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Anatomy of the Lens
Anatomy
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81. Overview
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1Exam pearl
- CLINICAL ANALOGY - Setting the Stage
- Think of the crystalline lens as a living optical fibre-optic cable - its differentiating lens fibre cells are drawn out into long, transparent fibres, packed in perfect geometric order.
- Differentiating lens fibre cells lose their nuclei and organelles, reducing light scatter.
- The lens is wrapped in the lens capsule, an optically clear basement membrane.
- Unlike a simple glass lens, it is metabolically active, capable of changing shape (accommodation), and avascular - it derives nutrients from the aqueous and vitreous humours.
- Lens structural features support transparency and refraction for image formation.
2. Gross Anatomy
Position & Dimensions
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1- The lens is a biconvex, avascular, transparent structure situated in the posterior chamber, immediately behind the iris and pupil, anterior to the vitreous. It is suspended by the zonules of Zinn from the ciliary body.
| Parameter | Value | Clinical Note |
|---|---|---|
| Equatorial diameter | 9–10 mm (adult) | Increases with age (9mm at birth → 10mm elderly) |
| Anteroposterior diameter | 3.5–5 mm | Increases with accommodation (~4mm → 4.5mm) |
| Anterior radius of curvature | 10 mm (unaccommodated) | Decreases (steepens) on accommodation |
| Posterior radius of curvature | 6 mm (unaccommodated) | Less change on accommodation vs anterior |
| Refractive index (cortex) | 1.386 | Lower than nucleus |
| Refractive index (nucleus) | 1.406 | Gradient RI - essential for spherical aberration control |
| Total power (in situ) | +18 to +20 D | Contributes ~1/3 of eye's total refracting power |
| Weight | ~255 mg (adult) | Increases with age due to continuous fibre addition |
| Parameter | Value | Clinical Note |
|---|---|---|
| Equatorial diameter | 9–10 mm (adult) | Increases with age (9mm at birth → 10mm elderly) |
| Anteroposterior diameter | 3.5–5 mm | Increases with accommodation (~4mm → 4.5mm) |
| Anterior radius of curvature | 10 mm (unaccommodated) | Decreases (steepens) on accommodation |
| Posterior radius of curvature | 6 mm (unaccommodated) | Less change on accommodation vs anterior |
| Refractive index (cortex) | 1.386 | Lower than nucleus |
| Refractive index (nucleus) | 1.406 | Gradient RI - essential for spherical aberration control |
| Total power (in situ) | +18 to +20 D | Contributes ~1/3 of eye's total refracting power |
| Weight | ~255 mg (adult) | Increases with age due to continuous fibre addition |
- Pearl: The lens contributes ~18–20D in situ; ~15D more in air (due to surrounding aqueous). Unlike cornea, lens has no endothelium or stroma - it is unique in structure.
Surfaces & Poles
- Anterior surface: Less curved (R=10mm), faces aqueous humour; covered by single layer of epithelium.
- Posterior surface: More curved (R=6mm), directly contacts vitreous face; NO epithelium posteriorly.
- Equator: Peripheral rim where anterior and posterior surfaces meet; site of zonular attachment and active mitosis.
- Anterior pole: Central point of anterior surface.
- Posterior pole: Central point of posterior surface.
- Axis: Line joining anterior to posterior pole.
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Keep reading Anatomy of the Lens
12 more sections are included with the Clinical plan.
- STRUCTURAL LAYERS - From Outside In
- LENS PROTEINS - Crystallins and the Molecular Basis of Transparency
- ZONULES OF ZINN - The Suspension System
- EMBRYOLOGY - How the Lens Forms
- Lens Biochemistry & Physiology
- AGE-RELATED CHANGES - From Clarity to Cataract