- For the Pacific island, see Wallis Island
Uvea is an island in the Pacific Ocean belonging to the French overseas collectivity of Wallis and Futuna.-Geography:...
.
The
uvea (
Lat.Latin is an Italic language originally spoken in Latium and Ancient Rome. Through the Roman conquest, Latin spread throughout the Mediterranean and a large part of Europe...
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
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
. 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. Its use as a technical term in
anatomyAnatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy and plant anatomy...
and
ophthalmologyOphthalmology is a branch of medicine which deals with the diseases and surgery of the visual pathways, including the eye, hairs, and areas surrounding the eye, such as the lacrimal system and eyelids. The term ophthalmologist is an eye specialist for medical and surgical problems...
is relatively modern.
Regions
The uvea lies between the corneosclera (outermost layer of the eye) and the
retinaThe vertebrate retina is a light sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
(innermost layer/in the back of the eye). It is traditionally divided into 3 or 4 regions, from front to back, the
irisThe iris is a membrane in the eye, responsible for controlling the diameter and size of the pupil and the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel...
,
ciliary bodyThe ciliary body is the circumferential tissue inside the eye composed of the ciliary muscle and ciliary processes. It is triangular in horizontal section, and is coated by a double layer, the ciliary epithelium. The inner layer is transparent and covers the vitreous body, and is continuous from...
,
pars planaThe pars plana is part of the uvea, choroidea, one of the three layers that comprise the eye. It is about 4 mm long and located near the point where the iris and sclera touch....
and
choroidThe choroid, also known as the choroidea or choroid coat, is the vascular layer containing connective tissue, of the eye lying between the retina and the sclera. In humans its thickness is about 0.5 mm. The choroid provides oxygen and nourishment to the outer layers of the retina...
. 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.
Histology
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
irisThe iris is a membrane in the eye, responsible for controlling the diameter and size of the pupil and the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel...
,
ciliary bodyThe ciliary body is the circumferential tissue inside the eye composed of the ciliary muscle and ciliary processes. It is triangular in horizontal section, and is coated by a double layer, the ciliary epithelium. The inner layer is transparent and covers the vitreous body, and is continuous from...
,
pars planaThe pars plana is part of the uvea, choroidea, one of the three layers that comprise the eye. It is about 4 mm long and located near the point where the iris and sclera touch....
and
choroidThe choroid, also known as the choroidea or choroid coat, is the vascular layer containing connective tissue, of the eye lying between the retina and the sclera. In humans its thickness is about 0.5 mm. The choroid provides oxygen and nourishment to the outer layers of the retina...
. The stroma also contains large nerves, which are branches of the posterior ciliary nerves. They enter the eye around the
optic nerveThe optic nerve, also called cranial nerve II, transmits visual information from the retina to the brain.-Anatomy:The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during...
, and run forwards in the uvea to reach their termination in the cilary body or iris.
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 humourThe aqueous humour is a thick watery substance filling the space between the lens and the cornea.-Locations:The anterior segment is the front third of the eye that includes the structures in front of the vitreous humour: the cornea, iris, ciliary body, and lens...
.
The inner aspect of the posterior uvea lies against
Bruch's membraneBruch's membrane is the innermost layer of the choroid. It is also called the vitreous lamina, because of its glassy microscopic appearance.It is 2–4 μm thick.-Layers:Bruch's membrane consists of five layers :...
, which separates it from the retina. On passing forwards beyond the
ora serrataThe ora serrata is the serrated junction between the retina and the ciliary body. This junction marks the transition from the simple non-photosensitive area of the retina to the complex, multi-layered photosensitive region...
, 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 planaThe pars plana is part of the uvea, choroidea, one of the three layers that comprise the eye. It is about 4 mm long and located near the point where the iris and sclera touch....
epithelium, the ciliary epithelium and the
iris pigment epithelium- Anatomy :The iris pigment epithelium is a two-cell thick layer of cuboidal cells lying behind the iris. Both layers are jet black due to the numerous large melanosomes which pack the cytoplasm of each cell. Towards the central axis, the IPE terminates at the pupillary margin...
.
Physiology
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 processesThe ciliary processes are formed by the inward folding of the various layers of the choroid, i.e., the choroid proper and the lamina basalis, and are received between corresponding foldings of the suspensory ligament of the lens.-Anatomy:...
, control of
accommodationThe accommodation reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object , comprising coordinated changes in vergence, lens shape and pupil size...
(focus) by the ciliary body, and optimisation of retinal illumination by the iris's control over the
pupilThe pupil is an opening located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils...
. Many of these functions are under the control of the
autonomic nervous systemThe autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...
.
Pharmacology
The pupil provides a visible example of the neural feedback control in the body. This is subserved by a balance between the antagonistic
sympatheticThe Sympathetic Nervous System is a branch of the autonomic nervous system along with the enteric nervous system and parasympathetic nervous system. It is always active at a basal level and becomes more active during times of stress...
and
parasympatheticThe parasympathetic nervous system is a division of the autonomic nervous system , along with the sympathetic nervous system and enteric nervous system . The ANS is a subdivision of the peripheral nervous system . ANS sends fibers to three tissues: cardiac muscle, smooth muscle, or glandular tissue...
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
pupilThe pupil is an opening located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils...
,
uveitisUveitis specifically refers to inflammation of the middle layer of the eye, termed the "uvea" but in common usage may refer to any inflammatory process involving the interior of the eye....
, 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
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 ophthalmiaSympathetic ophthalmia is a granulomatous uveitis of both eyes following trauma to one eye. It can leave the patient completely blind. Symptoms may develop from days to several years after a penetrating eye injury. See also the reviews by Damico et al. , Chu and Foster , and Friedlaender et al...
may represent 'cross-reaction' between the uveal and retinal antigens (ie, the body's inability to distinguish between them, with resulting misdirected inflammatory reactions).
Pathology
See
uveitisUveitis specifically refers to inflammation of the middle layer of the eye, termed the "uvea" but in common usage may refer to any inflammatory process involving the interior of the eye....
,
choroiditisChoroiditis is a condition where the choroid is inflamed.When it presents in combination with retinitis , the condition is known as chorioretinitis....
,
iritisIritis is a form of anterior uveitis and refers to the inflammation of the iris of the eye.-Types:There are two main types of iritis: acute and chronic. They differ in numerous ways....
, anterior uveitis,
sympathetic ophthalmiaSympathetic ophthalmia is a granulomatous uveitis of both eyes following trauma to one eye. It can leave the patient completely blind. Symptoms may develop from days to several years after a penetrating eye injury. See also the reviews by Damico et al. , Chu and Foster , and Friedlaender et al...
,
uveal melanomaUveal melanoma is a cancer of the eye involving the iris, ciliary body, or choroid . Tumors arise from the pigment cells that reside within the uvea giving color to the eye...
.
External links