The iris is the most anterior extension of the uvea (Figs 2-20, 2-21). It is made up of blood vessels and connective tissue, in addition to the melanocytes and pigment cells responsible for its distinctive color. The mobility of the iris allows the pupil to change size. During mydriasis, the iris is pulled into numerous ridges and folds; during miosis, its anterior surface is smoother.
The major structures of the iris are as follows:
The iris stroma is composed of pigmented cells (melanocytes), nonpigmented cells, collagen fibers, and a matrix containing hyaluronic acid. The aqueous humor flows freely within the loose stroma along the anterior border of the iris, which contains multiple crypts and crevices that vary in size, shape, and depth. This surface is covered by an interrupted layer of connective tissue cells that merges with the ciliary body.
Figure 2-20 Iris. A, Histologic section of the iris showing the sphincter muscle, typically found within 1 mm of the pupil border. The dilator muscle, derived from the anterior pigmented layer of the iris epithelium, is found in the mid iris. B, AS-OCT scan of the iris. AC = anterior chamber; Co = cornea; Ir = iris; Le = lens; Sc = sclera.
(Part A courtesy of Thomas A. Weingeist, PhD, MD; part B courtesy of Vikram S. Brar, MD.)
The overall structure of the iris stroma is similar in irides of all colors. Differences in color are related to the amount of pigmentation in the anterior border layer and the deep stroma. The stroma of blue irides is lightly pigmented, and brown irides have a densely pigmented stroma.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.