Cornea

• Cornea


  Smaller, anterior transparent sphere of the eye
• Sclera


  Larger, posterior opaque sphere of the eye
• The outer connective tissue coat of the eye has the appearance of two joined spheres
• The cornea has a radius of curvature of approximately 8 mm
• The sclera has a radius of approximately 12 mm
• The globe is not symmetric; its approximate diameters are 24 mm anteroposterior, 23 mm vertical, and 23.5 mm horizontal
• Cornea


  • Appears oval from the front as the sclera encroaches on the superior and inferior aspects
  • Anterior horizontal diameter is 11.75mm, and the anterior vertical diameter is 11 mm
  • Appears circular from behind, with horizontal and vertical diameters of 11.5 mm
  • Has an elliptic rather than a spheric shape, curvature steeper in the center and flatter near the periphery
  • Radius of curvature of the central cornea at the anterior surface is 7.8 mm and at the posterior surface is 6.5 mm
  • Central corneal thickness is 0.53-0.52mm, whereas the corneal periphery is 0.67-0.71 mm thick
• Cornea


  • Principal refracting component of the eye
  • Transparent and avascular to provide optimal light transmittance
  • Anterior surface covered by tear film, posterior surface borders aqueous-filled anterior chamber
  • Continuous with conjunctiva and sclera at its periphery
  • Composed of 5 layers: epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium
• Corneal function


  • To refract light and transmit light
  • Factors affecting corneal refraction: curvature of anterior surface, change in refractive index from air to cornea, corneal thickness, curvature of posterior surface, change in refractive index from cornea to aqueous humor
  • Total refractive power of the eye focused at infinity is between 60 and 65 diopters, with 43 to 48 D attributable to the cornea
  • Scattering of incident light is minimized by the smooth optical surface formed by the corneal epithelium and its tear film covering
  • Regular arrangement of surface epithelial cells and tear film filling irregularities provide a relatively smooth surface
  • Absence of blood vessels and correct spatial arrangement of components account for minimal scattering and distortion as light passes through
• Corneal epithelium


  • Outermost layer is 5 to 6 cells thick and measures approximately 50μm
  • Surface layer is two cells thick and displays a very smooth anterior surface
  • Consists of non keratinized squamous cells with flattened nuclei and fewer cellular organelles than deeper cells
  • Plasma membrane of surface cells secretes a glycocalyx component that adjoins the mucin layer of the tear film
  • Microvilli and microplicae on surface
  • Tight junctions (zonula occludens) join surface cells along lateral walls near apical surface, providing a barrier to intercellular movement
  • Desmosomes provide additional adhesion between cells
  • Surface cells are constantly sloughed off and replaced from layers below
  • Middle layer is 2-3 layers of wing cells with wing-like lateral processes
  • Innermost basal cell layer is a single layer of columnar cells with oval nuclei, attached to basement membrane by hemidesmosomes
• Corneal epithelial replacement


  • Maintenance of smooth surface depends on replacement of surface cells constantly shed into tear film
  • Cell proliferation occurs in basal layer, basal cells move up to become wing cells, wing cells move up to become surface cells
  • Stem cells in 0.5- to 1-mm–wide band around corneal periphery are source for renewal of basal cell layer
  • Slow migration of basal cells occurs from periphery toward center
  • Turnover time for entire corneal epithelium is approximately 7 days
  • Repair to corneal epithelial tissue proceeds quickly, minor abrasions heal within hours, larger ones often heal overnight
• Bowman's layer


  • Second layer of cornea, approximately 8 to 14μm thick
  • Dense, fibrous sheet of interwoven collagen fibrils randomly arranged in mucoprotein ground substance
  • Fibrils have 20 to 25 nm diameter, run in various directions, not ordered into bundles
  • Produced prenatally by epithelium, not believed to regenerate
  • Very resistant to damage by shearing, penetration, or infection
  • Corneal nerves passing through typically lose Schwann cell covering and pass into epithelium as naked nerves
• Corneal stroma


