Cornea Lecture Notes Review

Cornea

• Transparent, avascular structure forming the anterior 1/6th of the eyeball.

• Functions:

  • Protection: Part of the tough outer coat of the eye.

  • Optical: Provides approximately 2/3rds of the eye’s optical power.

Gross Anatomy

• Average Dimensions:

  • Diameter (white to white):

    • Horizontal: 11.7 mm

    • Vertical: 10.6 mm (Oyster, 1999)

Corneal Dimensions

• Diameter

• Radius of Curvature:

  • Anterior Central: 7.8mm (approximately 48.2 D)

  • Aspheric – flatter in the periphery

  • In young adult subjects, commonly vertical meridian steeper than horizontal (With-The-Rule or WTR astigmatism).

  • In older adult subjects, commonly horizontal meridian steeper than vertical (Against-The-Rule or ATR astigmatism).

  • Posterior Central: 6.5 mm (approximately -6.2 D)

• Thickness:

  • Central: 0.536 mm (0.47-0.59 mm)

  • Peripheral: 0.67mm (approximately 20% thicker)

Diurnal Variation in Dimensions

• Thickness:

  • Thickest in the early morning.

  • Overnight corneal swelling of approximately 5%.

  • Returns to baseline 1-2 hours after waking.

  • Relatively stable in thickness throughout the day.

• Curvature:

  • Anterior curvature flattest in the early morning (approximately 0.03 mm).

  • Posterior curvature steeper in the early morning (approximately 0.03 mm) (Read and Collins, 2009).

Variations in Normal Shape and Thickness

• Keratoconus

• Pellucid marginal degeneration.

Microscopic Anatomy

• Epithelium

• Anterior limiting lamina (Bowman’s layer)

• Stroma

• Pre-Descemet’s Layer (Dua’s Layer)

• Posterior limiting lamina (Descemet’s membrane)

• Endothelium (Remington, 2000; Pipe and Rapley, 1997)

Epithelium Details

• Anterior cell layer

• 50-60 μm thickness

• Stratified non-keratinized

• 5-6 layers thick (thicker at limbus ~ 10 cells)

• Forms a permeability barrier to water and ions and pathogens (Efron, 2002)

• Three distinct cell types:

  • Squamous cells

  • Wing cells

  • Basal (columnar) cells

• Basement membrane

Cell Types

• Squamous Cells:

  • Superficial cell layers

  • 2-3 layers of flattened cells

  • Barrier function

  • Tight junctions (Zonula occludens) near the apical surface

  • Interaction with tears

  • Microvilli, Microplicae (Oyster, 1999)

• Wing Cells:

  • Intermediate, 2-3 layers of cells

  • Irregular shape, concave inferior surface with lateral extensions (wings) (Oyster, 1999)

• Basal Cells:

  • Innermost, single layer columnar cells

  • Germinative layer of the cornea

  • Cells exhibit mitotic activity

  • New cells produced by mitosis transform from columnar to squamous as they migrate from the basal layer to the surface (surface cells eventually shed into tear film, aided by blinking) (Oyster, 1999)

Cell Junctions

• Tight Junctions:

  • Between adjoining squamous cells

  • Barrier to intercellular movement of substances

• Interdigitations:

  • Prominent infoldings with adjacent cells

  • Provides strong intercellular adhesions (Oyster, 1999)

• Desmosomes:

  • Adhesion between adjacent cells

• Hemidesmosomes:

  • Attachment of basal cells to underlying basement membrane and anterior stroma

• Gap junctions:

  • Allow for intercellular metabolic coupling

  • Fine channels between cells allow the passage of ions (Oyster, 1999)

Basement Membrane

• Basal lamina of epithelium synthesized and secreted by basal epithelial cells

• Thickness 0.5 – 1.0 μm

• Anterior clear zone (lamina lucida), posterior darker zone (lamina densa) (Hart 1992)

Complex Adhesion System

• Hemidesmosomes link basal cells via a series of anchoring fibrils to anchoring plaques in the anterior stroma (Efron, 2002)

Anterior Limiting Lamina (Bowman’s Layer)

• Thickness 8-14 μm

• Acellular region composed of randomly oriented fine collagen fibrils

• Formed and maintained by epithelial cells (Pipe and Rapley 1997; Oyster, 1999)

Stroma

• 90% of corneal thickness (~ 500μm)

• Composed of:

  • Collagen

  • Ground substance

  • Proteoglycans

  • Keratocytes

Stromal Collagen

• Collagen fibrils arranged in 250-300 layers (lamellae)

• Lamellae run parallel to the corneal surface and extend from limbus to limbus

• Within each lamella, fibrils are parallel with uniform size and spacing

• Adjacent lamellae tend to be perpendicular to each other

• Horizontal and vertical preferred orientation

• At the limbus, collagen lamellae take on a circumferential arrangement (Efron, 2002)

