Iris antomy

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Iris Anatomy

The iris is the most anterior part of the uveal (vascular) layer of the eye. It is a circular, pigmented diaphragm that projects inward from the ciliary body and contains a central aperture - the pupil - through which light enters the eye.

Position and Relations

The iris sits between the cornea anteriorly and the lens posteriorly, dividing the anterior segment of the eye into:

Anterior chamber - between cornea and iris, filled with aqueous humor
Posterior chamber - between iris and lens, also filled with aqueous humor

Its peripheral root attaches to the ciliary body, while its free inner edge forms the pupillary margin.

Gross Anatomy Diagram

Ciliary body and iris anatomy showing sphincter pupillae, dilator pupillae, iris, cornea, lens, ciliary body, posterior chamber, and scleral venous sinus

Fig. 8.113 - Gray's Anatomy for Students: Cross-section showing the iris in relation to adjacent structures

Layers of the Iris (Anterior to Posterior)

1. Anterior Surface (No Epithelium)

Unlike virtually every other surface in the body, the anterior face of the iris has no epithelial covering. Instead, it consists of a dense, matted layer of interdigitating fibroblasts and melanocytes exposed directly to the aqueous humor of the anterior chamber. Scanning electron microscopy reveals numerous ridges and crypts (Fuchs' crypts) on this surface.

2. Stroma

The bulk of the iris consists of loose connective tissue stroma containing:

Melanocytes - variable in number; their density determines eye color (see below)
Sparse collagen fibers
Blood vessels (sparse microvasculature)
Nerve fibers
Sphincter pupillae muscle - located in the stroma near the pupillary margin

3. Posterior Epithelium - Bilaminar (Two Cell Layers)

The posterior surface carries two layers of pigmented epithelium, continuous with the epithelium of the ciliary body:

Layer	Cell Type	Pigmentation	Special Feature
Posterior pigment epithelium (innermost)	Columnar epithelial cells	Very heavily pigmented - nuclei obscured	Basal lamina faces posterior chamber
Anterior pigment myoepithelium	Myoepithelial cells	Apical portions heavily pigmented	Basal portions extend radially as the dilator pupillae muscle

Intrinsic Muscles of the Iris

Histology and schematic diagram of iris structure showing sphincter pupillae, dilator pupillae, myoepithelial cells, posterior pigment epithelium, and stromal melanocytes

Histology Atlas - Fig. 24.8: Structure of the iris (schematic + photomicrograph)

Muscle	Arrangement	Cell Type	Innervation	Action
Sphincter pupillae	Circular fibers near pupillary margin, within stroma	Smooth muscle	Parasympathetic (CN III via ciliary ganglion)	Constricts pupil (miosis)
Dilator pupillae	Radial processes extending from myoepithelial cells of anterior pigment layer	Myoepithelial (unique - neuroectodermal origin)	Sympathetic (superior cervical ganglion, T1)	Dilates pupil (mydriasis)

Key point: the dilator pupillae is not a classic smooth muscle - it is formed by contractile processes of myoepithelial cells derived from the anterior pigment epithelium layer.

Histological Detail

H&E and SEM micrographs of iris showing anterior chamber (AC), stroma (S), dilator pupillae muscle (DPM), sphincter pupillae muscle (SPM), posterior pigmented epithelium (PE), posterior chamber (PC), and pupil (P)

Junqueira's Basic Histology - Fig. 23-10: Iris cross-section (H&E x140 and SEM x900)

AC = anterior chamber, PC = posterior chamber, S = stroma, PE = posterior pigment epithelium, DPM = dilator pupillae muscle, SPM = sphincter pupillae muscle, P = pupil

Eye Color

Eye color is determined by the number and melanin content of stromal melanocytes:

Few, lightly pigmented stromal melanocytes → light reflected from the densely pigmented posterior epithelium → blue iris
Increasing stromal melanocytes → progressively absorb and reflect longer wavelengths → green → gray → hazel → brown
The posterior pigment epithelium is densely pigmented in all individuals regardless of eye color.

Blood Supply and Innervation

Arterial supply: The major arterial circle of the iris (circulus arteriosus major) sits at the ciliary body root, formed by long posterior ciliary arteries and anterior ciliary arteries. From it, radial branches run toward the pupil and form the minor arterial circle (circulus arteriosus minor) near the pupillary margin.
Innervation: Sympathetic fibers (via the nasociliary branch of CN V1, traveling with the long ciliary nerves) innervate the dilator. Parasympathetic fibers (from CN III via the short ciliary nerves from the ciliary ganglion) innervate the sphincter.

Summary Table

Feature	Detail
Part of	Uvea (middle vascular layer)
Anterior surface	No epithelium; fibroblasts + melanocytes
Stroma	Loose connective tissue, melanocytes, blood vessels
Posterior surface	Bilaminar pigmented epithelium (myoepithelial + pigment layers)
Pupil constriction	Sphincter pupillae - circular smooth muscle - parasympathetic (CN III)
Pupil dilation	Dilator pupillae - radial myoepithelium - sympathetic (T1)
Eye color	Determined by stromal melanocyte density

Sources:

Junqueira's Basic Histology, 17e, pp. 1189-1190
Histology: A Text and Atlas, pp. 2392-2393
Gray's Anatomy for Students, p. 1090

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