The uveal tract is embryologically derived from mesoderm and neural crest cells. The neuroepithelium is derived from the optic cup. Firm attachments between the uveal tract and the sclera exist at only 3 sites: the scleral spur, the exit points of the vortex veins, and the 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 (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:
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.
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.