Proteoglycans

• Stromal collagen surrounded by and embedded in proteoglycan matrix

• Proteoglycan = central core protein + glycosaminoglycans

• Proteoglycans have a strong negative charge and strong affinity for water

• Tendency for proteoglycans to imbibe water into the cornea

• Water + proteoglycan → gel surrounding collagen fibrils (Oyster, 1999)

Keratocytes

• Flattened fibroblast cells

• Role: maintaining collagen fibrils and proteoglycans

• Lie between stromal lamellae

• The density of cells is greatest in the superficial periphery (Oyster, 1999)

Pre-Descemet’s Layer (Dua’s Layer)

• Region of the posterior stroma adjacent to Descemet’s Membrane

• Recently characterized and suggested as a new layer of the cornea

• ~10 µm thick, acellular, tough layer, consisting primarily of collagen fibrils (5-8 lamellae)

• May play an important role in the biomechanical strength of the posterior cornea (Dua et al 2013)

Descemet’s Membrane

• Highly elastic, basement membrane for endothelium

• Secreted by endothelial cells

• Appears as a thin, clear band adjacent to the endothelium

• Thickness increases throughout life

  • Birth: 3-4 μm

  • Adult: 10-12μm (Efron, 2002; Oyster, 1999)

Endothelium

• Mono-layer of squamous cells lining the posterior cornea

• Roughly cuboidal in cross-section

• Appear as a mosaic of polygonal cells

• Although endothelial stem cells have been identified, the endothelium has limited mitotic ability

• Active role in maintaining corneal hydration (Efron, 2002)

• At birth, cell density up to 4000 cells/mm^2

• Gradual decrease in cell density throughout life

• Remaining cells slide and expand to maintain continuous mosaic

• Endothelial mosaic becomes less regular:

  • Variation in cell size (polymegathism)

  • Variation in cell shape (pleomorphism) (Oyster, 1999)

• Age-related local thickenings in Descemet's membrane (Corneal Guttatta)

Corneal Innervation

• Richly innervated by sensory nerve fibres

• Long ciliary nerves → Nasociliary nerve →Ophthalmic nerve →Trigeminal nerve

• 50-80 pre-corneal nerve trunks enter the mid-stroma and move anteriorly

• Myelination is lost quickly (transparency)

• The degree of overlap of nerve fibres means poor localization of sensation (Oyster, 1999; Adler’s, 2011)

• Axons pass through Bowman’s (subepithelial plexus) and divide into fine branches between basal epithelial cells

• The majority of nerves terminate in the epithelium close to the surface

• ~325,000 nerve endings in the epithelium (Oyster, 1999)

• Avascular

• Anti-angiogenic factors present in the corneal epithelium prevent blood vessels from growing onto the cornea

Limbus

• Transition between cornea and sclera

• 1.5-2.0 mm width annular zone

• Corneal epithelium is continuous with bulbar conjunctival epithelium

• Corneal stroma continuous with scleral stroma

• The posterior termination of endothelium/Descemet’s membrane is called “Schwalbe’s Line” (Remington, 2000)

Limbus – Palisades of Vogt

• Radially oriented ridges of epithelium with fibrovascular papillae between them

• Concentrated in the superior and inferior limbus

• Location of limbal stem cells (Efron, 2002; Pipe and Rapley 1997)

Epithelial Cell Replacement

• Rapid turnover of epithelial cells

• Basal cells divide, migrate anteriorly, differentiating into wing cells then superficial cells. Superficial cells then shed into tear film

• Epithelium replaces itself ~ every 10 days

• Renewal of cells supplemented by continued slow migration of new basal cells from periphery to centre

• Parent cells = stem cells @ Palisades of Vogt (Oyster, 1999)

Functions of Layers

• Epithelium:

  • Mechanical barrier

  • Optical surface

  • Barrier to diffusion of H2O and drugs

• Anterior limiting lamina, stroma:

  • Protection/mechanical strength

  • Shape

  • Transparency

• Posterior limiting lamina, endothelium:

  • Barrier

  • Metabolic role (hydration)

Tissue Mechanics

• Tensile strength:

  • High tensile strength results primarily from corneal stroma

• Extensibility:

  • The cornea exhibits non-linear elastic and visco-elastic properties

• Shear strength:

  • The cornea has low shear strength (Dupps and Wilson, 2006)

Clinical Measures of Corneal Biomechanics

• Corneal hysteresis (CH)

  • Measured with ocular response analyser (ORA)

  • Difference between inward and outward applanation pressures

  • Decreased CH in biomechanically compromised corneas (e.g., keratoconus, post-refractive surgery)

Regulation of Stromal Hydration

• Normal cornea is 78% H2O

• Hydration control is important for maintaining corneal thickness and transparency

• Stromal swelling pressure

• Epithelial and endothelial barriers

• Endothelial pump

• Evaporation

• Intraocular pressure (Arffa, 1997)

Stromal Swelling Pressure

• Proteoglycan ground substance has a tendency to draw water into the stroma → Stromal swelling pressure

• Primarily due to GAGs (keratan sulfate, chondroiton sulfate) (Waring et al, 1982)

