The Iris: Introduction

Figure 1: Photo of a human eye showing the iris and pupil (http://en.wikipedia.org/wiki/Image:Bluishgrayeye.JPG)

The Iris

The iris is an extension of the choroid partially covering the lens of the eye leaving a central circular aperture called the pupil (Junqueira and Carneiro, 2005).


Figure 2: Diagram of the eye showing position of iris (Click for bigger) (Junqueira and Carneiro, 2005)


Figure 3: Section of iris, showing inner layer (adjacent to anterior chamber), outer layer (adjacent to posterior chamber) and the dilator and constrictor muscles of the iris (PT stain, click for bigger) (Junqueira and Carneiro, 2005)

The layers of the pupil are as follows (from Junqueira and Carneiro, 2005):

• Anterior surface: rough and irregular, a discontinuous layer of pigment cells and fibroblasts
  
  
• A layer of poorly vascularized connective tissue with few fibers, and many fibroblasts and melanocytes
  
  
• A layer of loose connective tissue rich in blood vessels
  
  
• Posterior surface: smooth, covered by 2 layers of epithelium which also cover the ciliary body and its processes
  
  
• Inner epithelium: heavily pigmented with melanin granules, in contact with posterior chamber. The outermost cells have radial tongue-like extensions and contain overlapping myofilaments
  

The abundance of melanocytes and intensity of pigmentation in the iris prevents stray light rays from entering the eye except through the pupil, which would interfere with image formation (Junqueira and Carneiro, 2005).

The melanocytes found within the connective tissue of the iris are responsible for eye colour: if only a few pigment cells are present, the light reflecting from the black pigment epithelium in the posterior surface will be blue (Junqueira and Carneiro, 2005). As the cell number increases, the iris colour becomes greenish-blue, gray, and finally brown (Junqueira and Carneiro, 2005). Albinos, having very little pigment, have a pink eye colour due to light reflecting from iris blood vessels (Junqueira and Carneiro, 2005).

Myofilaments of the inner iris epithelium form the dilator pupillae muscle, which runs radially from the pupil to the iris margin (Junqueira and Carneiro, 2005). Smooth muscle bundles concentric with the pupillary margin form the sphincter pupillae muscle imbedded in the connective tissue of the iris (Junqueira and Carneiro, 2005). The dilator and sphincter muscles work in concert to increase and decrease the pupil diameter, respectively (Junqueira and Carneiro, 2005). Innervation of the dilator muscle is sympathetic, and the sphincter muscle, parasympathetic (Junqueira and Carneiro, 2005).

The iris, along with the ciliary region and the retinal pigment epithelium, is formed from the neurectoderm during embryonic eye development (Akagi and Takahashi, 2005).


Figure 4: Development of the eye. (A) Evagination of the primary optic vesicle. (B) Invagination of the surface epithelium and formation of the optic cup by invagination at the embryonic fissure. (C) Detachment of the lens from the surface epithelium. (D) The ciliary region and iris are formed from neuroectoderm, and the lens from the posterior cells of the lens vesicle. (E) The completed eye. IPE, iris pigmented epithelium; RPE, retinal pigment epithelium; CB, ciliary body. (Akagi and Takahashi, 2005)

Iris-derived cells have been induced to express rod photoreceptor genes and to take the form of retinal rod cells, suggesting a possibility of retinal repair grafts from iris tissue (Akagi and Takahashi, 2005).

Iris melanoma

Iris melanoma (cancer of pigment cells or melanocytes) is the most common malignancy of the iris, though the iris is the least common site of primary uveal (involving the iris, ciliary body or choroid) melanoma (Henderson, 2008). It also has a better prognosis than other uveal melanomas (Henderson, 2008).

Victims of iris melanoma have an average age in the mid-late 40's, and the disease tends to occur most often in persons with fair complexions and blue to gray iris colour (Henderson, 2008). Interestingly, 80% of iris melanomas arise in the inferior half of the iris (Henderson, 2008).

There are two main types of iris melanomas: a) circumscribed, consisting of a yellow/tan/brown pigmented spot, and b) diffuse, presenting as a unilateral darkening of the iris (Henderson, 2008). Circumscribed iris melanomas are the easiest to detect (Henderson, 2008).


Figure 5: Circumscribed iris melanoma at approximately 10 o’clock position. The brown tumor arises at the root of the iris and protrudes into the anterior chamber (Henderson, 2008)


Figure 6: Diffuse iris melanoma. The iris developed its brown and gray color during 1 year. The opposite eye was blue. (Henderson, 2008)

The tumour usually consists mostly of uniform spindle melanoma cells, and some contain epithelial cells (Henderson, 2008). More than 50% of iris melanomas are reported as "spindle cell" melanomas, which have the best prognosis (Henderson, 2008).


Figure 7: A) Circumscribed iris melanoma fills the anterior chamber. The tumor is cellular and bulky (X2 Mag.). B) A majority of the tumor was composed of relatively uniform spindle melanoma cells, most of which contain visible melanin (X40). C) A minority of the tumor contained epithelioid cells, characterized by their larger size, greater pleomorphism, eosinophilic cytoplasm, and larger nuclei with prominent nucleoli (X40). All taken using hematoxylin-eosin staining (Click for bigger) (Henderson, 2008)

A limited number of treatments for growing iris melanomas are available. Most discrete lesions are excised surgically, preserving vision (Henderson, 2008). Tumours involving the angle of the anterior chamber require partial resection of the ciliary body, which involves an increased risk of ocular morbidity (loss of an eye) (Henderson, 2008). The procedure is difficult because a small specimen must be properly oriented and uveal tissue is easily crushed by handling (Henderson, 2008). Radiotherapy is not often used to manage resectable iris melanomas because of an increased risk of vision complications (Henderson, 2008). Particularly large or diffuse tumours (involving more than 50% of the iris) usually require enucleation (removal of the eye) (Henderson, 2008). For patients for which this is undesirable (for example, one-eyed persons) there are possible alternatives to enucleation including the use of specially designed radioplaques for treatment of iris melanoma (Henderson, 2008).

References:

Akagi, T. and Takahashi, M. 2005 Photoreceptors Derived from Adult Iris Tissue: Prospects for Retinal Transplantation. Seminars in Ophthalmology 20(1);11-15

Henderson E., Margo C.E. 2008. Iris Melanoma. Arch Pathol Lab Med. 132:268-272

Junqueira L.C., Carneiro J. 2005. Basic Histology, Eleventh Edition. The McGraw-Hill Companies, Inc. United States of America