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:
Nutrition and gas exchange: uveal vessels directly perfuse the ciliary body and iris, to support their metabolic needs, and indirectly supply diffusible nutrients to the outer retina, sclera, and lens, which lack any intrinsic blood supply.
Light absorption: the uvea improves the contrast of the retinal image by reducing reflected light within the eye, and also absorbs outside light transmitted through the sclera, which is not fully opaque.
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.
