The uvea (Lat. uva, grape), also called the uveal layer, uveal coat, uveal tract, or vascular tunic, is the pigmented middle of the three concentric layers that make up an eye. The name is possibly a reference to its almost black colour, wrinkled appearance and grape-like size and shape when stripped intact from a cadaveric eye. Its use as a technical term in anatomy and ophthalmology is relatively modern.
The uvea lies between the corneosclera (outermost layer of the eye) and the retina (innermost layer/in the back of the eye). It is traditionally divided into 3 or 4 regions, the iris, ciliary body, pars plana and choroid. These distinctions are based on their different structures as seen under light microscopy, and continued use of these terms is appropriate in anatomical studies. For clinical use, the terms anterior uvea (ie, iris and ciliary body) and posterior uvea (ie, choroid) are now in common use, since diseases often spread beyond a single anatomical region of the uvea.
In general the uvea consists of a pigmented, highly vascular loose fibrous tissue. The pigment is produced and held in numerous dendritic melanocytes, similar to normal dermal melanocytes. The blood vessels show patterns which are specific to the region of the uvea, and are described in more detail under iris, ciliary body, pars plana and choroid. The stroma also contains large nerves, which are branches of the posterior ciliary nerves. They enter the eye around the optic nerve, and run forwards in the uvea to reach their termination in the cilary body or iris. These parts of the uvea also contain smooth muscle.
External and internal relations
These are described in more detail under the anatomic regions, as above.
Broadly, the outer aspect of the posterior uvea lies directly against the sclera, but at the root of the iris, the uvea is reflected sharply towards the central axis, so that its outer surface becomes the anterior surface of the iris, which is in contact only with the aqueous humour.
The inner aspect of the posterior uvea lies against Bruch's membrane, which separates it from the retina. On passing forwards beyond the ora serrata, Bruch's membrane and the retina are no longer present, and the inner relation of the uvea is a continuous epithelial sheet, represented in turn by the pars plana epithelium, the ciliary epithelium and the iris pigment epithelium.
The prime functions of the uveal tract as a unit are:
1. 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, cornea & lens, which lack any intrinsic blood supply.
2. light absorption. The uvea improves the contrast of the retinal image by reducing reflected light within the eye (analogous to the black paint inside a camera), and also absorbs outside light transmitted through the sclera, which is by no means opaque.
In addition, some uveal regions have special functions of great importance, particularly secretion of the aqueous humour by the ciliary processes, control of accommodation (focus) 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.
The pupil provides the neatest and most 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 drugs, even crude plant extracts, applied to the cornea. Pharmacological control over pupil size continues to be an important part of the treatment of some ocular diseases - see pupil, uveitis, acute glaucoma, chronic glaucoma.
The normal uvea contains 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 (ie, the body's inability to distinguish between them, with resulting misdirected inflammatory reactions).