Uvea
Horizontal section of the eyeball. The constituents of the uvea follow: iris labeled at top, ciliary body labeled at upper right, and choroid labeled at center right.)
Latin	tunica vasculosa bulbi
MeSH	Uvea

For the Pacific island, see Wallis Island.

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 reddish-blue or almost black colour, wrinkled appearance and grape-like size and shape when stripped intact from a cadaveric eye.^[1] Its use as a technical term in anatomy and ophthalmology is relatively modern.

Anatomy [link]

Regions [link]

The vascular middle layer of the eye. It is traditionally divided into 3 areas, from front to back, the iris, ciliary body, and choroid.

Physiology [link]

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 not fully opaque.

In addition some uveal regions have special functions of great importance, including 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.

Pharmacology [link]

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 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 metabolically-active process of secreting aqueous humour can also be reduced by drugs, which is important in treating both acute and chronic glaucoma.

Immunology [link]

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 (i.e., the body's inability to distinguish between them, with resulting misdirected inflammatory reactions).

Pathology [link]

See uveitis, choroiditis, iritis, Iridocyclitis, anterior uveitis, sympathetic ophthalmia, uveal melanoma.

External links [link]

• MedlinePlus Encyclopedia 002337
• Diagram at visionweb.com

References [link]

1. ^ eye, human."Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD 2009

v t e Sensory system – visual system – globe of eye (TA A15.2.1–6, TH 3.11.08.0-5, GA 10.1005) Fibrous tunic (outer) Sclera Episcleral layer • Schlemm's canal • Trabecular meshwork Cornea Limbus • layers (Epithelium, Bowman's, Stroma, Descemet's, Endothelium) Uvea/vascular tunic (middle) Choroid Capillary lamina of choroid • Bruch's membrane • Sattler's layer Ciliary body Ciliary processes • Ciliary muscle Iris Stroma • Pupil • Iris dilator muscle • Iris sphincter muscle Retina (inner) Layers Inner limiting membrane • Nerve fiber layer • Ganglion cell layer • Inner plexiform layer • Inner nuclear layer Outer plexiform layer • Outer nuclear layer External limiting membrane • Layer of rods and cones • Retinal pigment epithelium Cells Photoreceptor cells (Cone cell, Rod cell) → (Horizontal cell) → Bipolar cell → (Amacrine cell) → Retina ganglion cell (Midget cell, Parasol cell, Bistratified cell, Giant retina ganglion cells, Photosensitive ganglion cell) → Diencephalon: P cell, M cell, K cell Muller glia Other Macula (Foveola, Fovea centralis) • Optic disc (Optic cup) Anterior segment Anterior chamber • Aqueous humour • Posterior chamber • Lens (Capsule of lens, Zonule of Zinn) Posterior segment Vitreous humour Other Ocular immune system • Tapetum lucidum • Keratocytes M: EYE anat(g/a/p)/phys/devp/prot noco/cong/tumr, epon proc, drug(S1A/1E/1F/1L)