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 (between the cornea and iris) and the posterior chamber (between the iris and the lens/zonular fibers), and forms a circular aperture (pupil) that controls the amount of light transmitted into the eye. The iris comprises 4 layers:

• anterior border layer

• stroma

• muscular layer

• iris pigment epithelium (consisting of 2 layers: anterior and posterior)

Figure 12-1 Uveal topography. The uveal tract consists of the iris (red), ciliary body (green), and choroid (blue).

(Courtesy of Nasreen A. Syed, MD.)

Figure 12-2 Histologic appearance of a normal iris: the anterior border layer is thrown into numerous crypts and folds. The sphincter muscle (red arrows) is present at the pupillary border, and 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.)

The anterior border layer represents a condensation of iris stroma, particularly melanocytes, and is coarsely ribbed with numerous crypts (Fig 12-2).

The stroma contains blood vessels, nerves, melanocytes, fibrocytes, and 2 types of clump cells containing pigment: histiocytes containing phagocytosed pigment (type I, or clump cells of Koganei) and variants of neuroepithelial cells (type II). The vessels within the stroma are surrounded by a thick cuff of collagen.

The muscular layer is made up of the dilator muscle and the sphincter muscle. Both muscles are composed of smooth muscle cells and are under autonomic control. The dilator muscle is a flat, radially arranged muscle that extends from the peripupillary region to the iris root. This muscle is integrated with the anterior layer of pigment epithelium. The sphincter muscle is a thicker muscle present just around the pupil in a radial arrangement.

The posterior iris surface is lined with a double layer of cuboidal epithelium (anterior and posterior layers of the pigment epithelium) arranged in an apex-to-apex configuration. The cytoplasm of these epithelial cells is packed with melanin granules. The number of melanin granules in the epithelium does not vary with iris color; rather, iris color is determined by the number of melanin granules and their size within the melanocytes of the anterior border layer.

Figure 12-3 Normal ciliary body. The inner face of the ciliary body is lined with a double layer of epithelium: an inner, nonpigmented layer (red arrow) and an outer, pigmented layer (black arrow). Note the fibrovascular connective tissue interposed between the pigmented ciliary epithelium and the ciliary muscle fibers (between green arrowheads).

(Courtesy of Nasreen A. Syed, MD.)

Excerpted from BCSC 2020-2021 series: Section 4 - Ophthalmic Pathology and Intraocular Tumors. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.