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Lens anatomy
Anatomy & Optics
Key Points
- •The lens is a transparent, biconvex, avascular structure contributing +15-20 D (~1/3 of total refractive power); derived entirely from surface ectoderm; measures 9-10 mm diameter and ~4 mm AP thickness; grows throughout life without shedding cells
- •Lens capsule: thickest basement membrane in the body (type IV collagen); anterior 14-21 um, posterior 2-4 um (thinnest at posterior pole - site of surgical rupture); capsulorrhexis size 5-5.5 mm
- •Crystallins (alpha, beta, gamma) constitute ~90% of lens proteins and ~33% of lens weight - alpha-crystallins are molecular chaperones; post-translational modifications → protein aggregation → cataract
- •Accommodation (Helmholtz mechanism): ciliary muscle contracts → zonules relax → lens thickens (more convex) → increased power for near; presbyopia (onset ~40 years) from nuclear sclerosis and loss of lens elasticity
- •Zonular fibres are composed of fibrillin-1; Marfan = superotemporal subluxation; homocystinuria = inferonasal; pseudoexfoliation = most common cause of zonular weakness worldwide
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- Definition
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- Aetiology and causes
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