The Eyeball: Vascular Tunic

By | December 23, 2014

The vascular tunic (tunica vasculosa bulbi) is the thick middle coat of the eye, interposed between the retina and the sclera. It is commonly referred to as the uvea or uveal tract. The vascular tunic includes three contiguous parts, which, from posterior to anterior, are the choroid, the ciliary body, and the iris. Its functions are numerous and include regulating the amount of light entering the eye through the pupil; producing the aqueous humor, which maintains the intraocular pressure and bathes the structures of the anterior segment; suspending the lens via zonula fibers; changing the visual focus via the ciliary body muscle; limiting the amount of scattered light within the eye (inner pigmented portion of uvea); increasing the photic stimulation of the retina under low light levels (tapetum lucidum of the choroid); and providing nutrition to structures within the eye (ciliary body and choroid).

Choroid

The choroid is a pigmented vascular layer. It is continuous with the ciliary body anteriorly and completely envelops the posterior hemisphere of the eyeball, except in the region of the area cribrosa (region of the optic nerve head), where it is absent.

The choroid is further divided into layers, which, from the outermost inward, are the suprachoroid (lamina suprachoroidea), the vascular layer (lamina vasculosa), the reflective layer (tapetum lucidum), the choriocapillary layer (lamina choroidocapillaris), and the basal lamina (lamina basalis). The last is poorly developed in the dog.

The tapetum is a specialized reflective layer of the choroid. It is thought to increase the ability of the retina to function under low light levels. In addition to the reflection of incident light back through the overlying photoreceptor layer, it is also thought to increase photoreceptor stimulation by intrinsic fluorescence of its structure when stimulated by incident light. A tapetum is present in all domestic mammals except the pig. In almost all mammals it is located within the middle-sized vessel layer of the choroid, interposed between the choriocapillaris and the large-sized vessel layer. It is cellular (tapetum lucidum cellulosum) in all carnivores, whereas it is collagenous (tapetum lucidum fibrosum) in all herbivores. It occupies approximately one third of the superior area of the choroid. Central areas of the canine tapetum have been reported to contain 9 to 11 layers of tapetal cells, or 15 to 20 layers. Cell numbers diminish to a single layer peripherally and next to the optic nerve. The cells are layered in a step-wise manner that form a brick-wall appearance. The zincand cysteine-rich tapetal cells are packed with highly refractive, membranebound tapetal rodlets. These rodlets are oriented parallel to the retina and are thought to be responsible for the tapetum’s reflectivity. The unique structure of the tapetum makes it exquisitely sensitive to the toxic effects of a beta adrenergic blocking agent that may have no toxic effects whatsoever in Beagles that inherit an aplasia of the tapetum. Penetrating vessels are oriented radially and connect the choriocapillaris with the stromal vessels. The radial orientation of these penetrating vessels minimizes their potential interference on tapetal function.

In dogs, the tapetum is in roughly the shape of a rounded right triangle with the hypotenuse resting on a dorsal plane and the right angle situated dorsally. The medial angle is more acute than the lateral. In large breeds of dogs, the hypotenuse (inferior border) is usually inferior to the optic disc. In small breeds of dogs, the tapetum is relatively smaller and does not extend inferiorly to include the optic disc. In some toy breeds, the tapetum may be greatly diminished in area or may be entirely absent as a normal variation. A heritable lack of the tapetum has been described in the Beagle, in which tapetal cells are initially present but fail to develop normal tapetal rodlets.

The tapetum develops after birth. As the dog matures, the color of the tapetum changes from a slate gray to violet to red-orange at approximately 4 months of age. The color is generally uniform except at the junction of the tapetal and nontapetal choroid, which may be quite irregular and demonstrate considerable pleochromism. The distinctive coloration of the tapetum is due to the optical phenomenon of thin film interference rather than the presence of specific pigments. In other words, tapetal coloration is structural rather than pigmentary.

The vascular layer of the choroid is a plexus of arteries, arterioles, veins, and venules supported by a collagenous and elastic stroma and traditionally subdivided into an outer largesized vessel layer and an inner middle-sized vessel layer. The outer large-sized vessel layer is the terminal branches of the ciliary arteries and the vorticose veins. Most of the large choroidal vessels run parallel to the meridians and to one another with the choroidal veins fanning outward from the point at which the vorticose veins penetrate the sclera. Branches of these vessels form the middle-sized vessel layer, which leads to and empties the choroidal capillary layer, which in turn nourishes the outer layers of the retina.

