CHAPTER 12

Uveal Tract

Topography

The iris, ciliary body, and choroid constitute the uveal tract (Fig 12-1). The uveal tract is embryologically derived from mesoderm and neural crest. Firm attachments between the uveal tract and the sclera exist at only 3 sites:

scleral spur

exit points of the vortex veins optic nerve

Iris

The iris is located in front of the crystalline lens. It separates the anterior segment of the eye into 2 compartments, the anterior chamber and the posterior chamber, and forms a circular aperture (pupil) that controls the amount of light transmitted into the eye. The iris is composed of 5 layers:

anterior border layer stroma

muscular layer

anterior pigment epithelium posterior pigment epithelium

The anterior border layer represents a condensation of iris stroma and melanocytes and is coarsely ribbed with numerous crypts (Fig 12-2). The stroma contains blood vessels, nerves, melanocytes, fibrocytes, and clump cells. The clump cells are both macrophages containing phagocytosed pigment (type I, or clump cells of Koganei) and variants of smooth-muscle cells (type II clump cells). The vessels within the stroma have a thick collar of collagen. The muscular layer is made up of the dilator muscle and the sphincter muscle. Both are smooth muscle under autonomic control; however, the dilator muscle is unique in that it is derived from the anterior layer of pigment epithelium. The posterior iris is lined by a double layer of cuboidal epithelium arranged in an apex-to-apex configuration. The cytoplasm of these cells is packed with melanin granules. Iris color is determined by the number and size of the melanin pigment granules in the anterior stromal melanocytes.

Figure 12-2 Histologic appearance of the iris: the anterior border layer is thrown into numerous crypts and folds. The sphincter muscle (red arrows) is present at the pupillary border, whereas the dilator muscle (black arrows) lies just anterior to the posterior pigment epithelium. Normal iris vessels demonstrate a thick collagen cuff (arrowhead). (Courtesy of Nasreen A. Syed,

MD.)

Ciliary Body

The ciliary body, which is approximately 6.0–6.5 mm wide, extends from the base of the iris and becomes continuous with the choroid at the ora serrata. The ciliary body is composed of 2 areas:

the pars plicata, which contains the ciliary processes the pars plana

The inner portion of the ciliary body is lined by a double layer of epithelial cells, the inner nonpigmented layer and the outer pigmented layer (Fig 12-3). The zonular fibers of the lens attach to the ciliary processes. The ciliary smooth muscle has 3 types of fibers: longitudinal (Brücke muscle), radial, and the innermost circular (Müller muscle). These muscle groups function as a unit during accommodation.

Figure 12-3 Normal ciliary body. The inner face of the ciliary body is lined with a double layer of epithelium. The inner layer is nonpigmented (red arrow) and the outer layer is pigmented (black arrow). (Courtesy of Nasreen A. Syed, MD.)

Choroid

The choroid is the pigmented vascular tissue that forms the middle coat of the posterior part of the eye. It extends from the ora serrata anteriorly to the optic nerve posteriorly and consists of 3 principal layers:

lamina fusca (suprachoroid layer) stroma

choriocapillaris

The choriocapillaris is the blood supply for the retinal pigment epithelium (RPE) and the outer retinal layers (Fig 12-4).