Uvea of the Human Eye Anatomy and Role in Vision

The uvea is the eye's middle layer. It is located below the white area of the eye (the sclera). The iris, ciliary body, and choroid make up the iris. Many eye functions, such as adjusting to varying levels of light or object distances, are controlled by these structures. Uveitis is the inflammation of one or more of these structures.

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History and Etymology 

The name derives from the Latin word uva, which means "grape," and refers to the fruit's grape-like look (reddish-blue or almost black colour, wrinkled appearance and grape-like size and shape when stripped intact from a cadaveric eye). In actuality, it is a partial loan translation of the Ancient Greek name for the choroid, which literally means "grape-like coating." Its use as a technical term for a part of the eye dates back to the Middle Ages, but it exclusively referred to the choroid before that.

Uveal Tract of the Eye

The uveal tract is a layer of tissue that runs between the cornea and sclera on the outside and the retina on the inside of the eye. The iris is located in the front (anterior) of the uveal tract, whereas the choroid and stroma of the ciliary body are located at the back (posterior) of the uveal tract. The iris is exposed to both visible light and UV radiation, although the choroid and ciliary body are largely protected from both. Because the uveal tract is highly pigmented, the iris also serves as a light filter, preventing excessive light from accessing the lens and retina.

Melanocytes and pigment epithelial cells are two types of pigmented cells found in the uveal tract. The melanocytes in the iris are constantly exposed to UV light, which causes them to become malignant and produce a specific type of malignant tumour called uveal melanoma. Only iris melanoma is assumed to be produced by the harmful effects of UV radiation because the choroidal and ciliary melanocytes are insulated from UV light.

Structure and Function of the Uveal Tract

The outer layer (the transparent cornea and the opaque white sclera), the middle layer (the uveal tract, or uvea), and the inner layer (the uveal tract, or uvea) make up the human eye's wall (the retina). The parts of the eye uvea are made up of three parts: the iris in front, an intermediate ciliary body in the middle, and the choroid in the back. The uveal tract is a connective tissue that is highly vascularized and contains a variety of cell types.

The Iris 

The iris creates a diaphragm over the lens and is located in the anterior region of the uvea. The pupil is a hole in the centre of the iris that allows light to pass through. The amount of light that enters the eye determines the size of the pupil. It contracts when exposed to bright light and expands when exposed to dim light, allowing more or less light to penetrate the lens and posterior region of the eye.

The iris is made up of three layers. The first layer, which covers the anterior section of the iris, is made up of melanocytes and fibroblasts. The primary mass of the iris is found in the second section (iris stroma). The iris stroma is a layer of blood vessels embedded in loose connective tissues that contain fibroblasts and melanocytes, among other cells. Two layers of pigment cells make up the third layer (iris pigment epithelium). The anterior iris pigment epithelium is a smooth muscle cell layer that regulates pupil size. The posterior iris pigment epithelium is a strongly pigmented cell layer that covers the iris's posterior layer.

The colour of the human iris ranges from blue to dark brown, and the kind and amount of melanin in the melanocytes play a major role in determining the iris colour. Eumelanin (black to brown) or pheomelanin (yellow to reddish) are the two types of melanin found in the eye. Dark-coloured iris melanocytes (brown and dark brown irides) have a higher eumelanin/pheomelanin ratio and produce more melanin than light-coloured iris melanocytes. Iridal melanocytes in light-coloured irides (blue, yellow-brown, and green irides) contain less melanin, notably eumelanin. The pigment epithelium of the iris contains mostly eumelanin.

The iris serves a variety of purposes, including a light screening effect that can block off stray light rays. The size of the pupil limits the amount of light that enters the lens and the posterior segment of the eye by responding to the intensity of light entering the eye.

The Ciliary Body 

The ciliary body is related to the eye's outer layer and is situated between the iris and the choroid (the sclera). From the base of the iris to the commencement of the choroid, it is around 5-6 mm long. The ciliary body has a similar structure to the iris, which consists of the stroma and ciliary epithelium. The ciliary epithelium is made up of two layers of cells: pigmented and non-pigmented ciliary epithelial cells, which cover the inner surface of the ciliary body.

The ciliary body's primary job is to produce aqueous humour, which fills the region in front of and behind the iris. The zonule, which is linked to the ciliary body and the lens, governs the shape of the lens when the ciliary muscle contracts.

The Choroid

The choroid is the posterior component of the uvea, situated between the eye's outer (sclera) and inner (retina) layers. The choroid is a connective tissue layer that contains a significant number of blood vessels. The pigment epithelium cells in this region (retinal pigment epithelium) are considered as part of the retina rather than the uvea, in contrast to the iris and ciliary body. The choroid's primary role is to sustain and nourish the retina.

Function 

The prime functions of the uveal tract of the eye as a unit are:

• Nutrition and gas exchange: Uveal vessels deliver diffusible nutrients to the outer retina, sclera, and lens, which lack an intrinsic blood supply, and directly perfuse the ciliary body and iris to sustain their metabolic needs. (Because the cornea lacks any blood veins, it is oxygenated solely by direct gas exchange with the surroundings.)

• Light absorption: The uvea improves the contrast of the retinal image by absorbing outside light transmitted through the sclera, which is not totally opaque, and lowering reflected light within the eye (similar to the black paint inside a camera).

Furthermore, various uveal regions perform important tasks such as aqueous humour secretion via the ciliary processes, ciliary body regulation of accommodation (focus), and iris regulation of the pupil for optimal retinal illumination. The autonomic nervous system is in charge of many of these functions.

Pharmacology 

The pupil is a visible illustration of the body's neural feedback control. A balance between the antagonistic sympathetic and parasympathetic divisions of the autonomic nervous system helps to achieve this. Since the pupil is easily observable and can be easily manipulated by applying drugs — even simple plant extracts — to the cornea, informal pharmacological investigations have been conducted on it for ages. Controlling pupil size with drugs is still an important aspect of treating various ocular illnesses.

Drugs can also inhibit the metabolically active process of aqueous humour secretion, which is crucial in the treatment of both acute and chronic glaucoma.

Immunology 

The normal uvea of the eye is made up of immune-competent cells, primarily lymphocytes, and is prone to forming lymphocytic infiltrates in response to inflammation. Cross-reaction between uveal and retinal antigens (i.e., the body's inability to distinguish between them, resulting in misdirected inflammatory reactions) may be the cause of sympathetic ophthalmia, an uncommon condition.

Do You Know?

How Long Does It Take Uveitis to Heal?

The length of time you have uveitis depends on which region of your eye is affected. Anterior uveitis can be cleared completely in a matter of days to weeks with adequate therapy. Posterior uveitis, on the other hand, might continue for months or even years, affecting your eyesight permanently.