Thin contractile circular disk
Approximately 12mm diameter
4 layers
Cone-shaped: pupil margin more anterior and rests on lens (iridonesis in aphakia due to loss of support)
Thickest at collarette and thinnest at insertion into ciliary body

## Embryology

Develops from 14 weeks
Neuroectoderm: iris epithelium and muscles (unique)
Mesenchyme: stroma, tunica vaculosa lentis and pupillary membrane vessels
Long posterior ciliary and anterior ciliary arteries infiltrate
Pupillary membrane disappears at 6 months
Sphincter muscle develops first (14 weeks), then dilator (6 months)

Note: the dilator pupillae never fully separates from the anterior epithelium and is therefore always partly modified basal processes of these neuroepithelial cells

Posterior epithelium is initially amelanotic but becomes highly pigmented from centre outwards by 7-8 months. Simultaneously the anterior layer loses its pigment

Melanocyte migration to produce the pigmentation of the posterior iris epithelium is stimulated by sympathetic innervation, hence a pale iris is seen in congenital Horner’s syndrome.

## Anterior border layer

Thickened stroma with fibroblasts and melanocytes
Deficient in areas creating crypts so that stroma is in contact with AC

## Stroma

Connective tissue
Fibroblasts, melanocytes (which provide colour), collagen fibres (types 1 and 3), macrophages
Clumps of macrophages (clumps of Koganei)
Sphincter muscle lies within stroma at pupillary zone between collarette and margin.

Concentrically arranged

## Dilator muscle

Derived from anterior iris epithelium: basal processes rest in this layer
Occupies peripheral ciliary zone outside collarette
Radially arranged
Sympathetic activity controlled by alpha receptors

## Anterior iris epithelium

In contact with the anteriorly positioned stroma
Few melanin granules situated in the apical cytoplasm
Continuous with the outer pigmented ciliary epithelium

## Posterior iris epithelium

Cuboidal and larger than the anterior layer
Becomes heavily pigmented (contains melanin): although continuous with inner, non-pigmented ciliary epithelium and thence with the neuroretina
Apices of this epithelium oppose the apices of the myoepithelial cells

This double layer of epithelial provides the main blood-aqueous barrier

Extends anteriorly to form ruff

## Blood supply

Major circle of iris lies in iris root:

Comprises anastomoses between long posterior and anterior ciliary arteries

Branches extend concentrically
Minor circle formed at collarette
Capillaries characterised by numerous tight junctions (zonula occludens), non-fenestrated and with a thickened basal lamina and so do not leak (eg fluorescein): contribute{' '} blood-aqueous barrier
Note: capillaries in ciliary body do not have tight junctions but the barrier is instead maintained by the tight junctions in the ciliary epithelium

## Nerve supply

Parasympathetic supply to ciliary body and sphincter pupillae via short ciliary nerves from ciliary ganglion (oculomotor nerve)

Preganglionic fibres come from Edinger-Westphal nucleus

Sympathetic supply joins sensory fibres in the long ciliary nerves (branch from nasociliary of ophthalmic nerve). Preganglionic fibres from superior sympathetic chain.

Post-ganglionic fibres from superior cervical ganglion travel along internal carotid plexus to long ciliary nerves.