In the majority of dogs, the vascular layer of the choroid and the retinal pigment layer are darkly pigmented. The nontapetal region in these dogs appears dark brown or black. In dogs with amber, blue, or heterochromic irides, however, the choroid and retinal pigment layers are unpigmented or nearly so. In these animals, the vessels of the choroid can be visualized with the ophthalmoscope, and the nontapetal fundus appears red or striped (so-called tigroid fundus). The absence of a tapetum lucidum or choroidal pigment or both is considered a normal variation that apparently does not affect the dog’s vision. Focal areas of choroidal hypoplasia, however, are serious ocular defects. They are a common manifestation of the Collie eye syndrome, a serious and widespread heritable defect in the Collie and Sheltie breeds.

Ciliary Body

The ciliary body (corpus ciliare) is the thickened middle segment of the vascular tunic, between the iris and choroid. The ciliary body consists of a ciliary ring and ciliary crown. The ciliary ring (orbicularis ciliaris) is the posterior flat portion of the ciliary body adjacent to the anterior border of the pars optica retina and continuous with the choroid. The ciliary crown (corona ciliaris) is the raised portion of the ciliary body anterior to the ciliary ring and adjacent to the iris. The ciliary processes are developed on the ciliary crown. Some texts refer to the ciliary ring as the pars plana and the ciliary crown as the pars plicata. The ora ciliaris retinae is the line that demarcates the boundary between the pars ciliaris retinae and the pars optica retinae as well as separating the choroid from the ciliary body. Anterior to the ora ciliaris retinae, at the ciliary crown, is the elevation of the ciliary body into approximately 100 small, flat, parallel, regular processes. These increase rapidly in height and coalesce as they pass anteriorly along the meridians, forming tall, thin folds rising up from the base plate of the ciliary body. At the root of the iris (margo ciliaris), these folds lose their outer attachment to the sclera via the ciliary body musculature and iridocorneal angle and arc centrally toward the lens as the short, blunt, free ciliary processes (processus ciliaris). There are approximately 76 major processes in the canine eye, which vary from 2 to 4 mm in length. Minor ciliary folds and processes are interposed between the major processes, although this relationship is not constant; two major processes may occur together without an intervening minor process. The minor processes are neither as tall nor do they approximate the lens as closely as the major processes. The posterior surface of the ciliary processes is almost entirely covered by fibers of the ciliary zonule. The major and minor processes together form the ciliary crown. The width of the ciliary body from the tips of the ciliary processes to the ora ciliaris retinae is greater on the lateral aspect of the globe.

Like the choroid, the ciliary body is highly vascular. Radial ciliary arteries branch directly from the posterior margin of the greater arterial circle of the iris. Initially, these arteries pass through the peripheral iris for 0.1 to 1 mm. At the anterior margin of the ciliary processes branches are given off that course inward to the anterior edge of the processes. Each ciliary process is supplied by a single arteriole traveling posteriorly throughout its length and sending capillary arcades to its margin from where they drain outward into venous sinuses at the base of the process. After giving off branches to the processes, the ciliary arteries pass posteriorly to supply a dense network of capillaries associated with the ciliary muscle fibers, and finally collateralize with the choroidal vasculature. The blood flow from the radial ciliary arteries thus has four possible destinations. It can branch anteriorly to become a radial iris artery; it can branch interiorly to become an afferent arteriole supplying a ciliary process; it can supply the fine capillary network of the ciliary muscle; or it can shunt through the region of the ciliary body and ramify with the choroidal vasculature. The ciliary body is drained by the choroidal and vorticose veins.

The inner nonpigmented and outer pigmented epithelia of the two-cell-thick pars ciliaris retinae produce the aqueous humor. The production of aqueous involves both ultrafiltration and active transport processes. The canine ciliary body receives both adrenergic and cholinergic innervation, which is thought to play a role in the regulation of aqueous production and outflow. The topical application of a β-adrenergic antagonist inhibits the aqueous humor formation.

