Uvea


The uvea, also called the uveal layer, uveal coat, uveal tract, vascular tunic or vascular layer is the pigmented middle of the three concentric layers that make up an eye.

History and etymology

The originally medieval Latin term comes from the Latin word uva and is a reference to its grape-like appearance. In fact, it is a partial loan translation of the Ancient Greek term for the choroid, which literally means “covering resembling a grape”. Its use as a technical term for part of the eye is ancient, but it only referred to the choroid in Middle English and before.

Structure

Regions

The uvea is the vascular middle layer of the eye. It is traditionally divided into three areas, from front to back, the:
The prime functions of the uveal tract as a unit are:
In addition, some uveal regions have special functions of great importance, including secretion of the aqueous humour by the ciliary processes, control of accommodation by the ciliary body, and optimisation of retinal illumination by the iris's control over the pupil. Many of these functions are under the control of the autonomic nervous system.

Pharmacology

The pupil provides a visible example of the neural feedback control in the body. This is subserved by a balance between the antagonistic sympathetic and parasympathetic divisions of the autonomic nervous system. Informal pharmacological experiments have been performed on the pupil for centuries, since the pupil is readily visible, and its size can be readily altered by applying drugs—even crude plant extracts—to the cornea. Pharmacological control over pupil size remains an important part of the treatment of some ocular diseases.
Drugs can also reduce the metabolically active process of secreting aqueous humour, which is important in treating both acute and chronic glaucoma.

Immunology

The normal uvea consists of immune competent cells, particularly lymphocytes, and is prone to respond to inflammation by developing lymphocytic infiltrates. A rare disease called sympathetic ophthalmia may represent 'cross-reaction' between the uveal and retinal antigens.

Clinical significance

See uveitis, choroiditis, iritis, iridocyclitis, anterior uveitis, sympathetic ophthalmia, and uveal melanoma.