting crystals in carnivores (i.e., the tapetum cellulosum in the dog and cat). Microscopically, the tapetum is interposed between the branching vessels in the choroid and the single layer of the choriocapillaris beneath the retina. The thickness of the tapetum varies, being multilayered at its center and thinning to a single cell (or lamella) at its periphery and adjacent to the optic nerve. The tapetum develops late in animals born with immature eyes (e.g., dogs and cats); it is usually completely developed by 4 months of age. Animals born with mature eyes (e.g., ungulates and equines) have a well‐developed tapetum at birth. Histologically, the tapetum cellulosum is composed of rectangular‐ shaped cells, iridocytes, with a species‐dependent variability in number of cell layers (Fig. 2.66; Table 2.10, pg 83). The carnivorous tapetum lucidum (TL) consists of layers of cells, called iridocytes, which vary in number, size, and composition. A. The dog. B. The cat. (Original magnification: 200×.) The tapetal layer is thickest centrally and thins toward the periphery until the tapetum cellulosum is replaced by regular choroidal stroma. From the underlying choroidal stroma, numerous small vessels penetrate the tapetal layer to form a single‐layered capillary bed, known as the choriocapillaris network, on the inner surface of the tapetum. The most striking ultrastructural feature of tapetal cells in the dog and cat is the presence of numerous slender, electron‐ dense rods in the cytoplasm. These rods in the cat are densely packed, uniform in size, circular in cross section, and circumferentially arranged into highly organized groups (Fig. 2.67). Tapetal cells of a cat. The main cytoplasmic components are the rods; mitochondria (M) are infrequent. The rods are uniform in a group concerning spatial orientation, but more than one group is often present within a cell (arrows). The sides of adjacent cells have wide intercellular spaces with scattered collagen fibrils (C), whereas the ends of the cells are in close apposition. N, nucleus of tapetal cell. (Original magnification, 7750×.) Within each cell, there may be several groups of rods, often oriented at right angles to each other but still maintaining their long axis parallel to the retinal surface. The rods are membrane‐bound and pack the cytoplasm so that the remaining organelles are limited to the perinuclear in small numbers in the tapetal cells on the scleral side of the tapetum), and tapetal rods have the ability to bind calcium at a similar level to melanin; this suggests that, in all likelihood, the iridocyte is a modified melanocyte. A high level of zinc (zinc cysteine) is present in canine iridocytes, whereas feline iridocytes contain abundant riboflavin and zinc In ungulates, closely and regularly arranged collagen fibers comprise the tapetum, which is often referred to as a fibrous tapetum. The fibrous tapetum is basically acellular, except for an occasional fibrocyte. The collagen fibrils are organized into well‐ordered lamellae that branch and interconnect with adjacent lamellae at the same level, parallel with the retinal surface. The collagen fibrils within each layer vary little in diameter (approximately 80 nm) and have a regular spatial arrangement similar to that seen in the corneal stroma (see Fig. 2.26). Small blood vessels, typically capillaries, penetrate the tapetum at right angles to the long axis of the iridocytes in carnivores, and to the collagen lamellae in herbivores, directly interconnecting the medium‐sized blood vessels with the choriocapillaris (Fig. 2.68). Capillaries (C) vertically interconnect mediumsized blood vessels with the choriocapillaris (CC) in the cat. The iridocytes (I), or tapetal cells, line up evenly next to these capillaries. The end of each iridocyte is bordered by mitochondria (m). F, fibrocyte. (Original magnification, 7200×.) When observed ophthalmoscopically, these end‐on vessels are sometimes called the “stars of Winslow.” In the dog and cat, these vessels are separated from the iridocytes by a small amount of loose connective tissue and a lining of fibrocytes (see Fig. 2.68). In nonmammalian species, the tapetum lucidum is either located within the retina (teleosts and some reptiles) or within the choroid (elasmobranches). The tapetum lucidum is consistently absent in birds. With a retinal tapetum lucidum, either lipid‐ or guanine‐laden material lies within the cytoplasm of the RPE. Among teleosts, movement of melanin granules within the RPE occurs depending on the environmental illumination, with the melanin granules migrating toward the vitreous humor (forward) during illumination or toward the sclera (backward) during dim light or darkness. The movement of melanin granules causes the reflective material within the RPE to become masked or occluded. In a phenomenon termed retinomotor movement, RPE cell processes and the photoreceptors experience positional rearrangements based on changes in ambient lighting. The RPE processes move between and over the reflective cells within the tapetum during illumination, masking the reflective material. Then in low light conditions, this movement is reversed, exposing the reflective material and enhancing the ability of the photoreceptors to process light. Choriocapillaris The choriocapillaris is the innermost layer of choroidal vessels, forming a thin layer of capillaries separated from the RPE by a basement membrane complex known as Bruch’s membrane (Fig. 2.69). Choriocapillaris in the dog. A. Note the numerous fenestrations (F) in the endothelium. Bruch’s membrane in most places consists of only the fusion of the pigment epithelium (PE) and the endothelial basal lamina (dark arrows), but small amounts of the collagenous zone are visible when they separate (open arrow). Note how the tapetum (T) is buffered from the capillary by a layer of collagenous tissue (C). Note also the lack of pigment in the PE over the area of the tapetum. (Original magnification, 12,400×.) B. The choriocapillaris (CC) forms the innermost layer of the choroid, being a wide capillary bed intimately associated with the RPE. I, iridocyte. (Original magnification, 3000×.) The lumen of the choriocapillaris is fairly