Uvea is the medical name for the middle skin of the eye, also commonly known as the tunica media bulbi. Its name is derived from the Latin term for grape, which the uvea is said to resemble when dissected.
What is the uvea?
The uvea is the pigment-bearing layer of the eye and thus responsible for the different eye colors. This depends on the strength of the pigmentation, which varies from person to person individually and is further genetically determined. Blue or pale gray or green eyes are the result of a rather weak pigmentation. A strong pigmentation, on the other hand, makes the eyes appear brown. The pigment-forming cells themselves, the so-called melanocytes, are only a few micrometers in size. They are not fully formed until after birth, which explains the mostly blue eyes of babies. Within the eyeball, the uvea lies directly below the non-transparent sclera. Unlike the inner eye skin, which lies beneath the uvea, the sclera is highly scattering. The uvea, on the other hand, protects the eye from this scattering radiation. It is penetrated by the optic nerve in the posterior region and is open at the front as the pupil
Anatomy and structure
The median eye skin is composed of the iris, ciliary body, and choroid, which perform various tasks as part of the eye’s function. The tissue itself, is comparable to the soft meninges. Directly behind the lens is the iris, often called the iris skin, which separates the posterior from the anterior chamber of the eye. It consists mainly of blood vessels, smooth muscle cells, the pigment cells and the pupil opening. It is joined by the ciliary body, which is framed by the ciliary epithelium. The corpus ciliare or ray body is directly connected to the lens via zonula fibers and can thus provide for a change in lens curvature by contraction or relaxation of its ciliary muscle, respectively. The third component of the uvea is the choroid, medically called the choroid. It surrounds almost the entire vitreous body of the eye and is the most intensively perfused tissue in the human body. Components of the choroid are various vessels, connective tissue cells (fibrocytes) and the pigment-forming melanocytes already mentioned. Furthermore, the structural protein collagen is detectable.
Function and tasks
The tasks of the three individual elements, iris, ciliary body, and choroid, differ, and thus no specific function can generally be attributed to the uvea. The main function of the iris is to adjust the pupil and thus control the incidence of light. Like an aperture in photography, the pupil is dilated or contracted with the help of two muscles, thus increasing or decreasing the incidence of light. The movement of the two muscles is controlled by the autonomic nervous system. Deliberate activation is not possible. Under stress, in darkness or when looking into the distance, the incidence of light is increased by dilating the pupil. Under fatigue, in bright surroundings and when looking close up, the pupil contracts. The ciliary body performs two functions. First, it is responsible for the production of aqueous humor. It produces about 2 microliters of water per minute, which first fills the posterior chamber of the eye. The water then flows into the anterior chamber, where it washes around the cornea and lens. Both, and additionally the vitreous body, are supplied with nutrients by this water. Furthermore, the eye needs the produced aqueous humor to maintain the eye pressure. The second task of the ciliary body is performed by its muscle. Through its direct connection with the lens, it controls its precise curvature and makes it possible to adjust visual acuity according to the distance of the object. The choroid supplies the underlying retina with the oxygen and nutrients it needs. As part of the central nervous system, this layer of nerve cells relies on the supply from the choroid.
Diseases
The possibilities of disease of the uvea are many. They can be congenital or occur during life. In most cases, medical treatment is inevitable to prevent late effects, especially blindness. A common inflammation is uveitis. This disease, commonly known as iris, is characterized by pain, reddened eyes, sensitivity to light and reduced visual acuity.Due to these symptoms, there is a risk of confusion with conjunctivitis. Treatment is usually with an ointment containing cortisone. While uveitis can affect different areas of the uvea, iridocyclitis affects the iris and ciliary body. This inflammation is also manifested by pain and visual disturbances. In addition, sluggish pupillary reactions and changes in eye color are common. Iridocyclitis caused by viruses or even certain rheumatic diseases can lead to glaucoma or cataracts. One of the most serious diseases is choroidal melanoma. It develops due to degenerated melanocytes and in many cases is discovered too late or only by chance. However, early detection is important given its tendency to spread widely. The risk of occurrence of the most common eye tumor is highest between the ages of 60 and 70. Genetically, a disease of the uvea is caused by albinism, which is characterized by missing pigment cells. These are then also completely missing in the uvea and so only the blood vessels of the choroid are visible in the eye. The eye of an albino, which is simultaneously affected by visual impairment, therefore appears red.
