The uvea plays a fundamental role in vision and overall eye health. This middle layer of the eye is positioned beneath the sclera, which is the white outer part of the eyeball. The uvea also contributes to maintaining the eye’s internal environment and supporting its delicate structures.
Anatomy of the Uvea
The uvea is comprised of three distinct components: the iris, the ciliary body, and the choroid. These parts collectively form the vascular layer of the eye. The term “uvea” itself comes from a Latin word for grape, reflecting its appearance when removed from the eye, with the pupil resembling the stem scar.
The iris is the colored part of the eye, a thin, circular structure made of connective tissue and muscle that surrounds the pupil. Its color is determined by the amount of pigment it contains. Behind the iris lies the ciliary body, a ring-shaped structure not visible externally. The ciliary body extends posteriorly from the iris. The choroid forms the posterior portion of the uvea, situated between the tough outer sclera and the light-sensitive retina at the back of the eye. It is a thin, highly vascular membrane.
Functions of the Uvea
Each component of the uvea performs specific functions important for clear vision. The iris acts like a camera’s diaphragm, controlling the size of the pupil. This regulation of pupil size manages the amount of light entering the eye, which helps adapt vision to varying light conditions and enhances visual clarity.
The ciliary body has two primary functions. It produces aqueous humor, a clear fluid that nourishes the lens and cornea and helps maintain the eye’s internal pressure. The ciliary body also contains muscles that change the shape of the lens, allowing the eye to focus on objects at different distances, a process known as accommodation. The choroid, a highly vascular layer, provides a rich blood supply to the outer layers of the retina, delivering oxygen and nutrients necessary for its proper functioning. It also absorbs excess light, which helps prevent internal reflections within the eye.
Common Uveal Conditions
The uvea can be affected by various conditions, some of which can impact vision. Uveitis is an inflammation of the uvea, which can manifest as inflammation of the iris (iritis), the ciliary body, or the choroid. This inflammation can be caused by infections, autoimmune diseases, or injuries.
Ocular melanoma is a type of cancer that originates in the eye, with approximately 85% of cases starting in the uveal tract. This condition involves the abnormal growth of melanocytes, the cells that produce pigment. While rare in adults, it is the most common primary intraocular tumor. Other conditions can involve abnormalities of the iris, such as aniridia, which is the absence of an iris, or coloboma, a gap or hole in the iris or other eye structures.