Crystalline lens

• Crystalline lens


  An avascular, transparent elliptic structure that aids in focusing light rays on the retina
• Crystalline lens


  • Located within the posterior chamber, anterior to the vitreous chamber and posterior to the iris
  • Mechanism that causes an increase in lens power is accommodation, which allows near objects to be focused on the retina
  • Posterior lens surface is attached to the anterior vitreous face by the hyaloid capsular ligament, a circular ring adhesion
• The lens is biconvex, with the posterior surface having the steeper curve
• The anterior radius of curvature measures 8 to 14 μm, and the posterior surface radius of curvature measures 5 to 8 μm
• The centers of the anterior and posterior surfaces are called the poles, and the lens thickness is the distance from anterior to posterior pole
• The thickness of the unaccommodated lens is 3.5 to 5 mm and it increases 0.02 mm each year throughout life
• The lens diameter is the nasal-to-temporal measurement and in the infant is 6.5 mm; the diameter reaches 9 mm during the teenage years and does not change significantly from that
• The equator is the largest circumference of the lens at a location between the two poles
• Refractive power of the unaccommodated lens

  Approximately 20 diopters (D)
• Factors affecting refractive power of the lens

  • Surface curvatures
  • Refractive index
  • Change in index between the lens and surrounding environment
  • Length of the optical path
• The power of the lens increases in accommodation, with the maximum accommodative amplitude, 14 D, reached between ages 8 and 12 years
• Accommodation decreases with age, approaching zero after 50 years
• Embryologic development of the lens

  1. Lens vesicle forms

  2. Posterior cells differentiate and elongate, forming the primary lens fibers

  3. Center of the sphere fills as fibers grow and reach the anterior cells

  4. Adult lens has no posterior epithelium because it was used to form these first lens fibers
• Lens capsule


  • Transparent envelope that surrounds the entire lens
  • Thickness varies with location, being thinnest at the poles and equator and thickest in an annular area around the anterior pole
  • Consists primarily of collagen, no elastic fibers but highly elastic due to lamellar arrangement of fibers
  • Encloses all lens components and helps to mold the shape of the lens
  • Zonular fibers insert into the capsule, merging with it in an area from the equator to near both poles
  • Provides some barrier function preventing large molecules from entering the lens
  • Anterior lens capsule is produced by the anterior epithelium and thickens with age, posterior capsule changes minimally
• Lens epithelium


  • Adjacent to the anterior lens capsule is a layer of cuboidal epithelium
  • No posterior epithelium present as it was used during embryologic development to form the primary lens fibers
  • Basal aspect of epithelial cells adjacent to the capsule, apical portion oriented inward toward the center
  • Lateral membranes of epithelial cells joined by desmosomes and gap junctions
• Germinal zone


  • Band of cells in the pre-equatorial region that lies just anterior to the equator, location of cell mitosis
• Lens fiber formation


  1. Cell division continues throughout life, each newly formed cell elongates with basal aspect stretching toward posterior pole and apical aspect toward anterior pole

  2. Fibers stretch toward the poles from all aspects of the lens periphery

  3. Elongated cell loses all cellular organelles and becomes a lens fiber

  4. Anterior end of lens fiber insinuates itself between the epithelial layer and underlying lens fibers

  5. All fibers formed from mitosis in the germinative zone are called secondary lens fibers
• Lens fibers


  • Lens fiber production continues throughout life, with new fibers being laid down outer to the older fibers
  • Fiber length is approximately 8 to 10 nm
  • Each fiber has a long crescent shape, with broad sides parallel to the lens surface and narrow sides at an angle to the surface near the lens poles
  • Lens fiber cytoplasm contains a high concentration of proteins called crystallins, which contribute to the gradient refractive index
  • Crystallin concentration varies from 15% in the cortex to 70% in the nucleus
  • Cytoskeletal network of microtubules and filaments provides structure and stability
  • Fibers joined by desmosomes
  • Junctions along the lateral membranes have a distinct morphology comparable to gap junctions but with different protein connexon forming the channels
• Ciliary zonules


