Overview
This lecture covers the anatomy, development, functions, and clinical aspects of the eye's lens, including its structure, associated ligaments, and surgical relevance.
General Anatomy and Location of Lens
- The lens is a transparent, biconvex crystalline mass located between the iris and the vitreous body.
- It sits in the patellar fossa, a depression on the anterior surface of the vitreous.
- The lens is attached by a circular ligament called the Wieger's (hyaloidocapsular) ligament.
Spaces and Ligaments
- The retro-lental or Berger's space is found behind the lens where attachment to the vitreous is loose.
Functions of the Lens
- The lens transmits, refracts, and focuses light onto the retina.
- It absorbs ultraviolet (UV) light below 350 nm, protecting the retina.
- Provides about 35% of the eye’s total refractive power and aids in accommodation.
Measurements and Physical Properties
- Anterior surface radius of curvature: 10 mm; posterior surface: 6 mm.
- Equatorial diameter: 6.5 mm at birth, increases to 9–10 mm in adulthood.
- Thickness: 3.5 mm at birth, up to 5.5 mm in adults; increases 0.2 mm/year.
- Refractive index: 1.39; refractive power: 16–17 diopters.
- Accommodative power: 14–16D at birth, decreases with age (presbyopia).
Development and Layers of the Lens
- Lens develops from the surface ectoderm via invagination to form the lens vesicle.
- Lens capsule: Thin, transparent, collagenous, no elastic fibers but highly elastic; thickest basement membrane in the body.
- Thinnest at the posterior pole (prone to rupture during surgery).
Lens Epithelium
- Only anterior epithelium remains after 3 months gestation; posterior epithelium forms primary lens fibers.
- Anterior lens epithelium: Cuboidal, nucleated cells, most metabolically active part.
- Epithelium zones: central (stable), intermediate (rarely divides), and germinative/equatorial (actively divides to form secondary lens fibers).
- Clinical significance: Metaplasia can lead to certain cataracts; posterior subcapsular cataracts can form from dysplasia.
Lens Fibers and Nuclei
- Primary lens fibers form embryonic nucleus (center), secondary fibers form fetal, infantile, adult nuclei, and cortex.
- Lens fibers meet at Y-sutures: upright anteriorly, inverted posteriorly.
- Sutural cataracts may occur along these sutures.
Suspensory Ligaments (Zonules of Zinn)
- Zonules anchor the lens to the ciliary body, arising mainly from the pars plana and ora serrata.
- Zonules are divided into parts: pars orbicularis, zonular plexus, zonular fork, and zonular limbs (anterior, equatorial, posterior).
- Spaces: Canal of Hannover (between limbs), Canal of Petit (behind zonules).
Surgical Anatomy and Clinical Points
- During cataract surgery, hydrodissection separates cortex from capsule, hydrodelineation separates nucleus from epinucleus.
- Lens hardness is graded by color; modern assessment uses LOCS III system comparing lens opacities to standard images.
Key Terms & Definitions
- Patellar Fossa — the depression in the anterior vitreous where the lens sits.
- Wieger’s Ligament — circular ligament anchoring the lens to the vitreous.
- Berger’s Space — potential space behind the lens.
- Lens Capsule — thick, transparent, basement membrane around the lens.
- Anterior Lens Epithelium — single layer of cuboidal cells on the lens’ front.
- Primary Lens Fibers — fibers formed from posterior epithelium during embryonic development.
- Secondary Lens Fibers — fibers formed after embryonic period from equatorial epithelium.
- Zonules of Zinn — suspensory fibers attaching lens to ciliary body.
- Y-Suture — suture line formed by meeting lens fibers.
- Hydrodissection — surgical separation of cortex from capsule.
- Hydrodelineation — surgical separation between nucleus and epinucleus.
- LOCS III — standardized system for grading lens opacities.
Action Items / Next Steps
- Review the development of the lens and associated diagrams.
- Study lens accommodation and refractive properties.
- Watch recommended videos on eye development, accommodation, and slit-lamp examination.