Iris

• 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

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.