  • Middle layer, approximately 500μm thick, 90% of total corneal thickness
  • Composed of collagen fibrils, keratocytes, and extracellular ground substance
  • Collagen fibrils have uniform 25- to 35-nm diameter, run parallel forming flat bundles called lamellae
  • 200 to 300 lamellae distributed throughout stroma, lie parallel to corneal surface
  • Fibrils within each lamella run in same direction, adjacent lamellae lie at angles
  • Lamellae in anterior 1/3 are thinner, branch and interweave more than deeper layers
  • Keratocytes (corneal fibroblasts) are flattened cells between and within lamellae, in corkscrew pattern
  • Ground substance contains proteoglycans with hydrophilic, negatively charged glycosaminoglycans
  • Regular arrangement of components and small fibril diameter contribute to stromal transparency
• Descemet's membrane


  • Basement membrane of endothelium, constantly thickens throughout life
  • 5μm thick in children, increases to approximately 15μm by age 40
  • Consists of two laminae: anterior banded layer secreted during embryonic development, posterior nonbanded layer secreted by endothelium throughout life
  • Exhibits elastic property, will curl into anterior chamber if torn
  • Very resistant to trauma, proteolytic enzymes, and some pathologic conditions
• Cornea


  • Scatters less than 1% of the light that enters it
• Descemet's membrane


  Basement membrane of the endothelium
• Descemet's membrane formation


  1. Produced constantly

  2. Thickens throughout life

  3. Doubles by age 40 years

  4. 5μm thick in children

  5. Increases to approximately 15μm over a lifetime
• Descemet's membrane


  • Consists of two laminae
  • Anterior lamina is a banded latticework of collagen fibrils
  • Posterior lamina is nonbanded and homogeneous
  • Exhibits an elastic property
  • Very resistant to trauma, proteolytic enzymes, and some pathologic conditions
  • Can be regenerated if damaged
• Schwalbe's line


  A thickened area of collagenous connective tissue at the termination of Descemet's membrane in the limbus
• The method of attachment between Descemet's membrane and the neighboring layers is poorly understood
• The adhesions between Descemet's membrane and the endothelium are not the typical hemidesmosomes but some variation
• Endothelial cells


  • Polyhedral: five-sided and seven sided cells can be found in normal cornea, but 70% to 80% are hexagonal
  • Arranged in a regular endothelial mosaic
  • Extensive interdigitations join the lateral walls
  • Gap junctions provide intercellular communication
  • Tight junctional complexes located near the cell apex
• Endothelium


  The innermost layer of the cornea, adjacent to the anterior chamber, composed of a single layer of flattened cells
• Endothelial cells are 5μm thick
• Endothelial cell density decreases normally with aging from 60,000 cells/mm2 in newborns to 2,400-3,000 cells/mm2 at age 80 years
• Endothelial cell function


  • Allows the entrance of nutrients from the aqueous humor
  • Moves excess water out of the cornea to maintain proper hydration
  • Metabolic pump mechanisms move ions across cell membranes to balance fluid movement
• Endothelial cells do not divide and replicate
• Endothelial cells possess proliferative capacity but are in an arrested phase in the cell cycle
• The minimum cell density necessary for adequate endothelial function is in the range of 400 to 700 cells/mm2
• Disruptions to the endothelial mosaic

  • Endothelial cell loss
  • Increase in cell shape variability (pleomorphism)
  • Increase in cell size variability (polymegathism)
• Excessive loss of endothelial cells can disrupt the intercellular junctions and allow excess aqueous to flow into the stroma, which the endothelial pumps may be unable to compensate for
• Corneal hydration


  Maintained by factors including: epithelial surface cells restricting fluid entry from tear film, pumps in epithelial and endothelial plasma membranes, and water channels called aquaporins in the endothelial cell membrane
• The cornea is 80% water and 20% solid
• The solid portion of the cornea is 19.8% organic matter and 0.2% inorganic salts
• The corneal epithelium is 70% water, with proteins, lipids, enzymes, and electrolytes making up the remaining 30%
• The corneal stroma is 75-80% water, with the remainder being collagen, proteins, and salts
• Descemet's membrane is 73% collagen
• The corneal endothelium contains enzymes
• Corneal metabolism


  • Oxygen is obtained from the precorneal tear film and aqueous humor
  • Mean total corneal oxygen consumption is 9.5 ml O2/cm2/hr
  • Glucose and amino acids are obtained for cell metabolism and protein synthesis
• Factors affecting corneal transparency

  • Epithelium and tear film
  • Arrangement of stromal lamellae
  • Corneal avascularity
  • Corneal hydration