![]() Color: The color of the iris varies in the individual according to the melanin content of the melanocytes (pigment cells) in the stroma and epithelial layer. Eyes with a high melanin content are dark brown, whereas eyes with less melanin are grayish-blue.A faded surface structure can be a sign of inflammation (see iridocyclitis). Surface: The normal iris has a richly textured surface structure with crypts (tissue gaps) and interlinked trabeculae.Pupil dilation is sometimes sluggish in preterm infants and the newborn because the dilator pupillae muscle develops relatively late.The pupillary portion contains the sphincter muscle, which is supplied by parasympathetic nerve fibers, and the dilator pupillae muscle, supplied by sympathetic nerve fibers. These muscles regulate the contraction and dilation of the pupil so that the iris may be regarded as the aperture of the optical system of the eye. The collarette of the iris covering the minor arterial circle of the iris divides the stroma into pupillary and ciliary portions.The anterior surface of the lens and the pigmented layer are so close together near the pupil that they can easily form adhesions in inflammation. The posterior layer is opaque and protects the eye against excessive incident light. The posterior ectodermal pigmented epithelial layer.Structure and function: The iris consists of two layers: Sensory supply is provided by the long and short ciliary nerves. Venous blood drains through four to eight vorticose or vortex veins that penetrate the sclera posterior to the equator and join the superior and inferior ophthalmic veins. The anterior ciliary arteries originate from the vessels of the rectus muscles and communicate with the posterior ciliary vessels. ![]() They form the major arterial circle at the root of the iris and the minor arterial circle in the collarette of the iris. The long posterior ciliary arteries course along the interior surface of the sclera to the ciliary body and the iris. The short posterior ciliary arteries enter the eyeball with the optic nerveĪnd supply the choroid. ![]() Neurovascular supply: Arterial supply to the uveal tract is provided by the ophthalmic artery. Position: The uveal tract lies between the sclera and retina. While the aetiology of age-related macular degeneration is complex and is as yet unresolved, it is likely that accelerated ageing-like changes in the RPE play a fundamental role in the development of this condition.Structure: The uveal tract (also known as the vascular pigmented layer, vascular tunic, and uvea) takes its name from the Latin uva (grape) because the dark pigmentation and shape of the structure are reminiscent of a grape. The uveal tract consists of the following structures: This pigment has been shown to be highly phototoxic and has been linked to several oxidative changes, some leading to cell death. The ageing characteristics of the RPE suggest that in addition to cell loss, pleomorphic changes and loss of intact melanin granules, significant metabolic changes occur resulting, at least in part, from the intracellular accumulation of lipofuscin. Through the expression and activity of specific proteins, it regulates the transport of nutrients and waste products to and from the retina, it contributes to outer segment renewal by ingesting and degrading the spent tips of photoreceptor outer segments, it protects the outer retina from excessive high-energy light and light-generated oxygen reactive species and maintains retinal homeostasis through the release of diffusible factors. The retinal pigment epithelium (RPE) is a single layer of post-mitotic cells, which functions both as a selective barrier to and a vegetative regulator of the overlying photoreceptor layer, thereby playing a key role in its maintenance.
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