The ciliary body muscle (m. ciliaris) consists of numerous smooth muscle fascicles located in the outer portions of the ciliary body. Both circumferential fibers (fibrae circulares) and meridional fibers (fibrae meridionales) are present in primates (which possess an extensive accommodative range), whereas only meridional fibers are present in the dog. The meridional fibers originate from the scleral ring (annulus sclerae) on the inner surface of the sclera posterior to the iridocorneal angle. The anterior-most muscle fibers form tendinous endings with the posterior uveal trabeculae of the iridocorneal angle. The meridional fibers insert in the stroma of the ciliary body near the ora ciliaris retinae. In blue-eyed dogs, contraction of the ciliary body muscles occurs following stimulation of M5 muscarinic receptors, whereas contraction of the ciliary muscle in brown-eyed dogs may require activation of M5 and M3 receptors. The ciliary body muscles lack functional adrenoreceptors. Exogenous norepinephrine inhibits contracture of the ciliary body muscle via activation of alpha2-adrenoreceptors of parasympathetic axons, which in turn inhibit release of acetylcholine. When the fibers of the ciliary body muscle contract on parasympathetic stimulation, they decrease tension on the zonular fibers supporting the lens. With the release in tension, the lens becomes more spherical as a result of the inherent elasticity of its capsule. The more spherical lens has a shorter focal distance, and close objects are now brought into critical focus on the retina, a process called accommodation. It is not known whether the ora ciliaris retinae actually moves anteriorly during this process, or whether the zonule fibers are relaxed because of a sphincterlike action of the ciliary body muscle that decreases the diameter of the ciliary body at the ciliary crown. The extent of the accommodative ability of the dog is inferior to that of primates, in which the ciliary muscle is far better developed. Accommodation in dogs has been reported to be only 1 to 3 diopters, which has been verified using the technique of videoretinoscopy.

Zonule

The lens is fixed in position by a delicate suspensory apparatus, the zonula ciliaris. The zonule is composed of a highly ordered array of zonular fibers (fibrae zonulares) that are aggregates of fibrils 10 nm in diameter, similar to those described in elastic tissue by Greenlee et al. (1966). The zonule lies posterior to the iris and ciliary body and separates the posterior chamber from the vitreous body. It is not visible in the intact eye unless the iris is very widely dilated or the lens is subluxated.

The apices of the ciliary processes may be in contact with the equator of the lens, but they are not attached to it by zonular fibers, as is commonly illustrated. The majority of the zonular fibers, in fact, originate from the pars ciliaris retinae just anterior to the ora serrata. They pass anteriorly, closely adherent to the surface of the ciliary body. Small auxiliary fibers arise from the epithelium of the ciliary folds and serve to anchor the main fiber bundles. As the small folds of the ciliary body unite to form the major ciliary folds, the zonule fibers also converge until they completely cover the sides of each major ciliary process. These fibers continue centrally beyond the apices of the processes, span the circumlental space, and insert primarily on the anterior lens capsule near the equator. They are thus designated the anterior zonular fibers.

The zonular attachments to the posterior lens capsule are not as well developed. Two subsets of fibers compose the posterior fiber group. Where a minor ciliary fold is present, fibers arise from its surface and the surrounding epithelium and insert on the posterior lens capsule. Fibers also originate from the valleys between ciliary folds. These arc toward the major ciliary processes, cross the face of the anterior fibers obliquely, and insert on the posterior lens capsule.

The anterior face of the vitreous body bulges forward between the ciliary folds and into the spaces between the zonular fiber bundles (spatia zonularia). The zonular fibers are under tension when the ciliary muscle is relaxed.

Iris

The iris is the most anterior segment of the vascular tunic. It is a thin circular diaphragm, which rests against the anterior surface of the lens. The central opening in the iris, the pupil (pupilla), is circular in the dog. The size of the pupil is variable and serves to regulate the amount of light reaching the retina. The diameter of the pupil is smallest when the intensity of illumination is greatest. The periphery of the iris (margo ciliaris) is continuous with the ciliary body and trabeculae of the iridocorneal angle.

In the fetus, the iris is not fenestrated and thus completely covers the anterior surface of the lens. The central portion of the iridial anlage (membrana pupillaris) contains vessels that nourish the growing lens (see section on development). Normally the pupillary membrane atrophies so that only remnants of these vessels are present when the eyelids open at approximately 2 weeks of age. Remnants commonly persist, especially on the dorsal pupillary margin, until the age of 4 to 5 weeks. Abnormal persistence of the pupillary membrane into adult life is a heritable condition in Basenji dogs.

The anterior surface of the iris is lined by a discontinuous layer of flat fibrocytes. Large intercellular spaces are evident, and communication between the anterior chamber and the underlying stroma can be traced. The stroma (stroma iridis) contains fibroblasts, collagen, myelinated and unmyelinated axons, smooth muscle fibers, melanocytes, and blood vessels. Mast cells have also been reported to normally reside within the anterior uveal stroma of the canine eye. Melanin is the only identified pigment in the dog iris. Blue-eyed dogs have a paucity of pigment restricted to the posterior pigmented epithelium of the pars iridica retinae of the iris. The blue color is caused by the differential absorption and selective reflection of light by the iris tissue itself and the posteriorly located melanin. In darkly pigmented irides there is an accumulation of pigment-laden melanocytes within the anterior iris stroma.

