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Chapter 160 – Anatomy of the Uvea

Chapter 160 – Anatomy of the Uvea

 

KAY L. PARK

 

 

 

 

 

DEFINITION

• The uvea is a pigmented, vascular structure consisting of the iris, ciliary body, and choroid.

 

KEY FEATURES

• Supplies blood to most of the eye from anterior and posterior ciliary branches of the ophthalmic artery.

 

ASSOCIATED FEATURES

• Produces aqueous humor in the ciliary processes.

• Controls near accommodation by contraction of ciliary muscles, which relax the zonular fibers to the lens.

• Increases aqueous outflow by contraction of ciliary muscles, which open the trabecular meshwork.

 

 

 

INTRODUCTION

The uvea (from the Latin uva, meaning grape) is a pigmented structure that primarily lies between the retina and the sclera and constitutes the vascular portion of the eye. Its blood supply comes from the ophthalmic artery, which nourishes most of the eye through branches of the anterior and posterior ciliary arteries. A separate branch of the ophthalmic artery, the central retinal artery, supplies the inner retinal layers and part of the optic nerve. The uvea also has secretory and mechanical functions including production of aqueous humor, improvement of aqueous outflow, and control of near accommodation. The uvea may become involved in disease processes through inflammation, known as uveitis, neoplasia (e.g., melanoma), and growth of abnormal vessels, known as choroidal neovascularization.

IRIS

The anterior portion of the uvea is called the iris ( Fig. 160-1 ). It is composed primarily of vascular stroma as well as melanocytes, nerves, clump cells, collagen, and hyaluronidase-sensitive acid mucopolysaccharides. [1] The vascular supply to the iris originates in the anterior and long posterior ciliary branches of the ophthalmic artery. These branches join in the ciliary body to form the major arterial circle before entering radially into the iris. The vessels lack an internal elastic lamina and are lined by nonfenestrated endothelial cells.

The anterior surface of the iris is composed of a fibroblast cell layer folded into many ridges and crypts, with a pupillary aperture located slightly inferonasal to the center.[2] Eye color is determined by the number and degree of melanin granules in the stromal melanocytes.[3]

Muscular and pigment epithelial structures are located in the posterior portion of the iris. Smooth muscle is tightly arranged in a circle to form the pupillary sphincter and is primarily innervated by parasympathetic nerves coming from the third cranial nerve nucleus. The radially oriented dilator muscles extend from their cell bodies in the anterior pigment epithelial layer and are innervated by the sympathetic nervous system.

In contrast to the irregular surface of the anterior iris, the posterior iris epithelium is velvety smooth. Here, heavily pigmented columnar cells are arranged apex to apex in two layers, absorbing incident light that does not enter through the pupil.[4] These two layers are continuous with the pigmented and nonpigmented layers of the ciliary body.

CILIARY BODY

The anterior portion of the ciliary body, the pars plicata, is composed of approximately 70 ciliary processes ( Fig. 160-1 ). The ciliary processes are arranged in a radial fashion and consist of vascularized stromal cores surrounded by two layers of epithelium—the inner pigmented layer and the outer nonpigmented layer. The cells of these two layers are arranged apex to apex with tight junctions between them.[5] The zonula occludens near the apices of the nonpigmented epithelial cells form the blood-aqueous barrier. The nonpigmented epithelium is also the site of aqueous secretion. Arterioles regulating blood flow through the ciliary body influence the rate of aqueous formation.

The middle portion of the ciliary body, the pars plana, is a flat structure, 4?mm in length, located between the pars plicata and the ora serrata. [6] Surgical access through the pars plana requires entry approximately 3–4?mm behind the limbus to avoid trauma to the lens or to the retina. The inner layer of the pars plana is composed of pigmented epithelial cells that are uniformly cuboidal in shape and continuous with the retinal pigment epithelium; the outer layer is made up of nonpigmented epithelial cells that are columnar in shape near the pars plicata and more cuboidal as they near the ora serrata. The nonpigmented epithelium secretes acid mucopolysaccharide, one of the main components of the vitreous.

 

 

Figure 160-1 Iris and ciliary body. The iris is lined posteriorly by its pigment epithelium, and anteriorly by the avascular anterior border layer. The bulk of the iris is made up of vascular stroma. Considerable pigment is present in the anterior border layer and stroma in this brown iris. (Courtesy of Dr. RC Eagle, Jr.)

 

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Figure 160-2 Choroid. The choroid is composed, from outside to inside, of the suprachoroidal (potential) space and lamina fusca, the choroidal stroma (which contains uveal melanocytes, fibrocytes, collagen, blood vessels, and nerves), the fenestrated choriocapillaris (the largest capillaries in the body), and the outer aspect of Bruch’s membrane. (From Yanoff M, Fine BS. Ocular pathology, ed 5. St. Louis: Mosby; 2002.)

