Structure
The iris lies on the anterior surface of the lens and acts as a contractile diaphragm to the centrally located aperture called the pupil.
From an anterior view, the iris can be divided into zones, the large zone adjacent to the ciliary body (ciliary zone) and a smaller internal zone adjacent to the pupil (pupillary zone). These two zones join at the collarette. Additionally, the iris may contain large depressions called crypts (crypts of Fuchs) (Standring, 2016).
The iris contains several cellular layers. From anterior to posterior these include the anterior border layer, the stroma, and two pigmented epithelial layers.
The anterior border layer is modified from the stroma layer and contains an increased number of fibroblasts and underlying melanocytes. It blends with the double epithelial layer covering the posterior surface of the iris at the pupillary rim, and with the trabecular connective tissue of the iridocorneal angle (the angle formed by the iris and cornea allowing for aqueous humor to drain).
The stroma is simply composed of fibroblasts and melanocytes nested in a loose connective tissue. It contains blood vessels and nerves as well as smooth muscles cells that form the sphincter pupillae.
The posterior surface of the iris is covered by a double layer of pigmented epithelium, which give rise to a person’s eye color. This double layer is continuous with the double-layered epithelium of the ciliary body. At the pupillary rim, the epithelium curves onto the anterior surface of the iris, forming a ruffled margin, the pupillary ruff.
The anterior layer of the double-layered epithelium of the iris (confusingly called the anterior epithelium even though it lines the posterior surface of the stroma of the iris) is a sort of myoepithelium, giving rise to the dilator pupillae muscle. It is continuous with the pigmented epithelial layer of the ciliary body.
The posterior layer of the double-layered epithelium of the iris is more heavily pigmented and is continuous with the non-pigmented layer of ciliary epithelium. It is contact with the anterior surface of the lens.
Key Features & Anatomical Relations
The iris lies between the lens and the cornea and is bathed in aqueous humor. Anteriorly, the pupillary zone of the iris is in contact with the lens, however, because of the convex shape of the lens, the ciliary zone of the iris moves away from the lens forming a space called the posterior chamber of the eye. The space between the cornea and the iris is referred to as the anterior chamber. The posterior surface of the iris has numerous folds that channels the movement of aqueous humor from the posterior chamber to the anterior chamber.
Function
The amount of light entering the pupil is regulated by the iris and is controlled by two involuntary muscles, the sphincter pupillae and the dilator pupillae muscle. Additionally, the double-layered pigmented epithelium of the iris restricts light reaching the retina.
References
Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series: Elsevier Limited.
