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Anatomy of the Iris and Ciliary Body
Anatomy
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121. Clinical Analogy - Setting the Stage
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1Exam pearl
- Think of the anterior uveal tract as a sophisticated aperture-diaphragm assembly in a precision camera. The iris is the aperture ring - controlling the amount of light reaching the film (retina) by dynamically altering its central opening (pupil) in response to ambient illumination and near-work demands. The ciliary body, wrapped around the equator of the lens, is the auto-focus motor - it adjusts lens curvature for near vision (accommodation) while simultaneously functioning as the aqueous humour manufacturing plant. Unlike any camera, however, this biological system also regulates intraocular pressure, modulates immune privilege, and participates in blood-ocular barrier maintenance - making it a uniquely elegant structure whose anatomy must be mastered in forensic detail for surgical safety and examination success.
2. Gross Dimensions & Zones
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1- Diameter: ~12 mm (horizontal)
- Thickness: varies - thinnest at iris root (~0.5 mm), thickest at collarette (~0.6 mm)
- Central aperture = pupil: 2–4 mm in bright light
- Miosis: pupillary constriction; pupil diameter varies with illumination and near fixation
- Mydriasis: pupil diameter may reach approximately 8 mm in darkness
- Two zones divided by the collarette (minor arterial circle region):
- Pupillary zone: smaller internal zone adjacent to the pupil; contains the sphincter pupillae muscle and pupillary ruff
- Ciliary zone: larger peripheral remainder of the iris; contains contraction furrows. Fuchs' crypts occur near the collarette and iris root. Schwalbe's folds are radial contraction folds near the pupil and structural folds extending toward the ciliary body.
- Iris root: thinnest part - site of iridodialysis in blunt trauma
Histological Layers (Anterior → Posterior)
| Layer | Composition | Key Points |
|---|---|---|
| Anterior border layer (not a true epithelium) | • Fibroblasts + melanocytes • no basement membrane | • Absent over crypts - crypts allow aqueous contact with iris stroma • no tight junctions |
| Stroma | • Loose collagen, melanocytes, clump cells, smooth muscle (sphincter), nerves, vessels | • Iris colour influenced here (stromal melanin content) • clump cells = clump cells of Koganei (pigmented macrophages) |
| Anterior pigment epithelium (APE) = myoepithelium | • Dilator pupillae (neuroectoderm) • pigmented cells | • Unique: dual nature - pigment + smooth muscle • Dilator pupillae innervated by sympathetics |
| Posterior pigment epithelium (PPE) | • Densely pigmented cuboidal epithelium | • Responsible for pigment ruff at pupil margin • absorbs stray light |
| Layer | Composition | Key Points |
|---|---|---|
| Anterior border layer (not a true epithelium) | • Fibroblasts + melanocytes • no basement membrane | • Absent over crypts - crypts allow aqueous contact with iris stroma • no tight junctions |
| Stroma | • Loose collagen, melanocytes, clump cells, smooth muscle (sphincter), nerves, vessels | • Iris colour influenced here (stromal melanin content) • clump cells = clump cells of Koganei (pigmented macrophages) |
| Anterior pigment epithelium (APE) = myoepithelium | • Dilator pupillae (neuroectoderm) • pigmented cells | • Unique: dual nature - pigment + smooth muscle • Dilator pupillae innervated by sympathetics |
| Posterior pigment epithelium (PPE) | • Densely pigmented cuboidal epithelium | • Responsible for pigment ruff at pupil margin • absorbs stray light |
Sphincter & Dilator Pupillae - Comparative Anatomy
| Feature | Sphincter Pupillae | Dilator Pupillae |
|---|---|---|
| Location | Pupillary zone stroma | Between anterior pigment epithelium (APE) & stroma (myoepithelial) |
| Origin | Neural ectoderm (optic cup) | Neural ectoderm (optic cup) |
| Width | ~0.8 mm band | Extends to root |
| Arrangement | Circular (ring) | Radial spokes |
| Innervation | Parasympathetic (oculomotor nerve [cranial nerve III] → short ciliary nerves via ciliary ganglion) | Sympathetic (superior cervical ganglion → long ciliary nerves) |
| Neurotransmitter | Acetylcholine → M3 receptors | Noradrenaline → α1 receptors |
| Action | Miosis (pupil constriction) | Mydriasis (pupil dilatation) |
| Pharmacology | • Blocked by atropine, tropicamide • Activated by pilocarpine, physostigmine | • Activated by phenylephrine, adrenaline • Blocked by thymoxamine |
| Feature | Sphincter Pupillae | Dilator Pupillae |
|---|---|---|
| Location | Pupillary zone stroma | Between anterior pigment epithelium (APE) & stroma (myoepithelial) |
| Origin | Neural ectoderm (optic cup) | Neural ectoderm (optic cup) |
| Width | ~0.8 mm band | Extends to root |
| Arrangement | Circular (ring) | Radial spokes |
| Innervation | Parasympathetic (oculomotor nerve [cranial nerve III] → short ciliary nerves via ciliary ganglion) | Sympathetic (superior cervical ganglion → long ciliary nerves) |
| Neurotransmitter | Acetylcholine → M3 receptors | Noradrenaline → α1 receptors |
| Action | Miosis (pupil constriction) | Mydriasis (pupil dilatation) |
| Pharmacology | • Blocked by atropine, tropicamide • Activated by pilocarpine, physostigmine | • Activated by phenylephrine, adrenaline • Blocked by thymoxamine |
Exam pearl
- Exam Pearl - Dilator Pupillae Embryology:
- Both sphincter and dilator arise from neuroectoderm of the optic cup (NOT neural crest).
- The dilator is a myoepithelial smooth muscle derived from neuroectoderm of the optic cup; neuroectodermal smooth-muscle development in the iris is unusual - frequently asked in viva.
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Keep reading Anatomy of the Iris and Ciliary Body
14 more sections are included with the Clinical plan.
- Iris Vasculature
- Iris Angle - Anatomical Landmarks (Gonioscopy)
- Gross Anatomy & Dimensions
- Ciliary Body Vasculature
- Zonules of Zinn - Anatomy & Surgical Relevance
- Aqueous Humour - Formation, Composition & Flow