wide, allowing red blood cells to pass through two to three abreast. The endothelial lining of the choriocapillaris possesses numerous circular fenestrations, which are often arranged in rows. External to the endothelium is a basement membrane forming the external layer of Bruch’s membrane. Bruch’s membrane is poorly developed in many domestic animals when compared to primates. In most diurnal species that lack a tapetum lucidum, including pigs and primates, Bruch’s membrane is pentalaminated, consisting of basal laminae of the RPE and choriocapillaris endothelia, two adjacent layers of collagen, and an intervening band of elastic fibers. In animals with tapeta, and especially with cellular tapeta, Bruch’s membrane is reduced to a trilaminate structure consisting of two basal laminae and a layer of collagen. Lens The crystalline lens is a transparent, avascular structure that focuses light onto the retina. It is suspended within the eye by zonules arising from the ciliary body epithelium (i.e., pars plicata) and attaching circumferentially to the lens capsule at the lens equator. The lens is also held in place posteriorly within a shallow depression in the anterior vitreous (i.e., the patella fossa), and the iris rests against it anteriorly. In many mammals, birds, and reptiles, the lens is biconvex; the degree of convexity (i.e., shape) is able to change during accommodation because of the elasticity of the capsule and the pliability of the lens fibers. In young mammals, the lens is quite soft, with only a small, central, denser nucleus. The lens grows throughout life, with newly formed fibers added continuously to the outermost cortex, causing compression of the central, older zone of lens fibers. This results in a hardening of the central nucleus (i.e., nuclear sclerosis), which reduces accommodation ability as the lens ages. The refractive power of the lens is less than the cornea because the change of refractive index is much greater at the air–cornea interface than at the aqueous–lens and lens–vitreous interfaces. Contraction of the CBM reduces tension on the lenticular zonules, thereby changing the shape of the lens resulting in an alteration of the dioptric power. Of the roughly 60 diopters of total refractive power of the eye, the lens contributes approximately 13–16 diopters in humans. In dogs, the dioptric power of the lens contributes approximately 40 diopters. The remaining refraction is provided by the cornea. Some mammals, including monotremes, marsupials, some herbivores, aquatic (primarily marine), placentals, and many nocturnal types such as mice and rats, have no known accommodative mechanism. Others, such as ungulates, accommodate weakly, having poor near vision; as a result, they rely more on other senses to detect near objects. The lens is proportionately larger in domestic animals than in humans. The dog lens has a volume of approximately 0.5 mL and averages 7 mm in thickness at the anteroposterior axis, with a 10‐mm equatorial diameter. The proportion of lens volume to entire globe volume ranges from 1 : 8 to 1 : 10. The equine lens, on the other hand, has a volume of approximately 3 mL, 12–15 mm average anteroposterior axis thickness, an approximately 21‐mm equatorial diameter, and a lens–globe ratio of 1 : 20. Lens volumes of sheep, cattle, and pigs fall between these volumes, thicknesses, and diameters (Table 2.11, pg 85). The lens consists of an enveloping basement membrane called the lens capsule, an anterior epithelium, and lens fibers occupying two main zones: the nucleus and the cortex (Fig. 2.70). Composite drawing of the lens, capsule, attachments, and nuclear zones. The lens epithelial cells line the anterior capsule. At the equator, these dividing cells elongate to form lens cortical cells (fibers). As they elongate anteriorly and posteriorly toward the sutures, their nuclei migrate somewhat anterior to the equator and form the lens bow. Zonular fibers (zf) attach to the anterior and posterior lens capsule and to the equatorial capsule, forming pericapsular or zonular lamellae of the lens. (Source: Modified from Hogan, M.J., Alvarado, J.A. & Weddell, J.E. (1971) Histology of the Human Eye. Philadelphia, PA: W.B. Saunders. Reproduced with permission of Elsevier.) Lens Capsule The lens fibers are completely enclosed within a thick, PAS positive capsule, which is the exaggerated basement membrane of the lens epithelium. It has elastic properties but no elastic fibers. The thickness of the capsule varies by region, with the thinnest being the posterior pole. The canine lens capsule thickness is 8–12 μm at the equator, 50–70 μ anteriorly, and only 2–4 μm posteriorly. Most of the capsule thickening occurs during the first year of life (Table 2.12, pg 85). As in most basement membranes, the main component of the lens capsule is type IV collagen. Anterior Epithelium Lining the anterior capsule is a monolayer of lens epithelial cells that continuously produce new basement membrane (i.e., capsule material). The cells are cuboidal to squamous axially at the anterior pole of the lens, become columnar near the equator, then elongate into slender hexagonal lens fibers. Nuclei are lost as lens fibers mature and move centrally. The lens epithelium lines only the interior aspect of the anterior surface of the capsule postnatally. The cell apices face the outer lens fibers, being attached to the underlying cortical fibers by tight junctions (zonula occludens) and macula adherens. The posterior lens epithelium forms the embryonic primary lens fibers and, thus, is absent under the posterior lens capsule later in life. Mature lens fibers become dependent on the anterior epithelium for maintaining a critical level of dehydration, which allows the soluble proteins to be functionally effective, and for providing a healthy level of reduced glutathione. The lens epithelium is highly susceptible to damage caused by factors such as changes in local oxygen concentration, exposure to toxins, X‐ray irradiation, and ultraviolet (UV) light damage. Nocturnal (and arrhythmic) animals might be less able to compensate for exposure to bright light, as free radical scavenging and other defense mechanisms might differ between species.