Two antagonistic muscles regulate the diameter of the pupil: the m. sphincter pupillae and the m. dilator pupillae. Both are derived from the outer layer of neuroepithelium of the pars iridica retinae. They both receive parasympathetic and sympathetic innervation. The sphincter muscle is a sheet of circumferentially arranged smooth muscle fibers near the pupillary margin. It is the larger of the two muscles. The dilator of the pupil is composed of radially arranged smooth muscle fibers that form a meshwork through which the collagen bundles of the iris stroma are looped. The dilator is posterior to the sphincter muscle. Both muscles are welldeveloped in the dog when compared with other domestic species.

The sphincter is innervated by postganglionic parasympathetic axons whose cell bodies are located in the ciliary ganglion. Preganglionic axons reach the ciliary ganglion via the oculomotor nerve. Postganglionic axons reach the iris via short ciliary nerves (branches of the nasociliary nerve). Parasympathetic activity is mediated by the activation of adenylate cyclase via M3 muscarinic receptors. An accessory ciliary ganglion has been described in a variety of mammalian species including the dog. This ganglion is located on a short ciliary nerve, has been shown to carry parasympathetic axons in the cat, and has been postulated to mediate pupillary constriction in association with convergence and accommodation of the eyes. The sphincter fibers also receive sympathetic innervation, which inhibits contraction of the muscle fibers. Recent evidence suggests that prostaglandins may also contribute to the tonus of the sphincter muscle (and to a lesser degree the dilator muscle) in the dog. The prostaglandins apparently act directly on these muscles rather than through the release of cholinergic neurotransmitters. The neuropeptide substance P is variably effective in inducing miosis in many mammals, but it does not induce contraction of the iris sphincter in the dog. Interestingly, substance P is inactivated by aqueous humor.

The dilator muscle is innervated by sympathetic and parasympathetic axons. The preganglionic sympathetic neuronal cell bodies are located in the intermediate gray column of the first three thoracic spinal cord segments. Their axons course cranially in the vagosympathetic trunk to synapse on cell bodies of postganglionic axons located in the cranial cervical ganglion. From the ganglion, the postganglionic axons pass rostrally through the tympanooccipital fissure with the internal carotid artery. They enter the cranial cavity through the foramen lacerum and join the ophthalmic nerve on the ventral surface of the trigeminal ganglion. They are distributed with the ciliary branches of the ophthalmic nerve. Sympathetic stimulation causes contraction of these fibers, causing pupillary dilation, and parasympathetic stimulation inhibits contraction. Other endogenous compounds, such as prostaglandins, also have been shown to play a role in regulating the state of pupillary dilation, typically in concert with the reduction of intraocular pressure. This may be why dogs exposed to a natural photoperiod exhibit significant circadian rhythms of intraocular pressure, which peaks during diurnal hours. This circadian rhythm disappears under conditions of constant light.

The blood supply of the iris arises primarily from the two long posterior ciliary arteries. These arteries follow the medial and lateral meridians from the area cribrosa to the iris. They are visible in the episcleral space from the optic nerve to approximately the equator of the eyeball and are useful landmarks for orienting an isolated eyeball to the horizontal plane. At the equator, the long posterior ciliary arteries pass deep to the sclera and continue anteriorly to the base of the iris. Some branches may be given to the choroid, where they anastomose with the choroidal arterioles. In the ciliary margin of the iris, the medial and lateral posterior arteries each divide into superior and inferior branches. This bifurcation may occur 2 to 3 mm posterior to the base of the iris. The branches run circumferentially in the iris, forming the undulating greater arterial circle of the iris (circulus arteriosus iridis major). The position of the circle approximates that of the equator of the lens. The circle is not directly completed at the superior and inferior points. In most specimens, however, a smaller vessel completes the circle near the base of the iris. The formation of the greater arterial circle of the iris in the dog is similar to that in humans but differs significantly in that the greater arterial circle of humans is within the ciliary body. In the dog, the greater arterial circle of the iris is often visible (especially in blue irides) and commonly protrudes slightly from the anterior iridial surface.

Fine radial arterioles originate from the arterial circle. These originate either directly from the circle or from radial ciliary arteries at a point close to where they branched from the major arterial circle and started posteriorly. Arterioles run either centrally toward the pupil to supply the iris stroma and musculature or peripherally to anastomose with the ciliary vessels supplying the ciliary processes. A lesser arterial circle near the pupil is not present in the dog. Venules begin blindly at the pupillary margin and course posteriorly to pass deep to the major arterial circle and into the ciliary vasculature. Incision of the iris in the dog results in profuse hemorrhage.

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