Adjacent to the pars plicata, the ciliary body is composed of three layers of smooth, nonstriated, muscular tissue. The outer longitudinal layer, which makes up the bulk of the tissue, attaches to the scleral spur; the radial and circular muscles originate from the middle and inner ciliary body, respectively. The three layers act as a unit and are innervated primarily by the parasympathetic nervous system with synapses in the ciliary ganglion. Contraction of these muscles relaxes the zonular fibers to the lens, allowing the lens to move forward and assume a more spherical shape for accommodation at near. Miotics also contract the ciliary muscles, opening the trabecular meshwork to increase aqueous outflow.

CHOROID

The choroid, located between the retina and the sclera, extends from the scleral spur anteriorly to the optic nerve posteriorly ( Fig. 160-2 ). Scleral attachments at vortex veins located near the equator account for the characteristic shape of choroidal detachments.[7] The choroid is composed of vessels, melanocytes, and fine connective tissue. The number of pigmented melanocytes in the outermost layer determines the degree of pigmentation in the choroid. Lighter-skinned individuals have less pigmentation in their choroid than those who have darker skin.

The choroid nourishes the outer retina and a portion of the optic nerve. It is the sole vascular supply to the foveal area, manifesting as a cherry-red spot in a central retinal artery occlusion. A cilioretinal branch from the choroid may also supply the fovea in about 10–15% of the population. Blood flow in the choroid is very high, with an oxygen concentration in the venous compartment that is only a few percent less than in the arterial. Drainage of the choroid occurs through four to seven vortex veins.

 

The choriocapillaris is a continuous vascular sheet of capillaries that lies beneath Bruch’s membrane and the retinal pigment epithelium. The architecture of the choriocapillaris differs from one area to the next. In the peripapillary and submacular regions, a dense honeycomb arrangement of capillaries is seen. In the remainder of the posterior pole and near the equator, lobular structures with central postcapillary venules and peripheral precapillary arterioles are found. Finally, peripheral to the choriocapillaris, the pattern of vascular networks is more elongated. Throughout this heterogeneous structure making up the choriocapillaris, angiographic dye has been shown to distribute homogeneously within lobular networks. Differential pressure gradients within the choroidal blood flow may explain these two patterns of lobuli in the choriocapillaris, anatomic versus functional.[8] [9]

The choriocapillaris, the largest capillaries in the body, is composed of vessels that are 40–60?mm in diameter and contain multiple fenestrations. Small molecules, such as fluorescein, readily pass through these fenestrations and are seen as the choroidal blush on fluorescein angiography. The middle and large choroidal vessels are not fenestrated. Indocyanine green dye has been used more recently to visualize the choroidal circulation because, unlike fluorescein, it is mostly bound to protein and does not leak from the choriocapillaris. Moreover, the longer wavelengths used by indocyanine green (with light absorption and emission at 766?nm and 826?nm, respectively, versus 485?nm and 520?nm for fluorescein) allow superior penetration of hazy media, blood, and retinal pigment epithelium. This makes indocyanine green particularly useful for identifying occult choroidal neovascularization, such as in patients with age-related macular degeneration.

 

 

REFERENCES

 

1. Fine BS, Yanoff M, eds. Ocular histology: a text and atlas, 2nd ed. New York: Harper & Row; 1972:168–212.

 

2. Hogan MJ, Alvarado JA, Weddell JE, eds. Histology of the human eye. Philadelphia: WB Saunders; 1971:202–59.

 

3. Apple DJ, Rabb MF, eds. Ocular pathology: clinical applications and self-assessment, ed 4. St. Louis: Mosby–Year Book; 1991:201–6.

 

4. Kardon RH. Anatomy. In: Tripathi RC, ed. Fundamentals and principles of ophthalmology, Section 2. San Francisco: American Academy of Ophthalmology; 1995–1996:49–54.

 

5. Green WR. The uveal tract. In: Spencer WH. Ophthalmic pathology: an atlas and textbook, 4th ed, Vol 3. Philadelphia: WB Saunders; 1996:1439–60.

 

6. Aiello AL, Tran VT, Rao NA. Postnatal development of the ciliary body and pars plana. A morphometric study in childhood. Arch Ophthalmol. 1992;110:802–5.

 

7. Guyer DR, Schachat AP, Green WR. The choroid: structural considerations. In: Ryan SJ, ed. Retina, ed 2. St. Louis: CV Mosby; 1994:18–31.

 

8. Fryczkowski AW, Sherman MD, Walker J. Observations on the lobular organization of the human choriocapillaris. Int Ophthalmol. 1991;15:109–20.

 

9. Fryczkowski AW. Anatomical and functional choroidal lobuli. Int Ophthalmol. 1994;18:131–41.

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One comment on “Chapter 160 – Anatomy of the Uvea

  1. […] Chapter 160 – Anatomy of the Uvea | Free Medical Textbookinflammation of the uvea (or uveal tract), the middle layer of tissue surrounding the eye that consists of the iris, ciliary body, and choroid. Uveitis can affect people at any age, but onset usually occurs in the third and fourth decades of life. … The forward portion of the uvea, the ciliary body and iris, is… […]

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