Iris

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Iris



P.K.Ghatak, MD

No.59


In a conversation, the word Iris generally refers to Irish, the people from Ireland. If the talk is around flowers, then Iris means a lovely delicate Iris flower.

In biology the Iris is a part of the eyeball, a very delicate screen like structure which regulates the amount of light rays can enter through the center aperture, Pupil to the retina for vision.

This screen is like the diaphragm of a single lens reflex camera; and both have many common features. In fact, the design of a camera is based on the eyes.

Anatomical structure of human iris:


The iris is thinnest at the periphery where it is attached to the ciliary muscles, and thick in the middle because the circular and radial muscle layers criss-cross in the middle of the iris The central aperture of the iris is the pupil of the eye. Contraction of circular muscles narrows the pupil and radial muscles enlarges size.

Histology of iris.

Structerally the iris consists oftwo distinct parts, anterior and posterior parts are not only structurally distinct and also embryiologically.

Anterior Part of iris:

The anterior part consists of ridges, valleys and lakes. It is made of collagen fibers, sphincter pupillary muscles, abundant vasculatures and several varieties of immunocytes, and a few melanocytes. The anterior part is not covered by epithelial cells.

The anterior part of the iris is derived from the Neural crest. The cells from the anterior part secrete Growth Factor-beta (TGFb). The TGFb is an immune regulator and acts like Cytokine and active locally.

The receptors of sex hormones are located in this part.

The posterior part of iris:


Two layers of heavily pigmented melanocytes and dilator radial muscles constitute this part of iris. This part of iris is derived from the neuroectoderm.

Melanin:

Chemically melanin are 5 different types . Only two out of 5 are present in the iris namely – Eumelanin and Pheomelanin.

Eumelanin gives black and brown colors, and Pheomelanin gives red and yellow colors.

Human eye color can vary from black, brown, blue, gray, green, amber, hazel red and colorless – albino. And rarely red and violet colored iris are seen in the albino group. All these colors are obtained by various combinations of Eumelanin and Pheomelanin. The color of iris is determined by inheritance.

The vascular uveal layer of the eye is made by the iris, ciliary body and choroid. Iris has no lymphatic vessels or lymph nodes.

Nerve supply:

The people are very sensitive to pain and other sensory modalities. Pain fibers are carried by the ophthalmic division of the 5th cranial nerve.

The sphincter pupillae muscle is innervated by fibers from the Edinger-Westphal nucleus of 3rd cranial nerve.

The dilator muscles of iris are innervated by the Superior Cervical Ganglion of the sympathetic chain. Both these nerve fibers hitchhike on the 3rd cranial nerve to reach the iris.

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Footnote:







  •  Irises (most common): Used in general and informal contexts.

  • Irides (technical): Used in medical contexts, derived from the Latin root.














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