Epithelial and Endothelial Barriers

• Counteract tendency of stroma to absorb fluid

• The epithelial barrier primarily comes from tight junctions between surface cells

• The endothelium is a less effective barrier (leaky) (Waring et al, 1982)

Endothelial Pump

• Enzymes in the lateral plasma membrane catalyse movement of ions from the stroma to the aqueous → Osmotic gradient draws water out of the stroma

• Active transport of HCO3- ions (also Na+, K+)

• Temperature reversal

• Metabolic poisons (inhibit enzymes) (Waring et al, 1982; Phillips and Speedwell, 1997)

Pump-Leak Model

• Stromal hydration maintained if: Pump rate = leak rate (Efron, 2002; Waring et al, 1982)

Evaporation

• From tear film → hypertonicity of tears draws water from cornea (epithelium) (Arffa, 1997)

Intraocular Pressure

• Typically only influences hydration at very high or very low IOP (Arffa, 1997)

Thickness/Hydration Relationship

• Linear relationship between thickness and corneal hydration

• ↑ Hydration → ↑ Thickness

• Hydration of the cornea depends on its metabolic integrity

• Monitoring changes in corneal thickness is a way of assessing the corneas' metabolic integrity

Corneal Metabolism

• Pathways:

  • Energy provided by breakdown of carbohydrates

  • Three metabolic pathways:

    • Anaerobic glycolysis

    • Tricarboxcylic acid (TCA) cycle (Krebs cycle)

    • Hexose monophosphate shunt (Efron, 2002)

• Oxygen:

  • Corneal metabolism is dependent on O2

  • Source: Atmosphere, Aqueous, Limbal vessels

    • Open eye: O2 from the atmosphere

    • Closed eye: O2 from palpebral conjunctival vessels, aqueous humour

• If oxygen levels are reduced, it leads to corneal swelling

• The minimum oxygen requirement to prevent corneal swelling is ~10%

• Corneal swelling → stromal haze/reduced corneal transparency

• Hypoxia can also alter endothelial cell morphology → endothelial blebs (Holden et al 1984)

• Supply of nutrients:

  • Primarily glucose

  • Glucose concentrations:

    • Tears: 2 mg/100 ml

    • Aqueous humour: 30 mg/100 ml

    • Plasma: 80 mg/100 ml

• Removal of wastes:

  • CO2 + Lactate

    • CO2 diffuses easily across the epithelium and endothelium

    • Lactate diffuses slowly across the endothelium

    • Hypoxia → Lactate accumulating in the cornea (Efron 2002)

Transparency of the Cornea

• Absorption:

  • The cornea transmits ~ 90% of light in the visible spectrum (high transmission)

  • Light <300nm is absorbed

  • Light >1400nm is absorbed

• Light scattering:

  • Lack of BVs and myelinated nerve fibres helps reduce scatter

• Lattice theory (of Maurice):

  • The regular arrangement of collagen fibrils of uniform diameter and spacing leads to corneal transparency

  • Light scattered from one fibril is cancelled by destructive interference (Hart 1992)

• Cellular layers:

  • Transparency due to the relatively uniform refractive index and regular arrangement of cells

  • Epithelial oedema → increased light scatter

Innervation and Sensitivity

• Innervation:

  • Dense innervation

  • Sensory, Reflex (Blink, Lacrimation) and Trophic functions

• Sensations:

  • Mechanical stimuli usually evoke pain

  • Thermal (non-noxious cold, noxious heat) and chemical sensory channels thought to exist (Phillips and Speedwell, 1997)

• Corneal sensitivity:

  • Measured with an aesthesiometer

  • Sensitivity greatest at the corneal apex

  • Sensitivity reduces:

    • With age

    • Iris colour (dark irides)

    • With contact lens wear

    • Post LASIK

    • Diabetic neuropathy (Efron 2002; Phillips and Speedwell, 1997)

• Regional corneal touch thresholds (mg/mm2):

  • Corneal apex: 20

  • Corneal periphery: 40

  • Conjunctiva (fornix = least sensitive): 70-200

  • Lid margin: >20

• Corneal sensitivity decreases with contact lens wear over time.

Corneal Wound Repair

• Epithelium:

  • Normally undergoes rapid turnover

  • The cornea modulates cellular behaviour in response to injury → rapid repair

  • Damage of cells triggers:

    • Mitosis temporarily shuts down

    • Migration of adjacent cells to cover the damaged area (sheet-like movement)

    • The increased rate of mitosis resulting in landslide-like movement of cells to repair the epithelial defect (Suzuki et al 2003; Hart 1992)

• Stroma:

  • Less regenerative capacity than the epithelium – heals slowly

  • Collagen synthesized to repair damage

  • Newly synthesized collagen lacks the regular arrangement of original collagen → Scarring (loss of transparency) (Oyster, 1999)

• Endothelium:

  • Cells have limited capacity to undergo mitosis

  • Repaired by cell expansion and migration (Oyster, 1999)