Wednesday, October 12, 2005


What is the iris? The iris is the pigmented diaphragm separating the anterior and posterior chambers. The iris is a component of the uveal tract. The iris stroma contains melanocytes, fibroblasts, and blood vessels arranged in a loose network.
What are the boundaries of the iris? It is joined to the ciliary body at the iris root. The anterior surface, also known as the anterior border layer, is composed of a condensed layer of fibroblasts, melanocytes, and collagen fibrils and borders the anterior chamber.The posterior surface of the iris is composed of two pigmented epithelial layers and borders the posterior chamber.
What are the layers of the iris? The iris contains pigmented cells and muscle and is composed of four layers: the anterior border layer, the stroma, the dilator muscle layer and the posterior epithelium. (Click to enlarge photo)
What is the structure of the anterior border layer? The anterior border layer consists of a dense packing of pigmented or nonpigmented cells similar in appearance to the cells present throughout the remainder of the stroma. Absence of cells produces the so called crypts in the border layer. What is histology of the stroma?
(The iris stroma is a loose fibrocollagenous support issue associated with spindle- shaped fibroblasts (stromal cells), blood vessels, nerves and macrophages (clump cells of Koganei) containing phagocytosed melanin pigment; at the pupil margin is the circumferentially arranged smooth muscle of the sphincter muscle of the pupil.
What is the composition of the posterior epithelium of the iris? The posterior epithelium is composed of two layers of cells which are densely pigmented with melanin. The posterior boundary of the iris stroma, peripheral to the sphincter muscle, is demarcated by another sheet of smooth muscle, the dilator muscle. The fibers of the dilator muscle are derived from, and remain in continuity with, the cuboidal pigmented cell bodies which make up the anterior layer of iris pigment epithelium.

What is the blood supply to the iris? The iris is supplied from the major arterial circle that is located in the ciliary body. Rupture of this vessels with ciliary muscle tears is frequently the injury that cause hyphemas. The blood vessels of the iris run in a radial direction. The anterior border layer contains very few vessels. Vessels on the surface of the border layer are abnormal and suggest ischemia in the eye such as in diabetic retinopathy, branch vein occlusions, neoplasms. Iris blood vessels appeared sheathed and have a characteristic loose appearance. They are nonfenestrated and have pericytes.

What is the innervation to the iris? The nerves of the choroid and iris are the long and short ciliary; the former being branches of the nasociliary nerve, the latter of the ciliary ganglion. They pierce the sclera around the entrance of the optic nerve, run forward in the perichoroidal space, and supply the blood vessels of the choroid. After reaching the iris they form a plexus around its attached margin; from this are derived non-medullated fibers which end in the Sphincter and Dilator pupillæ. Other fibers from the plexus end in a net-work on the anterior surface of the iris. The fibers derived through the motor root of the ciliary ganglion from the oculomotor nerve, supply the Sphincter, while those derived from the sympathetic supply the Dilatator.
What determines individual eye color? Eye color is determined by the relative number of melanocytes in the stroma. Few cells give a blue color, whereas many melanin- containing cells produce a dark brown color; gray and green are the intermediate colors.

What is the function of the pupil? It has a circular aperture (pupil) that can be opened and closed by the action of groups of smooth muscle. Contraction of the pupil reduces the amount of light entering the eye and thereby reduces the glare from light scattered from the periphery of the lens.
The dilator muscle layer is composed of the contractile processes of the myoepithelial cells of the inner layer of the posterior epithelium; it extends from the base of the iris to the sphincter muscle.

What is the appearance of the iris in cytology preparations? Normal iris may appear in intraocular washings from incidental ocutome cutting of the iris in an attempt to remove vitreous or lens fragments in the anterior chamber. Normal iris also may appear in fine needle aspiration specimens of iris neoplasms. In general, normal iris epithelium is so densely pigmented that cellular details are obscured. Iris stroma is characterized by the fine reticular meshwork of very cohesive and vascularized stroma.

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