  • Transparent, stiff, non-elastic fibers 0.35 to 1.0 u in diameter, made of microfibrils 8-40nm in diameter
  • Different types of zonular fibers: wavy fibers near vitreous, thin flat fibers, and fine circular fibers
  • Fibers arranged in orbiculo-post capsular, orbiculo-anterior capsule, cilio-posterior capsular, and cilio-equatorial configurations
• Epithelium-fiber interface (EFI)


  • The border between the apical membrane of the anterior epithelium and the apical membrane of the elongating fiber
  • Nutrients and ions are exchanged across the EFI
  • Gap junctions usually found on the lateral cell membrane, not the EFI
  • Minimal coupling between epithelium and fibers in the central zone, but increases toward the germinative zone
• Division of the lens

  • Embryonic nucleus (primary lens fibers)
  • Fetal nucleus (fibers formed before birth)
  • Adult nucleus (fibers formed between birth and sexual maturation)
  • Lens cortex (fibers formed after sexual maturation)
• The lens cortex has the lowest and the embryonic nucleus has the highest index of refraction
• As the lens fibers reach the poles they meet with the other fibers in the lens sutures
• Division of Lens


  1. Primary lens fibers from elongating posterior epithelium form embryonic nucleus

  2. Cell mitosis begins in pre equatorial region, new cells elongate to form secondary lens fibers

  3. Fibers laid down around embryonic nucleus before birth are fetal nucleus

  4. Fibers formed between birth and sexual maturation are adult nucleus

  5. Lens cortex contains fibers formed after sexual maturation
• Lens Sutures


  1. Fibers reach poles and meet other fibers, forming anterior and posterior sutures

  2. Fibers formed during embryologic development meet in three branches, forming Y sutures

  3. As lens grows, sutures become asymmetric and dissimilar
• Biomicroscopic stratification of the lens

  • Capsule
  • Superficial cortex: alpha C1, beta C2
  • Deep cortex: C3 & C4
  • Nucleus
• Zonules (of Zinn)


  • Threadlike fibers that attach the lens to the ciliary body
  • Primary zonules attach to the lens capsule
  • Secondary zonules join the primary zonules or connect processes to each other or the pars plana
• During Accommodation


  1. Lens thickness increases anterior to posterior

  2. Lens thins along the equator

  3. Anterior lens surface moves forward, anterior chamber becomes shallower

  4. No change in position of posterior pole
• Lens Transparency


  • Absence of blood vessels
  • Few cellular organelles in light path
  • Orderly arrangement of fibers
  • Short distance between components of differing indices relative to wavelength of light
• Lens Physiology


  • Metabolic activity in anterior epithelium to maintain cell & fiber function
  • Pre equatorial region has high level of mitotic activity
  • Nutrients obtained from surrounding aqueous with small contribution from vitreous
  • Free radicals disrupt cellular processes and cause damage, leading to high molecular weight aggregates
• Types of Zonular Fibers

  • Thick 1um and wavy
  • Thin and flat
  • Fine and run circular course
• Main Fibers of Ciliary Zonules

  • Orbiculo-posterior capsular fibers
  • Orbiculo-anterior capsular fibers
  • Cilio-posterior capsular fibers
  • Cilio-equatorial fibers
• Zones of Suspensory Fibers

  • Pars orbicularis
  • Zonular plexuses
  • Zonular fork
  • Zonular Limbs
• HYALOID Zonule


  Single layer of zonules connecting anterior hyaloid at border of patellar fossa with pars plana & pars plicata
• HYALOCAPSUALR Zonule


  Circular band of zonular fibres
• CIRCUMFERENTIAL Zonule Girdle


  Anterior Ciliary & Posterior Ciliry
• Biochemical Composition of Lens

  • Water (65% of wet weight)
  • Proteins (34% of total weight)
  • Minor proteins: glycoprotein, phosphoprotein, lipoprotein & fluorescent proteins
  • Soluble proteins: alpha crystallins, beta crystallins, gamma-crystallins
• Proteogenic Amino Acids


  • Alanine
  • Leucine
  • Glutamic acid
  • Aspartic acid
  • Glycine
  • Valine
  • Lysine
  • Arginine
• Non-Proteogenic Amino Acids


  • Taurine
  • Alpha amino butyric acid
• Carbohydrates in Lens


  Glucose, fructose, glycogen, sorbitol, inositol, ascorbic acid gluconic, glucosamine